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Garg0yle 2A3 DC

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Posted on January 19, 2015 at 11:36:40
Garg0yle
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 Now that the honeymoon is over with the BB-Proof 2A3 and have had some time to really listen to it. I have identified a few things I would like to change.

As mentioned in the original BB-proof thread, it didn't quite have the authority or snap of my 6BQ5 amp. (Which is derived from a Alex Kitic RH84 with a beefed up solid state power supply.)
The BB-Proof does however sound better in the mid to upper frequencies.

The bass deficiency and loss of dynamics was somewhat unexpected, if only for the reason that the BB-Proof has huge Hasimoto OPT's while my 6BQ5 amp has small OPT's pulled from a console.
I was hoping that would have somewhat offset lack of capacitance in the BB-Proof.

I say somewhat unexpected, as I did have concerns about the power supply, but I wanted to at least try it out first before being critical.

While the 2A3 has less power then my 6BQ5, It's not really an issue in my application as the 6BQ5 wasn't pushed hard at all with my speakers.
I had considered the different the damping factor to between the two amps as a possible reason, so I was going to re-port my speaker cabinets. After some thought I decided to hold off on that.

I had pondered the etiquette of making changes to a previous design, it appears at least from Alex Kitic's point of view in an unrelated thread I read that he does not wish to have modified versions of his schematics referred to by their original name. This example made sense to me for obvious reasons.

So this amplifier will branch off separately from Paul Joppa's BB-Proof 1.2 variant of a "Monkey on a stick" topology.

Immediate issues are:

-It really needs a stiffer power supply.
-I am debating the value of the Ultrapath arrangement. The problem with the Ultrapath and split capacitor as I get droopy voltage at the top of the 2A3 as well as the cathode. The sag is significant at the cathode, about half as much B+ sag. While at first glance it appears to soften the B+ voltage variation, however both points are undulating up ad down at different rates. I'm not certain to the extent of negative effects on the driver stage at this point, but it can't be good.
-I will see if the plate-choke capacitor on the 6J5 is actually needed once I smooth out the voltages.




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RE: Garg0yle 2A3 DC, posted on January 19, 2015 at 12:17:38
rage
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this is ironic I think.

is there no cap after the choke?

are you using the 5h 135ohm choke?

 

RE: Garg0yle 2A3 DC, posted on January 19, 2015 at 12:27:25
drlowmu
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 The 2A3 is a "direct access to the music" tube. As such, you need to double the capacitance of the FINAL cap, and add another L/C section as in L1/C1/L2/C2. Make the second L at least 10 HY so it stores energy to charge the caps, and 200 Ohms or higher will get you the bass and dynamics you are missing. Have fun !!

Jeff Medwin

 

RE: Garg0yle 2A3 DC, posted on January 19, 2015 at 12:32:41
Garg0yle
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 The chokes I am using are 7.5H and 110 ohms /160ma per channel. Somewhat similar though to the BB-proof, just a little more induction and a little less resistance then the specified Hammond which has a lot more availability.

The drawing takes a little getting used too, but if you look at the other side of the schematic you will see is effectively reservoir capacitors.

That "second" capacitor is split, the top half being the "ultrapath" that terminates at the cathode of the 2A3 above the resistor, as opposed to being terminated to ground below cathode resistor as in more traditional setups. (Like the 6J5)
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RE: Garg0yle 2A3 DC, posted on January 19, 2015 at 12:44:10
rage
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 I'd put another cap after the choke and downsize that ultrapath cap.

Something like 100-300uf after the choke and then the ultra path cap I'd think 40uf would be acceptable.

I'd also eliminate that cap from the cathode to ground.. The 100uf 250v cap.

If you don't like your next round of power supply tweaks maybe you could try one of Jeffs latest creations and give us a report after you've heard it. :)

 

RE: Garg0yle 2A3 DC, posted on January 19, 2015 at 12:52:32
Caucasian Blackplate
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 That's a neat design, somehow I missed it!

My immediate reaction as to where to spend money would be the OPT (which the OP already did) and that plate choke. The MQ-010 would be a fun substitution, and the adjustable bias trim will let you mess with different DCR plate chokes without penalty.

This, of course, goes against the original intent of the schematic, which was almost certainly to provide a relatively easy design to build with parts that aren't super expensive or hard to find.

 

RE: Garg0yle 2A3 DC, posted on January 19, 2015 at 13:14:21
Paul Joppa
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 The capacitor from 2A3 cathode to ground should really be 150uF, since is is sitting at 40% of B+. That would give "matching" voltage fluctuations. I stayed with 100uF so that all the caps would be the same, and none of them too big to make film caps practical. (See the Rev 1.1 diagram.)

If you remove that cap entirely, there will be a big loss in filtering, so a cap from B+ to ground would be needed, I think.

Certainly an added stage of filtering would reduce ripple; I didn't do that in order to maintain the necessary high voltage. Substituting a solid-state rectifier would provide some extra voltage, if that is needed. Cheaper than a new power transformer!

Do you have specs (especially inductance) for the Hashimoto transformer?

If the driver plate choke DC resistance is changed, the 2A3 cathode resistors may need to change to maintain the target voltages and currents.

Since I put this up as a community project, I have no problem with revisions. I think it would help everyone if changes are labeled, just so we can track the effect of variations.

 

RE: Garg0yle 2A3 DC, posted on January 19, 2015 at 13:30:03
Garg0yle
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 Drlowmu

Chokes not only filter residual ripple well, they have directional momentum.
In this application the momentum helps to stabilize the B+ against transient demands.

When the voltage tries to drop, the magnetic field in the choke starts to collapse, releasing energy that opposes the voltage drop.

So while you think of my 110 ohm choke as a resistance, when called upon in action it is not that way at all.


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RE: Garg0yle 2A3 DC, posted on January 19, 2015 at 13:59:27
Garg0yle
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 "The capacitor from 2A3 cathode to ground should really be 150uF, since is is sitting at 40% of B+. That would give "matching" voltage fluctuations. I stayed with 100uF so that all the caps would be the same, and none of them too big to make film caps practical. (See the Rev 1.1 diagram.)"

Makes sense.

"If you remove that cap entirely, there will be a big loss in filtering, so a cap from B+ to ground would be needed, I think."

I would just assume leave it below the ultrapath since I can balance it out and or increase the total capacitance.

"Certainly an added stage of filtering would reduce ripple; I didn't do that in order to maintain the necessary high voltage."

Actually the ripple is pretty good.(just to clarify I view "ripple" the bit of AC left over from the rectifier and not voltage variations caused be the load.)
So maybe I can get by with just added C.

"Do you have specs (especially inductance) for the Hashimoto transformer?"

13H/100mA on the 2.5K Ohm tap.
I think the primary should be 99 Ohms, I'm not sure if that is the 3.5K tap though.

"Since I put this up as a community project, I have no problem with revisions. I think it would help everyone if changes are labeled, just so we can track the effect of variations."

OK




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RE: Garg0yle 2A3 DC, posted on January 19, 2015 at 14:05:02
Triode_Kingdom
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 "When the voltage tries to drop, the magnetic field in the choke starts to collapse, releasing energy that opposes the voltage drop."

That's actually the opposite of what happens. If the amplifier creates a sudden demand for current, the choke "bucks" it and it's output voltage goes down. This is why it's critical that the last cap have sufficient storage. Note that SE amps make different demands, as their average current draw doesn't change significantly.

 

RE: Garg0yle 2A3 DC, posted on January 19, 2015 at 14:10:36
Al Noakes
 I'm finding much smaller value chokes in LCLC arrangement gets good results and is quiet enough , I think this is the best sounding HT supply for SE . I would not suggest the low-value chokes and caps that Jeff does but I would suggest steering well clear of cap-input supplies , even if ultrapath is used . I don't like ultrapath , it always seems more noisy and requires more reservoir capacitance for a given level of ripple which seems to cancel any benefits . YMMV . Why not just stack the supplies ? One for the 2A3 with a slow warmup rectifier and another for the driver stage with SS or DH rectifier ?

Al

 

RE: Garg0yle 2A3 DC, posted on January 19, 2015 at 14:40:11
thermionic addictions
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 When John Broskie discussed this circuit he suggested sending
only part of the ultrapath capacitance to the PS, around 20%
and the rest to ground.

It would be easy to try.

Ca

 

RE: Garg0yle 2A3 DC, posted on January 19, 2015 at 15:16:09
Garg0yle
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 Triode_Kingdom, here is how I understand it.

What I am describing is the similar to what generates a spark in an automotive ignition coil. A steady current passes through the coil, when the flow is interrupted an large surge of energy is released.

Power absorbed by an inductor is the product of the voltage across it as well as the current flowing through it.
By changing the voltage dropped across the inductor it should be altering the current through it.

Here is an exaggerated view using an inadequate 1H choke in spice.
You can see red choke voltage drop while it lifts the green B+.




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RE: Garg0yle 2A3 DC, posted on January 19, 2015 at 15:35:34
Paul Joppa
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 Thirteen henries is moderately low for a 2.5K transformer; that can give you perceptibly weak bass, especially at high levels. The Hammond 125ESE is spec'd at 10 henries. I like to go for 20 henries at 2500 ohms; beyond that you get big transformers with questionable treble - in my experience at least.

I totally agree that better power supplies make better music. A shunt regulator for this amp would be a beautiful thing. But in series feed you need ore current through the shunt reg than through the amp itself, so the regulator will dissipate at least 30 watts - no longer a simple, inexpensive project!

Incidentally, I did not design this for parallel feed even though I personally prefer it. A lot of people seem uncomfortable with it. But it does provide a good measure of power supply isolation.

 

"~ultrapath", posted on January 19, 2015 at 16:03:06
Paul Joppa
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 I probably should not have included the tricky last power supply capacitance, but should instead have kept it conventional, with a cathode bypass capacitor and the last power supply capacitor going to ground.

I did not realize that Ultrapath was still controversial - thought that ship had sailed! I really didn't want this design to be controversial or incorporate more than the one tricky bit of direct coupling. So I won't stand behind that particular aspect.

 

RE: Garg0yle 2A3 DC, posted on January 19, 2015 at 16:38:39
Garg0yle
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 Al Noaks

"I'm finding much smaller value chokes in LCLC arrangement gets good results and is quiet enough , I think this is the best sounding HT supply for SE . I would not suggest the low-value chokes and caps that Jeff does but I would suggest steering well clear of cap-input supplies"

I am pretty limited to the CLC configuration I have now, I considered the choke input, but it drops the voltage to much with what I have.

"I don't like ultrapath , it always seems more noisy and requires more reservoir capacitance for a given level of ripple which seems to cancel any benefits . YMMV ."

It's a bit early for me to form a concrete opinion one way or the other with the ultrapath just yet. I don't seem to have an issue as far as noise but I do agree I could use a little more C.

"Why not just stack the supplies ? One for the 2A3 with a slow warmup rectifier and another for the driver stage with SS or DH rectifier ?"

I had not considered that. Just to clarify, did you mean using one transformer or two?



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RE: Garg0yle 2A3 DC, posted on January 19, 2015 at 16:42:51
Garg0yle
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 Thanks, I was literally going to look for that page tonight!
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RE: Garg0yle 2A3 DC, posted on January 19, 2015 at 17:07:05
Garg0yle
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 "Thirteen henries is moderately low for a 2.5K transformer; that can give you perceptibly weak bass, especially at high levels. The Hammond 125ESE is spec'd at 10 henries. I like to go for 20 henries at 2500 ohms; beyond that you get big transformers with questionable treble - in my experience at least."

The OP transformers do have a 3.5K - 18H @ 80mA taps along with variable 25W cathode resistors.

"I totally agree that better power supplies make better music. A shunt regulator for this amp would be a beautiful thing. But in series feed you need ore current through the shunt reg than through the amp itself, so the regulator will dissipate at least 30 watts - no longer a simple, inexpensive project!"

That's OK, I'm confident something can can be worked out with what you have here. IMO

"Incidentally, I did not design this for parallel feed even though I personally prefer it. A lot of people seem uncomfortable with it. But it does provide a good measure of power supply isolation."

I was somewhat interested in parallel, although I already had these OPTs, so just as well for myself!
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RE: "~ultrapath", posted on January 19, 2015 at 17:18:16
Garg0yle
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 "I probably should not have included the tricky last power supply capacitance, but should instead have kept it conventional, with a cathode bypass capacitor and the last power supply capacitor going to ground. "

I think it is pretty slick idea.
As you mentioned earlier my OPTs may not be optimal which could be contributing to my specific situation.

"I did not realize that Ultrapath was still controversial - thought that ship had sailed! I really didn't want this design to be controversial or incorporate more than the one tricky bit of direct coupling. So I won't stand behind that particular aspect."

I'm kinda lost in this thread already lol, is this in reference to Al Noaks suggestion?
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Chokes and Caps, posted on January 19, 2015 at 19:14:19
Triode_Kingdom
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 "A steady current passes through the coil, when the flow is interrupted an large surge of energy is released."

Yes, that's the same mechanism, but you're describing what happens when the load suddenly draws *less* current. Under those conditions, the field collapses, and voltage at the output of the choke spikes. The opposite occurs if the load draws more current (an action that can possibly drain the last capacitor). The magnetic field then grows larger, and the choke's output voltage momentarily dips. Both these actions are the inverse of what we want.

To expand on this concept, there is a camp on this forum that eschews large filter caps. Regardless of the reason for that preference, reducing the values of filter caps (particularly the last cap) requires that the choke's constant current mechanism be similarly reduced. Otherwise, output voltage will swing wildly in response to the amplifier's changing current demands. The downside to these Low-C, Low-L designs includes reduced energy storage and less effective filtering, and they can be significantly more susceptible to line noise, ripple, audio-band resonances and intermodulation effects. The latter is sometimes mistaken for increased bass response and "speed."


 

RE: Chokes and Caps, posted on January 19, 2015 at 20:22:22
Garg0yle
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 "Yes, that's the same mechanism, but you're describing what happens when the load suddenly draws less current. Under those conditions, the field collapses, and voltage at the output of the choke spikes. The opposite occurs if the load draws more current (an action that can possibly drain the last capacitor). The magnetic field then grows larger, and the choke's output voltage momentarily dips. Both these actions are the inverse of what we want."

I will have to digest this some more. However I am still seeing the output side of the choke swinging both positive and negative with respect to B+, one of those swings would have to drop the current flowing through the choke a bit releasing stabilizing emf similar to what we agree happens in an ignition coil?
I won't get to hung up on it, I will ponder and reconsider it in the future.

"To expand on this concept, there is a camp on this forum that eschews large filter caps. Regardless of the reason for that preference, reducing the values of filter caps (particularly the last cap) requires that the choke's constant current mechanism be similarly reduced. Otherwise, output voltage will swing wildly in response to the amplifier's changing current demands. The downside to these Low-C, Low-L designs includes reduced energy storage and less effective filtering, and they can be significantly more susceptible to line noise, ripple, audio-band resonances and.."

Actually I am working on some sims at the moment that correlate with this, I will my findings shortly.

"..intermodulation effects. The latter is sometimes mistaken for increased bass response and "speed."

I could see how little random high energy blips of bass could be heard. Inter-modulation with the source material would cause peaks when a transients hits on one of the wiggling B+ peaks.

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RE: Chokes and Caps, posted on January 19, 2015 at 21:17:44
Garg0yle
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 Similar to what Triode_Kingdom was talking about, I was able to find a bit of a sweet spot of capacitance for my choke.

The spice model is somewhat arbitrary (Has SS diodes, generic PT) but it gives me some insight as to what I can expect.
The test is at 30Hz.

I started of increasing the bottom capacitor below the ultrapath in 50uF increments up to the 20% ultrapath ratio that thermonic addictions recommended. (100uF/400uF)
These C increases lowered the wiggle in the lower capacitor almost proportionately from 2.8V to 0.8V. It did not however help the B+ which stayed around 7.5V.

Next I doubled the ultrapath which cut the B+ wiggle almost in half to around 4V

Making the ultrapath 300uF over a 400uF is the point of diminishing returns as 400/400 did not get any better.

So the 300uF ultrapath over 400uF yeilds wiggle of around 2V and 0V respectively.

While 700uF total seems like a lot of capacitance, due to the series connection it is much less.
**EDIT:The 400uF in series with the 300uF ultrapath would yeild around 171.5 uF as a "second" cap.

Paul Joppa was right on with the ~40% ultrapath ratio he recommended earlier.




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RE: Garg0yle 2A3 DC, posted on January 19, 2015 at 21:40:20
Paul Joppa
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 What counts is the ratio of inductance to impedance, which is the same on both taps.

 

RE: Garg0yle 2A3 DC, posted on January 20, 2015 at 00:46:06
Al Noakes
 Stacked supplies require 2 transformers or one transformer with two sets of HT windings . Makes for a much more flexible driver stage plus there is a marked reduction in heat and efficiency as there is no large value cathode resistor throwing heat into the air . By the way : resistors are evil

Al




























 

RE: Garg0yle 2A3 DC, posted on January 20, 2015 at 06:36:56
drlowmu
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 Al,

I once did ( about in 2009 ) a two stage AVVT AV32B DC amp with two separate high voltage windings on one PT ( 9 Ohms DCR ) doing each stage, and it sounded better, more coherent and dynamic with just ONE transformer winding doing both stages, which I postulate MAY have to do with the amp being directly coupled.

That I recall - very well. YMMV, it was just MY one experience, but it cured me of separate supplies on DC two stage amps !!! TOTALLY opposite of my non-DC amp experiences !!

Jeff Medwin

 

RE: Garg0yle 2A3 DC, posted on January 20, 2015 at 09:14:31
Al Noakes
 Different from my experience but who am I to question yours ? I would suggest the original poster try both methods and decide for themselves before committing to a chassis . This amp appears to be a 'monkey' choke loaded design . In a stacked setup , a resistively loaded driver will pass the signal through both supplies rather than a choke-loaded design where it will pass through one . I find stacked designs attractive especially WRT flexibility and excessive cathode resistor heat . I especially dislike components doing nothing apart from throw waste heat into the air , the only part that should do that is the HT bleeder

:)

Al

 

RE: "~ultrapath" Paul, first off thanks for your contributions and support of the DIY community..., posted on January 20, 2015 at 09:54:31
Ivan303
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 As far as the Ultrapath, it is hardly controversial.

Please do post any upgrades to the circuit as you make them so those of us considering the project can keep up to date.

And again, thanks for your contributions!






First they came for the dumb-asses
And I did not speak out
Because I was not a dumb-ass

 

RE: Garg0yle 2A3 DC, posted on January 20, 2015 at 12:19:10
Garg0yle
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 "I would suggest the original poster try both methods and decide for themselves before committing to a chassis . "

You raise some valid points Al Noaks.
As you can see though, test flights have already commenced!

Cheers.




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RE: Garg0yle 2A3 DC, posted on January 20, 2015 at 13:07:46
rage
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 Beautiful amps! Best cake pans I've seen yet.

 

RE: Garg0yle 2A3 DC, posted on January 20, 2015 at 13:42:29
Al Noakes
 Very nice ! I think Rage is right about your pans :) Are these making noises ?

Al

 

RE: Garg0yle 2A3 DC, posted on January 20, 2015 at 14:47:28
Garg0yle
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 Thanks guys.

Yes they are making all kinds of noises!
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RE: Another Lowmu backflip, posted on January 21, 2015 at 08:46:50
mach1
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 Last I heard, any choke over 10 ohms DCR was decidedly lo-fi, now you're saying 200+DCR is needed.

Don't you realise nobody belives a thing you post any more? Your backflips are legendary.

 

RE: Another Lowmu backflip, posted on January 21, 2015 at 16:47:26
Garg0yle
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 The good Doctor was being sarcastic. lol
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RE: Garg0yle 2A3 DC, posted on January 21, 2015 at 17:31:34
Garg0yle
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 "What counts is the ratio of inductance to impedance, which is the same on both taps."
-OK Gotcha.


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Plate capacitor, posted on January 21, 2015 at 17:51:42
Garg0yle
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 OK so now I modeled the plate capacitor.

When 100uF plate capacitor is used with the 100uF ultrapath and 100uF cap below it, (as per BB Proof Rev 1.2 drawing), the wiggle on the cathode of the 2A3 is 2.62V

Without the 100uF plate capacitor the wiggle is 2.75V (As mentioned in the previous simulation.)

Unless someone can point out additional attributes that justify the use of the 100uF plate capacitor on the 6J5, I would say it is not worth it.
The 100uF is better combined into the other positions. (Ultrapath and lower series capacitor.)

I ponder the idea of paralleling 100uF caps for lower ESR to offset the series ESR, or to just get some bigger electrolytics.




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RE: Garg0yle 2A3 DC, posted on January 21, 2015 at 18:44:10
Garg0yle
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Rage had in another thread, what would happen to a B+ voltage over a certain amount of time. Based on the in those sims in that discussion.

I did a sim that shows what happens in that context, but with this power supply.

This is a 30Hz sine showing a 10 second interval.
Once the B+ recovers from the attack, the voltage stabilizes fine.




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RE: Chokes and Caps, posted on January 21, 2015 at 18:51:05
Garg0yle
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 I made an EDIT to the post I am quoting.

It should have read 171.5uF as the equivalent series capacitance.
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RE: Garg0yle 2A3 DC, posted on January 22, 2015 at 07:57:56
rage
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 looks quite a bit different than the other supply.


the other one was essentially ringing after the transient, right?

 

RE: Garg0yle 2A3 DC, posted on January 22, 2015 at 08:46:27
Garg0yle
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 I thought you interested in seeing the long term stability of the B+ in a stiffer power supply that has more capacitance.

"gargoyle -what happens if you extend out beyond 4 seconds and allow the 150uf cap to discharge?"

I may have misunderstood you.

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RE: Garg0yle 2A3 DC, posted on January 22, 2015 at 08:47:32
rage
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 no we're on the same page... sorry a little under the weather.

 

RE: Chokes and Caps, posted on January 22, 2015 at 15:19:24
Garg0yle
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 I was able to find some bigger caps locally today.

I got a 270uF/400V ultrapath and a 330uF/250V lower series capacitor.

They are a little bit smaller then I would have liked, I wanted 300uF/400uF, but the big stuff was shy on the voltage ratings.

I am going to start the swap tonight.

As I mentioned earlier, the 6J5 plate choke seems to not really be needed.
I may or may not take mine out, depending if I need the room or not.
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RE: Chokes and Caps, posted on January 22, 2015 at 20:49:42
Garg0yle
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 OK got the new capacitors in. They worked out really well.

-They gave the low end more definition, as it was a little smooth and whispy before. This really helps out at lower listening levels.

-They also give the transients the kick in the pants I was after. Percussion instruments have better attack now, they sound nice and jumpy.

Those where the two main objectives I was after, so it was a definite success.

As an added bonus surprisingly the capacitors did a lot to warm up the mid-range. I was not expecting this. I was able to raise the L-pads back up on my speakers. I have some small horns in my system, they sound sandier now, not as glassy as before. It's nice.

It's too bad it's so late, I will have to wait until tomorrow to really put the pedal down!






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Delayed reaction, posted on January 29, 2015 at 10:06:20
Garg0yle
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 Can anyone identify this anomaly at the start and end of this waveform?

As I start to lower the source frequency below ~500Hz I am starting to notice a voltage offset. The swing seems to be fine at first, but as I go lower the relative short rise of the first swing, seems to coincide with a bit of an extra swing at the end of the waveform. It does start to compress the first swing a bit at lower frequencies.

I think I have isolated it to tye 6J5 area. My 6J5 model is not be great, I guess I could figure out how to use a 6SN7 model to try in place, thought I would ask first.

Does this have something to do with the influence from the plate choke on the 6J5? (Paul Joppa specified a 150H hammond (159H would be 20Hz -3db)) I tried playing around with the values a bit, it doesn't seem to correct much so I am not sure.

I am not going to get too excited about it, just wondering if I am seeing this correctly or if it's due to an obvious simulation error.

Below is my current schematic and a screen shot of the 6J5 plate connection.








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RE: Delayed reaction, posted on January 31, 2015 at 19:42:12
Garg0yle
Audiophile

Posts: 859
Joined: December 1, 2014
 OK well I have figured out that once the plate choke for the 6J5 gets up over 10H (series resistance being the same), causes the phase shifting I see at lower frequencies.

This of course assumes my model is accurate enough.

It makes me wonder if I would be just as well to use a resistor instead of the Hammond 156C.

With just the resistor I am seeing 3mV of ripple at the 6J5 plate vs the 1mV ripple of the choke. This seems like fairly minor difference.


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RE: Delayed reaction, posted on January 31, 2015 at 21:43:02
Garg0yle
Audiophile

Posts: 859
Joined: December 1, 2014
 I decided to try the 6J5 circuit with a 10K plate resistor instead of the choke, 1.5K cathode resistor, 150u cathode bypass cap. (These values were found by trial and error if they seem odd.)

On the surface it seems I have smoothed out the response and phase issue.

The 30Hz transient looks good now. It also has the same swing here as at 1Khz.
The choke setup seems to be a less linear, having higher swing at 1K.

You may have noticed in the previous sims that the 6J5 plate was not sitting at 135V. I can't seem to balance the 180V, 135V and 3.4V bias readings at the same time.
Hopefully it's just a glitch in my modeling, I will have to measure the amps and see what is happening.

I will have to digest this for a bit, to see how I can get this result combined with the parameters of Paul Joppa's original BBProof.




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RE: Garg0yle 2A3 DC, posted on July 28, 2015 at 13:24:32
Garg0yle
Audiophile

Posts: 859
Joined: December 1, 2014
OK I have this phase issue sorted out.

First, I would like to correct myself on my original hum measurement. I gapped out and did not short the inputs. When I had them on the bench the other day I did short the inputs and got 1.2mV and 0.2mV respectively. Most times I get around 1.4mV, seems to be right at the limit of adjust-ability with my rheostats.

So while I did gain back some bass (comparable to my RH84), I wasn't quite
satisfied with the performance of this amp, considering I have much nicer
iron on these amps.

I decided to model the RH to see whats doing. Sure enough it has much nicer
phase then the choke loaded BB Proof amp.
The choke load presents two issues.
1. I already have -3db in the bass region of my speakers, I can't afford to
lose any more therefor need an amplifier flat out to 20Hz.
2. The group delay of the choke load messes with the low group delay of my
speaker cabinets. It makes them sound a little slower and boomier.

The choke load had to go.

So here are the plots, I scaled them equally at -3db for comparison.
1. The Garg0yle/BB-Proof. You can see the bass rolloff and the ugly 66
degree phase shift.
2. The RH84 does better, only down -1db bass at 20Hz and a phase shift of
less then 10 degrees.
3. My amplifiers now have flat response past 20Hz and around 5 degree phase
shift over the audible band.

I did test the BB-Proof on the bench and did get the same rolloff as spice.
I used Audacity on my tablet as a signal generator. First I verified that
the tablet would go down to 20Hz flat with a 100K load.
The tablet does not swing 2.7 volts so I tested at a lower level.

20Hz 5.8
30Hz 7
40Hz 7.5
50Hz 7.8
60Hz 8.2
70Hz 8.2
80Hz 8.2
90Hz 8.6
100Hz 8.8
500Hz 9.4
1000Hz 9.4
5000Hz 9.4






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RE: Garg0yle 2A3 DC, posted on July 28, 2015 at 13:26:44
Garg0yle
Audiophile

Posts: 859
Joined: December 1, 2014
 I had debated changing driver tubes, parallel 6SN7, different
configurations etc. Rather then speculate, I put the scope on the speaker
terminals to see what kind of voltage I actually need.
95% of my listening is well under 1W, more like 240-500mW. This tells me I
can get away with chopping of some power in the name of quality.

At first I debated whether this would be a waste of a 2A3, but in reality
it's not as even if I could push 3.5 watts, I would never need it. 45s may
be an option, but I don't feel compelled to switch.

The BB-Proof was running my 2A3s a little hot, due to my higher voltage as
well. and lower OPT voltage drop.

So I had to rework the voltages to fall inline with my iron. I am now
running the 2A3s at ~52.5mA/275V. It's a juggle, lowering the current
raises the B+ etc. so this is where I settled.
The 25watt 2A3 cathode resitor is now 4Kohm instead of 3Kohm.

Next the driver stage, I was tempted to abandon the BB-proof concept
(Applying negative bias to the 2A3 during start up.), just go a more
traditional route and connect the 6J5 to the regular B+, as it would have
let me use a bigger plate resistor for more swing.
However I persevered and after 20 or so versions later, I settled on a
configuration that works.
Basically it has similar operating points as the BB-Proof, 6mA/~130V on the
plate, but now uses an 8Kohm plate load and a higher B+ of 180V instead of
157V. As Paul Joppa said, this is a good operating point. It works with the
voltages of the amp at least.
I did throw around a few different choke values, but nothing jumps out as
even being close to acceptable, performance, they are garbage lol.

The output transformers are now 3500K instead of 2500K.
B+ is now 465V

Bias on the 6J5 is now stabilized using a 1500uF/50V cathode bypass
capacitor. FWIW I do not consider this to be in directly in the signal
path.

The 390 ohm partial bias resistor is now 100ohm to allow for the larger
plate resistor.
With the 6J5 removed in the BB-Proof, the 2A3 saw 191V @ 74mA, -28 bias
resulting in 14watts dissipation.
With the 100ohm, I have 110V @79mA, -8 bias 8.7 watts dissipation.
While I have a touch more current under this fault mode the voltage is less
resulting in a lower wattage, so I should be BB-Proof.

The operating points were chosen to get the lowest distortion. Both tubes
have around 2.5% each of mostly 2nd harmonic at full output. the 2A3 bias
is around -52V.
The amps put out 1watt to the speaker terminals.

These amplifiers are now stunners. They are now the upgrade I was truly
wanting. They go lower and have better bass then my RH84 (with smallish
OPTs had) They sound "right" as the phase is much much better, this made a
huge difference in the bass department. Transients are much better not
having that delayed bass.

I have a square wave of the BB-proof as well, but I will post that after I
square wave test these amps for comparison.

I will clean up my schematic and post it here shortly.

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RE: Garg0yle 2A3 DC, posted on July 28, 2015 at 13:28:03
Garg0yle
Audiophile

Posts: 859
Joined: December 1, 2014



Here is what I am using.

The voltage and wattage ratings are the same as the BB-Proof Rev 1.2, with
the addition of the 1500uF/50V 6J5 cathode bypass capacitor.

Note: the real 6J5 cathode resistor is still a 1Kohm potentiometer, not a
fixed resistor as the screenshot shows.
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RE: Garg0yle 2A3 DC, posted on July 28, 2015 at 13:29:48
Garg0yle
Audiophile

Posts: 859
Joined: December 1, 2014
Here are the curves.
BB-proof on the left, my new amp on the right.

I turned down the volume on the BB-proof to show the distortion comparable
to the maximum output of my new amps.

The first row are the 2A3s. Pretty straight forward, the new setup has a
2.75% distortion vs 3.5% in the BB-proof at around ~1 watt.

Now for the 6J5, I had to model the BB-Proof showing best and worst case
scenarios, 20K and 3700 ohms depending on how the choke is reacting.
However these models do not show the distortion caused by the phase shift
from the choke, so in reality the 2.75% distortion of my new amps with a
regular plate resistor is more accurate then the lower distortion implied
in the BB-Proof curves.

Also I had to turn down the input to 1V to match the output of my new amps
at 2.7v input. If you feed the BB-Proof with 2.7v, you are looking at about
0.74% to 3.7% depending on how the choke reacts. Again, this is misleading
as it assumes static resistors and does not demonstrate the horrendous
phase shift caused by the choke. (Actually it does if you compare the light
blue load lines, the vertical load-line with the 3700 ohm resistance shows
lack of voltage swing compared to the flatter load-line with 20K plate
resistance.)




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RE: Garg0yle 2A3 DC, posted on July 28, 2015 at 13:30:45
Garg0yle
Audiophile

Posts: 859
Joined: December 1, 2014
 Here are some square waves.

I used the 1000Hz generator on my scope because the tablet does not put out
a nice square wave.

This is a 5V square wave, so it is over-driving the amplifier with twice
the input voltage.
I debated dropping the voltage down to ~2.7, but hey lets see what they can
do at 5V.

Not too shabby.




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RE: Garg0yle 2A3 DC, posted on July 28, 2015 at 18:26:24
Mr_Steady
Audiophile

Posts: 2042
Location: North Florida
Joined: August 19, 2014
 Way cool!

Glad to see the update.


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Big speakers and little amps blew my mind!

 

RE: Garg0yle 2A3 DC, posted on July 28, 2015 at 19:04:00
Garg0yle
Audiophile

Posts: 859
Joined: December 1, 2014
 Cheers.
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