Strange Vox AC30 problems

[Jimmy74]

Hi folks I've got some really strange problems with a late 70's Vox AC30 reverb amp, the problem was and still is a loud background hum. There are actually too types of hums that happen at the same time, one is sort of like bad filtering low frequency hum and the other hum is a noise hum sort of when you touch the tip of the guitar lead. I guess I should start off with what's been done:

1) Converted to tube rectifier GZ34
2) Replaced the 2 big 390r resistors with a MM choke
3) Replaced both OT and PT with MM Toneclones
4) Replaced all filter caps, lowering the main filter from 100+60uf to 50+50uf to avoid frying the tube rectifier
5) replaced all electrolytic, poly and ceramic disc caps
6) replaced 98% of all resistors
7) replaced all the tubes with JJ pre's, Golden Lion power tubes and sovtek rectifier
8) fitted in a separate filter node for the reverb
9) changed out some of the shielded wires
10) went through all the ground points following the schematic and everything looks fine
11) voltages off all the tube pins are ok too
12) lifted the ground lug on the hum pot and grounded the heater CT off the PT

Photos of the amp here:
vox ac30 limited - a set on Flickr

the last 6 photos are what the amp looks like now. The original schematic is there too. This is a schematic with what's been done:

Flickr Photo Download: ac30 conversion to choke & reverb mod

Strange things that happen:
1) when increasing the normal channel volume I get another type of hum
2) when increasing the high-cut, the hum becomes less hissy
3) when passing my hand over or touch the wires on V11 the low frequency hum disappears
4) the 100n cap off pin 6 on V12B and pin 2 on V5A which is the PI tube, reading voltage to ground on the side of the cap that goes to pin2 on V5A, the hum totally disappears, this happens also when reading voltage off pin 7 on V5. And also when I get close to pin 7 on V12B or to the 3.3Mohm resistor off that pin, the hum gets louder.

Apart from all the hums, the amp works fine, reverb, tremolo and all 3 channels are working correctly.... the only original things left are the pots, jacks, tube sockets, wiring and PCB boards, reverb tank and reverb driver transformer.

any help at all would be fantastic

[nickb]

Try pulling all the tubes expcept the outputs and rectifier. If the hum is still bad you might have made an error in the rectifier/filter wiring.

[R.G.]

Reminds me of the antique dealer that was selling George Washington's axe - it's all original, but it's had three new handles and two new heads.

You're perceptive to note that you have multiple hum sources. Before you get too deeply into other things, did this amp have the hum problems before it was rebuilt?

The right way to go about this is to disable various parts of the signal path to see what makes the hum diminish, along with some prescriptive measures.

Prescriptive measure 1: Ground the thing correctly. Regardless of the rest of the grounding, the + line from the rectifier and the CT of the power transformer go directly to the first filter cap, nowhere else. The negative from the first filter cap goes off to a clean ground on a single wire. No signal grounds connect to the negative terminal of the first filter cap. Violate this and you *will* have 2x power line frequency buzz.

Prescriptive measure 2: Loosen, clean, and re-tighten the chassis connections of every single pot, switch, and jack if you have not already done so.

Prescriptive measure 3: Wire the third wire safety ground correctly, to its own bolt on a sanded-clean place on the chassis that's not used for anything else, and use toothed washers.

Now to the debugging. You probably have three hum sources going on, from the description. You have some line-frequency from grounding issues, and possibly transformer pickup. You probably are receiving AC line spikes. And you either have a shielding/wire routing problem or the thing is already oscillating, as witness the wave-your-hands variations.

I would do the following.
1. Take all the tubes out. Every one. Turn power on. Any hum in the speaker? Probably not, but bad transformer orientation can let the OT pick up stray field from the PT. Ditto choke. And you replaced all three. If any hum, fix it before proceeding.
2. Put in the power tubes. Hum? If not, good. If so, track it down. Putting in the power tubes ups the power used to about 85-90% of full use, so this tests the power supply too. Cure heater hum, power supply ripple, etc. here.
3. Put in the PI. Find the ripple, cure it.
4. Work your way back through the amp, fixing hum issues as they arise.

The reverb section is a likely hummer.
Don't ground both ends of a shielded cable. Ground the quiet end, usually the sending end. Otherwise you can create ground loops where none existed.
Get a scope on the audio at the power tube grids and see if it's oscillating.

[Jimmy74]

Reminds me of the antique dealer that was selling George Washington's axe - it's all original, but it's had three new handles and two new heads.

You're perceptive to note that you have multiple hum sources. Before you get too deeply into other things, did this amp have the hum problems before it was rebuilt?

The right way to go about this is to disable various parts of the signal path to see what makes the hum diminish, along with some prescriptive measures.

Prescriptive measure 1: Ground the thing correctly. Regardless of the rest of the grounding, the + line from the rectifier and the CT of the power transformer go directly to the first filter cap, nowhere else. The negative from the first filter cap goes off to a clean ground on a single wire. No signal grounds connect to the negative terminal of the first filter cap. Violate this and you *will* have 2x power line frequency buzz.

Prescriptive measure 2: Loosen, clean, and re-tighten the chassis connections of every single pot, switch, and jack if you have not already done so.

Prescriptive measure 3: Wire the third wire safety ground correctly, to its own bolt on a sanded-clean place on the chassis that's not used for anything else, and use toothed washers.

Now to the debugging. You probably have three hum sources going on, from the description. You have some line-frequency from grounding issues, and possibly transformer pickup. You probably are receiving AC line spikes. And you either have a shielding/wire routing problem or the thing is already oscillating, as witness the wave-your-hands variations.

I would do the following.
1. Take all the tubes out. Every one. Turn power on. Any hum in the speaker? Probably not, but bad transformer orientation can let the OT pick up stray field from the PT. Ditto choke. And you replaced all three. If any hum, fix it before proceeding.
2. Put in the power tubes. Hum? If not, good. If so, track it down. Putting in the power tubes ups the power used to about 85-90% of full use, so this tests the power supply too. Cure heater hum, power supply ripple, etc. here.
3. Put in the PI. Find the ripple, cure it.
4. Work your way back through the amp, fixing hum issues as they arise.

The reverb section is a likely hummer.
Don't ground both ends of a shielded cable. Ground the quiet end, usually the sending end. Otherwise you can create ground loops where none existed.
Get a scope on the audio at the power tube grids and see if it's oscillating.

thankyou for all the fabulous tips!! Just to answer a few questions, I don't know if the amp was doing it before all these mods were applied, my customer did complain about unsupportable hum though I can't be 100% certain we're talking about the same hum I'm getting now. The amp did come to me with desperate need of new filter caps, but the owner wanted to apply the mods to get it closer to a 60's AC30, so I did all the modding and replacing parts in one big job.

The CT for the HV is grounded all alone, whereas the CT for the heaters is grounded together with the "shield" wire from the PT at the first filter cap ground point.... so I will rewire both of these wires to another ground point.

You said this "the + line from the rectifier and the CT of the power transformer go directly to the first filter cap, nowhere else"..... the + line from the rectifier meets one choke wire at a solder point on the output board and then follows through to the first filter cap where it meets the CT from the output transformer.

I don't have a scope, but I used a tester to see if there was any AC off pin 6 on V12 and was getting around 2v. I will follow through your advice step by step and then get back to you when I'm done.

thanks

[R.G.]

The CT for the HV is grounded all alone, whereas the CT for the heaters is grounded together with the "shield" wire from the PT at the first filter cap ground point.... so I will rewire both of these wires to another ground point.

It helps to think of it this way. The first filter cap doesn't have a ground point - it has two and only two wires connected to the (-) side. One is from the CT of the power transformer. The other is to the circuit ground. Anything else carries the possibility of contaminating the ground with the current pulses which go through when the power AC is rectified.

You said this "the + line from the rectifier and the CT of the power transformer go directly to the first filter cap, nowhere else"..... the + line from the rectifier meets one choke wire at a solder point on the output board and then follows through to the first filter cap where it meets the CT from the output transformer.

This is likely a source of some of your buzz. Maybe not, but it violates one principle of power supply wiring: never share any part of the wires to the transformer with wires from the rest of the circuit. Especially ground, but it can happen with the + side as well.

The rectifiers conduct in short, sharp pulses at the peak of the AC waveform. These pulses are five to hundreds of times the DC current the power supply provides. They're big enough to make the copper wire act like a resistor and have a pulse voltage on top of the average DC the filter cap makes.

For extra credit, you actually should twist the two high voltage wires and the CT from the transformer together when you run them to the rectifier, and then the rectifier output with the CT from the rectifier to the first filter cap. This minimizes the loop area of the current path, and minimizes transmission from the loop in the antenna sense.

I don't have a scope, but I used a tester to see if there was any AC off pin 6 on V12 and was getting around 2v.

I think that's bad.

I will follow through your advice step by step and then get back to you when I'm done.

Let me know. At least some of what I've told you is probably wrong, but we can work it out.

Hey... wasn't there a song... ?

[Jimmy74]

Prescriptive measure 3: Wire the third wire safety ground correctly, to its own bolt on a sanded-clean place on the chassis that's not used for anything else, and use toothed washers.

Which wire are you refering to?

Debugging:

1. Take all the tubes out. Every one. Turn power on. Any hum in the speaker? >No hum
2. Put in the power tubes. Hum? If not, good. If so, track it down. Putting in the power tubes ups the power used to about 85-90% of full use, so this tests the power supply too. Cure heater hum, power supply ripple, etc. here.
> No hum
3. Put in the PI. Find the ripple, cure it.
> Very faint hum which I think is just heater hum
4) Working backwards so putting in V12 and that sure makes all the hums come out. At this point I tried turning up the volumes, the high cut attenuates the "noise" hum, the normal channel volume seems a little scratchy but no increase in hum. The reverb volume gives out a loud squeal when the reverb tank is not connected, when it is connected the reverb volume gives out a louder "noise" hum.

Everything has been swapped out around the PI, V12 and V11 except the 3.3Mohm resistor, the pots, tube sockets and wiring. I have also swapped out the shielded wires.

So it could be anything in the PS, PI or reverb section?? When taking the reverb driver out of the circuit (HV supply) the hum is still there.

thanks

[Wakculloch]

I had the same problem recently and if it is the model with the reverb circuit in the power amp chassis, that is where the hum is coming from. Sorry to say i never did get the hum to totally stop, but poor grounding did seem to make the problem worse.

[R.G.]

Prescriptive measure 3: Wire the third wire safety ground correctly, to its own bolt on a sanded-clean place on the chassis that's not used for anything else, and use toothed washers.

Which wire are you refering to?

The one marked "E" at the power transformer primary.

3. Put in the PI. Find the ripple, cure it.
> Very faint hum which I think is just heater hum

We'll work on this inner layer of the onion later.

4) Working backwards so putting in V12 and that sure makes all the hums come out. At this point I tried turning up the volumes, the high cut attenuates the "noise" hum, the normal channel volume seems a little scratchy but no increase in hum. The reverb volume gives out a loud squeal when the reverb tank is not connected, when it is connected the reverb volume gives out a louder "noise" hum.

Now for some divide and conquer. Remember to turn the amp off and unplug it before adding/removing shorts.
I.Short The reverb control wiper to ground at the bottom end of the 4.7K resistor on the cathode of V12B. Turn the reverb control up halfway. This completely eliminates anything coming from the reverb circuit. What does that do to the hum?
II. Leaving the reverb shorting wire in, add another temporary short to ground at the lower end of the same 4.7K on the cathode of V12B from the junction of the 470K, 3.3M, and 220K from the normal volume pot. What does that do to the hum?
III. Leaving the previous two shorts in place, connect pin 2 of V5 to ground at the grond end of the 47K in its cathode circuit through a 1uF or larger capacitor. This removes all the contribution to V5 from V12.

The difference between II. and III. is how much noise and hum are coming from just section B of V12. If there's not much, you have eliminated Section B as a problem. If it's huge, we need to find where it's coming from.

I think it will be not much. So -
IV. Remove the short and cap from step III and move the reverb shorting wire from the reverb pot wiper to instead short V12 pin 2 to ground. This lets V5 amplify whatever's coming from V5B, but adds back in whatever comes from V12A, but not what's coming in from the reverb tank. Wobble the reverb volume control min to max. How much does the hum/noise change here?

V. If the preceding was "not much" then remove the short you put in for II and mess with the normal volume control. How much noise?

So it could be anything in the PS, PI or reverb section?? When taking the reverb driver out of the circuit (HV supply) the hum is still there.

Can't tell yet. Still dividing and conquering.

[Jimmy74]

All this dividing and conquering is to be done with the power tubes, rectifier V5 and V12 in? Or do I put all the tubes in?

thanks

[Jimmy74]

Ok I've drawn up the first three tests modding the original schematic:

I.Short The reverb control wiper to ground at the bottom end of the 4.7K resistor on the cathode of V12B -> the red line

II. Leaving the reverb shorting wire in, add another temporary short to ground at the lower end of the same 4.7K on the cathode of V12B from the junction of the 470K, 3.3M, and 220K from the normal volume pot. -> the blue line

III. Leaving the previous two shorts in place, connect pin 2 of V5 to ground at the grond end of the 47K in its cathode circuit through a 1uF or larger capacitor. ->the purple line

A couple of questions:
1) In test I do I have to completely take out the 470k resistor off the reverb wiper or do I ground the lower end of the 470k resistor?
2) In test II I just have to ground that junction or do I have to take out any components?
3) In test III what type of cap does it have to be? Electrolytic, poly or ceramic? Do I have to remove the 100n cap too?

Also let me know if the mods I've done to the schematic are correct

Flickr Photo Download: ac30 reverb & PI tests

thanks

[Jimmy74]

TEST I = the hum got much louder, took out the 470k resistor and grounded the wiper to the ground of the 4.7k on pin 8.... the wire coming from the wiper of the reverb pot is shielded therefore when shorting to ground both ends of the shielded wire are grounded to the same point. The reverb pot doesn't have any effect at all on the hum.

TEST II = nothing changes from TEST I

[R.G.]

Hmm. I typed in an answer, but it didn't show up.

Ok I've drawn up the first three tests modding the original schematic:
I.Short The reverb control wiper to ground at the bottom end of the 4.7K resistor on the cathode of V12B -> the red line
II. Leaving the reverb shorting wire in, add another temporary short to ground at the lower end of the same 4.7K on the cathode of V12B from the junction of the 470K, 3.3M, and 220K from the normal volume pot. -> the blue line
III. Leaving the previous two shorts in place, connect pin 2 of V5 to ground at the grond end of the 47K in its cathode circuit through a 1uF or larger capacitor. ->the purple line

A couple of questions:
1) In test I do I have to completely take out the 470k resistor off the reverb wiper or do I ground the lower end of the 470k resistor?

You don't need to remove parts for any of the tests I outlined. Just stick in a length of wire to short the points to ground. In the case of test III, you need the capacitor to keep from upsetting the DC balance inside the amp. That would not hurt the amp, but it won't tell you anything either.

2) In test II I just have to ground that junction or do I have to take out any components?

Just ground the junction. No components need removed.

3) In test III what type of cap does it have to be? Electrolytic, poly or ceramic? Do I have to remove the 100n cap too?

No components need removed. The "shorting" cap can be any kind, and the value is not critical, but 1uF is sufficiently bigger than the existing parts to ensure a good AC short. Be sure you get 100V or so, as the voltage at that grid is about 60V as I remember.

TEST I = the hum got much louder, took out the 470k resistor and grounded the wiper to the ground of the 4.7k on pin 8.

I'm not sure what you did here. The 470k should not have been removed. Only the reverb-pot-wiper end tied to ground. You get much the same thing by turning the reverb knob all the way CCW, but if the ground is open between the reverb pot and the rest of the circuit, that tells you nothing. Running the wire to the indicated point ensures that the PI sees no signal from reverb.

the wire coming from the wiper of the reverb pot is shielded therefore when shorting to ground both ends of the shielded wire are grounded to the same point. The reverb pot doesn't have any effect at all on the hum.
TEST II = nothing changes from TEST I

Again, still confused about what you actually did.

[Jimmy74]

Ok I did as you said and fitted the 470 resistor back in, and did the tests from scratch:

TEST I - No difference in hum and reverb pot doesn't seem to do anything at all
TEST II - No difference from TEST I
TEST III - Hum is cancelled out leaving only a faint "heater hum". I used what I could find a 2.2uf 250v poly cap

So what's next?

thanks

[R.G.]

TEST III - Hum is cancelled out leaving only a faint "heater hum". I used what I could find a 2.2uf 250v poly cap

OK. Not what I expected. Oh, well.

What this test tells us is that messing with the inputs to V12B doesn't help any, but shorting its output does. There's only a few ways that can happen.

One of those is that the "ground" is not the same voltage between V5 and V12.
I would first measure the voltage difference between the "ground" end of the 4.7K resistor on the cathode of V12B and the "ground" end of the 47K resistor on the cathode circuits of V5, both with the meter set to DC and with it set to AC.

Any voltage difference over a millivolt or two is a problem that needs corrected.

Also, a good thing to try that I didn't have you try earlier is to install a temporary shorting wire from pin 7 of V12 to the ground end of the 4.7K resistor on its cathode. Does the hum vanish when you do this?

[Jimmy74]

OK. Not what I expected. Oh, well.

What this test tells us is that messing with the inputs to V12B doesn't help any, but shorting its output does. There's only a few ways that can happen.

One of those is that the "ground" is not the same voltage between V5 and V12.
I would first measure the voltage difference between the "ground" end of the 4.7K resistor on the cathode of V12B and the "ground" end of the 47K resistor on the cathode circuits of V5, both with the meter set to DC and with it set to AC.

Question: Do I have to take out all the previous ground mods before taking the readings? I guess that these voltage readings are taken by placing the tester leads across the resistor?

Any voltage difference over a millivolt or two is a problem that needs corrected.

Also, a good thing to try that I didn't have you try earlier is to install a temporary shorting wire from pin 7 of V12 to the ground end of the 4.7K resistor on its cathode. Does the hum vanish when you do this?

Question: I suppose I should take out the TEST III short before placing the short from pin7 V12 to the 4.7k resistor?