Early 90s Vox AC30 Boost channel Noise?

[jwhitmore]

Hello all, this might well be a stupid question but it's the first one, so at least there's that. I want to build a Valve Amp, but decided as I don't actually own a Valve amp that perhaps I should maybe dip my toes in and buy one. So a friend of a friend was not using an early 90s AC30 so we agreed a price and I took it off their hands. Now it does sound amazing but even if you've nothing plugged into the amp and adjust the Boost channel's volume there's some serious thumping noise out of it.

So this noise issue is it normal? As I've never owned or used a Valve Amp before I'm not sure what to expect. I've tried cleaning the POT in question with contact cleaner but it's not making any difference, but I seems to be a fairly well sealed POT so I might not be getting any contact cleaner down the shaft to the contacts. The noise only occurs when adjust the Boost Volume when you stop turning it settles down again.

I guess I could take the PCB out, desolder the POT in question and replace it, but being new to this that might be the wrong direction to take. I had a look at the schematic I found [1] thinking I'd put a scope onto it and try make sure of the source of the problem. Looking at the schematic I found the 'Brilliant POT' in the center of the right hand column in the schematic, but the track from the POT to the gate of the Valve 7 is a PCB track so not sure I'd get to it. As I have the underside of the Valve socket facing me I could solder a test wire to it just to be able to fix the scope lead to it. Having said that maybe if I removed the Valve in question and put the scope in from the top and see what the signal on the gate looks like. Again is that constructive? Sorry this is a bit vague and hand wavey, but where do you start.

Apart from all that the Amp is working fine, at least as far as I can tell. Seriously loud ;)

[1]  https://www.thetubestore.com/lib/thetubestore/schematics/Vox/Vox-AC30-93PR-Schematic.pdf

[pdf64]

A soundclip of the issue would be really helpful.

[PRR]

> there's some serious thumping

At the same rate as the Tremolo? If you change the Trem Rate switch S-M-F, does the thump rate change?

[jwhitmore]

Sorry never thought to attach a recording.

So in this recording I've got nothing connected to any of the six inputs on the AC30 and all three volumes are at zero. In the recording I only sweep the boost volume from zero to max and back again to zero. I do that twice. The thump is at the bottom of the range just before it gets to zero.

I can't here any change in this when I adjust the trem/vibrato settings.

Thanks for your help, should have thought of the audo attachment. I couldn't attach an mp3 file so it's compressed into a .zip hope ye can get to it.

[sluckey]

That is not normal but since it only occurs when you set the pot to zero, does it really matter?

[pdf64]

Ahh, right, so it's not a repetitive thump, just a single thud in response to the pot being turned down quickly.
My guess is that the valve in V7, dealing with the top boost circuit, has excessive grid current; with the pot VR4 set high, maybe up to 0.25VDC appears at the grid / wiper / hot track end?
And the cathode voltage at R9//C7 probably changes similarly between VR4 being set at min and max?
And if the control is turned slowly, there's quite a bit of 'rustling'? Due to the operating point shifting.

[jwhitmore]

Thanks a million pdf64 what you're describing sounds close, but I don't think I have to turn up or down quickly. I'm turning up and down gradually and getting the effect, but I might be splitting hairs there. My speed of turning up and down is a bit subjective. But rustling is the perfect description of the sound as I'm turning the volume pot.

So I put the Scope onto your suggestions. Yellow trace, channel one was on the center pin of the POT VR4 and Blue trace, channel two onto the business end of R9.

The two channel are on the same zero level, on the scope, and appear to follow each other even if at different voltages. I'll attach the two traces. One with the volume POT being turned up and the other with the volume being turned down. In both cases there a lot of rustling and the thump as you get close to zero volume.

Thanks again for taking the time to read, listen to the issue and respond. That goes for everyone, even reading this. Feels good to have a scope on the gate of a valve in an AC30. It's the little things

[pdf64]

Well done!
So how about with a different valve in V7?

[jwhitmore]

Not sure there's much difference in terms of noise, there's a slight difference in voltage level on the POT, which is closer to the cathode. So there is a difference. Can the amp be killing Valves. Guess that's down to Anode Voltage?

The valve I used is a 12AX7 that I'd messed about with making a pre Amp out of. So it has never done much.

[pdf64]

So with the vol up full, what’s the anode, grid and cathode VDC?

[PRR]

And what is the DC voltage at all three legs of the pot?

[jwhitmore]

This is going to get more colourful with all four channels on the scope:

Channel 1, Yellow       Pin 1 of the POT  it's on x10 probe so 50V/Division
Channel 2 Light Blue  Pin 2 of the POT
Channel 3 Purple        Pin 3 of the POT
Channel 4 Dark Blue   Business end of R9 / Cathode of Valve

First trace again is volume being turned up, second volume going down:

[jwhitmore]

Should maybe have said I'm not sure about the convention for the pin numbers on the POT. I'm just numbering based on the left to right order that I'm looking at. One pin is solid on ground voltage which agrees with the schematic. I've called that Pin 3 but call it pin 1, pin 2 is the middle pin,

[PRR]

And what is the DC voltage at all three legs of the pot?

[jwhitmore]

The DC Voltage at the legs of the pots kinda seems to depend on where you have the Pot set. If the volume is set to zero on the brilliant volume pot, with no input connected to the amp then one leg of the pot is tied to ground. That pin ain't going to change through out the volume range, but the other two change.

Back to Zero volume and One leg is at zero volts. Then because the wiper, middle leg, is turned to zero volts that pin is at zero volts as well. The last pin is sitting at about 170V give or take.

[sluckey]

All three pot lugs should have zero volts present. C9, 470pF, is faulty, allowing dc voltage from the previous tube plate to leak onto the pot. Replace it.

[PRR]

....The last pin is sitting at about 170V...

Slucky said what to do. I might note that, while small mica caps *were* always 500V or better, lately I have seen 50V units. Which won't block 170V for long. I'd just take the cap out and see if it fixes the DC and the thump (that there isn't another sneaky flaw).

[jwhitmore]

OMG that makes perfect sense when it's pointed out. New to this but obviously yes C9 is a filter cap between stages and should be removing any DC. What was I thinking.

So as suggested I removed the Cap C9 and powered up the Amp again and total silence. Ye are Legends! I now have to order a 500V 470pF mica Cap so it might be a while before ye hear from me again, but I'll let you know.

[Willabe]

New to this but obviously yes C9 is a filter cap between stages and should be removing any DC. What was I thinking.

It's not a filter cap, it's a coupling cap, sometimes called a blocking cap because it blocks dcv like you said.

Filter caps are in power supplies.

[PRR]

Silver Mica isn't what it used to be. Today a good 470pFd 500V Ceramic is a very safe bet. (Not the same kind of "ceramic" as the 0.05uFd blobs that even _I_ can hear.)

[pdf64]

Polypropylene caps are available in the sub 1nF range at high voltage, in a radial ‘box’ pcb format. And pretty cheap.

[jwhitmore]

Oops sorry coupling cap. I ordered up a Mica one and got the old one replaced and we're golden.

Some of the valves appear to be micorphonic, certainly seem to be picking up vibrations when I tap here and there, but apart from that I'm happy. I'll put a set of preamp valves in a shopping basket and get time some time.

Thanks again for all your help.

[SoundmasterG]

Oops sorry coupling cap. I ordered up a Mica one and got the old one replaced and we're golden.

Some of the valves appear to be micorphonic, certainly seem to be picking up vibrations when I tap here and there, but apart from that I'm happy. I'll put a set of preamp valves in a shopping basket and get time some time.

Thanks again for all your help.

AC30's tend to get microphonic valves in the preamp. There's a lot of vibration in that amp when cranked. The preamp valves go through holes in the chassis and plug into the PCB. Not much you can do except to use short plate 12AX7'/ECC83's and hope for the best. Some people try those rings around the glass, and I've heard of others trying to put stuff around the holes in the chassis or widen the holes so the tubes won't rattle against them, but I think it is just the nature of the beast. I've had my 1993 Korg AC30 since new and it has microphonic tubes. I just live with them. THe noise that gets me is Ghost notes, and it took forever to get rid of that. In my case it was cone cry.


Greg

[pdf64]

All receiving valves will tend to be microphonic to some degree.
It’s only a problem if it causes, err, a problem.
The earlier in the signal path, the more critical it becomes that the valve is that stage has a low degree of microphonics. eg a particular ECC83 that rings when used in a combo’s input stage may be fine, in, eg its phase splitter, or if not, then maybe V2 or V3 of a 2204 head.