Trying to ID and tune up a stereo HIFI amp

[sonicmojo]

Hi guys,

I have a little extra time being stuck at home and pulled this beast off my shelf. Someone had given it to me years ago and it is a homemade stereo amp of unknown origins (says William Abel Design but no google info).

It has dual 5AR4 rectifiers, 6AU6 tubes, and had a quad of EL34s in it when I got it.

I'd love to get it working right. It does have a slight hum but I've never done any serious debugging on something like this. My cursory check of all the filter caps by paralleling another on each one didn't seem to help the hum so I figure the issue may be elsewhere.

My question is do any of you HIFI guys have a clue as to what this may be based on so I could grab a similar schematic and try to understand it a little better?

Thanks!

[Stevem]

Try looking at some Fisher, Scott, Dynaco or Laffayette schematics.
Of course with time on your hands you can always bite the Bullet and just draw out a schematic should you really think you need one.

[xtian]

Wow, that thing is awesome! You should definitely re-cap it.

[sonicmojo]

Try looking at some Fisher, Scott, Dynaco or Laffayette schematics.
Of course with time on your hands you can always bite the Bullet and just draw out a schematic should you really think you need one.

Thanks, I will look at those. I traced as far as the input RCAs to the first preamp tubes.... The nice thing is since it is stereo and duplicate on each side, it is only half as complicated as it looks....

[sonicmojo]

Wow, that thing is awesome! You should definitely re-cap it.

It is cool and those block transformers are so heavy! Caps for sure, I suppose as the first step. The mallory cap on the board is dated 1977. I'll order some radials for that bank in the row. I put tubes back in it and don't get the same hum I was getting the last time I checked but am using different tubes. It is silent on idle.

If I play music through it, it sounds like it has good power but is distorted. When I inject a sine signal through it, I think I am hearing some ghost octaves. It is that way on both channels. It is very possible some of the filter caps are causing that but I am also concerned about those .22uF Vitamin Q oil caps so may pull a few and measure.

The pots between the speaker jacks are marked AC and DC on each side. I assume these are a tuning adjustment but I don't know how they are to be used.....research...

[sonicmojo]

Progress. I managed to squeeze my bias probes up under the low clearance chassis lip and made some adjustments to the bias supply. The bias was super cold and was causing the distortion. I know realize the "DC" pots on the front make fine tune adjustments to the bias for each side. I'll have to figure out what the "AC" trimmers do. It is hard to deduce where things connect under the board.

Now that the bias is better, it has a small but audible hum but it sounds pretty good otherwise! New filter caps ordered....

I don't think the hum I am hearing is filter cap related so I will keep poking in the meantime....try some different tubes, check ground connections, probe, etc.....

There is a multi position selector switch on the far right side that ties to the input jacks, power tube cathodes, and front panel meter. I assume it acts as some sort of adjustment for the meter but I don't know what it does. Each power tube cathode has an 8.2 ohm resistor to ground. I can't find anything out there that looks similar to this but will keep digging.

[xtian]

The "AC" trimmers are probably heater balance...

[sonicmojo]

The "AC" trimmers are probably heater balance...

I'll check that, thanks.

One channel sounds different than the other. It seems I have a couple volts of AC and DC on the input grid for one of the channels. Not sure what would be causing that....but I think it is a problem on that side. Swapping tubes around did not help. From what I can tell, it is a direct connection from the input jack to the 10K grid stopper/470K junction.