Tube Amp Archive

It sounds good, I need to go through the pots and jacks and clean/replace where needed, there’s some crackling here and there. I’m pleased with it though. I haven’t gotten too deep into what the waveform stuff on channel 2 does so that will be interesting.

That was exactly it! Replaced c63 and I’ve got proper PI measurements. Most importantly, the amp now works! Thank you for all your help Martin!

The two amps are the same. Both SG-100. Mine just doesn't have the reverb or phase shift circuits. So as far as plate 1, the voltage drops at R82 down to 19V and is still that at the plate. The resistor seems fine so I'm suspecting an issue with c63, I'll check that out this afternoon. I haven't rea...

I’m thinking there may be a problem in the PI circuit. I’m getting 19VDC on plate 1 (Pin 1) and 1VDC on plate 2(pin 6). I’m not sure what it is supposed to be, but I feel like it should maybe be higher than that?

Yes, AC at grids, leaks and stoppers. Pop and hum in the speaker when those are probed, much louder than the the guitar signal through the speaker.

I do not know how find the voltage ratio. I checked DC resistance and had about 44 between center and each plate. I also removed the 100 ohm resistors that were between the heater and cathode on the tubes and I can now hear it, but it is very quiet. Channel and master up to 10 and it’s quieter than ...

You were right, there was a short at the cap going to AC. I now have the tubes biased properly. Good measurements throughout the power section. Thank you so much for your help! Now I have the issue of no sound going to the speakers. I am going to probe the preamp section, but I’m hearing pinging noi...

Before it blows I get no reading across c51. Also no reading through the rest of the bias circuit. Must be a wiring error somewhere. When you say you get no reading across C51, that's DC? Is your modification putting AC voltage on it? Yes. I just checked and I’m getting 17VAC across c51. That expla...

With all modifications removed I’m getting about-16 to -21V. It will not run 6550s or EL34s as is. I tried each and could not get the 6550s current below 100mA, which is the limit of my probe. EL34s I got down to 67mA, which is still far too high. I have rechecked the modification several times and ...

Put another capacitor in and got some readings, nothing at c51. 23v at c53 and c54, was 33 without the modification. Didn’t blow this time but was very hot when I turned it off.

I disconnected all of my alterations and it looked good. Nothing blew, -33v at c51-. Let it run for about 5 minutes, everything good. Reconnected the modification and c51 blows again. I’ve triple checked electrolytics and diodes. I’m stumped. The cap blew before I got a measurement on it to see if i...

If you can understand the poor layout here, this is how I arranged everything, using existing unused traces. Maybe you see something dumb I did that I’m not noticing? Also, D3 and D4 should be 1n4007s? I put those in. The ones that were there were unlabeled, but looked really bad so I replaced them.

Diodes are oriented correctly

I’m pretty sure they are. I will double check though

I’ve done the conversion for KT88, I’m using 6550s though. Any idea what could be blowing C51? Also, since I got it without power tubes I don’t know if it was in working condition before I started messing with it. Since The changes go directly into c51, the simplest answer is that I likely did somet...