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

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  • q9522678
    replied
    So have spent a couple of hours doing some measuring and investigating to try and further narrow things down.

    Below are V1 and V2 voltages noting we are looking are the preamp. These are a little low in some areas but not sure if they’re low enough to be concerning.

    Also pin 2 of V2... should there be any DC voltage here at all?

    V1
    1 - 175vdc
    2 - 0vdc
    3 - 1.2vdc
    4 / 5 - -1.2mvdc
    5 - 2.35v
    6 - 174vdc
    7 - 0vdc
    8 - 1.23vdc
    9 - -1.3mvdc

    V2
    1 - 152vdc
    2 - 1.4vdc
    3 - 2.4vdc
    4 /5 - - 1.3mvdc
    6 - 147vdc
    7 - 0vdc
    8 - 0.98vdc
    9 - -1.3mvdc

    Plate voltage on pin 3 of power tubes - 488vdc

    Bias is interesting. The amp has two test points. The reading between Test Points 1 and Test Points 2 is 4.6vdc using my Fluke multimeter. It is my understanding that this should be ideally 0.08 to 0.12vdc

    However junction of R9 and R10 reads -47vdc which is in line with what the schematic says.

    Using a euro tubes style bias probe the plate voltage is the same as above but”cathode current” is 2.7mv, which seems to support the reading at R9 / R10 and is in the ballpark in terms of recommended bias.

    Seems the bias test points might not be accurate. The 1ohm resistors that were here were well and truly underrated and all burnt up, which scorched the board as well. I replaced with 2W 1ohm resistors and triple checked all traces for continuity and seemed to be ok. Will double check the work here.

    In terms of power tubes, I fitted a new matched quad of Ruby (Shuguang) EL34BHT - these are meant to be great for high voltage applications. Given I happened to have two matched quads of this type sitting around I tried both and hum remained. This amp came with EL34s fitted and according to the manual can handle 6L6 and EL34. So simply for interest also tried a matched quad of Ruby 6L6GC and this also made no difference to the hum. So I think safe to say no issue with the power tubes.

    Of note, the bias pot makes a significant difference to hum level, but probably not surprising. At the recommended bias setting hum is quite loud. At colder settings it is lower but still quite noticeable.

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  • Helmholtz
    replied
    No real change with guitar (early 70’s tele) plugged in and guitar volume increase
    Only the hum level with guitar volume at zero matters. The guitar is only used to short the amp's input via the vol pot.

    Leave a comment:


  • q9522678
    replied
    Hi John,

    I tagged a 10UF / 450v across C9 in case this was the culprit.

    Unfortunately made no different to the hum at all.

    Open to any other suggestions.

    Leave a comment:


  • q9522678
    replied
    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!

    Leave a comment:


  • q9522678
    replied
    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.

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

    Leave a comment:


  • Enzo
    replied
    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.

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  • q9522678
    replied
    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.

    Leave a comment:


  • Enzo
    replied
    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?

    Leave a comment:


  • q9522678
    replied
    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?

    Leave a comment:


  • Enzo
    replied
    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?

    Leave a comment:


  • q9522678
    replied
    Originally posted by q9522678 View Post

    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.

    Leave a comment:


  • q9522678
    replied
    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.

    Leave a comment:


  • Enzo
    replied
    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?

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  • q9522678
    replied
    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..)

    Leave a comment:

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