Hey everybody, hope you are well. This is my first post here and I'm hoping you can help me figure out an issue I'm trying to resolve with a buddy's HD 130.
So the story goes like this - my friend bought this amp a while back and at some point some years back, he ordered a matched quad of tubes. He said he tested it out and it apparently worked fine. He put the amp in his basement where it sat undisturbed until about 2 weeks ago. When he went to play it, he said he got no sound out of it, just a loud hum. That's when he brought it to me. I'm certainly no expert but since I've built a few amps in the past and fixed a few others, he asked me to take a look at it. I was able to get intermittent sound out of it, but it was very "blatty" and gated in sound.
SInce the caps had never been changed, I thought I would start there and I ended up changing out the two 100uF power caps and the four 150uF caps that are used to bias the opamps in the preamp section. I did not change the smaller electrolytic caps in the power amp section (a dubious mistake as I'll now have to pay shipping again on more capacitor if I decide to change them). After making these changes, I fired up and got the following sound from the amp.
https://www.youtube.com/watch?v=2fggAvd81LQ
I might be mis-remembering but I do think this was an improvement from my first test, though it's obviously not "fixed".
Reading a little more, I found a post describing how to bias the transistors in the power section using the trimpot. Based on the notes, I was attempting to get as close to 25mV across the 3.9 ohm resistors as possible. Interestingly, I could not get the value to budge much (if at all) from approximately 20.8mV at either end of the trim pot. In fact, adjusting it seemed to have no effect on the value at all.
Can anyone provide some suggestions on things to try or check next? Are there any common points of failure on these? I did also check the voltages at the 1N4745 diodes and got -16V and +15.75V, so it appears to at least be in the ball park of what the opamps are expecting to see.
Thanks!
Just adding a comment so I
Just adding a comment so I can flag the topic for updates via email - didn't see that option when I initially posted
Sounds like a leaky cap
Sounds like you could have a leaky cap between the preamp and the driver stage. The two diodes in the bias circuit will let the bias regulator voltage travel between approx. 1.2V and 0.6V. This voltage will be present where R58 and R59 and C43 meet. If any stray DC voltage is applied to the other ends of R58 or R59 respectively, you will lose the ability to adjust the bias. Some voltage checking in this area is definitely in order.
Sincerely
Lars Verholt
Good Advice
Good advice Lars. Something is clamping the bias voltage too low. With the amp off, check the pot (TR2) to see if it changes resistance when adjusted. Also check to see if one or both diodes (D15,16) have failed. Once the bias network is fixed, other advice would be to clean all of the panel pots, especially the master volume pot to get rid of the scratchiness. Since it was sitting in a basement for who knows how long, it would be prudent give all of the input and speaker jacks, tube sockets and IC sockets a good cleaning with something like DeOxit. Good Luck and let us know what you find. -mgriffin
Thanks for the ideas Lars and
Thanks for the ideas Lars and mgriffin. I will give these ideas a shot as soon as I get a chance and report back with an update.