AC30 Normal channel ground loop fix

[Big Bear]

Hi all,

I have a VOX AC30TBX on the bench and I am experiencing hum/buzz with the normal channel volume control, which disappears with the knob at it's mid point.

After a long browse of the internet it seems as though this is an issue of early 90's models and is quite common. Although there are many threads on this matter on different forums there is no modification details anywhere. I have contacted VOX but I am yet to hear any response.

Does anyone have a definitive solution to this?

I have read some posts saying there is a fix, and some saying there is no chance of sorting this out.

It may be that it is just bad PCB design and there is no solution, but I would be very surprised if this ever got passed QC as it is really bad on this amplifier. Maybe there is another problem elsewhere...

Hopefully this might help future searchers too.

Thanks in advance for any help.

Cheers

Charlie

[Big Bear]

Most detail response from any thread so far is this;

"The newer AC30's I've seen with pc boards and a "star" ground near the PT have several problems regarding hum/buzz. There is a an issue of bad grounding on the PC board as well as the lack of shielding to the input grids of the Norm & Brilliant channels. To fix it requires cutting some traces and running jumpers to the star ground as well as changing the location of the input resistors, grounding the resulting unused traces and adding shielded wire to the 1st stage preamp tube grids. I recently had a new HW series AC30 with a noise problem. All it required was a couple of tie-wrap blocks and bundle ties to hold one wire in place on the chassis."

I read somewhere that VOX sent out modifications when asked, so I am hoping that there is a schematic from them....

If not, I will go through this process.

[Big Bear]

In my efforts to track down this ground loop I must of done something very stupid because now there is huge amounts of hum.

It is in the preamp somewhere, with all the preamp valves out it is not present, but with v6 in the hum is there.

Spent the last 5 hours trying to track it down but no joy yet. The amp works, it amplifies as it should but there is this horrible loud hum. Sounds much like a disconnect ground but I have checked and checked what might of changed and it all seems OK.

Grounding the grids of v6 kills it. Measuring about 40v on the grids though which seems high.

Not hugely experienced with guitar amps so would really appreciate some advice, any pointers to stop me losing my mind on this too....?

Thanks in advance, hopefully it is something stupid and I can get back to the actual reason for this thread!

[Chuck H]

The trouble is that with your limited experience we don't want to suggest cutting PCB traces and re routing grounds because, as you've seen, it's easy to get lost and end up doing more harm than good.

The first order of business is to fix what was broken in your efforts. That is, get the amp back to "normal". But we don't know what you did. And yes, 40V on the grids is high. Anything more than a couple of mV is high. And you must take meter the standing meter calibration into account.

For now, or at this point, all I can offer is that your theory on ground loops was probably accurate, But the problem isn't at the volume control. At center adjustment the hum is minimized. That tells me that there is a loop at either end of the adjustment and at center the ground loops are being grounded by each others ground. If that's not too confounded. Or the loops may be out of phase and therefor are canceling each other when equalized. Without having the thing on my bench I can only speculate. I did a short search for a PCB layout but came up dry. Just images of the board itself. And that won't do for accuracy.

I hope you find your mistake and hear from KORG on the matter.

[Big Bear]

Thanks for your response. I am seeing 100hz hum on the output which suggests a power supply problem I guess. Actually there is a half wave rectified signal on the grids of the output valves, which must be coming from the pcb board. Could you suggest a cause for that?

My experience is with pro audio and HIFI but have not worked on many guitar amps so am a little lost. All I have changed to this amp was replacing heater wire and I experimented with the normal pot a little.

If there is 40v on the grids of v6 and 50 on the cathodes, and the coupling caps from the previous stages are working fine what might cause this? I can't see where this voltage is coming from.

Any help most appreciated!

[bob p]

Got a Schematic?

I know that we're supposed to have them, but I was looking for various AC30 schematics the other day and I noticed that many (most?) of the links to the current AC30 schems in our sticky thread have gone 404.

If you could link us up that might make it easier to get help.

[Big Bear]

I've been using this one, its the only one I could find but it is not 100% accurate.

http://www.voxamps.com/downloads/circuits/ac30ltd.jpg

[Chuck H]

Thanks for your response. I am seeing 100hz hum on the output which suggests a power supply problem I guess. Actually there is a half wave rectified signal on the grids of the output valves, which must be coming from the pcb board. Could you suggest a cause for that?

My experience is with pro audio and HIFI but have not worked on many guitar amps so am a little lost. All I have changed to this amp was replacing heater wire and I experimented with the normal pot a little.

If there is 40v on the grids of v6 and 50 on the cathodes, and the coupling caps from the previous stages are working fine what might cause this? I can't see where this voltage is coming from.

Any help most appreciated!

It sounds to me like there is a ground lifted. The amp may still operate somewhat as long as the positive/negative relationships still happen, even if incorrectly. The voltages don't have to be "injected" on the tube grids. Without proper 0V reference and bias, voltage can ride on the grids via the plate voltage as the tube conducts. The half wave rectified voltage is confusing. It's pretty important to have the right schematic.

[Big Bear]

It sounds to me like there is a ground lifted. The amp may still operate somewhat as long as the positive/negative relationships still happen, even if incorrectly. The voltages don't have to be "injected" on the tube grids. Without proper 0V reference and bias, voltage can ride on the grids via the plate voltage as the tube conducts. The half wave rectified voltage is confusing. It's pretty important to have the right schematic.

The schematic is accurate apart from the valve rectifier as far as I can tell.

The 47k joint cathode resistor is grounded, and it's dropping almost 60v! So there might be a lifted ground somewhere else that is causing this?

[bob p]

I can't get too enthusiastic about looking at a schematic that isn't the right schematic for the amp. Hopefully Korg will help you out that way.

I must of done something very stupid because now there is huge amounts of hum.

A dramatic increase in hum right after PCB manipulation suggests that you might have caused damage / lost continuity in a ground path. I'd start retracing every ground path to make sure you haven't damaged one.

[Jazz P Bass]

I may be wrong but did not the 'Original' Vox AC30 have a hum problem on Normal Channel?
For whatever reason putting the volume at 1:00 removed it.
Who uses Norm Ch anyway?

[Big Bear]

I will check over the ground paths again, the obvious points to me are grounded but there is clearly something wrong.

As far as the schematic is concerned it is the closest one I can find online, and all values appear to be correct. It definitely isn't the version I have infront of me though, I don't know whether this is because it was discontinued because of the pcb fault or not. The owner (my neighbour) tells me it is 1/100 made, I assume they only made 100 because it wasn't correct... ?

As for the old AC30s I wouldn't know - I can't play guitar so even testing amps with guitars is painful!

Thanks for your help guys. I would love to sort this issue out and get back to sorting the ground loop problem!

[km6xz]

Korg issued a couple mods of the AC30 but not sure which version you have in front of you. Is there a large resistor marked R70 mounted perpendicular to the edge of the pc board? If so, the ground side of that resistor needs to be grounded with a wire to the star ground point at the chassis ground point.
Additionally the pc ground plane needs to be cut on the mounting tabs for the Brilliance pot.

[Big Bear]

I have the VOX AC30TBX Ltd edition - big purple monster.

Not in front of the amp right now but yes there is a wire grounding the EL84's shared cathode resistors. I was curious about that one at first as it made no difference with it in or out of circuit but I figured that the builders knew what the were doing so left it in.

Regarding the Brilliance pot, I believe this pot is grounded by a jumper lead under the pcb. Should I remove this? I know that this isnt the cause of my hum issues but unfortunately there is more than one problem with this amp. Thanks for the tip!

[Big Bear]

Back after a long week at the Musik Messe and am tackling this amp again.

Unfortunately I did not hear back from Korg or Vox about a schematic, but I have gone through the values and the only differences in my amp compared to the posted schematic are the 100K grid leak resistors on V6 are 1M in this amp. I have traced all ground points and they all seem to be fine, so do not understand what might of changed in this amp. Something must of changed because it did work, I am just lost for ideas.

Any suggestions would be really appreciated as I do not know where to look next. Are there any proceedures that I could try?

As I stated before - the only changes I made were changing the heater wiring, and I lifted the ground of the Normal Volume pot to experiment with the ground placement, but then soldered it back.

Thanks in advance!