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Thread: Strange Vox AC30 problems

  1. #71
    Supporting Member Alex R's Avatar
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    WOW this amp is so sensitive to heater balance! I tried a new 220R trimmer instead of the 500R and took the wiper to various grounds, and to the power valve cathodes just in case that helped... tiny changes, not much to speak of. There is SO much hum when you move the trimmer around, and you can't quite dial it out. Why so much heater hum I wonder?

    I notice the schematic seems to show some kind of resistor in series with the heaters just after the trimmer. The amp on my bench doesn't have one of those. What's that about?

  2. #72
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    I have no idea what that resistor is, the one on my bench doesn't have that resistor either. Would you be able to post some photos of the amp you have?

    thanks

  3. #73
    Supporting Member Alex R's Avatar
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    I'm not sure if it's a resistor or not, it runs off the edge of the copy I have. Sending pics isn;t easy from here - what did you want to know?

  4. #74
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    you can register Welcome to Flickr - Photo Sharing or some other photo hosting site and copy the links to the photos here on the forum. Anyways I'd be interested in having a look at the whole circuitry of your amp.

    thanks

  5. #75
    Supporting Member Alex R's Avatar
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    ac30vrev.jpg

    I have tried to attach a later schematic for the early 90s AC30 reverb. This one uses DC on the preamp heaters. Maybe that's the way to go?

  6. #76
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    The amp I have doesn't seem to have the same problem as yours does, the hum trim pot was not doing anything on this amp, except maybe raising the heater hum but not doing anything to the hum already present. So I completely took the heater hum trim pot out and grounded the heater CT of the power transformer. but that didn't make any difference either, it all seems to be concentrated in the V12-V5 area. One thing I did notice, that when the chassis is placed inside the cab on the shield plated panel, the hum slightly decreases.

  7. #77
    Supporting Member Alex R's Avatar
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  8. #78
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    As I noted in the thread you linked to, DC filaments can be very quiet.

    They have two problems, one which happens 100% of the time and one which may sometimes happen, perhaps rarely.
    The 100% problem is the expense and extra circuits to make 6.3Vdc out of what you have in tube amp already. What everyone wants to do is simply rectify and filter the existing 6.3VAC winding. Mother Nature, with Her passion for details, intrudes when you do this.

    Most "6.3Vac" heater windings are not exactly that. They are designed to sag to 6.?? volts under load. This is great for running AC heaters, but it complicates making DC out of it. It's really often as much as 7Vac in new-manufacture amps. It can be more than that if you have an old amp designed for AC line voltages of 110Vac or 112Vac and run it on the modern AC line at 125Vac, which happens, a lot.

    Let's do the nominal case first. 6.3Vac has a peak value of 1.414 times 6.3Vac
    (N.B. for the reviewers: yes, the AC power line is not a perfect sine wave, and other electronics distort the peaks, especially the rectifiers inside the same tube amp; but you have to start somewhere)
    That means the peak voltage it makes is 6.3*1.414 = 8.91Vpk. Doing the rectification with silicon diodes subtracts two diode forward voltages from this, about 0.7 to 0.9V depending on the diode. So that drops back the 8.91V to 7.5V or down to 7.1V depending on the diodes and currents. This is too high for putting directly to the heaters. (N.B. This is an opinion, but there is some reasoning behind it; available on request.) but not high enough for the common, cheap three terminal regulators. So it has to be dropped a little.

    And you have to filter it. This needs surprisingly large filter caps to keep ripple down. No point in removing 60Hz hum and getting 120Hz hum in return, is there? You'll need perhaps 2000uF to 4700uF or more, depending on the current in the heaters, to keep this quiet. There is a competing-requirements issue here, where bigger caps make the voltage smoother, but pull bigger, sharper pulses of current through the rectifiers, increasing loading on the transformer winding and causing more voltage drop in the winding and diodes and more heating in the transformer, not to mention possibly causing RF pulses from diode turnoff.

    One choice is to measure your filament currents in that amp, and put a power resistor or diode in series with the DC. This works fine as long as all the tubes are plugged in. But if you run the amp with one or more tubes, especially power tubes, removed, this lowers the load on the rectifier-filter and lets the voltage to the remaining tube heaters rise.

    Another is to regulate. However, the common-as-dirt three terminal regulators need 2V across them to regulate. They can't work properly with only 7.5V or less available and still get to 6V. One solution to that is to use a low-drop-out (LDO) regulator, which can work with only a fraction of a volt across it. This works. But now you have a regulator to set up and heat sink.

    This is a good example of a place not to even consider a zener/shunt regulator. Those have their place, but not for high current/low voltage supplies with marginal overhead.

    The problems with making the DC are the PITA.

    The rare problem I mentioned is that run on DC, heater filaments **can, may, possibly** under some circumstances evaporate faster at one end/polarity than the other. AC operation minimizes this because the ends flip polarity all the time. DC operation maximizes it because they're always the same. So heater wear out can, may, possibly be an issue with DC if something else doesn't kill the tube first.

  9. #79
    Supporting Member Alex R's Avatar
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    Thanks RG for the detail and the helpfulness. In the end I got the hum down by elevating the heaters with a voltage divider on the B+. That v12b section is very hummy indeed! Oddly it is quieter (ie in this amp, less sensitive to heater imbalance) with everything connected back on the grid than it is with just a 1M ground reference tacked in. Don't get that at all - but I achieved a pretty quiet amp for the customer so I moved on. The addition of DC heaters in the later design does seem to suggest that the hum problem in these amps might be heater-related. Probably the poor grounding layout you were talking about correcting before I hijacked the thread...

  10. #80
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    Quote Originally Posted by Alex R View Post
    Thanks RG for the detail and the helpfulness. In the end I got the hum down by elevating the heaters with a voltage divider on the B+. That v12b section is very hummy indeed! Oddly it is quieter (ie in this amp, less sensitive to heater imbalance) with everything connected back on the grid than it is with just a 1M ground reference tacked in. Don't get that at all - but I achieved a pretty quiet amp for the customer so I moved on. The addition of DC heaters in the later design does seem to suggest that the hum problem in these amps might be heater-related. Probably the poor grounding layout you were talking about correcting before I hijacked the thread...
    My pleasure. I feel like I've been groping around in the dark, though. If your experience is any guide, the V12b section may be Jimmy's problem to - it sure seems like it - and we may have to go to DC elevation. I sure hate to do that though. It complicates the insides of the amp a lot. Generally you can get to very good results without it, but hey, what's necessary is necessary.

    It bothers me that I still can't come up with a good theoretical basis for why that one section should be so sensitive. I thought I had it with the 3.3M grid resistor, but then it's not a problem in other amps so much. I thought I had it with location and magnetic radiation, but that seemed to not be true, either.

  11. #81
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    Quote Originally Posted by R.G. View Post
    This tells me that the power supply to the plate of V12 is not causing the hum. Removing the tube leaves the plate resistor to the power supply for V12 and the 100nF cap for the coupling to the grid of V5 in place. So if the hum was coming from the power supply for V12, the hum would still be there.


    I don't think you mentioned this before. Maybe you did and I missed it. Let me be sure I understand. If you put your voltmeter lead on ground (where?) and touch the V5 grid side of the cap between the plate of V12B (pin 6) and grid of V5 (pin 2), the hum disappears?

    Can you try that again and be very sure? Also, note exactly where the ground lead of your meter goes.
    It's not the same thing, but it's very interesting. I'm doing some "simulation" in my head about this, but I can't pin down exactly what it might say. One thing it may say is that your meter is enough of a load to pull V5 pin2 down so far that the phase inverter is no longer amplifying and any hum is removed by the saturation condition, not that the hum isn't still there. But I can't quite nail all that down yet.
    Yes I'm 100% sure about that, sorry for getting back so late but I have been ill. Anyways I did re-do the test and when putting the lead on V5 pin 2 or pin 7 the hum disappears, it doesn't matter where I put the ground lead of the meter, on the chassis or on the main ground point, the hum disappears.

    thanks

  12. #82
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    Quote Originally Posted by R.G. View Post
    Could you pull the V5 pin 2 connection from the 100nF cap loose and listen for hum?
    I unsoldered the 100nf cap (which is actually a 47nf cap.... the original cap was 47nf) and I get a low frequency hum (not noise hum) sort of like bad filtering. This hum also disappears when putting the tester leads on pins 2 or 7 on V5 across to ground.

  13. #83
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    Quote Originally Posted by Jimmy74 View Post
    Yes I'm 100% sure about that, sorry for getting back so late but I have been ill. Anyways I did re-do the test and when putting the lead on V5 pin 2 or pin 7 the hum disappears, it doesn't matter where I put the ground lead of the meter, on the chassis or on the main ground point, the hum disappears.
    No problem. I hope you're feeling better.

    And this is another issue to work. That's good - I've run through most of the usual suspects.
    (1) what kind of meter are you using? The input resistance of the meter could be turning V5 off so no signal at all gets through. We have to eliminate this from being a red herring. Another way to test this is if you have two meters, put one of them plate to ground on V5 and use the other to probe V5p2 and V5p7. You'd look for a radical change in the plate voltages. Unbalancing the PI diffamp could run it into saturation where the hum is still there, but can't get through the PI to the power tubes.
    (2) Temporarily tack a 100nF cap across the 47K to ground in the cathode circuit of V5. What happens?
    (3) Do you have any 10M resistors? If you do, temporarily tack one from each of pin 2 and 7 to ground at the bottom of the 47K resistor in the V5 circuit. What happens?
    (4) Where do the wires from pin 2 and pin 7 run? Do they parallel heater wires, power transformer wires?

    Quote Originally Posted by Jimmy74 View Post
    I unsoldered the 100nf cap (which is actually a 47nf cap.... the original cap was 47nf) and I get a low frequency hum (not noise hum) sort of like bad filtering.
    This probably is causing low frequency oscillation by raising the source impedance of the "grounded" side of the PI diffamp.

  14. #84
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    Just to answer your questions:
    1) I don't have 2 meters, and the meter I have now is a cheapo but has served me well since I bought it.
    2) I can try this.
    3) I wouldn't know where to get 10M resistors (I couldn't find a 3.3M resistor and had to put a 1M resistor in series with a 2.2M resistor)
    4) No these wires run far away from heater wires, Pin 2 is very close to the board where it is soldered and pin 7 runs far away from anything else. These 2 wires then follow through to the reverb board using shielded wires (which I did replaced).

    I also tested the heater voltages and they seem balanced enough (with the heater CT going to the main ground point) at 3.22vAC.

    thanks

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    placing a 100n cap across the 47k resistor for V5 didn't change anything.

    thanks

  16. #86
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    Quote Originally Posted by Jimmy74 View Post
    Just to answer your questions:
    1) I don't have 2 meters, and the meter I have now is a cheapo but has served me well since I bought it.
    The reason I asked is that some digital meters have an input impedance of 1M, some 10M, and it's not necessarily a matter of price. An analog meter may be much lower. In any case, connecting the resistance of the meter across the grid-to-ground path can pull down one side of the PI diffamp, and cause it to not pass any signal, including the hum.
    2) I can try this.
    placing a 100n cap across the 47k resistor for V5 didn't change anything.
    OK. That's another thing eliminated.
    3) I wouldn't know where to get 10M resistors (I couldn't find a 3.3M resistor and had to put a 1M resistor in series with a 2.2M resistor)
    If you have two or three 2.2M resistors, put them in series and put equal numbers of them from each of V5p2 and V5p7 to the ground end of the 47K. This test the theory that somehow one of them floating is picking up hum. Pulling them both down at the same time with 4.4M to 6.6M should tell us whether it's the pulldown on the grid or the imbalance of measuring one grid and not the other.

    Actually, you might try putting a big (i.e., a couple of 2.2M resistors in series) from ONE of V5P2 or V5P7 at a time and seeing if this kills the hum. If it does, then use the meter to measure the two plate voltages on V5 and see if they're still balanced.

    4) No these wires run far away from heater wires, Pin 2 is very close to the board where it is soldered and pin 7 runs far away from anything else. These 2 wires then follow through to the reverb board using shielded wires (which I did replaced).
    OK. That's not it.

    I'll go back and review, but to save me some time, does the hum vanish when you pull out V12, or is it still there with V12 pulled out?

  17. #87
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    I'll have to order those resistors ( I don't have any here atm.) My meter is digital. And yes working backwards first putting the power tubes in then V5 and then V12 the hum appears only when V12 is in the circuit.

    thanks

  18. #88
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    Quote Originally Posted by Jimmy74 View Post
    I'll have to order those resistors ( I don't have any here atm.) My meter is digital. And yes working backwards first putting the power tubes in then V5 and then V12 the hum appears only when V12 is in the circuit.
    Hold up on the resistor buy, then. I was chasing down that maybe it was a hum problem in V5, not really V12.

    I'm rapidly running out of easy-to-test ideas. Just to catch the audience up to date:
    - Hum is not a tube swap issue; always change tubes first!
    - Hum disappears when signal at the grid of V12B is shorted.
    - Hum disappears when V12B is pulled out of the socket.
    - Hum disappears when either grid of V5 is probed with a DVM.
    - Hum is unaffected by shorting signals of any or all of the stages in front of V12B are shorted to ground.
    - Hum is unaffected by changing grounding scheme changes, within reason.
    - practically everything except the chassis is new.
    - it does not appear to be a wire routing problem or magnetic induction problem from simple tests; still could be, but a lot more work would be needed to find that.

    The fact that it disappears when V12 is pulled seems to eliminate V5 in spite of it going away when V5p2 or V5p7 is probed.
    The fact that eliminating the signal from all previous stages makes no change seems to eliminate them as hum sources. It also seems to eliminate V12A, the reverb return section. The usual suspects of control wiring hum and reverb wiring hum are eliminated by the signal shorting tests, I think. All indicators point to V12B alone.

    We've poked around with re-grounding the heater CT. We've messed with balance on the CT. No change.

    Here's all I can think of that we haven't messed with.
    1. Could be a bum socket for V12 that's letting heater AC get to one of the grids. That could be dirt or contamination on the socket, leftover soldering flux, etc., or could be just a leaky socket. Lotsa work involved to change out the socket for the test. Might not show up on metering between pins because grid impedance is so very high.
    2. Could be a wire routing or magnetic induction problem I can't see because I don't have the amp here to mess with.
    3. Could be that this particular tube position really, really needs DC elevated heaters.
    4. Could be that this particular tube position really, really needs DC powered heaters.
    3. Could be something else I just haven't thought of. In fact, that's dead certain now that I think about it!

    At this point, if I were you, I'd think about the following? I say "think about" because these tests get expensive in terms of labor time. How bad would it be to:

    1. Elevate the heaters maybe 12V above ground by connecting the heater CT to a +12Vdc source that maybe does not yet exist in that amp?
    2. If that fails, power the heaters for V12 from an external DC 6V power source (6V lantern battery works) to test whether DC heaters fixes it?
    3. If that fails, trade out the socket for a new, known good socket and be very careful about how it's soldered in and cleaned after soldering?

    Or, probably better, yet, yell for Enzo to review what we've done so far.

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    I think I'll go straight for number 3 because n1 is impossible and number 2 would be costly. I know there's a lot of science about creating a DC rectifier for heaters, but what about getting it done for just one tube? 4 diodes and a couple of smoothing caps... would that be possible?

    thanks

  20. #90
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    Quote Originally Posted by Jimmy74 View Post
    I think I'll go straight for number 3 because n1 is impossible and number 2 would be costly. I know there's a lot of science about creating a DC rectifier for heaters, but what about getting it done for just one tube? 4 diodes and a couple of smoothing caps... would that be possible?
    Number 2 (power heaters from an external 6V dc source) is a test procedure only. It lets you test out whether DC heaters on that one tube fixes the problem. I meant for this to say "disconnect the V12 heaters from the 6Vac heater stuff in the amp; find a 6V lantern battery and use clipleads to hook it to the heaters of V12 only, making sure that one end or the other of the battery is grounded, preferably the negative one; make sure the battery brings V12 heaters up to an orange glow, then turn on the rest of the amp and see if the hum is changed."

    If the hum vanishes, you can be sure that the heaters on V12 are somehow leaking into the audio signal, and can do further work on cleaning up the heater supply. If nothing changes, it was not the heaters at all, and you can ignore heaters as a cause until the rest of the problem gets resolved.

    Only if number 2 is a success should you think about number 1; this isn't as hard as it looks. A 12V zener powered through a resistor to B+ and maybe a capacitor is all you need. But number 2 (which I should have put first! Doh!) is a necessary first step to see if it will help.

    Number 3 is one thing I'd do if number 2 causes no change. 2 eliminates heaters as a cause if the hum is still there. That pretty much forces it to be either the socket or wiring/induction hum.

    I think...

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    I was looking through the photos of how the amp was originally and I noticed that both of the original transformers had bell covers on the inside-the-chassis side too. I had to make the mounting hole bigger for the MM power transformer, I can't do anything about it, but the MM OT was a perfect replacement, and is very close to the V12 & V11 tube sockets, do you think that fitting on the original inside-the-chassis bell cover would be worth trying?

    thanks

  22. #92
    Senior Member Enzo's Avatar
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    I just read all 91 posts, and believe me this is tough to wrap my head around starting cold.

    Fortunately I have a bottle of rye next to me.

    A couple thoughts came along while reading. I see terms like heater hum, ground hum, other hums. A real important question is this: is the hum 60Hz or 120Hz (in the USA and other 60Hz mains places) or 50Hz versus 100Hz for our 50Hz mains friends. I don't recall the answer to that going by. Power supply ripple will be the higher frequency. You likely have both.

    About the meter killing the hum when touched to a tube. Since we don;t really know if the amp is oscillating, we don;t know if the meter ompedance is quashing said oscillation. And another thing, hum can come from multiple sources. Touching a meter lead to a grid generally CAUSES hum, the lead wire is an antenna. However, and this related to waving the hand over things etc, If there is for exampl 200mv of hum signal in a circuit, and your meter lead introduces 200mv of hum itself, if those two sources happen to be out of phase with each other, the hums CANCEL. SO it is possible applying the meter probe to the tube is introducing a reverse phase signal to cancecl the hum.

    At one point you said the hum balance control did nothing. HArd to check in circuit, but removed from the circuit, you might check it for continuity through the resistive element and see that the wiper makes contact throughout travel.

    Shunting a tube grid to the bottom of its cathode resistor ought to silence anything coming into it. I am concerned that at various steps on this thread the communicatiion didn;t seem to be 100% on this aspect.

    Now I'm hungry.

    If you ever see a complaint that hum is present at zero on the volume but goes down as you turn up the volume until it minimizes at like 4, then goe sup from there. What happens there is there are two sources of hum. One is in the latter part of the system, and the other is coming from earlier stages, but is out of phase. SO at zero volume, yoou hear the later stage hum, then as the volume is advanced, the early stage hum starts to cancel out the later. When the two are at the exact same amount - 4 in my example - they cancel out. They come from two completely separate sources there, but they cancel out.

    SInce you have no scope, google up "RF PROBE" and look at some of the results. It is a very simple thing made with a diode and a cap, and I allows your meter to read the presence of very high frequency signals, way above the meters normal response. Electrically is is what we call a detector, it is what makes an AM radio work. If an amp oscillates above audio frequencoes, you can't hear it at all, but it is making the amp work, and usually results in a hummy audio signal. Just a possibility.
    Education is what you're left with after you have forgotten what you have learned.

  23. #93
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    Quote Originally Posted by Enzo View Post
    I just read all 91 posts, and believe me this is tough to wrap my head around starting cold.

    Fortunately I have a bottle of rye next to me.

    A couple thoughts came along while reading. I see terms like heater hum, ground hum, other hums. A real important question is this: is the hum 60Hz or 120Hz (in the USA and other 60Hz mains places) or 50Hz versus 100Hz for our 50Hz mains friends. I don't recall the answer to that going by. Power supply ripple will be the higher frequency. You likely have both.
    Ok I'm in the friendly 50hz side of the world, as I explained I'm not sure as to what hz the hums I'm getting, one hum seems to be noise hum (like when you touch a guitar lead while it's plugged in) and the other hum is sort of what you'd expect when the amp has bad filtering, it's a low hum. I did notice this, as I explained, both hums do decrease when the chassis is mounted inside the cab (probably because the speakers and shielded mounting board help in some way).

    Quote Originally Posted by Enzo View Post
    About the meter killing the hum when touched to a tube. Since we don;t really know if the amp is oscillating, we don;t know if the meter ompedance is quashing said oscillation. And another thing, hum can come from multiple sources. Touching a meter lead to a grid generally CAUSES hum, the lead wire is an antenna. However, and this related to waving the hand over things etc, If there is for exampl 200mv of hum signal in a circuit, and your meter lead introduces 200mv of hum itself, if those two sources happen to be out of phase with each other, the hums CANCEL. SO it is possible applying the meter probe to the tube is introducing a reverse phase signal to cancecl the hum.
    The hum cancels out when I touch the grid points on V5 with the meter lead but only when the ground lead touches the chassis or goes to the main ground point. If I do not ground the meter, and touch the grid pins, the hum does increase as you said.

    Quote Originally Posted by Enzo View Post
    At one point you said the hum balance control did nothing. HArd to check in circuit, but removed from the circuit, you might check it for continuity through the resistive element and see that the wiper makes contact throughout travel.
    I did fit in a new hum trim pot because the original one looked a bit tattered up, and when turning the pot, it made no difference to the hums I was getting, but increased, what my logical guess was, and made evident heater hum. The noise hum decreases when turning up the CUT pot. The normal channel volume pot (associated with V4B-V12B) when turned up makes another noise hum arise. To a lesser point this also happens with the reverb pot.

    Quote Originally Posted by Enzo View Post
    Shunting a tube grid to the bottom of its cathode resistor ought to silence anything coming into it. I am concerned that at various steps on this thread the communicatiion didn;t seem to be 100% on this aspect.
    R.G. didn't we try this on both V5 and V12?

    Now I'm hungry.

    Quote Originally Posted by Enzo View Post
    If you ever see a complaint that hum is present at zero on the volume but goes down as you turn up the volume until it minimizes at like 4, then goe sup from there. What happens there is there are two sources of hum. One is in the latter part of the system, and the other is coming from earlier stages, but is out of phase. SO at zero volume, yoou hear the later stage hum, then as the volume is advanced, the early stage hum starts to cancel out the later. When the two are at the exact same amount - 4 in my example - they cancel out. They come from two completely separate sources there, but they cancel out.
    I can't remember which pot seems to to create this situation, probably the reverb pot or the normal channel volume pot.

    Quote Originally Posted by Enzo View Post
    SInce you have no scope, google up "RF PROBE" and look at some of the results. It is a very simple thing made with a diode and a cap, and I allows your meter to read the presence of very high frequency signals, way above the meters normal response. Electrically is is what we call a detector, it is what makes an AM radio work. If an amp oscillates above audio frequencoes, you can't hear it at all, but it is making the amp work, and usually results in a hummy audio signal. Just a possibility.
    I will certainly try this, but would it help in an audio ssignal where hum is created in more than 1 point?

    thanks

  24. #94
    Senior Member Enzo's Avatar
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    Hum is not universal or monolithic, it can have multiple sources, each requiring its own countermeasures. For example hum from AC heaters and hum from power supply ripple. Running heaters on DC will do nothing to improve power supply ripple hum, and increasing filtration on the power supply will do nothing to prevent heater hum. SO I don't know if you have any RF oscillation or not, but if it is one contributing factor, it would seem worth a few small cokmponents to se if it is involved or not.

    One needs to be careful not to confuse two out of phase hums cancelling and a single hum being attenuated. That is why I brought up examples as I did. We need to know if your meter probe, for example, is actually killing the hum in question or is it introducing hum of its own that happens to be put of phase and thus negating the existing hum.
    Education is what you're left with after you have forgotten what you have learned.

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    I found this one on the net, would it be what I'm looking for:

    N5ESE's Classic RF Probe

    would a 1kv ceramic disc cap be ok?

    thanks

  26. #96
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    Enzo, thanks for taking a look.

    What I've done so far is to exclude things. Here's what I think we did, subject to understanding back and forth posts correctly. These are not in time order, but reflect what I was thinking as I told him to do things and backtracked as needed.
    1. had him short incoming signals to ground to run down where the hum occurred
    - shorting volume control wipers to ground ahead of the 3.3M reverb isolation resistor did nothing
    - shorting grid of reverb driver V12A to ground did nothing
    - shorting reverb return cable to ground did nothing
    - shorting reverb depth pot wiper to ground did nothing
    All of these are ahead of the 3.3M reverb isolation resistor and 470K reverb return mixing resistor. Shorting the grid of V12B to ground stopped the hum - and all other audio signals, too, of course.
    2. removing V12 or shorting its plate signal to ground killed the hum, as did apparently even measuring the voltage on the plates of V5. In retrospect, that's a good vote for RF oscillation, as you noted immediately. I hadn't picked that up as a cause.

    I took all that as an indication that a majority of the hum he was referring to was coming in on V12B. So I went hunting for what the cause was.
    3. Heater AC is always a good test. Since he thought this was an unusual amount of hum and it was a PCB amp, I started to go after heater induced hum, especially since he'd said he dinked with it. When he described the grounding scheme, a couple of things looked bad for rectifier hum, so I temporarily diverted into grounding scheme to try to get rid of rectifier pulse ripple. No net result from that.
    4. Back at heater hum, moving the heater CT around to ground points, even theoretically bad ones, made no change. He says it's resistor CT grounded, so unless he's confused about the resistors being same value, that's not an issue. I haven't explicitly asked jimmy to test the resistor values, so that's a possible.
    5. I asked about location of wires, moving wires, AC heater wires, etc. His reports indicated not much there, although it's possible he missed the one which was doing it.
    6. I asked about the tube being close to transformers and inductors; he said no. But then his recent posts say V12B is close to the un-belled end of the PT, too. So it's possible it's still a loop-induction issue.
    7. Along with the heater CT grounding, I also had him move the grounding of V12 to the V4 star point. No change.
    8. I hit a fair number of miscellaneous stuff along the way, like reverb tank wires, shielding, and grounding.

    Pretty much removing signal to grid of V12B stops it, or removing the plate by removing the tube stops it. That's why I focused on V12B. I went back to worrying about V5 out of exhausting a lot of possibilities and that funny "probing the grid stops it" thing.

    @jimmy: Enzo is dead correct on hum. Hum fundamentally has three major paths to get into your amp: conduction, electrostatic radiation, and magnetic radiation. And every place that picks up hum can add it to the audio signal, sometimes indirectly. More confusingly, each source of hum may have a different phase, and the rectification system can generate the three paths of hum but at double the power line frequency, all on its own.

    That last is why you listen for the frequency of the hum. If it's power line frequency, the rectifiers and filters are not involved. If it's double the power line frequency, then the rectifiers and filters are involved because that's the only place to get 2X power line frequency that's readily available. Some people can tell this by ear, but some are so taken by the exact octave that they need an oscilloscope.

    So yes, hum in an amp is often a complex mixture of the sum of hums (!?), and getting a really quiet amp is like peeling an onion. You remove the outer, most offensive hum, then attack the next layer. You expect many layers, and have as a goal getting enough of them removed. Fatigue usually stops complete removal.

  27. #97
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    just a couple of things to clear up in what R.G. wrote:

    I firstly ungrounded the heater hum pot and grounded the PT heater CT, and turning the trim pot made no difference to hum at all. So I forwarded to taking that trim pot out altogether and leaving the PT heater CT connected to ground. From this I found out that 3 things did not make any difference to the hum I was getting:

    1) NO PT heater CT connected to ground, heater hum trim pot grounded. Turning the hum trim pot made another hum come out which I supposed was actual heater hum

    2) Heater hum trim pot ungrounded (ground lug lifted) and PT heater CT grounded, the heater trim pot made no difference to hum while turning it.

    3) Heater hum trim pot completely taken out and PT heater CT grounded

    One other thing to correct, the unbelled end of the PT sits very close to the rectifier tube socket (as were all tube rectified AC30's)
    The unbelled side of the OT and the choke itself both sit very close to the V11 and V12 tube sockets as you can make out in the photos and looking at the tube layout on the schematic.

    The original transformers were belled on both sides

  28. #98
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    I don't know if I mentioned this but I did notice something strange in the original cab, there was a wire coming off one of the speakers and bolted on one of the mounting screws of that speaker and a wire going from that point and bolted on to a mounting screw on the other speaker. I didn't replicate this with the new celestion alnicos, should I?

    thanks

  29. #99
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    This is such an interesting and instructive thread. Thanks to R.G. and Enzo for helping to show the proper troubleshooting processes and techniques. I've got a humming Bogen CHB100 that I've been modding, and I'm hoping some of the techniques in this thread will help me pinpoint what is going on. I hope the issue with the Vox can get resolved eventually.

    Greg

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    I'm thinking of getting this scope:

    Oscilloscope ST-16B :: Oscilloscopes :: Measurement and Testing :: Tools :: Electronic Parts :: Banzai Music

    would it be good enough for working on tube amps?

    thanks

  31. #101
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    I'd say that a 20Mhz scope would be more adequate, and a dual trace scope can be very useful. I use a couple 100 Mhz dual trace Tektronix 2235 scopes myself, and they are very useful and give a nice and bright trace. I'm not sure where you're located but I do have an HP scope that I wouldn't mind selling. It is a dual trace and I think at least 50 Mhz but I'd have to go look again. I'm sure that you could find scopes on ebay or around where you are locally that would work just fine though, but if you're interested in knowing more, send me a PM and I can get you details. Equally as important as the scope itself is that you have a good 10:1 probe to use with it btw.

    Greg

  32. #102
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    PM sent

  33. #103
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    This is like reading a serialized novel ..can't wait for the next installment !
    Just a small point Jimmy74 - when you fitted the new power transformer did you reuse the old bell covers ?
    I was wondering if the power transformer needs rotating 180 deg ?
    I just felt compelled to post this.
    OC

  34. #104
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    Reply sent. I'll see if I can get you more detail on the scope in a day or two if you want it. A bit busy here lately.

    Greg

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    I had to make the chassis hole bigger for the new MM PT, because it is quite a fair bit bigger than the original one, therefore I couldn't use the original bell covers for the PT.

    Thanks

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