Strange Vox AC30 problems

[R.G.]

Question: Do I have to take out all the previous ground mods before taking the readings?

No.

I know it's a bit pedantic, but we have to be careful with terminology, or we'll get mixed up. Those were not "ground mods" in the sense that they did not change what the grounding does. They just forced various signal voltages to be equal to the local ground voltage, whatever that is. Let's call them "temporary signal shorts" instead. And let's not call them "mods". Otherwise, the ever-present cadre of mod-hungry amp beginners who sift the internet ever-finer with google will be screaming about amp mods not doing anything.

And no, you don't have to remove them first. They were intended to connect places with voltages that should already be at 0V dc to ground, which should be 0V dc. Only any signal voltages were zeroed out. So for this small, tiny step, they do not need removed. Or they could be removed. Makes no difference to the testing.

I guess that these voltage readings are taken by placing the tester leads across the resistor?

No.

This test is looking for the possibility that what should be "ground" and the same voltage of 0.000000V in both places is in fact messed up some way and the two places are not at the same voltage. This could theoretically cause a condition which would fit what you have described as the problem.

So you do not use tester leads across a resistor, you touch one lead to the ground side of two *different* resistors.

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

You can if you want, but it makes no difference, as explained above.

I have a great deal of apprehension that what I'm trying to tell you is not getting across, and as a result the chance for error is high. I'm trying not to have to go do a schematic drawing for each test, as that's going to be a huge PITA and time taker.

[Jimmy74]

Ok I did take out the short for TEST III (since TEST III is already cancelling hum, if I leave it in I wouldn't be able to tell what this last short is doing) and placed in the last short you mentioned which I'll call TEST IV and the hum disappeared. That way I'm grounding both sides of the 470K resistor on the reverb wiper and both sides of the 3.3M resistor as shown in this updated schematic:

Flickr Photo Download: ac30 ground short tests

TEST I = RED LINE
TEST II = BLUE LINE
TEST III = PURPLE LINE (absent in this last test)
TEST IV = ORANGE LINE

I'll now go on to taking the voltage readings on both resistors and also, leaving in TEST IV, I'll try taking out TEST II and TEST I and putting them back in 1 at a time to see what happens.

Ofcourse I follow your advice on how to name what we are actually doing

thanks

[Jimmy74]

Voltage tests in AC and DC for both resistors give 0.00V. I did do these voltage tests without the power tubes in

[R.G.]

Ok I did take out the short for TEST III (since TEST III is already cancelling hum, if I leave it in I wouldn't be able to tell what this last short is doing)

Good point!

and placed in the last short you mentioned which I'll call TEST IV and the hum disappeared. That way I'm grounding both sides of the 470K resistor on the reverb wiper and both sides of the 3.3M resistor as shown in this updated schematic:

OK. Good info. Here's what we've learned.
- hum is not coming from the output of the normal channel or the Vibrato channel; the blue-line short forces this signal to be zero, and it makes no difference.
- hum is not coming from the reverb output nor the wiring to/from the reverb pot; the red-line short forces this to be zero, and that makes no difference.
- hum IS coming from the plate of V12B into the phase inverter; the purple line short causes this to be zero, and this changes hum a lot.
- hum is affected by the grid of V12B, because the orange-line short forces it to be zero, and this changes the hum a lot.

I'll now go on to taking the voltage readings on both resistors and also, leaving in TEST IV, I'll try taking out TEST II and TEST I and putting them back in 1 at a time to see what happens.

OK. The critical voltage will be between the south end of the 4.7K (where the red, blue, and orange lines meet) and the south end of the 47K.

It is possible that there is a voltage between these two places. If the voltage at either AC or DC readings on your meter (test it both ways) is more than a couple of millivolts, install one more temporary shorting wire, this time between the two "ground" ends of the 4.7K and the 47K. This will force the two points to the same voltage, and should change the hum all by itself, with Test I, II, III and IV removed; if it's the problem, that is.

[Jimmy74]

Taking out TEST I and TEST II leaving TEST IV all by itself is getting rid of hum, but you have already arrived at this conclusion so there's no use trying TEST III all by itself. Maybe you skipped my last post "Voltage tests in AC and DC for both resistors give 0.00V. I did do these voltage tests without the power tubes in". Should I go ahead and shorting the 2 resistor ground ends anyway or do these results mean the problem is elsewhere?

thanks

[R.G.]

Taking out TEST I and TEST II leaving TEST IV all by itself is getting rid of hum, but you have already arrived at this conclusion so there's no use trying TEST III all by itself.

Correct.

Maybe you skipped my last post "Voltage tests in AC and DC for both resistors give 0.00V.

Sorry - I missed that one, possibly it came in while I was typing a reply.

I did do these voltage tests without the power tubes in". Should I go ahead and shorting the 2 resistor ground ends anyway or do these results mean the problem is elsewhere?

Put the power tubes back in, then try shorting the two ground ends.

Summing up, something around V12B is where the problem is. It's not coming in on the inputs to V12B, and it's there on the output. So all that's needed is to find and eliminate whatever it is that's causing the hum in V12B.

This could be:
- Ground potential is different between V12B and the tube it drives. This is what the ground test/shorting wire both tests and solves if it's true. However, the 0.00V difference says this may not help. It's still worth doing just to be sure.
- Could be a bum tube. Just to be absolutely sure, swap with another tube. You did this before, but this is a make-doubly certain step before getting even deeper into it.
- Could be the socket condition. Swapping to another tube probably wipes any corrosion off as well. The grid pin socket could be loose or not making good contact. It's worth cleaning it with Deoxit and a pipe cleaner, and retensioning the contact. Even if that's not the problem, someday it will be.
- Could be the wiring to/from the socket. Check the heater wires going into the socket to ensure that the heater wires are cleanly wired to the heater pins and twisted together leading up to/from the socket. Also check the physical routing of the wire from the grid pin of V12B through the amplifier. If it's too close to heater wires, AC power wires, etc., then it will pick up hum capacitively. You could test this by running the amp and moving the grid wire around with a wooden chopstick or skewer while listening to hum. Notice that this is as dangerous as petting a rattlesnake in terms of you getting hurt or killed.
- It is possible that the power supply to this section has excess ripple, but this is an unlikely possibility, given what else has happened.

We have the problem trapped. All that remains is to find its true nature, and then kill it.

[Jimmy74]

Correct.


Sorry - I missed that one, possibly it came in while I was typing a reply.


Put the power tubes back in, then try shorting the two ground ends.

Summing up, something around V12B is where the problem is. It's not coming in on the inputs to V12B, and it's there on the output. So all that's needed is to find and eliminate whatever it is that's causing the hum in V12B.

This could be:
- Ground potential is different between V12B and the tube it drives. This is what the ground test/shorting wire both tests and solves if it's true. However, the 0.00V difference says this may not help. It's still worth doing just to be sure.

Took out all the TEST shorts and shorted both resitor ground ends together, but the hum is right back

- Could be a bum tube. Just to be absolutely sure, swap with another tube. You did this before, but this is a make-doubly certain step before getting even deeper into it.
- Could be the socket condition. Swapping to another tube probably wipes any corrosion off as well. The grid pin socket could be loose or not making good contact. It's worth cleaning it with Deoxit and a pipe cleaner, and retensioning the contact. Even if that's not the problem, someday it will be.

Tried these too, atleast all the tube socket pins are bright now!

- Could be the wiring to/from the socket. Check the heater wires going into the socket to ensure that the heater wires are cleanly wired to the heater pins and twisted together leading up to/from the socket. Also check the physical routing of the wire from the grid pin of V12B through the amplifier. If it's too close to heater wires, AC power wires, etc., then it will pick up hum capacitively. You could test this by running the amp and moving the grid wire around with a wooden chopstick or skewer while listening to hum. Notice that this is as dangerous as petting a rattlesnake in terms of you getting hurt or killed.

I did do that before writing up on this forum, as I said the noise hum seems to be produced around that 3.3Mohm resistor as noise does increase when passing your hand near by. Heater wirings are all twisted together and properly soldered to each pin and are bent down towards the chassis and away from the signal and HV wires all through the amp.

- It is possible that the power supply to this section has excess ripple, but this is an unlikely possibility, given what else has happened.

So what we've narrowed down to is a probable defective 3.3Mohm resistor or something along the chain going to pin 6 V12?

thanks

[R.G.]

I did do that before writing up on this forum, as I said the noise hum seems to be produced around that 3.3Mohm resistor as noise does increase when passing your hand near by. Heater wirings are all twisted together and properly soldered to each pin and are bent down towards the chassis and away from the signal and HV wires all through the amp.

So what we've narrowed down to is a probable defective 3.3Mohm resistor or something along the chain going to pin 6 V12?

We've eliminated the other issues. Still could be something else, but it seems now that the most likely candidate, as you note, is the 3.3M.

If it were me, I would next replace the wire from the 3.3M to the grid of V12B with shielded wire, grounding the shield. This will stop any capacitive hum pickup from the wire, and may be all you need. This carries with it the possibility of the shield causing too much capacitance and dulling the normal signals.

If you can, try a length of shielded wire. If this is a big improvement, we can work on the treble.

[Jimmy74]

I did use a short length of shielded wire from pin 7 V12 to the 3.3M resistor, I grounded the shield on the side that's soldered to the tube pin, to the centre hole of the tube socket which is grounded together with shielded wire that goes to the reverb footswitch/tank section. But this test doesn't seem to do anything to the hum which is still present. I did also try in a nother set of power tubes but it's obvious that the problem isn't there either.

What's next? Should I try replacing the 3.3M resistor? I'll use a 2.2M + 1M resistor in series and see what that does. Also what exaclty does that 4uf 350v cap, at the junction with the 6.8k resistor, do? I did replace it with a 4.7uf 450v cap.

thanks

[Jimmy74]

I changed that 3.3M resistor out for a 2.2M+1M in series but still am getting hum. Should I try a 5.6M resistor?

[R.G.]

OK. Well that wasn't it. Your replacing the resistor eliminates a far-too-high resistance, and the shielding should have at least changed the hum if it was capacitive pickup.

Next check, wiring and layout.

Where, physically, does the 3.3M resistor and its paralleling cap sit in the box? It would be best if the V12B side was as close to the tube socket pin as you can physically get it. Likewise the 470K from the reverb pot. These components being in series to the grid pin make the impedance of that line very high, so it's susceptible to hum pickup. I would try moving the 3.3M, 470K, and 10pF cap as close to the tube pin as I could get them. This will not hurt in any case.

Also, the ground wire routing can make a difference. Well, all wire routing can make a difference. Where physically inside the chassis, does the ground wire from V12 A and B run?

Also, how are the volume pots for normal and brilliant channel and the reverb pot grounded, both the signal ground shown on the schematic and the case/bushing ground of the physical mounting?

Where is the power supply and added choke physically compared to where the V12 tube and components sit in the chassis?

Also what exaclty does that 4uf 350v cap, at the junction with the 6.8k resistor, do? I did replace it with a 4.7uf 450v cap.

It's a power supply filter, to keep the current drawn by the other stages from changing the B+ voltage that V12 gets.

Here's another thought.

Test V: Remove all the shorting wires from the previous tests. Also, remove the 3.3M and its 10pF cap. Still hum? This tests whether the hum comes in on the 3.3M.
Test VI: if it does still hum, leave the 3.3M resistor out and remove the 470K to the reverb pot, and temporarily add a 470K resistor from V12B pin 7 to the ground side of the 4.7K resistor on its cathode. This tests whether the tube, with its existing power supply and grounding, hums all on its own.

By doing these tests, we're excluding things that could cause hum, and getting down to what is causing hum. At some point, we'll hit it.

[Jimmy74]

OK. Well that wasn't it. Your replacing the resistor eliminates a far-too-high resistance, and the shielding should have at least changed the hum if it was capacitive pickup.

Next check, wiring and layout.

Where, physically, does the 3.3M resistor and its paralleling cap sit in the box?

Check this photo:
Flickr Photo Download: 26052010117

The last 3 parts on the very left of the board right next to the V12 tube socket in the order from closest to the tube socket -> 3.3M resistor, 10pf cap and 470k reverb wiper resistor. You can also see how the reverb footswitch/tank shielded wire is grounded to the center hole of the V12 tube socket

Also, the ground wire routing can make a difference. Well, all wire routing can make a difference. Where physically inside the chassis, does the ground wire from V12 A and B run?

I think you can see the ground wire for V12 in the above photo going to the center hole of the socket.

Also, how are the volume pots for normal and brilliant channel and the reverb pot grounded, both the signal ground shown on the schematic and the case/bushing ground of the physical mounting?

check this photo:
Flickr Photo Download: 26052010112

There's a metal bar running across all 3 volume pots & tremolo switch which is connected to ground via the green wire that you can see just above the electrolytic cap in the middle of the board. Something strange here: I tested the small electrolytic caps and they are grounded on both sides, I mean the positive side is grounded too.... is this normal? Or is it because I grounded the shielded wire between Pin 7 V12 - 3.3M resistor?

Where is the power supply and added choke physically compared to where the V12 tube and components sit in the chassis?

The Power transformer is on the other side of the chassis and is far away, the choke sits underneath the V11 tube socket, it is physically very close to the V11 socket, here's the photo:

Flickr Photo Download: 30032010012

I'll go on to doing TEST V and VI now

[Jimmy74]

Here's another thought.

Test V: Remove all the shorting wires from the previous tests. Also, remove the 3.3M and its 10pF cap. Still hum? This tests whether the hum comes in on the 3.3M.

I had already taken out all the previous shorts before placing the shielded wire from pin 7 to the 3.3M resistor. Anyways hum is still there.

Test VI: if it does still hum, leave the 3.3M resistor out and remove the 470K to the reverb pot, and temporarily add a 470K resistor from V12B pin 7 to the ground side of the 4.7K resistor on its cathode. This tests whether the tube, with its existing power supply and grounding, hums all on its own.

I also left out the 10pf cap for this test. Hum is still there.

[R.G.]

You can also see how the reverb footswitch/tank shielded wire is grounded to the center hole of the V12 tube socket

Hmm.The problem with "grounded to the center hole of the V12 tube socket" is that then the question is "where is that center hole connected to ground?"

The wrong answer for hum and noise is "the chassis". Or at least *A* wrong answer is that.

I think you can see the ground wire for V12 in the above photo going to the center hole of the socket.

I can. And it looks like all of the triodes have something like this for local grounding. This is a basic problem with hum and noise of all kinds.

I gave up and tried to mod the picture you posted. Look at this one:
Star Grounding. It's how to do it the right way. What you actually do will probably be a hybrid, but the first points for relieving hum are the most important.

Things to note:
1. The earth/safety ground connects to the chassis. This is exactly one, no more, wires. The connection to the chassis from the AC power line ground must be at a bolt on the chassis used for this purpose, and no other, and there must be a star washer under the wire's ring terminal (and the wire must have a crimped ring terminal!) to bit into the chassis and terminal.
2. The star ground point at B connects to the chassis with exactly one, no more wires. This is the multicolored wire between B and the earth ground. If you remove that one wire, there must be no continuity to the chassis. Although I show it going to the earth ground point, it may be better to connect the input jack bushing to ground and make this the only signal ground to chassis ground point for RF noise reasons.
3. The negative side of the rectifiers does NOT connect to the chassis. It has a single wire to the negative side of the first filter cap. This is quite important for minimizing rectifier buzz. The positive side of the rectifiers goes on one wire to the first filter cap.
4. The negative side of the first filter cap goes on a single wire to the star point B. You must not have the other signal wires connect to the negative side of the first filter cap, nor to the negative side of the rectifiers. This will positively cause hum issues.
5. The two red octopi at the two input channels show what you should do with local grounds. It is important that local grounds NOT be tied to chassis out at the tube. You collect the local grounds for a tube or two which are heavily connected, and make up that center dot. The center dot has a wire which goes to the star point B.
6. Decoupling caps should have their own wire to B.
7. On V5, V12, etc particularly:
V12's local grounds gather at point E, which is then run with a single wire to the star point B. V5's local grounds do the same at D. It's important that even if the tube socket offers you a tempting chassis ground point, you not connect those local grounds to chassis, but instead run them back to the star point.

The controls' outer cases are attached to the chassis by their bushing, and a common mistake is to "ground" lugs to the control's metal case. This can sometimes create hum where there was none. The grounded lugs of the pots go back to the local star point they're most involved with, or to the star point B.

This is a lot of work. And it's not the only way to get a quiet amp. But it's the only way you can say ahead of time that will for sure remove ground hum and noise.

In your case, you have to get the following done (I believe):
1. Get the local grounds off those tube socket center pins which connect to the chassis.
2. As part of that make a star point like B. Run the wire from B to the first filter cap negative as shown. Run the wire between the first filter cap negative and the minus side of the rectifiers, and neither of these wires can touch chassis.
3. Run the local grounds for V5 and V12 as shown, with local collection points at D and E respectively, then a single wire from each local collection point to B.

I consider this so important to hum reduction that I ran star grounding on the PCBs for our amplifier line. It turned out very, very low in hum, and the grounding was never touched again.

I've probably confused things. Questions?

[Jimmy74]

Well I will have a few questions while reorganising the grounding throughout the amp, but one thing I can say now that you probably forgot is that the amp is now tube rectified(GZ34) so there's no ground in the rectifier. Also maybe you missed what I found out during the last tests: "Something strange here: I tested the small electrolytic caps and they are grounded on both sides, I mean the positive side is grounded too.... is this normal? Or is it because I grounded the shielded wire between Pin 7 V12 - 3.3M resistor?"

thanks