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DHT 01a tube preamp

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Posted on February 13, 2025 at 16:27:18
mwhouston
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Some time back I built a 71a preamp with 100 year old Cunningham tubes. This 01a amp is similar but these tubes have more gain. Once more Cunningham tubes. Regulated DC of course on the heaters and a resistor divider to balance the voltage further. Has its own RFI/EMI power filter for a cleaner mains. Hammond power tranni.

Mundorf EVO Oils for output caps and remote control Alps Blue velvet pot. Fine silver plated copper wire for hookup and mostly carbon film resistors. Remote PS with double Pi filter for HT. 300uf of 400V filter caps. Had some hum issues but by reducing the gain managed to get this right down. Love the colour - Iron Stone - and the look. Excellent to listen to.
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RE: DHT 01a tube preamp, posted on February 14, 2025 at 08:02:44
GEO
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 Nice. I used that tube in an amp. KC1>01A>45/vt52/183. It was nice. I use an RE134 in my preamp.








 

RE: DHT 01a tube preamp, posted on February 14, 2025 at 08:44:08
FlaCharlie
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 That's a beauty! What brand of paint is Iron Stone?

You mention that you had hum issues that were solved by reducing the gain. What method did you use?

Could you post some pics of the insides and a schematic? This is, after all, a DIY forum where visitors are interested in such details not just eye candy.

 

RE: DHT 01a tube preamp, posted on February 14, 2025 at 10:30:03
GEO
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RE: DHT 01a tube preamp, posted on February 14, 2025 at 16:21:08
mwhouston
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Underneath PS.
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RE: DHT 01a tube preamp, posted on February 14, 2025 at 16:29:01
mwhouston
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 Standard plate loading with values from the 01a spec sheet. To limit output I put a series resistor of 330K to the output RCA with 100K to ground at the RCA. This meant the signal would be dropped 3 to 1 put still have an output - DC - resistance of 100K.
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RE: DHT 01a tube preamp, posted on February 14, 2025 at 22:02:15
mwhouston
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 The Iron Stone is Dulux spray can, Epoxy Enamel. Takes a week to dry hard but tough when dry. This colour has a nice finish and easy to apply.
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RE: DHT 01a tube preamp, posted on February 15, 2025 at 06:29:51
Tre'
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 "To limit output I put a series resistor of 330K to the output RCA with 100K to ground at the RCA"

That is a really high output impedance. I can't see how that wouldn't cause frequency response issues (loss of highs).

What cables and amp are you planning to drive with this preamp?

Tre'
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Output Impedance, posted on February 15, 2025 at 08:33:45
FlaCharlie
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 Is the 100k resistor to ground considered a factor in calculating the output impedance of the preamp or is it considered a factor in calculating the input impedance of the amp? Or both??

I'm not very technically oriented but, as I understand it, the dominant factor when it comes to output impedance is tube's Rp in parallel with the value of the load resistor. A load resistor value that's 3x to 5x that of the Rp is common, sometimes higher.

The Rp of the 01A is ~10k which, in parallel with a 50k load resistor, results in a value of 8.33k. Although other resistances may combine with this the resulting value is even lower as long as those resistances are not lower than the Rp. If the 100k to ground is part of the preamp calculation, it only lowers the output impedance. The result of 10k, 50k and 100k in parallel is 7.69k.

Since tube amps generally have an input impedance of 100k there shouldn't be a problem with a preamp whose output impedance is 10k or less. If used with a SS or Class D amp a lower output impedance would be indicated.

Or is the 100k to ground part of the input impedance of the amp? In parallel with a 100k input impedance of a tube amp, it would lower the input impedance to 50k and may create issues. I'm not sure if they would be audible or not, but I imagine they could be measured.

If it contributes to the amp's input impedance it seems like it would be better to use a 33k with a 10k to ground. This would have the same effect on the output voltage and only lower the input impedance to ~9.1k. But this assumes that a tube amp is being used.

It seems like a 33k / 10k combo would also be better if it affects BOTH the output impedance of the preamp and the input impedance of the amp since this would also lower the output impedance of the preamp further. The result of 10k, 50k and 10k in parallel is 4.55k.

A SS / Class D amp typically has an input impedance of 10k to 20k. In which case the 330k / 100k combo would be best.

That's how I see it but perhaps I'm missing something.

 

RE: Output Impedance, posted on February 15, 2025 at 09:22:04
Tre'
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 "The Rp of the 01A is ~10k which, in parallel with a 50k load resistor, results in a value of 8.33k"

I will assume your math of correct. Now add to that the 330k of series resistance he said he added. The output impedance is now 338.33k

The output impedance of the circuit is way to high to drive anything without losing high frequencies do to the cable capacitance plus the Miller capacitance of the first stage of whatever power amp it is connected to.

Tre'
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RE: Output Impedance, posted on February 15, 2025 at 10:46:24
FlaCharlie
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 Earlier you questioned the value of the 100k resistor to ground but now you're saying that the 330k that's in series actually creates more of a problem??

Perhaps I'm mistaken but I thought that the resistance in series (330k in this case) only affects the calculation of the -3db rolloff point.

If resistance in series is added then, if the amp has a volume pot on the input, wouldn't that also affect the output impedance of the preamp?

Say you're using a 100k pot. Whatever portion of the 100k that's in series with the grid of the amp's first tube would be added to the output impedance of the preamp??

If that were the case it seems like the use of a voltage divider, or an adjustable voltage divider aka a volume pot, would technically create an impedance mismatch in many cases. Perhaps most cases.

 

RE: Output Impedance, posted on February 15, 2025 at 11:54:11
Tre'
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 "Earlier you questioned the value of the 100k resistor to ground"

No, I never mentioned the 100k resistor. My concern is the 330k series resistance added to the output impedance of the stage.

"...I thought that the resistance in series (330k in this case) only affects the calculation of the -3db rolloff point"

The -3db rolloff point is, more of less, the only concern when it comes to the output impedance of a circuit (a preamp in this case).

"Whatever portion of the 100k that's in series with the grid of the amp's first tube would be added to the output impedance of the preamp??"

If the pot is mounted in the power amplifier, then the output impedance of the preamp is driving the cable capacitance.

The pot's output impedance is the output impedance of the preamp plus the series resistance of the pot with that value in parallel with the portion of the pot that shunts to ground from the wiper. That is the impedance value that is driving the Miller capacitance of the first stage of the power amp.

I was under the impression that mwhouston has placed the 330k and the 100k resistors in the preamp.

"If that were the case it seems like the use of a voltage divider, or an adjustable voltage divider aka a volume pot, would technically create an impedance mismatch in many cases. Perhaps most cases."

I tend to agree.


The stage needs to be able to properly drive the shunt capacitance. When there is no pot at the amplifier, the shunt capacitance seen by the preamp's final stage is the cable capacitance and Miller capacitance added together. Miller capacitance is the capacitance between the grid and the plate of the input tube of the power amp multiplied by the gain of that stage and then the capacitance between the grid and the cathode is added. The -3db point of the low pass filter created by the output impedance of the driving stage and the total shunt capacitance is easy to calculate. If we want 20kHZ to be unaffected by that filter, then the -3db point of that filter needs to be at least 200kHz.

A capacitor is a reactive device, It's reactance (impedance) goes down as the frequencies go up. At the highest of frequencies the impedance of the capacitance reactance will be very low. The device driving that capacitance (the preamp's final stage) has to be able to deliver the current that the reactance of the shunt capacitance will require otherwise the preamp stage will distort.

So a stage has to have a low enough output impedance so that the low pass filter created between it and the value of the shunt capacitance is well out of the audio band and that stage has to be able to deliver the current needed to drive the shunt capacitance's low impedance at high frequencies.

A 330k ohm resistor in series with the output impedance of the preamp will make these requirements hard (if not impossible) to meet.

Tre'



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RE: Output Impedance, posted on February 15, 2025 at 12:11:47
Chip647
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 You have to look at the circuit from the Pin 2 output of the 01a to the grid pin or input of the component you are driving.

Usually there is no resistance "in series" with the output, so there are only resistors to ground represent the "load". Typically you will have something like a 1 meg resistor grounding the pre amps output capacitor so that it won't store a nasty voltage if operated without a load. Then there is the input impedance of the thing you are driving. 25K to 250k ohms is typical.

In this case, the impedance contribution of the 1 meg safety resistor is negligible so you are left looking at the load presented by the component you are driving. If you have a 100k component load to drive it will be at the far end of the interconnect. You will be driving that interconnect with the output impedance of the preamp. Lower the better. The very conservative rule of thumb is 1:10, 10K output impedance to drive 100K input impedance. Adding the 330K series resistor adds to the output impedance of the preamp creates a big impedance miss match. You may actually like this tone control, though. If your hearing does not go much above 10khz it might actually sound more pleasing.

People here are pretty knowledgeable, more than myself, and I have learned a lot by listening to them. No one here wants to steer you in the wrong direction or be unhelpful.

 

RE: Output Impedance, posted on February 15, 2025 at 13:16:19
FlaCharlie
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 Hopefully I'm not hijacking the thread, I'm just trying to wrap my head around the interactions and the math.

So, if a volume pot or a fixed voltage divider is used to reduce the output signal (and associated noise in this case) of the preamp then putting the pot after the interconnect (on the amp) would not increase the output impedance of the preamp. Correct?

If the preamp is driving a SS or Class D amp does this affect the placement of the volume pot? I assume that there is no Miller capacitance or the equivalent involved in that case. Or is there?

Would the resistance in series have the same effect on the output impedance if there was no Miller involved, only the cable capacitance?


 

RE: Output Impedance, posted on February 15, 2025 at 13:40:12
Tre'
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 "So, if a volume pot or a fixed voltage divider is used to reduce the output signal (and associated noise in this case) of the preamp then putting the pot after the interconnect (on the amp) would not increase the output impedance of the preamp. Correct?"

That is correct.

"If the preamp is driving a SS or Class D amp does this affect the placement of the volume pot?"

No.

"I assume that there is no Miller capacitance or the equivalent involved in that case. Or is there? "

A transistor has a input capacitance they just don't call it the Miller capacitance but it is very much the same thing.

"Would the resistance in series have the same effect on the output impedance if there was no Miller involved, only the cable capacitance?"

Yes, the output impedance value has nothing to do with the total shunt capacitance value but if there were no Miller capacitance then the total shunt capacitance would be less so the -3db point of the low pass filter would be higher (more out of the way) but that filter would still exist and the output impedance of the stage is still what it is.

Tre'


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"Still Working the Problem"

 

RE: DHT 01a tube preamp, posted on February 15, 2025 at 14:43:47
mwhouston
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 No high frequency problems just reducing output level. Works well. Drives into a few different DIY tube power which also has a 100K input.

Cables are home made which is twisted wire wrap wire - fine copper and silver plated - with two earths. Only one earth terminated at the destination end. Cables are directional. The other earth scavengers holding noise back at the originating end.
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RE: DHT 01a tube preamp, posted on February 15, 2025 at 15:10:30
Tre'
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 Have you measured the FR of the preamp (connected to an amplifier) with and without the 330k series resistor?

If everything is fine then I have to say that somehow your circuit defies the laws of physics.

Tre'
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RE: DHT 01a tube preamp, posted on February 15, 2025 at 15:20:48
mwhouston
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 I don't need to connect to an amp just load the output of the preamp into 100K. I will check.
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RE: DHT 01a tube preamp, posted on February 15, 2025 at 15:40:39
Tre'
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 You do need the cable hooked up and the amplifier hooked up.

It is the shunt capacitance (in combination with the output impedance of the preamp) that will cause the highs to be attenuated.

The cable has shunt capacitance and the amplifier has shunt capacitance (that is called Miller capacitance). The capacitance from those two add together.

Tre'
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RE: DHT 01a tube preamp, posted on February 15, 2025 at 16:09:10
mwhouston
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 Capacitance of my cables is 175pF (0.175nF). Quite low.
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RE: DHT 01a tube preamp, posted on February 15, 2025 at 16:15:25
Tre'
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With an output impedance of 330K playing into 175pf the -3db point of the low pass filter will be 2756Hz. The -1db point will be at 1378Hz.

That is a massive lose of high frequencies and it is only going to get worse when you add in the Miller capacitance of the first stage of your power amp.

Tre'
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Correction, posted on February 15, 2025 at 18:29:18
Tre'
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Correction, with the 100k resistor in parallel with the 330k ohm resistor the output impedance is 76744 ohms so the -3db point of the low pass filter is 11850Hz. Better but still a lot of high frequencies lost in the audio band.

Tre'
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Poor Design for a Preamp, posted on February 16, 2025 at 09:51:58
Triode_Kingdom
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 "To limit output I put a series resistor of 330K to the output RCA with 100K to ground at the RCA"

Your later comments regarding HF response and testing indicate that you don't understand how this all works together. Below is the frequency response of the 300K/100K combination driving your 175pF cable. And this doesn't account for the additional output impedance of the 01A anode or input capacitance of the amplifier. In addition to high frequency loss, phase (dotted line) is badly shifted.






 

RE: Poor Design for a Preamp, posted on February 16, 2025 at 11:42:34
FlaCharlie
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 Sorry for asking more questions but I'm still confused and I'd like to understand how this calculation works. I do appreciate everyone's patience!

Here's a formula I was given a couple of years ago. It also considers the influence of a pot / voltage divider on the output. I mistakenly thought that the series resistance of the pot was only used to calculate the high pass value. Apparently, as was explained earlier, it's also a factor that's used to determine the output impedance.

(Tube's output impedance + series segment of pot or voltage divider) + (grounded segment of pot or voltage divider || input impedance of amp and/or other resistance to ground) = Overall output impedance

Tre's most recent calculation parallels the values used in the voltage divider, the 330k in series with the signal and the 100k that goes to ground and uses that to calculate the low pass frequency.

Why are they in parallel? According the formula above the 330k should be added to the impedance of the tube, not paralleled with the 100k that goes to ground.

Is the formula I was given incorrect or am I misinterpreting it somehow?

TK mentions that the tube's impedance should be added. The plate resistance of the 01A is 10k. As I understand it the tube's impedance is calculated by putting this in parallel with the tube's load resistor. If a 50k load resistor is used then the resulting impedance of the tube would be 8.33k.

Do we ignore the input impedance of the amp since it's on the other end of the cable? Or is it also part of the calculation?

 

RE: Poor Design for a Preamp, posted on February 16, 2025 at 12:17:02
FlaCharlie
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 Regarding the voltage divider . . . couldn't he just use a 33 ohm in series and a 10 ohm to ground? The voltage reduction would be the same.

The resulting output impedance would then be dominated by the impedance of the tube which, assuming a 50k load resistor, would be 8.33k.

If the formula I posted is correct (??) the output impedance would be:

(8.33k + 33 ohms) + (10 ohms || the assumed 100k input impedance of the amp) = 8363 ohms + 9.99 ohms = 8373 / 8.373k

Looks like this would move the rolloff point to ~108k if only the 175pf capacitance of the cable is included.

Am I missing something?

 

RE: Poor Design for a Preamp, posted on February 16, 2025 at 12:57:28
Tre'
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 You can't drive into a 43 ohm load to ground with a stage that has a 8.3k output impedance. That would rotate the load line for the tube way to the vertical and there would be very little gain and a whole bunch of harmonic distortion.
8.3k output wants to see a 83k load.

Tre'
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RE: Poor Design for a Preamp, posted on February 16, 2025 at 13:18:33
Tre'
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 The output impedance of a triode gain stage is the plate resistance in parallel with value of the plate resistor (the resistor between the power supply and the plate of the tube).

A triode needs to see a plate load resistor value of 3 to 5 times the tube plate resistance. (more if you have the supply voltage that will allow it or better yet a CCS)

A gain stage wants to see a load impedance 10 times the output impedance of the stage to prevent loading and rotating the load line for the triode towards the vertical causing a loss of gain and an increase of distortion.

When you drive a pot the total impedance of the pot is the load that is being driven by the preamp's output impedance (assuming there is no added resistance to ground following the wiper).

The output impedance of the pot (this is the value that is driving what ever follows the top) is dependent on the position of rotation.
If we assume that the impedance of that is driving the pot is zero (and of course it's not ), the output impedance of the pot at the -6db position is 1/4 of the value of the pot. So a 100k pot has an output impedance of 25k when the wiper is at the position with 50k above and 50k below it.

It would not take very much shunt capacitance to start disturbing the highs with a drive impedance of 25k.

So pots are a balancing act. Too low of a value and the source (the preamp in this example) has a hard time driving it, too high of a value and the pot has a had time driving what it is trying to drive.

All of the numbers that describe all of this are easily found on the internet.

Tre'
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"Still Working the Problem"

 

RE: Poor Design for a Preamp, posted on February 16, 2025 at 14:34:32
FlaCharlie
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 "You can't drive into a 43 ohm load to ground with a stage that has a 8.3k output impedance. . . . 8.3k output wants to see a 83k load."

OK, so in the case of this 01A preamp, he could use a voltage divider with 68k in series and 22k to ground. This would reduce the voltage about the same as the 330k/100k combo he's using and it would present a load of 90k to the tube and be no problem.

The value of 68k || 22k is 16.62k. If we plug that into the low pass calculator then the -3db point would be 54.75k. That should also not be an issue, right?

 

RE: Poor Design for a Preamp, posted on February 16, 2025 at 15:22:32
Tre'
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 "OK, so in the case of this 01A preamp, he could use a voltage divider with 68k in series and 22k to ground. This would reduce the voltage about the same as the 330k/100k combo he's using and it would present a load of 90k to the tube and be no problem."

Yes, you are getting this!

"The value of 68k || 22k is 16.62k. If we plug that into the low pass calculator then the -3db point would be 54.75k. That should also not be an issue, right?"

Well, almost. With a -3db point of 54.75kHz the -1db point will be 27.375kHz. That is out of the audio band but the phase is still being shifted by the filter. To get the filter completely out of the audio band the -3db point of the filter needs to be a full decade above 20kHZ.

***Also remember that these calculations do not take into account the Miller of the first stage of the power amp and that could be more capacitance than the cable.***

So to keep the filter totally out of the picture it needs a -3db point at 200kHz.

There are other things in a circuit that will not allow 200kHz bandwidth and there is not much we can do about it (200kHz would be almost impossible for an output transformer) but I don't see that as an excuse to not solve a problem that can be solved. These things stack up so there is no reason to exacerbate the problem.

My question would be, why does he need this gain reduction in the first place? Doesn't the preamp have a volume control in front of the tube stage?

Tre'
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RE: Poor Design for a Preamp, posted on February 16, 2025 at 19:58:38
mwhouston
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 I like your 22k//68K idea. Yet to put the preamp on the CRO. Gain was reduced because there was a hum issue.
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Yes, with DHTs hum can be an issue, posted on February 16, 2025 at 20:29:51
Tre'
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 Are you using AC for the 5 volt filament?

You might want to try a critical inductance input choke filter DC supply for a DHT.

Or maybe try Rod Coleman's Filament Regulators.

Tre'
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RE: Yes, with DHTs hum can be an issue, posted on February 16, 2025 at 20:34:15
mwhouston
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 Well filtered 6V then a 5V regulator. Also balanced 100ohm/100ohm network with Rk central tap. Hum is not in the filament voltage.
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RE: Yes, with DHTs hum can be an issue, posted on February 16, 2025 at 20:39:48
Tre'
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 I guess not. So it is either the B+ or a hum field.

What happens when you move the power supply chassis as far away as possible?

Do you have a schematic of the power supply?

Tre'
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"Still Working the Problem"

 

RE: Yes, with DHTs hum can be an issue, posted on February 16, 2025 at 20:51:24
mwhouston
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 It maybe an earth configuration issue. Ive used my design HT double Pi filtration in wellover 100 builds. Power amps and preamps. None have hums.
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RE: Yes, with DHTs hum can be an issue, posted on February 16, 2025 at 21:37:40
Tre'
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 I see the preamp PS is earth grounded. Is the power amp earth grounded?

Tre'
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"Still Working the Problem"

 

RE: Yes, with DHTs hum can be an issue, posted on February 17, 2025 at 07:45:40
FlaCharlie
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 "I like your 22k//68K idea."

Yeah, if you need to place the gain reduction solution on the preamp I figured that would be a good compromise.

On the other hand, if you could put it on the amp end of the interconnect that would seem to be even better.

If I'm understanding this correctly, that would lower the output impedance of the preamp and result in a higher -3db rolloff point. Right??

Either a simple two resistor voltage divider or a volume pot could be mounted on the amp itself, at the input. This would be great if the preamp was always used with the same amp. But this seems like an unlikely scenario.

A more versatile solution would be to build the voltage divider into an interconnect which would be used exclusively with this preamp. Or any other pre that's either too noisy or has too much gain.

One question. If the voltage divider was put on the amp end of the interconnect would it be better to use much larger resistance values than 68k in series with 22k to ground? Wouldn't the 22k to ground be in parallel with the amp's existing input impedance (let's assume 100k)? Or would the 68k, 22k and 100k all be in parallel? Either way that would lower the input impedance significantly (to ~15k) and make the amp harder to drive.

It that's the case than you could use a combo of 2.2 meg in series and a 750k to ground to get about the same voltage/noise reduction. Or would this create other issues??

Obviously, if the hum is the result of a ground issue then you could eliminate the voltage divider as long as the increased gain is not an issue.

 

Those are all good ideas but..., posted on February 17, 2025 at 09:06:57
Tre'
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 He could just fix the hum problem that started all of this and then he wouldn't need the attenuation at all. Impedance problem solved.

Tre'
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RE: Bartola, posted on February 17, 2025 at 11:08:37
deafbykhorns
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 I built something similar to the Bartola but coupled the output with a TVC to reduce the output impedance (Thanks to Dave Slagle).
Its the best preamp I've had in 40 yrs, it will make a Krell SS amp sound good....

 

I bet that works good, posted on February 17, 2025 at 13:34:15
Tre'
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 The Bartola is taking the output from the mu output connection on the lower device of the CCS plate load. That works like a cathode follower and should have a pretty low output impedance all by itself.

One thing I don't understand about his design, He has a bias voltage coming through P1. He could turn that up which would force him to increase the value of R4 to keep the same set current. That would be a good thing. R4 is not only the current set resistor but it is also acting as the "cathode" resistor if viewing the lower device (M2) as a cathode follower. The larger the value that resistor is, the more feedback there is for the device and that would lower the output impedance of the mu output.

The other thing I don't understand is this, "All schematics and designs are copyright Alejandro Moglia and Bartola Ltd. UK unless stated otherwise. All rights reserved. You are welcome to build the circuits presented here for your own personal entertainment. You may NOT build from information on this page for commercial profit without a royalty agreement with the author in place."

All a copyright prevents others from doing is coping the image of his schematic. Like making a copy of a copyrighted book and selling it. A copyright does not protect ideas. A circuit design is an idea. To protect a circuit design he would have to patent it. To patent it it would have to be new and unique. I'm not sure he's done anything that hasn't been done before.

Oh well, there are a lot of things I don't understand in this world.

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

 

RE: I bet that works good, posted on February 17, 2025 at 16:48:57
Triode_Kingdom
Audiophile

Posts: 10357
Location: Central Texas
Joined: September 24, 2006
 I worked for several years as a technical writer. You're correct regarding copyright. It's a violation to publish the same schematic in its original form (except under fair use provisions). However, if you redraw the schematic so that it's no longer the same document, all's well.

 

Yup! nt, posted on February 17, 2025 at 17:15:13
Tre'
Industry Professional

Posts: 17961
Location: So. Cal.
Joined: February 9, 2002
 .
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: DHT 01a tube preamp, posted on February 19, 2025 at 15:57:59
xaudiomanx
Audiophile

Posts: 3700
Joined: August 16, 2004
 I have one for sale but email me parsonally.

 

RE: Yes, with DHTs hum can be an issue, posted on February 19, 2025 at 16:42:17
mwhouston
Distributor or Rep

Posts: 471
Location: Southern Victoria
Joined: August 20, 2011
 All good ideas.
| retro-thermionic |

 

Back in the day when those tubes were being made, posted on February 20, 2025 at 10:05:55
Ralph
Manufacturer

Posts: 5096
Location: Minnesota
Joined: April 24, 2002
 The volume control changed the heater supply Voltage :)

 

RE: Those are all good ideas but..., posted on February 20, 2025 at 13:50:02
mwhouston
Distributor or Rep

Posts: 471
Location: Southern Victoria
Joined: August 20, 2011
 I tried for months but to no avail. This allows me to enjoy something I have spent a huge amount of time on.
| retro-thermionic |

 

"I tried for months but to no avail.", posted on February 20, 2025 at 18:16:22
Tre'
Industry Professional

Posts: 17961
Location: So. Cal.
Joined: February 9, 2002
 I hate when that happens.

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

 

RE: Yes, with DHTs hum can be an issue, posted on February 26, 2025 at 03:48:34
mwhouston
Distributor or Rep

Posts: 471
Location: Southern Victoria
Joined: August 20, 2011
 Finally got the preamp on the CRO. With the 300K/100K config, through the DIY shieldless cable (175pF), the upper 3db point was 19khZ into 100K. But, taking the 68K/22K suggestion lead to a much better upper 3db;- 37khZ. Not only that but the preamp was even quieter. One channel reading 0.0mV! Plaid all afternoon and throughly enjoyed it.

Gain though is only 3db but these are power tubes so what do you expect? It will deliver 4Vpp into 100K. My DIY SE UL EL34 amp has plenty of gain so no concern. Not only that the two are a perfect synergy. Love the sound of this 6N7 preamp and love the metal tubes.
| retro-thermionic |

 

RE: Yes, with DHTs hum can be an issue, posted on February 26, 2025 at 06:32:38
FlaCharlie
Audiophile

Posts: 965
Location: Gville, FL
Joined: June 1, 2003
 "With the 300K/100K config, through the DIY shieldless cable (175pF), the upper 3db point was 19khZ into 100K. But, taking the 68K/22K suggestion lead to a much better upper 3db;- 37khZ. Not only that but the preamp was even quieter. One channel reading 0.0mV! Plaid all afternoon and throughly enjoyed it.

Gain though is only 3db but these are power tubes so what do you expect? It will deliver 4Vpp into 100K. My DIY SE UL EL34 amp has plenty of gain so no concern. Not only that the two are a perfect synergy. Love the sound of this 6N7 preamp and love the metal tubes."

6N7? Metal tubes? Huh?? Isn't this thread about a 01A preamp?

 

RE: Yes, with DHTs hum can be an issue, posted on February 26, 2025 at 13:32:24
mwhouston
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Posts: 471
Location: Southern Victoria
Joined: August 20, 2011
 Looks like I got my threads mixed. But I've had the same problem over four different preamps based on the 01a, 71a, 6N7 and 12AX7 preamps. Forgive the mixup but the result (good result) hopefully will be the same.
| retro-thermionic |

 

RE: Poor Design for a Preamp, posted on February 26, 2025 at 17:55:07
mwhouston
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Posts: 471
Location: Southern Victoria
Joined: August 20, 2011
 Tre made the suggestion to use 68k+22k resistors to give me a dropping output level but not to limit upper frequency. Tried this ides with a 6N7 preamp I also had noise issues with and it worked well.Even with my DIY shieldless lead (175pF) into 100K upper 3db was 37KhZ. 6N7 are power tubes and Im not sure of they have a very high frequency range or not. Regardless I'm happy with 37K.

Applying the same 68k+20k idea to the 01a preamp gave me noise level outputs of 1.9mV and 0.9mV a big improvement over the ~4.0mV with the 300k+100k setup. At good volume only by putting my ear to my 93db sensitive 12" woofers do I hear the slightest hum then only in the left channel.

Good result for two preamps so far and two to go. Lucky I have a good collection of Rs to mod the others.
| retro-thermionic |

 

Confusion, posted on February 27, 2025 at 09:50:01
FlaCharlie
Audiophile

Posts: 965
Location: Gville, FL
Joined: June 1, 2003
 "Tre made the suggestion to use 68k+22k resistors to give me a dropping output level but not to limit upper frequency."

Actually, I was the one who suggested using the combination of 68k in series and 22k to ground and said it should improve the -3db point of the upper frequencies.

My earlier post: "OK, so in the case of this 01A preamp, he could use a voltage divider with 68k in series and 22k to ground. This would reduce the voltage about the same as the 330k/100k combo he's using and it would present a load of 90k to the tube and be no problem.

The value of 68k || 22k is 16.62k. If we plug that into the low pass calculator then the -3db point would be 54.75k. That should also not be an issue, right?"

Tre quoted me and commented about the corresponding -1db point, which he said would be 27.375kHz. He suggested that, from a technical standpoint, this would still not be enough to "get the filter completely out of the audio band".

I'm glad to hear that, from a practical standpoint, it seems like it improved things subjectively. I try to understand the technical end of things - hence my questions in this thread - but I'm not convinced that striving for technical perfection is always audible beyond a certain point. Sometimes "better than it was" is good enough and going to more extreme lengths is just a waste of time and money if it doesn't yield audible improvement. IMO, but that's just me.

I guess for an engineer technical perfection IS the goal so it's a valid pursuit regardless of whether it's audible or not.

I understand how it's easy to get confused though. AA's software is ridiculously out of date and totally lame. Unlike other sites like DIY Audio, Audio Karma, and others AA doesn't allow you to easily quote a previous post without rewriting it and adding quotation marks. As I have done above.

Unless someone makes the effort and adds a quote manually it's often impossible to tell which previous post someone is responding to.

I suspect that AA will never modernize their software. I don't know about the site as a whole but the forums I used to visit regularly years ago are pretty much dead in the water now. Some of them now go weeks without a post where, in the past, there were a large number of new posts and threads every day. Sad.

I still find some useful and interesting posts on AA but I rarely post here anymore because of the lack of activity. Fortunately, the search function yields a wealth of information from the time when AA was a vibrant forum.

 

RE: Poor Design for a Preamp, posted on February 27, 2025 at 10:19:33
FlaCharlie
Audiophile

Posts: 965
Location: Gville, FL
Joined: June 1, 2003
 "Tried this ides with a 6N7 preamp I also had noise issues with and it worked well.Even with my DIY shieldless lead (175pF) into 100K upper 3db was 37KhZ. 6N7 are power tubes and Im not sure of they have a very high frequency range or not. Regardless I'm happy with 37K."

The 6N7 was primarily intended as a Class B output tube. But the 6N7 data sheets (GE, TungSol) suggests multiple uses for the tube including use as a Class A driver and gives suggested operating points for that application. Since nobody in the DIY community seems interested in building Class B amps all the forum discussion on the 6N7 seems to be about using it as an input / preamp tube.

Apparently, there are some glass (G, GT) versions. I have some of the 6A6s which are glass. The 6E6, 79 and 53 are supposedly equivalents or near equivalents. There's also the 5694 which is equivalent or nearly but has separate cathodes.

At one point I was considering building a Japanese SE 2A3 design that uses them on the input but I was going to use the 6A6 version. I like using less common tubes but I don't really have a need for a preamp with that much gain (mu 35).

 

RE: Confusion, posted on February 27, 2025 at 14:08:29
mwhouston
Distributor or Rep

Posts: 471
Location: Southern Victoria
Joined: August 20, 2011
 Well thanks for the suggestion. Probably like you Im on a number of forums mainly an Australian one which is where I live. Id like to think we are all here to listen and to help where we can fostering friendship and help in a common field of interest.

Two preamps to re-mod. Ill post results.
| retro-thermionic |

 

RE: Poor Design for a Preamp, posted on February 27, 2025 at 14:13:55
mwhouston
Distributor or Rep

Posts: 471
Location: Southern Victoria
Joined: August 20, 2011
 I have priced on eBay some glass 6N7s and though not cheap well within my budget. They do look good though. But still having three boxes of tubes im trying not to buy more but use up what I have in new DIY gear and move on. I have a number of other metal tubes. All sound excellent in their respective amps.
| retro-thermionic |

 

Metal Tubes / Tube Sources, posted on February 28, 2025 at 10:43:52
FlaCharlie
Audiophile

Posts: 965
Location: Gville, FL
Joined: June 1, 2003
 Yeah, I don't remember if the 6A6s I got came from eBay or if I found them locally.

I'm less than 30 minutes from Radio Electric Supply (vacuumtubes.net) which is probably the biggest tube dealer in the world. They have warehouses full of tubes, many millions of them, though not all audio related, of course. They ship worldwide and have reasonable prices.

They're generally much cheaper than the "boutique" sellers who give flowery descriptions of how a particular tube allegedly sounds. As if a particular tube sounds the same regardless of which circuit it's used in or any other differences in the rest of the system. Of course, the "holy grail" tube$ are always what they just happen to have in $tock at any particular time.

The guys at RES (brothers, Roy and Dale) are not audiophiles, they just sell tubes. Hence the total lack of BS.

Another very large dealer is about 100 miles south of me in Orlando. That's vacuumtubesinc.com which also ships worldwide.

The only metal tube I've experimented with is the 6AG7. It caught my interest because it's incredibly easy to drive. I have a breadboarded PSE design that uses them with each channel driven by half of a 3A5, which is a dual DHT with a mu of 15.

It's another "inverted SET" (iSET) design which is a term coined by Andy Evans. I built another iSET amp which I call the Nuance. Details are in a thread on Audio Karma.

The "inverted" aspect refers to fact that instead of using DH output tubes driven either by higher mu indirectly heated tubes or multiple stages of somewhat lower mu tubes, the amp is only two stages with input tubes that are DH. The outputs are indirectly heated and to easy to drive, unlike the output tubes that are commonly used in SET amps.

The PSE 6AG7 is on the sidelines for now, though. I'm back to playing with low mu (10 or less) preamp tubes at the moment which is why your thread caught my eye.

 

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