So i'm stuck with this hum that won't quit. I did a lot of work on my V4 (aluminum knobs version). However the amp does put out some good sounding bass.
So here's what i've done so far :
- Changed every darn cap in the amp including large filter caps with the fliptops kit. (did the V4B coupling cap conversion)
- New JJ's 7027 matched quad
- All resistors seem good
- All tubes seem good, i tried replacements for every single one
- Tried poking around the boards and connections with a wooden stick, nothing happened
- Reheated almost every soldering point
- Deoxited every pot and jack (deoxit fader F5 for pots)
- Grounded AC installed
- Tone PEC 7 pin package replaced with discreet components (eq works fine)
I tried removing tubes (V1 and V2, then V201, then V3) to isolate the hum, but there seems to be hum induced in every stage... The only way that the amp quiets down is when i take out the phase inverter V4 tube. I also noticed that the hum balance pot doesn't change anything at all when i turn it. The post resistance reads good. I also tried changing the case orientation to see variations due to possible outside magentic fields but nothing changes. I tried shorting various points of the signal path to ground and it reduces the hum, with more reduction as i move closer to the power tubes (output of C7, input of C201, output of C10).
I'm working the same issue in the workshop thread V4 Hum Balance. These amps seem to develop ground issues over time.
The preamp (most of the amp except the echo board) is grounded via the input jacks. In many cases these jacks don't pass the ground to the front panel well but test fine with an ohm meter. The echo board is grounded by the footswitch jack (questionable design) which sometimes has the same jack issue. I replace all three jacks and the associated shielded cables/resistors.
If the star washers on the jacks are corroded, get new ones at you home improvement store. There will likely still be other hum issues but this is a good start. Follow my other thread. I will update it as time permits.
03-07-2016, 11:54 AM (This post was last modified: 03-07-2016, 11:55 AM by Liquids.)
Sucks to hear that two of you are having dramatic hum issues.
Hope to hear more about what resolves the issue.
One thing worth trying - if your amp has the 'ext in' jack. Pulling tubes is a basic, useful test. Pulling the PI and getting no hum isn't super clear to me. But if you can isolate things by disconnecting everything before the ext in, and input a 'signal' at the ext in point (even if one that's silent, as with out a volume control, it's pretty loud to play anything at the ext in!), without the prior preamp stage also feeding the point, for the early amp schematics, that means you're down to one tube stage, the PI, power supply/filter caps, and the power amp stages as being part of the hum.
Having replaced all the coupling and filter caps puts you in good position -- it's what this amp needs regardless. Once you can get it running cleanly without so much hum, you'll be happy you did all that, for sure. Also sets you up for knowing the amp inside and out, which is useful down the road.
Sorry I can offer more help. Do you have a 'scope to get a handle on just how much hum you're getting, peak to peak, at various points in the audio circuit, and in the filtering circuit?
Thanks Liquids, i just tried putting some signal through the ext in jack. With or without the prior V1 and V2 and V201 preamp tubes in place, i get significant noise reduction when i plug something in, (my phone). However, there is still some pretty loud humming and the remaining hum is more in the midrange audio spectrum. I tried swapping V3 and V4 with brand new ones and still get hum.
@AEB-1 : seems like the echo board is grounded by the footswitch jack, but also by one of the standoffs that has a tab screwed on and soldered to the board's ground bus. I tried unsoldering the wire going to the footswich jack with no improvement and the humming... I'm now gonna try cleaning the input jacks and changing the shielded wires to see if I get any improvement on the input section humming. But since i'm getting humming in every stage, i'm starting to suspect that the hum is coming either from the "E" power supply rail or from the heaters... Any idea why tweaking the hum control balance pot isn't doing any change at all on the hum ? Shouldn't it be at least increasing the hum when turned all the way up or down ?
another question : i don't have a scope but can i measure the power supply rail ripple measuring the ac voltage on the rails ? if yes, what readings should i be getting for a "clean" rail ?
I suppose that tells you something and is useful info to have...but what that tells you is not 100% clear, depending on other variables.
I forgot to say in my last post - I think very few of us landed a V series amp that was 'good to go' from the first moment. I love mine -- it's the ultimate tube amp, for me--but it took weeks and many sessions of chassis out/chassis in over a period of time before the amp was genuinely stable. The work is well worth it.
I 'unfortunately' have not had hum issues to the degree you two have, so I don't have that kind of experience with them. Steve/Hangman should chime in, in due time, and he is a wealth of info, and probably has run into this before, and have some more useful tips about what to look for I'd bet.
03-07-2016, 04:16 PM (This post was last modified: 03-07-2016, 04:20 PM by Liquids.)
(03-07-2016, 02:27 PM)goltigut Wrote: another question : i don't have a scope but can i measure the power supply rail ripple measuring the ac voltage on the rails ? if yes, what readings should i be getting for a "clean" rail ?
I don't know off hand.
It's simply good to know about how much ripple you have...it starts as AC, it's rectified to DC...not that you don't know that. But no power supply is going to be perfect.
Current drawn will be a factor, as is your target/actual measurable DC voltage level.
"The ripple voltage is often expressed as a percentage of the maximum DC voltage. A typical figure might be 10% for a push-pull amp or 5% for a single-ended amp, though this is highly dependent on individual circuit requirements of course."
seeing how/where it's smoothing out could be useful. Or not--we don't quite know the problem...but having the info won't hurt now or in the future (if you resolve the hum, then later run into other issues etc), and if you amp is already open for measuring where it's at right now, etc.
It can also help, as if you know where it's at on the power supply, but get really weird DC readings or ripple at various other places (at triode pins, etc) you'll be tracking down some of where the hum is being injected...though right now it sounds like the answer to that is "everywhere."
Obviously you will see most ripple at the power amp's plates, as it's considered "least critical." Next will be the screen tap...and so on, per the schematic. It should be decreasing with each r-c filter in the power supply chain.
Did you replace the 40-40-40 filter that filters the E supply? If you did, E is probably OK.
From the symptoms in your original post, I suspect the input jacks are not shorting the input when no plugs are inserted. Did you move any of the grounds from the main filters that might not be rewired correctly? I've had many Ampeg hum pots that are intermittent, especially if there was ever a shorted output tube. You may have a bad hum pot. With the amp off, measure from ground to each side of the pot while turning the shaft. Each side should go to ground. I usually change the hum pot it is not 100 percent solid.
03-07-2016, 07:46 PM (This post was last modified: 03-07-2016, 07:50 PM by goltigut.)
SOLVED !! The hum balance pot was shot!! AEB-1 you were right on the money. I actually had to remove the pot from the circuit to see that one side was not going to ground. I fixed the pot by putting a small dab of solder to connect the burnt wires together inside where the wiper swipes. Works a treat !! Now i get to enjoy that sweet feeling of victory after many weeks of trial and error ! Thanks to AEB-1 and everyone that quickly provided valuable information.
I think i'm gonna order a spare hum balance pot. Does anybody know where to get them other than fliptops. Their shipping fees to canada are pretty expensive.
Bad pot
fixed pot
