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NFB in a PP DHT amp?

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Posted on January 30, 2023 at 03:32:42
zacster
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 My amp is a modified ST-70, using 6B4G in place of the EL34, with the necessary changes, including cathode bias, and a Curcio CCDA driver board. So really not much of an ST-70 anymore.

But the amp still has the wiring in place for the NFB that would be pretty much standard with EL34, but when I did the mods many years back I put it onto an unused post on both sides. When I replaced the sockets on the driver board I got curious about trying NFB in the amp. The driver board has the input and circuitry to handle it, and uses an amount appropriate for the EL34.

Aside from just trying it since it would take about 15 minutes to do the mod with most of that time disconnecting/reconnecting the amp from the system, what do you all think it would do? Would I prefer the sound? Would it lower the gain? Would it really lower distortion?

Of course that's the whole purpose, lower distortion, but I don't find my amp's distortion to be objectionable. I've been reading up on it and most of what I read says that triode based amps don't need NFB, and that with NFB it will lower distortion, until it doesn't. It'll be more like solid state where you have low distortion up until clipping and then it becomes unlistenable, whereas without NFB you get soft(er) clipping.

I guess I could just try, it wouldn't be permanent. Or even run it through a switch, but that's more work.

Thoughts?

 

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RE: NFB in a PP DHT amp?, posted on January 30, 2023 at 06:49:56
PakProtector
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 For the most part I have found the output Z reduction the most important benefit about NFB. Hooking NFB up will reduce delivered gain. Will you like it? That is a fair question... :) Don't have an answer on that one...

Class A loading with PP leaves you half the a-a. This is not that...
You are down to load equals plate Z when one side cuts off. Output Z changes with power output in the no NFB applied case.

Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: NFB in a PP DHT amp?, posted on January 30, 2023 at 07:04:31
Triode_Kingdom
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 "I have found the output Z reduction the most important benefit about NFB."

^^^^ This!

NFB will also increase output power at the onset of distortion at low frequencies. The increase can be significant from a measurement standpoint (50% or more). Whether this is audible in a given system obviously depends on many factors.

 

RE: NFB in a PP DHT amp?, posted on January 30, 2023 at 14:17:59
stellavox
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 Maybe 25 years ago I happened to build up a few 6B4G amps on 70 chasses like you did. Made up my own driver pretty similar to Curcio's circuit. Used it for Years - made 5 others and sold them mostly to local friends. ABSOLUTELY FINE sound. You won't believe this but they did a nice job driving my ESL-63's for many years.

Did experiment with NFB. A few dB, maybe 5, REALLY tightened up the bass. However, try as I might I couldn't live with the resulting 60/20hz hum. Tried all kinds of re-laying / re-configuring the wiring. Gave up and left them alone.

Charles

 

RE: NFB in a PP DHT amp?, posted on January 30, 2023 at 14:37:12
zacster
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 I guess I'll just have to try it. When I did the tube sockets a few months back one result was that the hum disappeared. I'd hate to bring it back.

 

I'll just have to give it a try (nt), posted on January 30, 2023 at 14:37:47
zacster
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 .

 

RE: NFB in a PP DHT amp?, posted on January 30, 2023 at 15:05:43
Tre'
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 Each tube needs its own filament transformer (or winding). One tube of each pair can just use resistors to create the center tap for the cathode connection while the other tube of each pair gets a hum pot. With the filament transformers in phase, as you adjust the hum pot so that the hum contribution is equal (and in phase) for each of those two tubes the hum will cancel across the output transformer. CMR

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: NFB in a PP DHT amp?, posted on January 30, 2023 at 17:27:29
zacster
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 These tubes share a winding with the cathode resistor on the center tap. I used 4 2k2 resistors on each side, 12w each, arranged away from each other for maximum heat dissipation. It seems to work pretty well. I tried a 560R gold finned resistor pasted to the chassis and it overheated. This was all 10-15 years ago when I first put it together. You may have helped me on this.

 

RE: NFB in a PP DHT amp?, posted on January 30, 2023 at 18:02:20
Tre'
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 "These tubes share a winding with the cathode resistor on the center tap"
and that is a fine way to do it but it leaves you with no way to null the filament hum. I modified a pair of Heathkit w-5m amplifiers to use 6b4gs and also have both tubes (per amp) running off the same filament winding. I have a bunch of 6b4gs and just swap them around until the hum is acceptable.
I am thinking about changing the output transformers in those amps anyway so, I think I will get a couple of 6.3 volt filament transformer and start practicing what I am preaching.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: NFB in a PP DHT amp?, posted on January 31, 2023 at 05:26:16
stellavox
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 Tre et al,

I had no trouble using the stock 70 transformer. Used a choke input filter to lower the B+ for the 6B4's.

By switching/selecting outputs, was able to get the hum level down to very low levels (only heard right up next to the speaker) - same results for 6 different amps;

Schematic attached

 

RE: NFB in a PP DHT amp?, posted on January 31, 2023 at 07:06:32
Tre'
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 Where did you find space for a critical inductance input choke?

Swapping output tubes around is what I also did to get the hum down.


Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: on my rust-bucket MKII's I just bolted the choke on the side of the chassis, posted on January 31, 2023 at 08:33:45
Oldbean2
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 I figured I couldn't make them look any worse. And piece of aluminum where the PCB was, I use a single 417A driver and autoformer to split phase. To add to the ugliness, they're not even matched chassis, one is the old style with the larger board.

I should rebuild them someday in a more visually pleasing way, but they just sound so good...

Back for a bit again. Ignore me if you like.

 

RE: NFB in a PP DHT amp?, posted on January 31, 2023 at 15:36:30
stellavox
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 Put choke inside the chassis - used the "stock" 70 choke - worked fine

 

RE: NFB in a PP DHT amp?, posted on February 1, 2023 at 06:45:11
Tre'
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 That is not an input choke and its Henry value is no where near critical inductance. It would be more of a hash choke and can cause more problems than it solves.

A input choke is intended to have a "critical inductance Henry value", that is "the Henry value that causes each diode to stay on for its full half cycle". If the choke doesn't meet that requirement then the filter will not function like a choke input filter and the full benefit will not be realized.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

I have modified a lot of amplifiers over the years but I...., posted on February 1, 2023 at 06:53:46
Tre'
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 only did one dyna 70 mod to take 6b4gs.

I should say two. I took two dyna 70 chassis and made two mono block PP 6b4g amplifiers.

Removing one output transformer from each chassis left room for a critical inductance choke allowing for a proper choke input filter for the power supply.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: NFB in a PP DHT amp?, posted on February 2, 2023 at 04:28:08
stellavox
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 Stock choke worked just fine for my needs. Dropped the B+ to well within the safe limits for the B4's.

 

RE: NFB in a PP DHT amp?, posted on February 2, 2023 at 04:32:13
sony6060
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 As for feedback I built a DIY PP 45 amp with selectable 0dB, 3db, 6dB and 12dB feedback. I like the 3dB feedback most, but not a large difference in sound using any feedback value.

 

RE: NFB in a PP DHT amp?, posted on February 2, 2023 at 08:54:27
Eric Chan
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 If you can make your amp not using NFB, meaning no harsh and grainy sounding, the zero NFB amp has mush faster speed and clarity

 

Speed and Clarity, posted on February 2, 2023 at 09:30:24
Triode_Kingdom
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 "the zero NFB amp has mush faster speed and clarity"

My 211 SETs include about 5dB NFB. Frequency response at -1dB extends to 40kHz. How much faster do they need to be?


 

RE: NFB in a PP DHT amp?, posted on February 3, 2023 at 03:19:20
PakProtector
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 Ummmmm...I have to say that NFB does not need to result in such behaviour. I agree with not doing things to cause the characteristics you describe, and I would love to have you hear some amps with NFB... :) which are most certainly neither grainy or harsh.

They are not DHT either... LOL

cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

Feedback is usually misapplied. You might want to try this, posted on February 3, 2023 at 09:38:07
Ralph
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 Usually feedback in a tube amp is connected to the cathode of the input tube.

The tube isn't linear although it might be 'fairly linear'. As a result the feedback signal gets distorted before it can do its job.

As a result, higher ordered harmonics and IM are generated.

Traditionally the solution has been to 'add more feedback'. That kinda sorta works but you run into limits really quickly, especially with tube electronics which have more frequency poles than solid state.

This is why feedback has gotten a bad rap in high end audio

Assuming the amp has good linearity without any feedback, my suggestion is to do the feedback a different way. Instead of applying it to a tube, use a resistor divider network similar to how opamps use feedback. So there will be a series input resistor and the feedback resistor, which meet at the grid of the input tube.

To do this you might have to invert the phase of the primary winding of the output transformer if its equipped with multiple taps at the output.

Since resistors are far more linear than any tube or semiconductor, the feedback signal arriving at the input will be more accurate and so less of the artifacts I mentioned earlier will be generated.

Of course by adding feedback the amp will have less overall gain. You might have to add some voltage gain to make up for it. That might be accomplished by using a different input tube.

 

RE: Feedback is usually misapplied. You might want to try this, posted on February 3, 2023 at 16:47:56
Triode_Kingdom
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 "Usually feedback in a tube amp is connected to the cathode of the input tube. The tube isn't linear although it might be 'fairly linear'. As a result the feedback signal gets distorted before it can do its job."

and...

"Instead of applying it to a tube, use a resistor divider network similar to how opamps use feedback. So there will be a series input resistor and the feedback resistor, which meet at the grid of the input tube."

Why would the input tube appear to be more linear when the feedback is applied to its grid than when it's applied to the cathode?


"This is why feedback has gotten a bad rap in high end audio"

I think it's gotten a bad rap primarily due to SET owners who prefer the "richer" 2nd harmonic sound achievable with Class A. Far fewer owners of pentode push-pull amplifiers believe NFB is categorically bad, especially if they've ever compared the resulting distortion numbers.



 

RE: Speed and Clarity, posted on February 4, 2023 at 21:13:46
hahax@verizon.net
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 But SET amps are asymmetrical, they amplify the bottom half of the signal differently than the top half which is a main source of high second harmonics characteristic of their sound.

 

RE: NFB in a PP DHT amp?, posted on February 4, 2023 at 21:17:12
hahax@verizon.net
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 My one experience with reducing NFB was done by David Berning decades ago with a PP triode tube output amp. As NFB was reduced in steps from 15 dB to 0 dB the sound became more alive, more open and more dynamic and I preferred the 0 dB setting best.

 

RE: Speed and Clarity, posted on February 4, 2023 at 23:24:30
Triode_Kingdom
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 "But SET amps are asymmetrical"

How does that relate to the claim that NFB degrades speed and clarity?


 

RE: Speed and Clarity, posted on February 5, 2023 at 22:01:36
hahax@verizon.net
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 It means they are inherently non linear and That's a bad starting point. For what it's worth I once had a great demo, albeit for a PP triode amp with variable NFB and dynamics and aliveness improved with each reduction in NFB.

 

RE: Speed and Clarity, posted on February 6, 2023 at 08:21:20
zacster
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 Hmmm. Since I'm starting without NFB, maybe it'll be obvious I won't like it when I add it. Sometimes changes take all the life out of things. I tried EL34 in triode mode before I modded this amp to run 6B4G and I immediately switched it back. Triode mode sounded totally lifeless to me and I couldn't understand why anybody would use it. And yet DHTs in the same amp sound great. I won't know about NFB until I try it.

 

Why would the input tube appear to be more linear, posted on February 6, 2023 at 09:15:51
Ralph
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 Why would the input tube appear to be more linear when the feedback is applied to its grid than when it's applied to the cathode?

It won't.

But the feedback signal won't be mixed in the tube as per usual. Instead its mixed outside the tube where it will be distorted far less by means of a simple resistor divider network.

 

RE: Speed and Clarity, posted on February 6, 2023 at 09:25:10
Eric Chan
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 DHT is the most linear device ever invented by human being, even up to nowadays

Amp without NFB is fastest response and highest clarify as long as you can control the high order odd harmonic distortion

Furthermore single ended DHT zero feedback amps have best coherence, clarity, transient

That is why people like SET

 

Operating point, posted on February 6, 2023 at 09:32:59
fetko
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 What B+ voltage and current are you guys running the 6B4Gs at in the stereo 70? thanks.

 

RE: Speed and Clarity, posted on February 6, 2023 at 20:28:28
Triode_Kingdom
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 "Amp without NFB is fastest response and highest clarify"

Western Electric didn't think so. But like anything else, it has to be done "right."



 

RE: Why would the input tube appear to be more linear, posted on February 6, 2023 at 20:35:51
Triode_Kingdom
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 Just to be clear, you're saying that for a given amount of NFB (dB), the resistive mixing ahead of the grid produces improved IM and less upper harmonic energy compared to injection at the cathode. Is that correct? Have you made measurements of this effect or examined SPICE sims? It should be relatively straightforward to design this into amplifiers if it's indeed effective.





 

RE: Feedback is usually misapplied. You might want to try this, posted on February 6, 2023 at 22:05:33
Eric Chan
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 Ralph,

This idea of applying NFB seems better, like a single stage tube applying feedback from its plate going back to its grid.

My experience is , apart from local feedback (degeneration, cathode resistor or partial cathode resistor not bypassed)，any kind of loop negative feedback slow down and muddy the amp. Loop NFB definitely impact the clarity and transient response of an amp.

 

RE: NFB in a PP DHT amp?, posted on February 6, 2023 at 23:38:38
Eric Chan
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 May be my statement above was incorrectly interpreted.

I was saying if you can build an amp not using loop NFB without sounding harsh and grainy, then this non NFB amp can be more alive, fast and transparent.

While you employ loop NFB also at this amp. The sound will become muddy, restricted, slow, and dull. Loop NFB improve the technical aspects but impact the sound.

 

RE: Speed and Clarity, posted on February 7, 2023 at 00:05:52
Eric Chan
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 The reduction in speed and clarity mainly come from the processing when the output signal feeding back at 180 degree out phase to previous stage of application device.

The output signal produced by an amp is after processed amplification but this output signal has to offset the signal just newly arrived in the amp input terminal.

Charles Hensen once said, Using past signal to offset the current signal as to reduce the distortion, it does not exist in real world nature.

 

RE: Speed and Clarity, posted on February 7, 2023 at 00:16:15
Eric Chan
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 More precise speaking

Charles Hensen once said, Using past signal to compare the current signal as to reduce and offset the distortion, it does not exist in real world nature.

 

RE: Speed and Clarity, posted on February 7, 2023 at 00:25:38
Eric Chan
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 Except for degeneration or local feedback such as cathode resistor unsurpassed

The degeneration of a tube , the amplification and distortion offset are taking place at the same time. So this kind of feedback does not cause dynamic compression or muddy up the sound

Highest grade amplification does not use loop feedback

 

RE: NFB in a PP DHT amp?, posted on February 7, 2023 at 02:26:09
Eric Chan
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 Loop NFB help the amp to have lower output Z, this for sure

Without using loop NFB, we can use higher ratio output transformers to achieve. Say 845 originally used 6K OPT and now you switch to 11K OPT.more significant tighten bass will be.

For SE domain I always heard people complaining loose bass in SET. In fact loose bass can be tighten up by using higher ratio OPT and the more powerful, more linear, lower impedance driving tube to achieve


We build SE amp because of its simplicity not using split phase mechanism to get purer and more coherence sound

When simplicity get to simplest, we also want the SE to amplify the sound without restriction so no loop NFB is the key.

As little as 3dB loop NFB applied, the dynamic is significantly reduced and the transparency is impacted


 

RE: Operating point, posted on February 7, 2023 at 05:56:47
zacster
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 I have 390 at the plate and 75 at the cathode with 550R shared resistors, so 315 volts. I'm using Sovtek 6B4G though, a NOS tube probably couldn't run that hot. And the Sovtek are single plate where NOS were dual triodes internally strapped together. I've always wondered what USSR tube these actually are that were just rebranded as 6B4G. The 6A3 is the same Sovtek tube with a 4 pin base, and the 2A3 is the same with a 2.5v filament.

 

RE: Feedback is usually misapplied. You might want to try this, posted on February 7, 2023 at 07:20:44
Paul Joppa
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 Plate to grid feedback is also local, and is an example of what Ralph is suggesting.

 

RE: Feedback is usually misapplied. You might want to try this, posted on February 7, 2023 at 09:04:48
Eric Chan
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 Plate to grid feedback has time delay causing the same negative effect of loop NFB. The output signal which means the past signal used to compare the newly arrived current signal at input terminal as to offset the distortion. Even though the loop NFB is supposed to take place very fast. The past signal pattern is not the same as current input signal.

Any loop feedback has one unique output terminal and one input terminal, for instance in one tube the plate output terminal feedback to grid input terminal.

In cathode resistor unbypassed, there is no difference for input terminal and output terminal

Only the cathode resistor unbypassed does not cause the dynamic compression and impacting the clarity. It is because amplification and the distortion reduction and also the flattening freq response processes, they all take place at the same time in cathode resistor unbypassed. This is the only feedback mechanism I am willing to use.

 

RE: Operating point, posted on February 7, 2023 at 11:13:26
fetko
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 Thanks, tells me I'm roughly on the right track with the load lines I graphed. I've got only 2 NOS 6B4Gs at the moment, but a bunch of 6AV5GA, which are said to be a good approximation in triode connection. Different constructions have been reported to be all over the place in terms of plate dissipation and screen voltage handling though.

 

common misconception, posted on February 7, 2023 at 12:20:27
Ralph
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 Plate to grid feedback has time delay causing the same negative effect of loop NFB.

This statement is incorrect. What there is, is phase shift, which if looked at a particular frequency will look like a time delay. What is happening is the circuit is performing as filter theory suggests.

If the feedback is effective, it will suppress the 2nd and 3rd, which, depending on their phase, will alter how the perceived 'dynamics' of the circuit as well as the perceived soundstage (which might otherwise seem deeper).

On its face it seems like no feedback would be better in this situation. But is the object to reproduce the signal properly or add an effect?

 

Norman Crowhurst, posted on February 7, 2023 at 12:31:41
Ralph
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 wrote about this 60 years ago. You might want to read his comments on this, buried somewhere I can't recall, but probably the tome is on Pete Millet's website, link below.

Now Crowhurst did not say that mixing the feedback ahead of the circuit would improve things; what he said is that non-linearities at the feedback node contribute to harmonic, inharmonic and IM distortion which becomes a sort of noise floor of any amp using feedback in this manner.

So this is more a logic thing; if you linearize the feedback node you'll get less of these effects.

We can see this in opamps, where there are lots of transistors involved; the feedback is mixed ahead of the opamp itself. Opamps can be quite linear and very neutral; we hear them all the time in any modern recording. Of course they are often using quite a lot more feedback than most tube amps.

We've been using this technique of feedback in our amps for decades.

 

RE: Norman Crowhurst, posted on February 7, 2023 at 22:51:48
Triode_Kingdom
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 "if you linearize the feedback node you'll get less of these effects."

I don't disagree with that, but it's not clear to me that this statement is sufficient to validate a claim to a significant benefit. If you've been using this technique for decades, surely you've made comparative measurements at some point?


 

RE: NFB in a PP DHT amp?, posted on February 8, 2023 at 02:41:09
cellailuca
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 I built the same PP amplifier with and without NFB.
I prefer the NFB unit, a friend of mine prefers the unit without.


Omnes feriunt, ultima necat.

 

That is about it... :), posted on February 8, 2023 at 03:25:57
PakProtector
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 And of course go a step along this path and use the heater winding of the now-missing pair of EL34 and run a pair of damper diodes...

Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

You should try it, posted on February 8, 2023 at 10:11:58
Ralph
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 I don't disagree with that, but it's not clear to me that this statement is sufficient to validate a claim to a significant benefit.

We've always done it that way since there is no means of feeding feedback into the cathodes of our amps; they are fully differential.

I can't help but see your comment as contradicting itself to some extent. You agree that using a less distorted feedback signal is good but can't be sure that there's a benefit (trying not to commit a strawman here...)? Is my rewrite accurate?

 

dynamics and aliveness, posted on February 8, 2023 at 10:56:19
Ralph
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 Over time I've become convinced that when audiophiles use the term 'dynamics' they really mean 'distortion' (and in particular, higher ordered harmonic distortion). Effectively in 95% of audiophile conversation you can change out the word 'dynamics' for distortion without changing the meaning of the conversation.

When you have less higher ordered distortion (to which the ear is tuned to sense sound pressure) the presentation usually becomes less 'loud' and less 'dynamic'. The actual dynamic contrast should come from the recording, not the electronics. But people like dynamics and so electronics that produce masked higher ordered harmonics (such as a zero feedback tube amp) have a following.

When you realize its distortion messing with your physiology, it sort of wrecks it...

 

RE: You should try it, posted on February 8, 2023 at 16:20:18
Triode_Kingdom
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 The key word was "significant." It's just not clear to me where this falls in the big picture. It might be night and day, or it might be something none of us can hear. Point noted that your amps require it. I will indeed try this someday, but I've got too many projects in the hopper at the moment.



 

RE: Why would the input tube appear to be more linear, posted on February 8, 2023 at 18:01:55
Tre'
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 I am confused by "mixed in the tube"?

Of course the tube is non-linear, if it were linear then there would be no need for feedback to lower the distortion.

A little of the distorted output fed back into the input of the tube but out of phase so the circuit becomes self correcting. Why would returning the feedback signal to the cathode be any different than returning the feedback signal to the grid? (apart from phase)

Tre'

Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Why would the input tube appear to be more linear, posted on February 9, 2023 at 09:41:51
Ralph
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 I am confused by "mixed in the tube"?

Of course the tube is non-linear, if it were linear then there would be no need for feedback to lower the distortion.

A little of the distorted output fed back into the input of the tube but out of phase so the circuit becomes self correcting. Why would returning the feedback signal to the cathode be any different than returning the feedback signal to the grid? (apart from phase)
If returned to the grid, the feedback signal is only distorted by the feedback network itself, which is much more linear than any active device. Most of the time this might be a simple resistor.

But when returned to the cathode, the linearity (or lack of it) in the tube is now messing with the feedback signal, which, in order to do its job, simply has to be correct. Distorting it does not make sense- now its telling the amp to do something that isn't intended. Of course, the more feedback you use allows you to overcome this problem (Baxandall) to some extent. But then you run into limits associated with phase margin (tube amps tend to have lots of frequency poles) and if you run out of Gain Bandwidth Product, rising distortion with frequency (which is bad, something that most amps with feedback do BTW...I think this is part of why feedback has a bad rap in many corners of high end audio; zero feedback amps do not have risgin distortion with frequency).

Using a distorted feedback signal you get higher ordered harmonic distortion which might not be masked by lower orders. IMO/IME if that is so then the amp will sound harsher and brighter than the input signal.

 

In a conventional tradition tube amp this would be easy to do. , posted on February 9, 2023 at 09:46:10
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 If any of those projects involve tube amps, it would only take a few minutes to switch from one technique to the other.

 

RE: In a conventional tradition tube amp this would be easy to do. , posted on February 9, 2023 at 12:36:03
Triode_Kingdom
Audiophile

Posts: 10047
Location: Central Texas
Joined: September 24, 2006
 I currently have an opamp-based distortion test adapter breadboarded on my bench pending final construction. It will allow me to measure distortion in high-Z stages with large pk-pk voltage swings. After that, I need to finalize some minor machining and build side panels for my recently completed (nearly) 211 SETs. Then I need to rework the grounding on my Citation V. Once that's done, I can pull my capacitorless-output preamps out of mothballs and finish that project. If I'm still on the planet after all this, I might have time for a few "curiosity" projects like the one you're describing. :)


 

:) +1 nt, posted on February 9, 2023 at 13:47:13
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 -

 

RE: Why would the input tube appear to be more linear, posted on February 9, 2023 at 18:09:40
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 Maybe you could draw it for me.

Feedback is just some of the output of a circuit returned to the input of that circuit. Of course it's distorted, that is why one is applying it back to the input, to correct the distortion of the circuit as a whole.

You keep saying that the "tube is distorted". Yes, it is. You keep saying that returning a portion of the output to the cathode of the first stage will some how distort that portion of the output even more vs. returning it you the grid of the first stage but you haven't explained why you believe that.

Again, please draw it for me both ways so I can understand what you are saying, thanks.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

distorted signal, posted on February 10, 2023 at 10:32:52
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 When feedback is taken from the output, of course the assumption is that the output signal is distorted compared to the original.

I think where you might misunderstand what I've been saying here is that once you take that signal and feed it back, its really important to not distort it further. The feedback signal must have its integrity if it is to do its job correctly.

When you feed it into the cathode of the input tube and the input signal is coming in on the grid, the non-linearity of the tube distorts the feedback signal as it mixes in the tube. Higher ordered harmonics are generated as well as IMD.

If OTOH you mix the undistorted feedback signal with the input signal using a resistive divider network sans any active devices, the feedback signal will encounter far less distortion and you will have a more accurate corrected signal compensating for the distortion of the amplifier.

As I mentioned earlier, Norman Crowhurst wrote about this issue over 60 years ago, but IIRC he didn't offer an alternative. I'm simply pointing out that there is one. I assume that the resistive divider network method will introduce distortion of its own, but its hard to imagine it will add as much as a tube or transistor! I've yet to see any active device as linear as a resistor...

Do you still need to see diagrams?

 

RE: distorted signal, posted on February 10, 2023 at 14:18:50
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
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 No. I now understand what you are thinking.

I don't think you are correct. The feedback signal will see the distortion of the first stage either way.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Of course!, posted on February 10, 2023 at 14:31:49
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 But it will be more accurately modified to correct for the non-linearity.

 

RE: Of course!, posted on February 10, 2023 at 15:33:14
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 I don't see how and I don't understand why you haven't explained it.

Returning the feedback to the cathode of the first tube is mixing that feedback signal with the input using a resistive voltage divider. The feedback resistor with is the series resistor and the cathode resistor to ground is the shunt.

Why is that different than a feedback series resistor feeding the grid with a grid resistor to ground?

What makes one better (less extra distortion) than the other?

When answering think about how degenerative cathode feedback works. I have never heard anyone argue that cathode feedback (local current feedback) is distorted feedback. Quite the opposite. Most people think of local current feedback as feedback in it's purest form.

Edit, I should have added that voltage feedback connected to the cathode enters the circuit in the same way as local current feedback does but it is not local current feedback.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

have you ever seen this amp, posted on February 10, 2023 at 19:42:54
elblanco
Audiophile

Posts: 3486
Joined: August 20, 2004
 page 15

funky efp60 tube used

crazy specs

 

RE: distorted signal, posted on February 12, 2023 at 00:58:10
tube wrangler
Manufacturer

Posts: 2484
Location: USA
Joined: January 29, 2007
 Hi, Tre!

You are right to question all this.

This is all very simple-- adding onto or into a signal--
ANY time-delayed artifact simply smears music.

That negates any and all forms of NFB for music, period,
regardless of measurements or theories... making you very right
to ask questions.

Any non-linearity of an amplifying device, if not totally and
grossly severe, has VERY LITTLE effect on its ability to
handle actual music, unless the device is really pushed hard.

Arguers spending hours discussing all this should try some
Common-Sense:

If Bob says something to TED, and then we splice-in Ted's
response to Bob onto Bob's original statement, do we or do we
not now have a distortion of what Bob said in the
first place?

The best to all of you-- I'm always thinking of you all.

-Dennis-

 

Local feedback, posted on February 13, 2023 at 10:36:18
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 Why is that different than a feedback series resistor feeding the grid with a grid resistor to ground?

Its not that different other than you'd have only one feedback loop for the entire amplifier circuit.

 

RE: Local feedback, posted on February 13, 2023 at 18:26:32
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 Why are you now talking about "only having one feedback loop...."?

That doesn't have anything to do with my question.

Can we please stay on point?

My point is, if local current feedback can enter the circuit at the cathode without causing problems (and I think the evidence is that it can and does) then why can't loop feedback?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

On point, posted on February 14, 2023 at 08:53:52
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 I've been on point, but I've noticed that we don't seem to be communicating- I'm thinking I've had pretty succinct explanations...

My response had everything to do with your question! To delve deeper, if you are using local feedback with a resistor to the grid as you described, you would need to do that throughout the circuit. What I'm talking about is simply using one feedback loop rather than the number needed if its all local.

My point is, if local current feedback can enter the circuit at the cathode without causing problems (and I think the evidence is that it can and does) then why can't loop feedback?

This does not seem to be what you were asking earlier!

All I've been saying is mix the feedback outside an active device. Plain and simple. If you mix it inside an active device the feedback signal will be distorted by the device. This is not to say that there isn't a benefit, but it is to say the benefit will be diminished, and may cause harm (brighter, harsher) as well due to generation of higher ordered harmonics and IM (per Crowhurst, nothing controversial here).

Why this is important for tube amps is usually you simply can't apply enough feedback, and when you do the amp winds up being brighter and harsher than the input signal for reasons I explained- although I didn't mention the bit about frequency poles which are numerous in tube amps. So the trick seems to be to not damage the feedback signal; that way perhaps you won't need as much and the result will be more pleasant/accurate.

 

here is the famous CrowHertz article, posted on February 14, 2023 at 11:58:59
elblanco
Audiophile

Posts: 3486
Joined: August 20, 2004
 March 1958 Radio Electronics mag

 

Its a good one, but does not mention, posted on February 14, 2023 at 13:19:36
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 the effects of non-linearity at the feedback node.

 

RE: On point, posted on February 15, 2023 at 07:26:56
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 "If you mix it inside an active device"

This is the part I don't understand. Why is returning the feedback to the cathode considered "mixing inside an active device" but returning the feedback to the grid is not?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: On point, posted on February 15, 2023 at 09:15:30
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 Because the mixing occurs within the tube's influence, so is distorted. When you mix at the grid this does not occur.

 

RE: On point, posted on February 15, 2023 at 09:44:55
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002

You have not explained what you mean by "mixing within the tube".

Why is applying signal at the cathode of a tube any different than applying signal at the grid of a tube?

If applying signal at the cathode of a tube is "mixing within the tube" then why is applying signal at the grid of the tube not "mixing within the tube"?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Here's a drawing..., posted on February 15, 2023 at 10:16:01
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002



As you can see, the linearity of the tube distorts the feedback signal when the mixing occurs. When mixed at the grid this does not happen. Of course the signal becomes distorted in the tube, but the feedback signal is correcting for that, where it cannot when it itself has been distorted.

IOW when mixing via the cathode, part of the tube is outside of the feedback loop.

 

RE: Here's a drawing..., posted on February 15, 2023 at 10:25:28
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 Are you saying that the local current feedback signal is mixing with the loop feedback signal and that is what you are concerned with?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Of course!, posted on February 15, 2023 at 11:04:35
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 If you think about it, the local feedback isn't linear.

The feedback signal absolutely must not be distorted before it can do its job! Otherwise distortion will be generated.

 

Then why didn't you say so?, posted on February 15, 2023 at 11:24:19
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 Instead of explaining what you meant by "mixing within the tube" you just kept repeating it like it was some kind of mantra.

Whatever.

I'm glad we got that cleared up.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

:) I did, like several times... nt, posted on February 15, 2023 at 11:31:58
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 -

 

You never said it until I did, posted on February 15, 2023 at 13:33:44
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 Can you show me where you said that the problem was the loop feedback signal was mixing with the local current feedback signal at the cathode of the tube?

You can't because you never did.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

That's correct., posted on February 15, 2023 at 13:51:20
Ralph
Manufacturer

Posts: 4769
Location: Minnesota
Joined: April 24, 2002
 Instead what I said is that mixing via a resistive divider network is more linear than doing inside an active device. In that way I'm able to include transistors, where this problem is even worse.

 

RE: NFB in a PP DHT amp?, posted on March 14, 2023 at 04:30:33
PakProtector
Audiophile

Posts: 12364
Joined: May 14, 2002
 Is more than one way to do NFB. No loop for me.

Now, if we could separate drive signal from NFB signal...
-say, drive the cathode and apply NFB to g1.

Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

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