Vox AC30 and noise

Danny1969 · July 2023

Got 3 of my friends AC30's in at the moment. 2 of them look like early eighties, the 3rd one is a Korg ninieties model I think.

Of the eighties models One had a bad filter cap, another one a bad screen resistor and bad bypass cap. While I had that board out I replaced everything on it as the component count and cost was so low. Both these models run fine but there's a fair bit of hum and hiss even with nothing plugged into the inputs. It's not terrible but as these amps are used mic'ed up in a theatre environment they need to be quieter really between songs. Any tips, mods that might help with this issue ?

The 3rd one, the Korg model has an obvious problem, the AC heater feeds for the 12AX7's are just straight wires running across the board, not remotely twisted. I can lower the noise floor just by moving the wires away from the board. So I will replace these with a twisted pair and route them away from the most sensitive area of the PCB. Any other ideas that might help ?

ecc83 · July 2023

IMHO it is stupid to use valves on a PCB and not use DC heating for the pre amp stages but that is no help here!

When I serviced the Artisan 15&30 amps I found selecting the first ECC83 from a batch could lower hum by several dB.

Don't be offended but are you sure the input jack is shorting with nothing plugged in? I have had them strained and O/C.

Cathode bypass caps might be a bit low in value, they keep hum off the cathode.

Is the heater winding centre tapped? If not fit a 'humdinger' pot or at least a series pair of 100 Ohm Rs?

Dave.

DannyP · July 2023

If he has 3x AC30s, I assume he's deaf and doesn't mind the noise?

thermionic · July 2023

Psionic Audio on YouTube has videos on a few AC30s and improvements he made, might be worth looking to see if he has one on the 90s model (I think he has).

Cirrus · July 2023

IIRC there was a service bulletin regarding a ground loop in the preamp of the '90s Korg AC30s, and it's mentioned in one of the Psionic Videos. Something about cutting a PCB trace. I'm not at all familiar with the '80s models.

BigPaulie · July 2023

Is your friend Brian May?

Danny1969 · July 2023

BigPaulie said:

Is your friend Brian May?

Lol, no Shadows tribute

These amps are so unreliable you pretty much have to have at least 2 and preferably 3. I've never known anyone to gig one on a regular basis, like twice a week for more than a few months before something goes wrong. Normally valves and bad joints but also cooked caps and resistors.
These things get way to hot IMHO and some of the early ones I see are death traps in the way they have been earthed and had filter caps lashed in with tape over the years.

They sound great though.

impmann · July 2023

Danny1969 said:

BigPaulie said:

Is your friend Brian May?

Lol, no Shadows tribute

These amps are so unreliable you pretty much have to have at least 2 and preferably 3. I've never known anyone to gig one on a regular basis, like twice a week for more than a few months before something goes wrong. Normally valves and bad joints but also cooked caps and resistors.
These things get way to hot IMHO and some of the early ones I see are death traps in the way they have been earthed and had filter caps lashed in with tape over the years.

They sound great though.

True story about an early one...

I had a 1965 'Bass' model that one night turned into a firework on stage - sounded epic just before it detonated but when it let go, I had flames & smoke. Proper fucked.

Anyway... I took it to a shop I used to buy loads of vintage gear from. They had a great tech but they also had a sales dude who thought he could fix stuff. I told him not to touch it, as it was proper blown up and to give it to the proper tech... obviously he didn't listen and after I went, plugged it in and connected up his Les Paul and flicked the on switch....

Thankfully there was someone else in the shop to kill the power... you see, he had hold of the neck with his left hand, touching the strings whilst he flicked the metal mains switch with his right hand. He survived but wasn't well. The top of the switch was blackened even after it had been repaired.

I can't remember the exact fault but basically someone in the past had lashed up the power supply and when the amp blew up, there was a dead short from the power traff to deck meaning the mains switch was sat at high potential with zero chassis earth (probably to 'cure' an earth loop)... a proper death trap!!

AC30s were worth sod all for a long time, and often were 'repaired' by folks who didn't know what they were doing... this one was an example of this. Thankfully it didn't kill me (or the sales monkey). Thats why any vintage amp that comes my way gets properly checked over before I even plug it in... lots of vintage gear has 'vintage' repair work...

ecc83 · July 2023

I had an AC30 on the bench at B's for another purpose ('ticking' EL84s) and was not impressed! That sample ran the OP valves at nearly 18W apiece.

In fact, when the first samples of the Artisan 15s and 30s arrived we found that the EL84s were mildly red plating if viewed in low light. Not ALL brands of 84 but even those that didn't were bang on 12W or a touch above. The cathode resistor was increased slightly in value and I remember manning a switch while Ian thrashed the amps to see if it sounded better or worse at the safer anode current. He concluded that they were fine JUST inside book dissipation and so they went out into the world that way.

I don't recall the Artisans being particularly hard on OP valves since?

If anyone has one that seems to be so it must be a very very early one!

Dave.

ICBM · July 2023

Danny1969 said:

Any other ideas that might help ?

Use a more modern amp with a lower noise floor (and probably with better volume control as well). Sorry, probably not helpful!

ecc83 said:

I had an AC30 on the bench at B's for another purpose ('ticking' EL84s) and was not impressed! That sample ran the OP valves at nearly 18W apiece.

AC30s were designed in the days when Mullards were the standard EL84, and they will (just) take that sort of abuse and run reasonably reliably. Modern ones tend to die more quickly.

That said, Pete Cornish posted this pic of the valves when he was servicing Brian May's AC30s after a Queen tour, when they'd been thrashed to death presumably. That was just how things were done then! (Not helped by his AC30s having solid-state rectifiers fitted too, which increases the dissipation.)

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