Power section coupling cap mismatch

[Irkin_Bollikans]

Just got an old ptp amp of unknown manufacture. Among the many strange things I've found inside, I notice the coupling caps from the pi to the power tubes are mismatched severely. One is .02 and the other is .27. I've not seen this before and was wondering if there was a rhyme or reason. Swapping the suspect cap to a normal value seemed to cause an oscillation and guitar signal did not make it through. I can provide pictures if needed

[sluckey]

Schematic?

[Irkin_Bollikans]

Schematic?

  I have literally no idea what this is. Could be someone's home build from 40 years ago. I dont think I'm anywhere near the point of being able to draw one up. Still wet behind the ears

[Irkin_Bollikans]

Just tried to post my miserable first attempt at a schematic and the file is too large. Too bad, probably would have been worth a couple laughs

[Irkin_Bollikans]

I haven't found any schematics showing different value coupling caps in this location. Anybody have any clues as to why it may be like this?

[sluckey]

Post some hi-rez pics. Need to be big enough to see details. If file size exceeds the forum limit then upload to some image hosting website and post a link. You can also post your schematic to such a website.

[Irkin_Bollikans]

https://www.flickr.com/photos/192907383@N07/albums/72157719100147552.

Maybe this will work

[sluckey]

Only rated for 160V? That seems very low for a coupler between PI and output tube.

[Irkin_Bollikans]

Only rated for 160V? That seems very low for a coupler between PI and output tube.

 I agree 100%. But when I switched it out for a .02 600v I just got squeal with no guitar output. Did you see my awful drawing?

[Irkin_Bollikans]

Update- swapped the wrong cap with a bad cap. Lead fell off when I tried a second time. Subbed new one in and no squeal. I'm a little slow to catch on sometimes. Still puzzled about the 6v6 control grids being tied to the heater circuit, but I might need to start a thread on the peculiarities of this amp

[sluckey]

Your drawing looked fine to me. Lotta stuff missing but I get the idea.

Grids tied to filament. I bet if you look closely you'll find that that side of the filament is connected to ground.

[Irkin_Bollikans]

Your drawing looked fine to me. Lotta stuff missing but I get the idea.

Grids tied to filament. I bet if you look closely you'll find that that side of the filament is connected to ground.

Yes it seems that those three tubes use the filament for all grounds. Then after the pi, it goes to one filament wire with a 1.4 ohm resistor inline, knocking heater voltage down to 4.2v for the 6sq7 and 6sj7 preamp section. The heaters are again used as grounds. Maybe this is common and I'm just not getting it

[sluckey]

 They're not using the filament for ground. They simply connected one side of the filament to ground. And it became convenient to connect the grid resistor to ground at the filament pin. Long time ago it was common to tie one side of the filament to ground. Look at this simple schematic for an example...

     https://el34world.com/charts/Schematics/files/Fender/Fender_champ_5f1_schem.pdf

[Irkin_Bollikans]

Any idea for 4.2v on the heaters of the first two preamp stages?

[sluckey]

No ideas. What voltage do you actually measure with one probe on pin 2 and the other probe on pin 7 of the 6SJ7 tube?

[Irkin_Bollikans]

No ideas. What voltage do you actually measure with one probe on pin 2 and the other probe on pin 7 of the 6SJ7 tube?

Same 4.3v. It drops right before the 6sq7 which is the second preamp tube. When I tried omitting the resistor, I got loads of hum. I think I'll be rearranging some things in here but I'm going to take it slow as this is very different than other projects I've dealt with. Been scouring schematics for days trying to find anything close to no avail.

[sluckey]

I think you will have to draw a schematic (even if just a skeleton) and measure voltages for all filter caps, tube grids, cathodes, screens, and plates before we can make any headway. As to the original question, I believe the .27 cap is wrong and was probably put in the amp by someone that didn't know what they were doing. Also post some more hi-rez pics. Include some that show the entire amp.

[PRR]

.... the file is too large.

Indeed. And badly lit.

Image size-How to reduce it.

[sluckey]

...

[Irkin_Bollikans]

I think I did it


https://flic.kr/p/2kWzEKS

[Irkin_Bollikans]

If anybody sees anything obvious that would make this thing stay clean almost to full volume, please chime in. My other amp with a pentode in v1 has little headroom, so I was expecting a little more grit