Need help understanding this tube amp circuit

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bancika
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Need help understanding this tube amp circuit

Post by bancika »

Hi,

I'm trying to salvage an old Italian phonograph and convert it into a guitar amp. It's an odd push-pull design with ECL80 tubes that might be interesting for guitar, apart from an auto-transformer that will need an isolation transformer to make it safe.

I traced schematic of the amp minus the power supply. There's not much missing, the power supply uses EZ80 tube wired as half-wave rectifier, going into a 33uF cap, 1K series resistor and a 47uF cap (used to be also 33uF). That's the node that powers the whole amp. I replaced electrolytic capacitors and one resistor that was blown, but still can't get it to work. I'm getting motorboating sound though the speaker and no input signal comes through. Here's the clip
phono_noise.wav
Can someone please help me understand what's going on in the schematic? ECL80 have common cathode for the triode and pentode. All cathodes seem to be connected to the same node and share a 180R resistor and 220uF bypass cap. There's no conventional phase inverter, or at least I don't see it. There's one gain stage that feeds the first pentode, then the next gain stage that drops most of that signal using 270k/27k voltage divider and amplifies it back, I suspect only to flip the phase and drive the second pentode. There's some strange negative feedback circuit in the first triode with a pot labeled "Espans", I suspect short of "Expansion" in Italian. There's also a 270K resistor forming a feedback loop on the second triode. Screens are fed directly from B+, no screen resistors.

Can anyone confirm my analysis? Would you suggest any changes to make it better? Or tips to have it fixed?

I plan to take all voltages when I have some time to spare. Will update the schematic with those. B+ is around 220V DC.

Cheers,
Bane
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Cameron
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Re: Need help understanding this tube amp circuit

Post by Cameron »

Google ...self split phase inverter. A few guitar amps used this circuit.

Here's a thread on it ....

http://ampgarage.com/forum/viewtopic.php?t=22408

And a page with some links with more info...

https://dalmura.com.au/projects/Valve%20Info.html
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bancika
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Re: Need help understanding this tube amp circuit

Post by bancika »

Thanks. I've came across self-split PI before, but this doesn't seem to look like it. The second triode inverts the phase for the second pentode. So it's not self-split.
Or maybe I'm missing something.
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Malcolm Irving
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Re: Need help understanding this tube amp circuit

Post by Malcolm Irving »

I think your analysis is right. It’s an unusual design. I can’t claim to fully understand it, but here are some thoughts:
Once you have a big bypass cap, such as 200uF, you can regard the voltage at point A as constant. If the voltage needed to bias the push-pull pentodes is also suitable to bias the two triodes, the component count is reduced by sharing point A for all four tubes.

EDIT: The ecl80 has a common cathode, so sharing the cathode circuit between the triode and the pentode is obligatory.

V1a is a triode gain stage, with some variable negative feedback (for high cut?).
I think that V2a could be understood as follows (not entirely sure): If the voltages at pins 9 of the two pentodes are correctly out-of-phase with each other, the voltage divider formed by the two 270k resistors will lead to zero AC voltage on the grid of V2a. If pins 9 are not out-of-phase, the discrepancy will be amplified by V2a and ‘corrected’ by the AC voltage at pin 1 of V2a via the 22nF cap (negative feedback in other words).
Last edited by Malcolm Irving on Tue Nov 28, 2017 8:07 pm, edited 1 time in total.
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Re: Need help understanding this tube amp circuit

Post by Stevem »

And you have tested all the tubes for shorts?
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Malcolm Irving
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Re: Need help understanding this tube amp circuit

Post by Malcolm Irving »

The sound clip is definitely motor-boating. Did you replace the 200uF bypass cap along with the other electros? I think a bad cap there would be a likely cause.
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bancika
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Re: Need help understanding this tube amp circuit

Post by bancika »

Stevem wrote: Wed Nov 15, 2017 11:44 am And you have tested all the tubes for shorts?
no, I will do that.

@Malcolm, yes, I have replaced all three electrolytics with new ones.
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bancika
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Re: Need help understanding this tube amp circuit

Post by bancika »

update: checked all 3 tubes for shorts and they seem fine. I also measured voltages and they seem reasonable, although they fluctuate a bit all the time while measuring, like +-5% or so. Could be related with the motorboating issue. I also checked that the two 22nF coupling capacitors aren't leaky and they are not, as I measure 0V dc on the the other side.

Voltages are listed below. DC after the rectifier is ~260 and drops to 210 through a 1K resistor, so it seems like the whole circuit is pulling around 50mA. I guess that's reasonable. According to the data sheet, the pentode plates pull around 17ma and screens around 8mA.

Any other ideas?

Cheers,
Bane
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Re: Need help understanding this tube amp circuit

Post by JazzGuitarGimp »

What does the pot labaled EXP do?
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bancika
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Re: Need help understanding this tube amp circuit

Post by bancika »

I don't know, nothing at the moment :) Seems to control local negative feedback network on the first stage, some sort of tone control I guess.
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bancika
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Re: Need help understanding this tube amp circuit

Post by bancika »

Update. Still struggling to get it to run. Motorboating seems to be the only issue so far. I've played with it a little and have few more clues:

1) it's not the tubes. I bought a pair of NOS ECL80 tubes, replaced them and nothing changed
2) the input signal comes through the speaker, mixed with the motorboating sound
3) when I switch the mains off, there's a split second when it works fine without motorboating, probably as the capacitors discharge. Maybe this is a good clue? I've tried to record this and uploading the clip there
4) It's probably not the filter caps. I tried adding a capacitor in parallel with each of the capacitors and made sure that they charge up to the B+ voltage properly. They don't change anything.

Any comments would be appreciated!

Thanks,
Bane
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Malcolm Irving
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Re: Need help understanding this tube amp circuit

Post by Malcolm Irving »

The common point A couples all 4 valves together, as far as very low frequencies are concerned, and is a big suspect for causing motor-boating. This is because very low frequency oscillation is invariably associated with the long time-constants of high value caps.
Have you tried clipping an extra cap across the 200uF cap? For example if you put another 200uF in parallel, does it change the speed of the motor-boating?
Last edited by Malcolm Irving on Tue Nov 28, 2017 7:58 pm, edited 1 time in total.
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Malcolm Irving
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Re: Need help understanding this tube amp circuit

Post by Malcolm Irving »

If the 22nF cap between V2b and V2a was leaky it could also cause problems – might be worth replacing that one.
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bancika
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Re: Need help understanding this tube amp circuit

Post by bancika »

Malcolm Irving wrote: Tue Nov 28, 2017 7:32 pm Have you tried clipping an extra cap across the 200uF cap? For example if you put another 200uF in parallel, does it change the speed of the motor-boating?
Great tip, this solved the issue completely. Added a 220uF in parallel with the existing 220uF I put instead of the original 200uF...and the issue went away. I tested the cap, made sure it's not bad and it seems like doubling the capacitance was all that was needed. Now I'm wondering how it worked with the original one :)

The coupling caps are not leaking I think, I measured 0VDC on the output side.

Thanks for all the help! Now I need to get an isolation transformer (or just replace auto-transformer with a proper one).
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