Weird distortion.

[Dave]

A guy brought me an amp to work on that has some weird nasty distortion at all volumes. It's supposed to be a Plexi knockoff. Build looks decent. I asked him if he had ever heard the amp work right and sound good and he said yes. I dig through the amp, looking for the obvious things and there's nothing wrong (apparently). All components look good, test good. I can't find anything wrong. It's hard to follow the circuit because the builder ran a good portion of the wires under the board.


Finally, in frustration, I separated the preamp and power amp to see if I could find the issue that way. It is in the power amp. The tubes are good. I can't find any components that have stopped working correctly. I think there is a wiring issue and that the amp has never worked right. I don't know why the owner would tell me different, but I think that's it.


The only clue I have is that if you turn the presence control upscale the problem gets much worse.


Any ideas?


Dave

[Esquirefreak]

I think it's hard for anyone to help without a schematic or at least some pics.

/Max

[Dave]

I agree. If I had a schematic I would have posted it. It is supposed to be a one channel Plexi. It seems somewhat true, although I have found some slight changes. Pics won't help either because all the wiring is under the board. The board sits up high so as to make it difficult to see the tube sockets with a camera. Sorry... I was hoping that someone might say "Oh, that sounds like blah blah blah." and give me something to track down. I understand the need for schematics and pictures, but I can't do that this time.


The problem, again, is either the PI, or the power section. Tubes test good.



Dave

[Esquirefreak]

Bias numbers?

Sounds a little bit like one of the power tubes is drawing more or less current than the other. Or possibly a failing OT. Or just very low bias causing the tubes to cut off.

/Max

[Dave]

Both are valid points. I will recheck the bias, although I believe it is good. I had thought about a failing OT, but ignored that thought because of wishful thinking. Thanks for reminding me that checking that is better than continuing to chase ghosts.


Dave

[Dave]

400 on the plates
397 on the screens
-33 on both grids
Current draw 31mA and 33mA


That looks ok to me. Now I'm worried about the OT anything else you can think of before I perform major surgery?


Dave

[passaloutre]

Switch the blue and brown wires coming from the output transformer

[Dave]

"Flip the blue and brown wires from the OT"


I thought that the negative feedback circuit might be wonky because of the way the Presence control was behaving. So I tried disconnecting the NFB to see if it went away. No dice. Unless I'm mentally omitting something, that should have had the same effect (for testing purposes) as swapping the OT primaries, no?


Dave

[passaloutre]

Yes, disconnecting the NFB would also work to diagnose that.
 
I used to think that a reversed output transformer polarity would always result in that signature whine, but now I've come across a few examples where the results were a bit more cryptic. Since you mentioned power amp, pi, and presence control, I thought I had a slam dunk.

I suppose it could be some ultrasonic oscillation/feedback somewhere else.

What else have you done diagnostically? Next step is to make a voltage chart IMO.

[Dave]

I've traced out the circuit on the surface as best I could. All components seem to be good. The circuit is fairly true to a plexi circuit with the omission of the tremolo stage. I can't see everything, but everything seems to be connected correctly. Bias and voltages are good. Voltages to the preamp don't matter because I have it disconnected anyway. I have replaced all electrolytics. I'm getting mentally prepared to swap the OT as much as I don't want to.


On that note, I have seen OT's go out and create a short and I have seen them go open. I have never seen one work, but badly like making ugly distortion. Is that even a thing?


Dave

[Dave]

Alright, so I did a quick check of the OT without removing it.


It shows the following turns ratios...


4r  - 16 to 1
8r  - 7 to 1
16r - 10 to 1


I'm pretty sure that's not wired correctly, but I'll have to do the math to confirm. It shows 40 ohms across the primaries end to end. I didn't measure each one to the center tap.
Those numbers don't seem strange to me. Do they seem strange to you (other than the turns ratios being wired wrong)?


Dave

[Dave]

Alright, I just tested a Super Reverb OT for turns ratio to get a ballpark comparison. I guess I must have forgotten what the numbers are supposed to look like.


Super Reverb runs two 6L6 at similar voltages to the EL34 amp I am working on.
Turns ratio = 44 to 1 --- 44X44X2 = 3872


7X7X8 = 392


Now I know that EL34 OT's run a little fewer turns than a 6L6 of similar design, but not 10 times the difference in total impedance.


I guess maybe it's safe to assume now that the OT is shot?


Dave

[passaloutre]

The Hammond OT for "Plexi" type amps (https://www.hammfg.com/part/1750N) specifies a primary impedance of 3200R, so I'd expect a turns ratio to the 8R tap around 20:1. What are you using as an AC source for your measurents?

Sounds like you need a new OT. Or at least hook up a spare to prove it to yourself.

[Dave]

I run a variac into the primaries until I see 1 volt on the secondaries.


Dave

[elams1894]

With weird farty distortion, the majority of the time I find the phase inverter either has a bung tube, or the PI plate resistors are toast. Especially if they are only 1/2 watt. I did have a big amp (130 watt Rockit) once that puked an OT. The distortion before it blew was weak and anaemic, with 2 of the 4 power tubes red-plating. Took the 15" speaker with it. It certainly was a different tone (more a prolonged death cry) than the woofy-farty-cheese type distortion of a bung PI.

It killed me to part with the 15" alnico JBL, so lessons were learned. Red plating 6L6's now haunt my dreams.

[tubeswell]

400 on the plates
397 on the screens
-33 on both grids
Current draw 31mA and 33mA

13W is on the cool side for a 25W tube


What does a 1kHz output signal look like (on a scope)?

[Dave]

"What does a 1kHz output signal look like (on a scope)?"


Oooh, sorry... I already have half the OT unhooked and the board unscrewed.


Dave