Back to square one... success! Dumb mistake

[Randy Magee]

I patched in another transformer and to my dismay it too had the low volume very distorted sound, so I'm sure the OT is not my problem... Maybe I have a bad coupling cap that is throwing the bias of the next preamp stage off... I'm grasping at straws now... what else can I try?

[dartanion]

Have you checked all your voltages? Low distorted output can be related to a few different things. Faulty tubes. Faulty filter cap(s). Faulty coupling caps. Parasitic Oscillation. Faulty plate or cathode resistor. Checking voltages can help root out the problem area.

[Randy Magee]

dartanion, all my voltages are in the ballpark... tubes are a known working set...

[M Fowler]

Grounding of the pots perhaps especially volume pot?

[sliberty]

I'd suggest you follow a systematic approach. If this were my amp, I would use a "Weber probe" to check the output and tone at various points in the amp, starting with the input jack. At some point in the amp, the sound will go from good and loud to bad and quiet. This probing approach will help you zero in on what part of the amp is the offending stage.

[rdjones]

I'm tempted to suggest something shorted or not wired right on the secondary side of the output.
Like possibly pulling output from 8 and 4 instead of 8 and common.
A short will still let some signal reach the speaker.

rd

[marcoloco961]

I patched in another transformer and to my dismay it too had the low volume very distorted sound, so I'm sure the OT is not my problem... Maybe I have a bad coupling cap that is throwing the bias of the next preamp stage off... I'm grasping at straws now... what else can I try?

In your last thread you said "I traced a clean signal through the circuit all the way through the phase inverter... what are the odds having a bad OT right off the shelf?"

I think you are close to figuring this one out. It sounds like you have ruled out the OT and have traced a clean signal all the way through the PI. Mark Fowler was asking about the two signal wires ( the blue and the white ) to the power tubes. One attached above the board and the other below. Even though the solder joints might look good, re-flow them anywhere between the PI and the power tubes. You might consider moving the blue wire connection above the board also, maybe it is making some kind of contact with the chassis under the board where you cannot see it. Re-flow both connections on the power tubes and grid resistors from these wires also, anything in the signal path between the two points. I have seen solder joints that look great but have a cold solder on one leg of one of the parts causing a problem. Look under the board for possible solder that has "leaked" through the turret and sent the signal to ground to the chassis.

The only other thing I can think of is a parasitic oscillation of sorts, and you might need a scope to find the source of that.

I am betting it is something simple in this area. You will get it, and this is where you actually learn about what makes the beasts tick. Good luck, you will get to hear it soon, and it is worth the wait.


BTW, that is a very nice looking build, sweet work.

[passfan]

Flyback diodes on the output tubes could be bad ?? I think we've had a couple amps with these in backwards and I think this created the same condition.

[Gibsonman63]

Have you re-checked your voltages after your troubleshooting? I found on my last build that my jumpers under the board came unsoldered from my troubleshooting efforts. With a small mirror and a flashlight, you can take a peek under the boards to check that everything is still intact.

[Randy Magee]

I am going to revisit both boards and see where I end up....

[geetarpicker]

Flyback diodes on the output tubes could be bad ?? I think we've had a couple amps with these in backwards and I think this created the same condition.

Good call there. I'd lift the legs on one side of each of those and see what you get. Perhaps they were cooked during soldering...

[dartanion]

How about posting a couple of photos. Extra eyes helps solve problems.

[M Fowler]

https://tubeamparchive.com/viewtopic.ph ... sc&start=0

There are three or four separate threads on this.

[redshark]

Did you check every solder joint? Something similar happened to my rocket build. It was fine and one second after half the volume and distorted. How did I solve it? I took a chopstick and with the amp on at least half the volume nothing plugged in the input jack and of course a speaker cabinet plugged I took the chopstick and hit lightly every turret and every wire and every control until u hit one and listen to extra crackling noise, that means that there you have the problem and just resolder or reflow the joint...

[Randy Magee]

I lifted one end of the flyback diodes and no joy... I've chopsticked every joint on this with the amp up, but can find no short. I'm leaving in a short while to attend the King Biscuit Festival and will take a vacation through the weekend on this consarned amp...