Strange Vox AC30 problems

[Jimmy74]

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

[Enzo]

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.

[Jimmy74]

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).

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.

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.

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.

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.

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

[Enzo]

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.

[Jimmy74]

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

[R.G.]

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.

[Jimmy74]

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

[Jimmy74]

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

[soundmasterg]

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

[Jimmy74]

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

[soundmasterg]

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

[Jimmy74]

PM sent

[oc disorder]

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

[soundmasterg]

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

[Jimmy74]

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