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Thread: Vox AC30 Reverb Model Possible Choke Problem

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    Vox AC30 Reverb Model Possible Choke Problem

    Hello,
    I have a loudly humming Vox AC30 that I am working on and checked and substituted everything I can think of except the choke. The hum is present with only the phase invertor and power tubes in. When I remove the PI tube the hum gets much quieter. So I guess the question would be, would a bad choke possibly cause this issue? If so, other than sustitution, how would one test a choke? Any ideas would be great.

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    Senior Member redelephant's Avatar
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    Fixed it yet? I doubt it's the choke, but ya never know. Measure the voltage before and after the choke, u should have a voltage drop. U can also ohm it if u solder off one lead.
    Is the hum the same as u turn the master knob? Is it only the PI tube that makes a difference?
    I'd look at gnd connections (resolder them) and the heater connections. Chassie grounded?

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    Thanks for the reply. No, it's not fixed yet. I substituted the choke, output transformer, and all of the tubes for known good parts and no change. I studied a very in depth thread about Vox AC30s with reverb hum problems that was posted awhile back and it seems this amp is prone to this problem. The hum seems to come from around the V12 reverb tube or the phase invertor. I shorted the input grid of the phase invertor to ground with a 2 uf capacitor and the hum goes away. I fixated on the idea that the hum was coming into the into the phase invertor from V12. I now think that it could be comming from the phase invertor itself. I have checked all of the grounds in the amp pretty carefully. So I guess that leaves me with grid connections, although probing the grid connections with a chopstick when the amp is on shood make some noise if there is a bad one. I would think same with the heater connections. I may also try replacing the PI tube socket. The last thing I can think of is raising the ground(artificial hum balance, no center tap) of the heaters with positive DC. After that I am out of ideas.

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    Bent Member Chuck H's Avatar
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    Try moving the grounds for the PI. It's circuit has only one ground, the tail resistor. If that is on the same ground node as the main filters or power tube cathode try moving it somewhere closer to the preamp. If that doesn't help try moving the PI filter ground too. If that doesn't help try moving the ground reference for the reverb output also.

    If these amps ARE prone to hum it's not the amps fault. It's the layout and or ground scheme causing the problem. The amp doesn't have to hum as a rule for any reason.

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    Senior Member redelephant's Avatar
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    Quote Originally Posted by Charlie Wilson View Post
    I shorted the input grid of the phase invertor to ground with a 2 uf capacitor and the hum goes away. I fixated on the idea that the hum was coming into the into the phase invertor from V12. I now think that it could be comming from the phase invertor itself.

    I may also try replacing the PI tube socket.

    The last thing I can think of is raising the ground(artificial hum balance, no center tap) of the heaters with positive DC. After that I am out of ideas.
    I assume this is a new problem, something not there when amp was out of the factory. That means there is little wrong with the layout, and no need to find new grounds. I've only had newer production AC30s to work on, and the component quality was shockingly poor if I may say. They do sound great though. The cathode resistor is a bit on the low side and get the amp running a wee bit on the hot side. 68ohms is a better value if u ask me.

    If the hum goes away when shorting the input of the PI, it is not the PI or anything after. Something in front of the PI is placing hum at the PI's input. Have u gone stage by stage, from PI to input tube, and shorted each stage's input in turn?

    I seriously doubt replacing the socket will help with this kind of noise. U have power supply hum, if I get u right, and a bad socket does not give such a fault.

    Try changing the reverb tube with some other tube. Try removing the reverb tube, or solder off the output cap from the reverb circuit. If it get quiet, u know it's from the reverb circuit...

    Elevating the heaters to 25-80 volts is a very good mod, which always gives an improvement in hum. There is NO improvement in heater related hum from a DC regulated heater compared to an elevated heater. DC heaters are a waste in time and effort.

    Good luck. This is an amazing amp when it works (and if u don't mind the weight).

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    Hello, thanks for the reply. Sorry for the delayed response. The back story on this amp is that my friend bought it from a guy who said it did not work. It was brought to me and I found that a speaker wire had broken off a terminal. I repaired the speaker wire and the amp worked but with a loud hum. Someone has done a lot of work on this amp previously and it looks decent but I did have to resolder some components on the PC that were barely attached. Unfortunately those components were not the source of the hum. I guess the point is that I don't know what the original factory layout of this amp was and yeah I agree, it probably didn't make this much noise from the factory. What is driving me crazy is the hum goes away(completely) when I short the grid of the PI(pin 2) with a 2 uf cap but it does not go away when I lift the 100n(47n in this amp) cap that feeds the grid from the reverb output tube. So I thought maybe it is comming from the power supply of the reverb tube, so I disconnected the plate supply for the reverb output tube and removed both input and output reverb tubes and still there was hum. I have tried moving grounds every which way. I even tried raising the heater ground(artificall) 40v but that made the hum worse or maybe added 120hz hum. Between the input gid of the PI and the 100n cap that I tried lifting is a foot and a half long double wire. I guess the only thing left to try is replacing that wire or the PI tube socket which I agree is not a likely cause of the hum. Per the last post, I don't mind the weight but this amp is getting heavier and heavier

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    It's a long shot, but today I solved a PI hum problem that has been eluding me for a week. Turns out I had accidently used the wrong value for one of the output tube grid leak resistors, leading to very un-even gain in the 2 sides of the LTPI. Replaced with correct value, good symetry of PI gain, hum rejection improved, hum gone!
    So something to check might be all of the resistors that influence PI gains, grid leaks pre and post PI and plate load resistors, in case any of them is bad.

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    Senior Member redelephant's Avatar
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    good point, symmetry is important for cancellation of supply ripple.
    But it seems to me that since the hum goes away when shorting the PI input, the problem lays in front of the PI, and not in the PI or after. Troubleshooting by elimination is just as sound as using test instruments.
    If removing the coupling from the reverb did nothing, the fault is not in the reverb section. If removing the preamp tube, or the coupling cap from it, does nothing, the problem is not there either.
    That foot long wire may be the cause depending on how it's laying. Any wire more than a few inches long must be tightly twisted together with a gnd wire, 'twisted pair'. But low frequency hum can enter a twisted pair and even a shielded cable. Have u tried moving that wire around and listened for variation in hum level?

    It is very frustrating troubleshooting this type of gripe. Often the real cause is not even determined after parts have been R&R'ed to the point of ending up with a new amp. But it's not black magic, just that it may be poor connection and/or layout and after doing many things the fix is not always determined.

    Btw, which AC30 is this? There are many variations of this amp.

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    Senior Member redelephant's Avatar
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    I looked closer at a schem I found, and do you mean you remove the 100n cap that is common for both reverb and preamp? So in effect the PI output stage is isolated from the preamp.
    I can see how that must be frustrating. Jimboyogi might be on to something...something is throwing the PI off balance...Double check all resistor values in the PI and output tubes.
    Still, I'd wiggle that wire, unsolder it from the PI's input or something.

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    Thanks guys for the replies. I checked all the resistors associated with the power tubes and most of them have been replaced and all of them check O.K. Also, the 100n cap I lifted comes off the plate(pin 6) of V12(reverb output) tube and feeds the input grid(pin 2) of the PI tube. However, there is two feet of shielded wire between that cap and the grid of the PI tube. The PC board were the wire attaches before it goes to the PI tube is a pain in the you know what to lift. So, I clipped the wire and guess what. The hum went away. I then attached a new wire in a manner so that I could move it around and I could make the hum change alot depending on were the wire was. I got so many different variations of hum by moving the wire I could play a tune(think Hendrix and his curly cord). The wire I used for my experiment was not shielded but I am pretty shure even shielded wire will still need to be layed out just right to minimize the hum. Ath this point, I am pretty satisfied that I have found the source of the hum problem. Now I have to decided whether or not I will ever be able to get the amp quiet enough for the guy who brought it to me or if I should wire it like a stock AC30 and bypass the reverb altogether. The PC board would make that a bit of a nightmare. The model of this amp is an AC30 reverb circa 1978. Thanks again you guys for the ideas.

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    Senior Member redelephant's Avatar
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    Good to see u found the fault Hope u find a good route for that wire.

    I agree even shielded cables need careful conciderations, low frequency induced noise (hum) will enter those as well.

    In my builds I place all components so I get as short a signal path as possible. The input tube is placed about 1 inch from the input jack, and the layout follows that way all till the output. That helps eliminating such unexpected noise/oscillation problems u can easily run into with high gain circuits like this.

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    Supporting Member rf7's Avatar
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    Did that do the trick?

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    Quote Originally Posted by redelephant View Post
    If the hum goes away when shorting the input of the PI, it is not the PI or anything after. Something in front of the PI is placing hum at the PI's input. Have u gone stage by stage, from PI to input tube, and shorted each stage's input in turn?

    I seriously doubt replacing the socket will help with this kind of noise. U have power supply hum, if I get u right, and a bad socket does not give such a fault.
    I've got a 1965 Deluxe Reverb on the bench right now that is exhibiting very strange hum issues that seem similar to those at issue in this thread. It is tube sockets: they are conducting from the AC heater pins to the grid and/or plate (and/or cathode?). It is a very pronounced 120Hz [EDIT: 60Hz] when first turned on and as the amp warms up over 5 to 10 minutes it disappears. It may then be gone for the day as I turn it on once in a while to check, but it comes back the following day until the amp/socket is warmed (dried?) again. The PI tube was the worst, but it was also coming from the first preamp tube of the Vibrato channel and from the reverb return stage.

    How did I diagnose it? 1) when I turn it on and the hum is crazy loud I can put a heat gun on the tube socket in question (from inside the chassis) and the hum disappears within 10 seconds instead of 5 minutes; 2) if I blow on the tube socket through a straw just after the hum has disappeared it returns with a vengeance (moisture in my breath I assume is providing enough extra conductivity to restart the hum); 3) I have been slowly getting less and less hum by scrubbing the tube sockets in question with isopropyl alcohol and "no residue" electrical contact cleaner, then blowing it dry with compressed air. Each time I do this the hum is quieter, my breath through the straw makes less hum and there is a longer period of reprieve from the hum on startup (sometimes it's 2 days later that it comes back and then dissipates more quickly).

    I've not encountered this issue before and I don't know all the history of the amp other than that it had a replaced power transformer dated 1971 and the inside was covered in a dark coating of oily substance, almost certainly the result of smoke from a torching PT, judging by the smell inside. It has since been cleaned up a fair bit by me, and I've had the amp for about 2 years. It only started this hum recently and all the subject tube sockets seemed to do it around the same time. It may be that others have used various substances to "clean" sockets in the past. I've only cleaned them with a small spritz of Deoxit D5 and "no residue" contact cleaner. I do notice that although the sockets are dry and clean after my scrubbing with isopropyl, they may look a bit greasy again a day or 2 later, and that greasy look is associated with the return of the hum. I'm hoping that each scrubbing removes more of whatever is causing the conductance.

    Anyway, in terms of the PI tube, when there's conductance from the heaters to the grid and I ground the grid with a cap, the hum disappears, but the issue is with the PI tube socket.

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    Last edited by Iron Works; 08-20-2019 at 01:59 AM. Reason: error

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    Quote Originally Posted by Iron Works View Post
    Anyway, in terms of the PI tube, when there's conductance from the heaters to the grid and I ground the grid with a cap, the hum disappears, but the issue is with the PI tube socket.
    That's quite the shaggy dog story, and yes the residue from a roasting transformer will leave your amp smelling like the cigar club held their convention there. Plus leave all sorts of icky awful crud which infuses everywhere, and might cause weird symptoms besides. In any case... how about replacing that tube socket entirely? Maybe the leads going to it as well, because they might carry some of the cruddy remains of an immolated transformer & sort of "reinfect" even a new socket.

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    If it's 120Hz it is not heater hum (60Hz).
    If you are sure the culprit is the PI socket you should replace it. Or at least remove it and give it a thorough bath in isopropyl.

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    Oops I meant 60Hz. I just did it again by firing it up and blowing through a straw onto the tube socket (and the leads running to it). It hummed LOUDLY after a few seconds of blowing (OK I can hear the jokes in your mind about humming and blowing...). This might happen normally, given that it's quite humid here along the northwest coast (Vancouver, BC), and the amp did not hum on startup because I've had it running several hours today already (but it might tomorrow or the next day).

    I'm in the process of replacing the nasty stinky wiring one piece at a time and that tube - the third gain stage of the Vibrato circuit coupled with reverb return -- has all new wires except the heater wires. The new wiring has not seemed to help, but rinsing and scrubbing the sockets does. I'm hoping not to replace tube sockets if I can avoid it as I've not seen good replicas of the 1960's Fenders and the shields these days are rather thin and cheap feeling. Anyway I put a phone-app spectrum analyzer on it and it's definitely 60Hz. Surprisingly, once the amp is warm or if I hit 2 or 3 sockets with a heat gun for 4 seconds, this amp is quieter than most of this era.

    [EDIT: the humidity here today is reported to be 46% at 23 degrees Celsius]

    [EDIT: I did replace the PI tube socket when I first began suspecting sockets. After removal, I used a Fluke 75III set to ohms to measure from lug to lug. The readings ran from "open" through 9M, 5M and even 3.7M, suggesting that there was conductance there, and that's without 200 VDC or 6 VAC and the current capacity of an amp. After an isopropyl bath and scrubbing on that socket I got "open" readings across all lugs. Now it's 4 days later and I still get "open" readings but the socket looks a bit greasy so might pass higher voltages/current across lugs.]

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    Last edited by Iron Works; 08-20-2019 at 02:20 AM.

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    Makes sense.

    the humidity here today is reported to be 46% at 23 degrees Celsius
    That's rather low humidity.


    Seems that some hygroscopic deposit has covered most of your amp's innards. In this case it is very probable that also the board became conductive (won't inject heater hum but might cause other problems).

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Helmholtz View Post
    Seems that some hygroscopic deposit has covered most of your amp's innards. In this case it is very probable that also the board became conductive (won't inject heater hum but might cause other problems).
    That's what I thought too. I've had plenty of trouble with those black paper boards. Never the sockets. I would say that whatever is in there must not only be hydroscopic, but also prone to conductivity when wet because there's not much moisture in breath and if it can burn off in five or ten minutes in a warm amp then it's not taking much moisture to cause the hum. This is a problem if the amp sounds amazing because major repairs (like replacing the board and all sockets) might alter the tone to some degree.

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