I have an early 70s V4 that has been recapped and re-tubed. It plays awesome but with no input and the volume all the way down it has a hum (filament?) that I can get rid of with the hum balance pot. But then if I turn the volume up (no inputs) it hums loudly. I can cancel this by readjusting the hum balance but then with the volume turned back down it hums again.
I've tried multiple combinations of tubes. Currently using JJ-12AX7a, 12dw7 and NOS USA 12au7. Any ideas?
honestly, all of the Vt-22s and V4s I've ever worked on or owned were the same way. Its a little bothersome, but i just started adjusting it to the volume range I tended to play in.
I have other model ampegs with the hum balance that seem much less interactive, but they do the same thing.
I've done a little investigating. It's the first stage of the input 12ax7 V1 & V2. I disconnected the wires from c3,c6 to the control board and all is quiet. Hooked them back up and shorted the inputs to V1, V2 and still noisy. I unhooked the sensitivity wires from the main board and it gets better but not great. I put a scope on the output of c3 and definitely see trash when the hum balance is adjusted. I'm out of time for a few days but plan to take a hard look at the cathode circuits of v1,v2 when I get back.
02-15-2016, 01:17 PM (This post was last modified: 02-15-2016, 01:26 PM by Hangman.)
It could be an issue with the PCB. Ive found that many amps with a PCB have hum issues, and its often times because the heaters get delivered to the tube sockets through pcb traces, rather than twisted pair wires. it could be something as simple as the heaters inducing small voltage somewhere in the early preamp stage, which then gets amplified. I wish I still had my Vt-22 so I could test that hypothesis.
on the other hand, there is a reason people tend to use DC filaments for early gain stages.
The PCB layout may be contributing but I would expect the results to be more consistent from amp to amp. That said, Ampeg's PCB layout is pretty close to textbook except that they obviously couldn't twist the pairs. They did run them side by side on the PCB except directly across the tube socket.
If I don't come up with a better solution, I might sacrifice a PCB and convert the V1 filament to twisted pair. I'm also wondering if some 12ax7a are less susceptible to filament hum than others? Maybe the right tube fixes it? I don't remember hum being an issue when we used these new in the 70's and I doubt Ampeg could produce them in large numbers with hum problems.
I don't know if it is a gain difference but the second triode in the same tube with the same filament wiring seems quiet in both V1 and V2. First I'm going to try changing v1, v2 pairs with some NOS tubes and see if the hum gets better or not.
I'm all for less noise, truly. Not sure just how much hum you are getting - but with mine, once I am playing at any reasonable volume, it's not even there, and I have the bass boosted up not all the way, but pretty high.
I've never really even had the music stop and though "man that thing is humming"
Is it affecting recording/live playing in a true sense, or just that nuissance of "man I wish it were silent?"
I understand wanting it to be silent just the same - orville amps (steelyman) sells a replacement pcb or two that might be worth investigating if it's real bad. Other options above might work. AT some point though, there's going to be hum and hiss. Never met a tube amp (yet) that I didn't HEAR that it was now on, when it was on.
02-17-2016, 06:14 PM (This post was last modified: 02-17-2016, 06:17 PM by Hangman.)
I haven't found that the hum got any better with different brands/types of tubes. it was always present for me.
Here is a schematic for DC Filaments from a mesa Mark III they just used a bridge rectifier and a capacitor right on the 6.3 taps. you wouldn't have to add much to test it.
I'm short on time this weekend but I did the tube swap and no improvement. Then to be sure there isn't anything funny with the "E" supply, I added a filter capacitor at R6 and Gnd. Also no improvement. I'm more convinced it is filament to cathode hum. Next weekend I will sacrifice a board and add a DC rectifier/filter for V1, V2 filaments. The main problem is lack of physical room for a filter cap on the board.
02-21-2016, 11:54 AM (This post was last modified: 02-21-2016, 11:56 AM by Hangman.)
(02-20-2016, 09:10 AM)AEB-1 Wrote: I'm short on time this weekend but I did the tube swap and no improvement. Then to be sure there isn't anything funny with the "E" supply, I added a filter capacitor at R6 and Gnd. Also no improvement. I'm more convinced it is filament to cathode hum. Next weekend I will sacrifice a board and add a DC rectifier/filter for V1, V2 filaments. The main problem is lack of physical room for a filter cap on the board.
the cap doesn't have to be huge. the voltage rating can be like 16 volts, and you could use two caps, one on each socket... if you wanted to use smaller values.
The DC filament supply doesn't have to be completely free of ripple either. a little ripple won't be audible compared to the full AC heaters.
I am interested to see how this goes. I've always wondered if it would help, but never bothered to try.
I sacrificed a board, cut the filament traces, and installed a rectifier with capacitor. It made the problem worse due to fact that I couldn't get pure filtered DC. Even with 11000uf cap the dc had a hard notch which got bigger or smaller depending on the hum balance setting. It ended up with a raspy buzz rather than a hum. It did stop the additional hum that you get when you touch the 6800 ohm resistors on the sensitivity switches.
Restoring the original filament traces gets it back to the original hum levels.
I'm going to continue the investigation with the original filament layout.
