Low output and distortion.

[Ampzone]

Hello,

First post here on this great forum. Lots of threads to read that will keep me busy. Problem...recently came into possession of an DR504 50w type clone. I was told it had problems and did not work. So upon testing I checked the fuses...in place and good. So turned on the amp without tubes, it lit up, voltages were good heaters etc. Added tubes and powered up, no sparks, no HT or mains fuse popping, then set bias...no problem, power/preamp tube voltages were good. Sound though was only a very faint hiss/tinny buzz coming through the speaker.

Plugged in a guitar cable and played, output now was very low and heavily distorted. Bias was also running much higher with the cable installed (even without playing), also the power tubes flickered on and off. Not good...quickly shut it down. Powered back up with the cable removed, bias went back to normal, noise was less but very low, no tube flicker?

Order of attempted fixes:
Replaced power tubes, no change.
Replaced preamps tubes, no change.
Switched speaker cable and speaker, no change.
Checked bias cap/good
Resistors are in tolerance.
Checked input jacks/good
output jacks/good

Checked OT for shorts, none that I can tell.
Checked output/preamp pins for something loose/cold solder joint/none.
Removed and tested power supply filter caps/no change
High voltage readings are good throughout.

Plug the input cable back in and it's back low output/ distortion/runnaway bias and tubes flickering.

Something simple I am overlooking I bet. Starring me in the face. Any ideas?

Ampzone

[52 Bill]

Welcome to the place. I'm really not that good with numbers, so let me ask what a DR504 is?

What is the output tube bias voltage at idle with no signal? What is it when you add signal?

[Gtr_tech]

Hiwatt.

[Gtr_tech]

Yer prolly looking at an output transformer problem. Sub a known good one to verify.

[Ampzone]

Yer prolly looking at an output transformer problem. Sub a known good one to verify.

Thanks for the advice. I checked the OT for shorts. The readings I found did not indicate a short but I am not 100% sure. I have a couple Marshall style 100w OT's laying around, no 50w OT's for this 50w amp.

Ampzone

[Ampzone]

Welcome to the place. I'm really not that good with numbers, so let me ask what a DR504 is?

What is the output tube bias voltage at idle with no signal? What is it when you add signal?

At idle negative voltage around -40. The bias set appx 32ma. It jumpts to 80-120ma when you just plug the cable into the input.

ampzone

[Ampzone]

Yer prolly looking at an output transformer problem. Sub a known good one to verify.

Ok..replaced the OT with another. No sound from the speaker with this tranny. Guitar cord plugged in when touching the tip causes the bias to jump to appx 100ma. Tubes clink (internal sound) and filaments get brighter with contact on the cable. No sound though whereas before it was low output and distortion.

Hmmmm.

[Ampzone]

...to be on the safe side I am going to try another OT just in case this 2nd one (used) was faulty.

[überfuzz]

If it's a home-brew, you wrote clone, maybe you should start looking at the wiring. Do you know if it ever worked as it supposed to?

[Steve Conner]

Either the speaker output isn't connected to the OT properly (short, open circuit) or you have an instability/parasitic oscillation type problem.

Check your speaker jack wiring, speaker cables, cabinet etc.

"Filaments get brighter" is actually the screen grids glowing, when you see this you're at risk of burning out the tubes. It suggests the speaker wiring is open circuit.

[Ronsonic]

My bet is some sort of HF oscillation. Where it's from and why???? Time for some scope time.