Ampeg VT-120 Restoration / Repair - hum on channel C

[q9522678]

Hi All

Despite telling myself that my time is better spent playing Guitar rather than fixing amps after getting the Silvertone 1484 in working order (still going strong!) I managed to come by a USA built 1990 Ampeg VT-120 (inspired by Mesa Mk III) that needed some love. Unfortunately not a lot available online about these and people seems to trash them rather than try to repair them. Seems a pretty good amp in terms of specs although it weighs a tone.

The amp wasn’t working at all when I got it, but after replacing all electrolytics throughout, some burnt dropping resistors, bleeder resistor and screen resistors (with higher spec ones), resoldering every single joint in it and giving it a whole new tube complement it came back to life. The amp is a three channel amp with Channel C being clean, channel B slightly less clean and channel A being a full on gain channel. Appears to be not a lot of difference between channel B and C In terms of signal path.

What’s got me stumped is that the higher gain channels are whisper quiet with nothing connected and controls turned down. However channel C has a constant hum that is loud enough to be annoying.

Channel C and B are very similar and use much of the same circuitry. Channel B audio runs through a resistor and capacitor that Channel A does not. It also runs through a master volume and an additional optcoupler (same with channel A)- was thinking maybe this is what is making this channel quieter than channel C. Perhaps OC4 (and OC5 for channel A) are simply acting like noise gates and filtering the noise out? That being said, I can’t see these being this noisy from the factory. The hum is loud enough to suggest something is still amiss.

Also, the hum doesn’t change volume or sound with changing any of the controls and only affects channel C, which theoretically should narrow things down to those bits unique to channel C. I’ve also tried moving preamp Tubes around. No change (not was I expecting there to be). I thought if it were a grounding issue than it would surely carry through to Channel B and potentially channel A.

I have to say this is the first time I’ve experienced the clean channel being noisier than the dirty channels! Not something I’m going to die in a ditch over as can simply use channels A and B which are super quiet and functional, but open to any suggestions in terms of what this might be. Was hoping it was something quite simple (famous last words).

Schematic for the amp is here: https://ampeg.com/support/files/Sche...SCHEMATICS.pdf

Also found a very handy layout that had been marked up by hand in terms of components.

Was thinking coupling caps - but can’t see any DC where it shouldn’t be - and Channels B and C share the same caps and tubes. So would have thought any noise present on Channel C would be present on Channel B (and A).

I’m not familiar with Optocouplers but perhaps OC1 on channel C is not functioning appropriately (assuming these are installed to help filter out noise). Not sure how I would go about testing this. A quick poke with an audio probe shows the noise present through OC1, OC2 and OC3, which further confused me. At which point I decided post here.

Would appreciate any thought or insights on what might be the cause.

Thanks in advance!

[g1]

When you are comparing, you have the master set full up?

[q9522678]

Hi g1,

No, all controls are at zero across the board.

Are you suggesting channel C, without a master, is equivalent to Channels A and B with master dimed? If so that would make the channels sound more “balanced” in terms of hum!

[q9522678]

Just did a little experiment. Channel C and Channel B sound similar in terms of hum with the channel B master up.

Channel A is still quiet even with master dimed.

So I guess that broadens the search to channels A and B in terms of issues. Although I think faulty tubes are ruled out. I had read that these amps don’t have the best grounding scheme so I suppose could simply be a byproduct of that - but still surprised that they were this loud from factory in terms of hum (or perhaps I’m just used to super silent modern gear..)

[Enzo]

I have to say this is the first time I’ve experienced the clean channel being noisier than the dirty channels!

Actually not the way to look at it. You have a channel, and it is either noisy or it is not.

Optocouplers are simple. A light inside shines on a photocell inside. A photocell is a resistor that is sensitive to light. Shine light on it and the resistance goes down. Keep it dark and resistance is high. In this circuit they are not filters, they are just switches. Put one in series with a signal and it acts like a switch.

In the input stage we have the three channel paths, and each one runs through an OC. SO you shine a light on one of those OCs and that channel becomes "on".

Over in the master section, OC4, OC5 are switches for those master volume controls. When they are off, say OC5, there is a very high resistance so the B level control has no effect. SHine a light on OC5 and now the B control works because the bottom end now has a low resistance to ground.

There is only one path through the amp, really only the one channel. Within that circuit, we are switching things in and out. The different volume and tone circuits, the different master controls, and all the different tone and gain shaping things controlled by JFETs.

So put the amp on channel 1 and set controls midway. Hums, right? C gain has no effect, right? Isolate the problem. Pull V1, still hum or not? If hum goes wil can discuss that. Still hums? FIne, pull V2. Still hums?

[q9522678]

Hi again Enzo, nice to hear from you.

That’s what I love about this forum. I can labour over something for days with my limited electronics knowledge and then have it clearly and simply explained in a few succinct paragraphs. Thanks for taking the time to explain that. Make sense and demystifies the amp a little.

Yes - on Channel A with controls up half there is hum. The channel C gain does nothing in this config as you suggest. With preamp tube 1 pulled there is no hum.

[q9522678]

Channel A is still quiet even with master dimed

To clarify the above - with master dimed and gain at zero channel A is quiet. Once the gain comes up the hum comes up.

[Enzo]

OK then, so it is more like either channel makes hum when gain turned up. Pulling V1 stops hum, so we know the hum is happening in that first stage somewhere.

Certainly try a different tube in V1. Probably just swap with V4 or something. Any difference?

You hinted earlier, I can't hear it, but could be a normal amount of hum. We can still find where it comes from though even if so.

Select C and hum not affected, but does C gain still control guitar level?

Select A and A gain affects hum. And B channel?

OMG, that parts layout looks like my scan. The handwriting and position of the VT120 in the margin.

Does this hum occur when the amp chassis is back in the cabinet? POSSIBLE one of the VActrols is leaking light and your shop flourescents are causing this. There, turn off ALL the shop lights, does the hum remain?

[q9522678]

Enzo

I replaced all the tubes throughout as part of the refurb. I’ve tried moving the various 12AX7s around V1 and V2. Also tried a separate new 12AX7 and hum remained. So pretty sure tube is out as a cause (unless I’m very unlucky!).

In terms of being normal amount of hum, unsure as never heard one of of these new, but hums more than my ‘74 Ampeg VT-40, so guessing not necessarily normal. Certainly loud enough on Channel C to be annoying. On channels A and B it can at least be tamed with the volume. As a 120 watt amp - I guess when you’re pumping out 120 pure tube watts of guitar power you probably wouldn’t notice the hum! My initial gut feel is that the hum is louder than it should be. Don’t get me wrong - I don’t mind a little hum in tube amps - it’s part of the fun, but there is that threshold where it detracts from the enjoyment of listening to the amp and this is pretty much there.

With C selected hum remains. Gain control changes volume of the guitar. But hum is there even when gain is at zero.

With A selected and volume maxed but gain at zero there is no hum. However gain control brings in hum.

With B selected hum is not there with volume and gain turned down, but hum appears when volume turned up (unlike channel A where there is no hum with volume up and gain down). Gain on channel b also increases hum, along with volume.

The nature of of the hum is different on channel A, but I suspect this is simply because it’s going through different “high gain” circuitry. Sounds like “scooped mid” hum.

Parts layout could very well be yours. I found buried deep in the bowels of a message board somewhere. Has been very handy. Unsure why Ampeg moved away from labelling their boards like on the VT-22 / VT-40. These VT-120s aren’t very tech friendly given the dismantling you need to do to get to the underside of a board. Have been contemplating modifying the chassis to put an access plate in similar to the VT-40.

I’ve had the chassis in and out of the amp a few times and hum remains. Also tried with lights on and off. So don’t think it’s external electrical interference.

I guess where down to poking around 1st preamp stage - voltages and testing some components?

[Enzo]

OK, so really there is hum coming from the first stage, and is available to hear on all three channels once they are turned up. Pick a channel like B or C, we have hum. Now run the tone controls up and down. DO they affect the TONE of the hum? Channel A only has the one tone control plus the PULL BASS switch. SO does that affect the TONE of the hum?

[q9522678]

On channel C - gain brings in some additional hum, but hum is present regardless of where any controls are set. Low, mid and high controls make no difference to the tone of the hum. “Pull Bright” on high control makes no difference to volume of tone of hum. So other than gain on channel C, no other controls affect the hum.

On channel B - volume affects loudness of hum. At zero there is no real hum. With level above zero the gain control brings in more hum. Interestingly in channel B, the high control in channel C changes the tone of the hum. None of the other channel C tone controls seem to affect the tone of the hum.

On channel A - no real hum with volume down or up. Hum comes in when gain control increased. The high control on channel A changes the tone of the hum. The “pull low boost” brings in additional low hum (as advertised!). The ultra mid control on channel A changes the tone of the hum (So all controls on channel A alter the hum).

As an aside, the reverb control makes no difference to the hum but reverb is functional.

[Enzo]

We are trying to narrow down where it comes from. Leave the master volume controls at some mid point. We are only interested in the first stage controls - the ones called GAIN. So all three gain controls increase the hum. On A the tone controls affect the hum. Please note B and C use the same tone stack. SO it is not surprising the treble control for C also affects B. That leads me to think the hum comes from before the tone controls.

With a gain control up enough for hum, plug a guitar into the amp, and dial the guitar's volume control to zero. Does that affect the hum? I mean the volume control that is on the guitar. leave the master volume controls at mid point.

[Jon Snell]

Hang a 10u 450v cap across C9. If that is low value V1 will produce hum.

[q9522678]

Enzo - no probs. Got it, just trying to provide a complete picture of what’s happening. Apologies for any extraneous information. Only thought the high control changing the tone on channel B was interesting as it doesn’t work on channel C in terms of changing hum. I had read in the manual for the amp that B and C share tone controls.

No real change with guitar (early 70’s tele) plugged in and guitar volume increase. Only some minor noise introduced that I get from this guitar regardless of amp. The hum in the amp remains unchanged.

[q9522678]

Hi Jon

Thanks for the input. I’ll give that a crack in the morning. I did replaced C9 with a new cap as part of the complete electrolytic refresh - but always a chance it could be faulty. Cheers!