AC30/6 gone incredibly quiet?

[Loomer]

TL;DR - my AC30 is incredibly quiet and not the glass shattering amp it always was. What could be wrong with it and where should I start looking?

Longer version/more clues: So after years of non use due to its volume I finally lugged my Korg-era AC30 to a practice room to hear it in its full glory. I'd been using it with a dummy cam into Its at home and it's been working reliably, though I always felt it might need some attention as the tone wasn't 100% there, especially the early break up being a bit fizzy.

So I remember all too well how loud the thing was but upon turning it on... nope. REALLY quiet. Like, definitely couldn't keep up with a drummer quiet.

This reminded me the last time I'd played it through its speakers it sounded great until the volume dropped. This was over 10 years ago as the amp lived in Italy (where I'm from) and hadn't managed to take it back to the UK (where I've lived for over a decade). Since bringing it back I haven't had a chance to play it through the speaker but played plenty through the direct out box thingy.

I know it was revolved not long before the volume drop thing of 10 years ago but by a tech I don't necessarily trust a whole lot.

So if I were to inspect it by myself, where should I look first? It feels to me like this could be MANY different things, ranging from a dodgy valve to a busted OT but any input on possible causes and approaches to debugging would be hugely welcome.

One last thing, the tremolo/vibrato is SUPER fast, which I didn't remember being, but then again, I hardly ever used it so possibly just the way it is?

Thanks in advance for any thoughts you might be able to spare!

[pdf64]

Are all channels similarly affected?

Compare the voltages, report any discrepancies https://el34world.com/charts/Schematics/files/Vox/Vox_ac30volt_factory.jpg

[Loomer]

Are all channels similarly affected?

Compare the voltages, report any discrepancies https://el34world.com/charts/Schematics/files/Vox/Vox_ac30volt_factory.jpg

Thanks, will hopefully get a chance on Thursday. The fact mine has a valve rectifier shouldn’t change those voltages right?

[pdf64]

Are all channels similarly affected?

Compare the voltages, report any discrepancies https://el34world.com/charts/Schematics/files/Vox/Vox_ac30volt_factory.jpg

Thanks, will hopefully get a chance on Thursday. The fact mine has a valve rectifier shouldn’t change those voltages right?

Hopefully not much, but to cause the issue, any discrepancy should be significant, obvious.
Actually, the V DC reading (to chassis common) of each pin of each EL84 output valve may be enlightening.

[Loomer]

Are all channels similarly affected?

Compare the voltages, report any discrepancies https://el34world.com/charts/Schematics/files/Vox/Vox_ac30volt_factory.jpg

Thanks, will hopefully get a chance on Thursday. The fact mine has a valve rectifier shouldn’t change those voltages right?

Hopefully not much, but to cause the issue, any discrepancy should be significant, obvious.
Actually, the V DC reading (to chassis common) of each pin of each EL84 output valve may be enlightening.

I’ll have a probe at those - the volume drop is across all channels so would stand to reason the issue is at the power section... Or around the rectifier perhaps? The thought that the amp might be under fed crossed my mind as t would also could explain the fizzy breakup?

[Loomer]

So having taken a couple readings from the power tubes and the rectifier... The rectified voltage was 360 on the nose, and the only reading I could get on the power tubes was 340 (give or take a couple volts) and another ~15mv. I could not find the 10V the schematic posted above suggests.

Weird thing was, I switched it off and the standby light remained on even when unplugged (something I might not have normally seen because of the position of the amp) and after turning back on and taking another round of readings the rectified voltage read 430 and 428 at the power tubes pin 9 and that's how it's been since?! Standby light still stays on for about 5 mins and slowly fades, which suggests there's some voltages getting stuck somewhere?

Also been looking at this schematic, which is of a valve rectified model and perhaps closer to mine, thought it also suggests 450v (which is not a million miles form my reading) and 25 where the schematic posted above called for about 10v - that voltage I could not find while probing around

https://www.voxac30.org.uk/images/schematics/closeups/large_dallas_1974.jpg

[sluckey]

I could not find the 10V the schematic posted above suggests.

Measure resistance between EL84 pin 3 and chassis. Should be about 50Ω. What have you?

[pdf64]

‘Korg era’ has been over 30 years, which exact model is this?

[Loomer]

‘Korg era’ has been over 30 years, which exact model is this?

AC30-60-00, sadly haven't been able to find the exact schematic for it

[Loomer]

I could not find the 10V the schematic posted above suggests.

Measure resistance between EL84 pin 3 and chassis. Should be about 50Ω. What have you?

63ohm - assuming this is good news and within tolerance?

[pdf64]

…
Also been looking at this schematic, which is of a valve rectified model and perhaps closer to mine, thought it also suggests 450v (which is not a million miles form my reading) and 25 where the schematic posted above called for about 10v - that voltage I could not find while probing around

https://www.voxac30.org.uk/images/schematics/closeups/large_dallas_1974.jpg

There’s no valve rectifier on that 1974 schematic?
You may be getting the cap voltage ratings confused with idle voltage readings?
I can’t see any voltages noted on that schematic.

[pdf64]

‘Korg era’ has been over 30 years, which exact model is this?

AC30-60-00, sadly haven't been able to find the exact schematic for it

Is it the TB, or TBX, version, built by Marshall?
With 6 input sockets and vib/ trem.

[Loomer]

…
Also been looking at this schematic, which is of a valve rectified model and perhaps closer to mine, thought it also suggests 450v (which is not a million miles form my reading) and 25 where the schematic posted above called for about 10v - that voltage I could not find while probing around

https://www.voxac30.org.uk/images/schematics/closeups/large_dallas_1974.jpg

There’s no valve rectifier on that 1974 schematic?
You may be getting the cap voltage ratings confused with idle voltage readings?

Guilty as charged!And you're correct re the schematic, I had a bunch open and linked the wrong one!

I had thought 450v would be high and the earlier reading I got of 360 at the rectifier and ~340 at pin 9 was the correct one, but no idea why this has since shot up to 430?

[Loomer]

Is it the TB, or TBX, version, built by Marshall?
With 6 input sockets and vib/ trem.

It is indeed the 6 input AC30/6TB - my understanding is that this is a Korg era (and that's how I found this specific AC30-60-00 referred to online), but not sure if Korg and Marshall were one and the same at the time...!

This is the schematic I could find, though the model is AC30-60-02 https://www.drtube.com/schematics/vox/ac30-60-02-iss5.pdf

[pdf64]

Korg contracted Marshall to design and build the AC30 1994 - 2004.
Don’t use the standby switch.
http://www.voxshowroom.com/uk/amp/ac30tbx.html
Schematics
https://el34world.com/charts/Schematics/files/Vox/Vox_ac3093pa.pdf
https://el34world.com/charts/Schematics/files/Vox/Vox_ac3093pr.pdf

[sluckey]

The facts that the plate voltage is so high, cathode voltage does not exist, and even the STBY light taking a long time to go out, all point to the output tubes not conducting. You should have practically no sound from the amp. Is that so?

[Loomer]

The facts that the plate voltage is so high, cathode voltage does not exist, and even the STBY light taking a long time to go out, all point to the output tubes not conducting. You should have practically no sound from the amp. Is that so?

Well, you're correct - no sound coming out at all now?!
I'm pretty confident I never shorted anything while doing the first round of probing, so not sure why things would have changed like this since then?

[pdf64]

Heater fuse dodgy?

[Loomer]

Heater fuse dodgy?

Well, well, well... Any special precautions in taking this out or being at a low voltage point it shouldn't matter?

F3 is visibly gone, but T4 next to it (to its left in the photo) has no continuity either.

The mains fuse did blow a few weeks back, though I don't think this would have had anything to do with this (guessing it simply did its job and since got replaced)

Could this suggest the GZ34 might need replacing? And could that be at the heart of the whole issue?

[pdf64]

The 6.3V heater fuses F3&4 have got no relation to the rectifier. The rectifier’s heater fuse is F1.
A likely cause of F3 or 4 blowing is a bad EL84.

There’s no special precautions with the 6.3V heater fuses, but they are in close proximity to F1&2, which will probably stay at high voltage, as the cold valves can’t draw any HT current, and apart from the neon, there’s nothing else to bleed it down.

[Loomer]

The 6.3V heater fuses F3&4 have got no relation to the rectifier. The rectifier’s heater fuse is F1.
A likely cause of F3 or 4 blowing is a bad EL84.

There’s no special precautions with the 6.3V heater fuses, but they are in close proximity to F1&2, which will probably stay at high voltage, as the cold valves can’t draw any HT current, and there’s nothing else to bleed it down.

Much appreciated, thank you! F1 and F2 are holding onto 90v each, so not getting anywhere near those! The DMM's probes might make for a good help in prying 3 and 4 out.

Of course, going through the schematic I see now that the two that have blown are straight from the PT. I can also see with the DMM that all pins 4 and 5s share continuity - should this be the case? And if so, then why does each of them have an independent wire, one red and one black? My understanding is that the grounding for the heater should be pin 9, which according to the schematic goes to ground via a 100R resistor, but I don't get any resistance reading between it and the chassis?

Lastly - as I said above I don't feel like at any point during the first round of taking readings I shorted anything. But say I did, could this be the cause of the fuses blowing? And could the original lower level issue and the current fuse blown/high voltages/no volume issue be a separate one?

[Jalmeida]

All the tech minded here have you covered. My only contribution is:

1. Don’t use the standby switch(i went extreme and removed mine)

2. Keep a spare GZ34 around. They fail more often than the EL84 in my experiences.

[pdf64]

… I can also see with the DMM that all pins 4 and 5s share continuity - should this be the case? And if so, then why does each of them have an independent wire, one red and one black?

I think you’re referring to the EL84 sockets, and that this is with at least one 6.3V valve in a socket?
If so, the continuity is via the valve heaters, ie the heating filament inside each valve.

My understanding is that the grounding for the heater should be pin 9, which according to the schematic goes to ground via a 100R resistor, but I don't get any resistance reading between it and the chassis?

I think you may have misunderstood almost everything above.
Pin 9 of an EL84 is its screen grid, it’s at high voltage. Connected to their HT supply node via 100R resistors R73,75,77,79.
The schematic shows the centre tap CT of the mains transformer’s heater winding to be connected to 0V / circuit common / chassis / (ground).

Lastly - as I said above I don't feel like at any point during the first round of taking readings I shorted anything. But say I did, could this be the cause of the fuses blowing? And could the original lower level issue and the current fuse blown/high voltages/no volume issue be a separate one?

All the above is possible.
It’s a fact of life that probe tips slip at the most inconvenient moment, and damage can easily be caused.
I suggest to use clip on probe adapters, especially on output valve socket terminals.
Alternatively, use a file to get a sharp point on the probe, then sleeve off all but the very end.

[pdf64]

All the tech minded here have you covered. My only contribution is:

1. Don’t use the standby switch(i went extreme and removed mine)

2. Keep a spare GZ34 around. They fail more often than the EL84 in my experiences.

I think the bad standby arrangement implied in point 1 is a direct root cause of point 2.
Especially with early TB builds, which passed HT current via the GZ34 heater filament, ie the rectified DC output was taken from pin 2 rather than pin 8.

Fitting back up silicon diodes might prevent rectifier failure, at least it will keep the amp working and avoid blowing a fuse / damaging anything.
https://www.premierguitar.com/the-immortal-amplifier-mod

[Jalmeida]

All the tech minded here have you covered. My only contribution is:

1. Don’t use the standby switch(i went extreme and removed mine)

2. Keep a spare GZ34 around. They fail more often than the EL84 in my experiences.

I think the bad standby arrangement implied in point 1 is a direct root cause of point 2.
Especially with early TB builds, which passed HT current via the GZ34 heater filament, ie the rectified DC output was taken from pin 2 rather than pin 8.

Fitting back up silicon diodes might prevent rectifier failure, at least it will keep the amp working and avoid blowing a fuse / damaging anything.
https://www.premierguitar.com/the-immortal-amplifier-mod

I had read about the Diode idea when I picked my amp up. However I opted to removed mine because it can be reinstalled easily and the standby switch isn’t really needed. If I ever have to go back into the amp I may look into the diodes. Mine is the older CC2 with the Lyle Caldwell mods which is quite a bit different than the TBs. Not sure how that would change things for those diodes.

[Loomer]

On the question of the standby switch, I did find the amp wouldn’t fire up if I tried to turn the standby switch on immediately after power, and I’d need to wait something like 10-20 seconds before standby light and sound would come on - is this normal behaviour?

[sluckey]

That's normal. 10 to 20 seconds is about the amount of time for the GZ34 filaments to warm up enough to produce any B+. Your STBY light is powered from the B+.

[pdf64]

That's normal. 10 to 20 seconds is about the amount of time for the GZ34 filaments to warm up …

And the other (signal) valves too 

[Loomer]

Thank you all so much for your help so far. Will be picking up a few spare T6.3A fuses tomorrow and resume the investigation once those are in place, not sure there’s much point at the minute.

To fill in on the previous point about continuity between pins 4 and 5 on the sockets yes, valves are in place so that’ll be why 👍

Lastly, removing the standby switch and leaving it perennially on do exactly the same thing right?

[pdf64]

… removing the standby switch and leaving it perennially on do exactly the same thing right?

Yes.

Note the pin2 build error I mentioned earlier, I suggest it would be worth checking for that on your amp.

[Loomer]

Note the pin2 build error I mentioned earlier, I suggest it would be worth checking for that on your amp.

Ah, I’d failed to spot that earlier but will definitely check when I open it up to swap fuses, probably at the weekend 👍

[Loomer]

So, back at it after swapping the 6.3 fuses. Voltages are back to normal, just over 340v on all pin 9 of the power tubes and I get 10.6v at pin 3, so all in check there.

I haven't removed the EL84s so haven't checked the heater voltage, but since they glow I'm assuming that's all in check now. Or is there a way to measure with valves in the socket?

Amp did faintly crackle and pop a touch after start up which might be weird or completely normal to all but the ear of someone expecting issues...!

At least we're back to the original issue now: amp now plays is VERY quiet. With volume at noon on all channels you can talk over it where it would normally kill small animals and injure children because of its volume. Clean sound is good, but as you crank it the breakup is fizzy and very cardboard-y, for wanting of better words...

I'm very conscious that if I am to hit a dead end it simply means I should let a more trained eye on it, but if there's anything obvious worth checking I'll happily have a look at that first.

Regarding pins 2 and 8 on the rectifier - they both are connected but it's the ones at pin 8 that go to F1 and F2.

[pdf64]

Can you identify the 4 x 100R 1/2W screen grid resistors, that feed the EL84 pin9?
It would be useful to measure the V DC to the chassis (either leg) and dropped across the resistor (one probe tip on one leg, other probe tip on the other) for each.

Also V DC to the chassis for pins 1&6 for each valve V1 to V6.

[Loomer]

Can you identify the 4 x 100R 1/2W screen grid resistors, that feed the EL84 pin9?
It would be useful to measure the V DC to the chassis (either leg) and dropped across the resistor (one probe tip on one leg, other probe tip on the other) for each.

Also V DC to the chassis for pins 1&6 for each valve V1 to V6.

Yup, they are labelled as per the AC30-60-02 schematic posted above. They all read 337 or 338v to chassis (valve side) and voltage drops are as follow:
R73 - 0.40
R75 - 0.38
R77 and R79 - 0.37

Took readings across V1 - 6. A clarification, I'm guessing you meant the preamp tubes through to the PI. In the schematic they actually count down from the rectifier through to the vibrato channel. Using a V1-6 numbering and going L to R from the PCB I have:

                                      pin1  -  pin6  -  reference Vs
V1 (vibrato oscillator)       219      127       195 - 120
V2 (vibrato/trem preamp) 225      148       200 - 140
V3 (vib modulator/ECC82)145      144       75 for both?
V4 (normal/brill preamp)  175      169       170 - 170
V5 (tone stack)                298      173       290 - 180
V6 (PI)                            234      230       230 - 230

The role of each tube I could work out by comparing the R and C numbers around them - they seem to shift by one at some point, but since the schematic is a 02 version to my 00 I feel quietly confident that's how they're laid out.

This would mean that compared to the voltages you posted only the ECC82 is significantly out of whack.

[pdf64]

Thanks, sorry for forgetting about Marshall’s backwards valve numbering
All looks good, apart from, as you note, the ECC82 modulator. Is it definitely an 82 in there, not an 83?
Are you sure the speakers are properly connected, to each other and to the amp?

[Loomer]

Thanks, sorry for forgetting about Marshall’s backwards valve numbering
All looks good, apart from, as you note, the ECC82 modulator. Is it definitely an 82 in there, not an 83?
Are you sure the speakers are properly connected, to each other and to the amp?

Speakers are wired correctly (2 8ohms with +ive to first speaker +ive, -ive to second speaker -ive, jumper between second positive and first -ive running at 16ohm), but this got me taking the whole thing off the chassis and lo and behold, V3 (or 9 if you're Marshall) is an ECC83/12ax7.

Could that in any way be behind the whole issue? I can't see how it would be so I fear I've only discovered one further issue rather than solved the original...!

No 82 at hand at the moment, only a 12AY7/6072A, so will hang on rather than swapping for another wrong one...!

[Loomer]

And once again it's one step forward and two steps back...

Installed the ECC82, fired on, realised the cab wasn't hooked up (and speaker cable couldn't quite reach), so turned off and put the amp back together. Fired it up again, mains fuse gone immediately, standby lamp flashed a couple times with a buzz and off it went.

It happened once a couple of months ago, replaced it and it worked fine since, so didn't think too much of it. Replaced it, blown immediately again in the same fashion. Third time not lucky and I'm leaving it as is. Could it be the GZ34?

[pdf64]

Are you definitely not using the standby switch?
If you’re not using it, then yes, a new rectifier should fix the mains fuse blowing issue. But a couple of 1N4007 should also fix it too, and allow the existing GZ34 to work much longer. See the premierguitar page I linked to earlier in the thread.
Edit - Rob Robinette has a nice drawing (see V5 rectifier socket)


An 83 will work in the modulator circuit, but will tend to cause somewhat unpleasant distortion with high output guitars / heavy picking (the volume control is at the end of the channel’s signal path, more like a typical master volume. An 82 should make the voltages correct and allow the circuit to handle larger signal levels.

[Loomer]

Are you definitely not using the standby switch?
If you’re not using it, then yes, a new rectifier should fix the mains fuse blowing issue. But a couple of 1N4007 should also fix too, and allow the existing GZ34 to work much longer. See the premierguitar page I linked to earlier in the thread.
Edit - Rob Robinette has a nice drawing


An 83 will work in the modulator circuit, but will tend to cause somewhat unpleasant distortion with high output guitars / heavy picking (the volume control is at the end of the channel’s signal path, more like a typical master volume. An 82 should make the voltages correct and allow the circuit to handle larger signal levels.

I've used the Standby in the past and up to the conversation on this thread, but not been using it now as it chewed through fuses.
I took the GZ34 out and checked for shorts but it's all good - only pins 2 and 8 have continuity.

After reinstalling it I tried once more and it turned on, all tubes lit up but the standby light never came on. About 30 seconds in the GZ34 lit up twice with a loud buzz and the tube was gone.

Still worth trying a new rectifier as a last DIY attempt or time to send off to the doctors?

I checked the rectifier socket for held voltages and it had about 5 which I could quickly drain with a 10k to ground, so I'm thinking I shouldn't be needing any special precautions to solder onto its socket?

[pdf64]

… After reinstalling it I tried once more and it turned on, all tubes lit up but the standby light never came on.

The standby light is dependent on the GZ34 warming up and working correctly.
For it not to come on immediately is normal.

About 30 seconds in the GZ34 lit up twice with a loud buzz and the tube was gone.

Stop doing that to your poor amp, it’ll only cope so many fault current incidents before a transformer gets wrecked!

Fit the diodes, then retry it.

Still worth trying a new rectifier

Yes, but if the diodes are fitted,  that GZ34 may work fine.

I checked the rectifier socket for held voltages and it had about 5 which I could quickly drain with a 10k to ground, so I'm thinking I shouldn't be needing any special precautions to solder onto its socket?

Correct, the rectifier is shorting out rather than actually rectifying, so the caps can’t get charged in the first place.

[PRR]

....I took the GZ34 out and checked for shorts but it's all good - only pins 2 and 8 have continuity. ...

I had EL34/6CA5s in a burnt amp. Cold, the tubes had no short. I took an EICO 667 tube-"tester" away from a friend, and, nothing to lose, tested tubes. One tube showed short only AFTER 30 seconds of heat. Now that I knew the fault, I went ahead with a new transformer.

[Loomer]

Well, this is an old thread, but I finally got round to make some inroads but having a tech look at it.

It's bad news, as the output transformer is gone. The tech suggested it's probably due to very high HT (345+), plate voltage (334) and dissipation (18w) causing arcing/overload in the EL84s.

I was under the impression that, despite these readings being higher than what EL84s are meant to take and therefore being a red flag from an electronics persepective, this is not uncommon in AC30's and shouldn't necessarily cause such major issues, but I'm more than happy to stand corrected on that.

I'm curious to hear what the consensus is on that and whether I should seriously consider swapping out the power transformer too, as suggested by the tech, or if a new OT and perhaps a resistor to lower HT might be sufficient to have an AC30 doing what AC30's do without risking another such failure?

Not a case of mistrusting the tech (I don't) but the bill is adding up quite a bit to the point of being a good fraction of the value of the amp. I really love the amp and the tone I used to get out it and am keen to keep it, but seems sensible enough to try and get a couple perspectives before committing a few hundreds!

[tubeswell]

Yes, it is an old thread.


EL84s typically wear out faster than similarly rated octal types. To help avoid this happening again, ensure you change them out with a fresh quad when you start to experience volume drops/ weird noises.

[TitaniumValhalla]

I'm not an AC30 expert but I don't think those voltages are particularly abnormal for the circuit. For EL84's, it's on the hot side, but most EL84 guitar amps are similar. The Blues Junior has higher voltages. Of course it is known to abuse EL84s. Dissipation is really the enemy and of course heat with those little 9-pin bottles. As for whether to replace the PT to drop the voltage, that seems like way overkill - why not look at a rectifier with more voltage drop if that is a concern? I would hope your tech would at least propose that or grab one from his stash to test it with.

[HotBluePlates]

... my Korg-era AC30 ...

... very high HT (345+), plate voltage (334) and dissipation (18w) causing arcing/overload in the EL84s.

I was under the impression that, despite these readings being higher than what EL84s are meant to take and therefore being a red flag from an electronics persepective, this is not uncommon in AC30's and shouldn't necessarily cause such major issues ...

Although I once had a Korg-era AC30, I sold it back in ~2000.

Right now, I have several 1960s AC30s.  None of them have 330v plate-to-cathode (they're all lower than that).
Although some have the 50Ω cathode resistor, and one has the 82Ω cathode resistor, none of them operate the EL84s over 12w plate dissipation.


Some modern EL84 amps run the output tubes way hot, maybe for perceived tonal reasons.  But my experience with 1960s Vox amps is the line that they're all "running super-hot and on the verge of melt-down" is just B.S.  However, one may have to select tubes that don't run over-hot in a given cathode-biased amp.

[Loomer]

Well thanks all once again, if a little belated!

The amp is home and playing fine and with just shy of 310v on the plates. As HBP says, I think the whole “EL84 amps are basically on fire” thing is more a byproduct of modern power sockets delivering way more than they did back in the day, the difference being multiplied by the input transformer. I’m glad we lowered the voltage which, contrary to what someone said above, could not be achieved with a different rectifier since the 5U4 drawn 3A, aka 50% more than a GZ34, a demand the power transformer could not cater for. Because the AC30 draws around 200mA you’re left with no real options from that standpoint since the 5R4 would have been overkill.

NOS 6P14P’s are in place and I’m looking forward to enjoying it again! Current draws are 36mA, 35.2, 35.8 and 34.8 - are there close enough not to be considered a good match? I know much is said of matching/not matching for tone, but I’m simply looking at it from the perspective of them sharing a cathode resistor and wanting them playing evenly and nicely to minimise the chance or problems. For the record, this along with plate and cathode voltages leaves the amp biased at around 85/90%, so pretty standard if not a hair on the cool side.

One quick question for a few extra pairs of eyes - the tubes glow nicely with a hint of blue (seems very common in Russian valves!) and I can see some glow peeping through the gaps in the plates - that is not redplating is it? I’d be seeing the whole plate go bright wouldn’t I? Photo attached for my own peace of mind..! 🙂

[tubeswell]

The tubes in the photo look normal to me

[pdf64]

Yes, the anodes are the outermost of the internal electrode structures, and aren't glowing in any of those, just getting the orange glow from the heaters (and apparently also the cathode, which seems somewhat unexpected?).
Also a bit of blue glow, which looks lovely and isn't indicative of an issue.