Need schematic and advice for fixing vintage AC30

[rockman627]

This Vox AC30 is blowing fuses. I open it and am surprised to find a vintage circuit board. But it is very strange to me.What year is it and what schematic goes with it? How is it there is no standby switch? Plus I do not see either a tube or solid state rectifier. I am going through Rob Robinette's routine for troubleshooting blowing fuses but all I can do is remove the power tubes one by one, which I did to no avail. Serial number is 7404D1003. If you know anything about this Vox please help.
Oh yeah, is that a death cap from the fuse holder to ground?

[Jon Snell]

What is the exact model number ... having a turret board wiil date it around 1986 ish.
"How is it there is no standby switch? " Because we don't use them as they are of no advantage. "Plus I do not see either a tube or solid state rectifier" whoops.
Oh dear, please do not let it get destroyed by unskilled techies.
Take a look under the wooden base and you will find the rectifier diodes and clues if the mains transformer has been abused or not.
VOX has never fitted a "Death Cap" because they are dangerous, in any case it is an earthed (grounded) three pin leaded chassis and wouldn't require one to reduce the mains hum.
One can see it is not original and has been fiddled with ... shame.

[Helmholtz]

From the readable marking on one of the red ecaps (says Feb. 70) it seems the amp is from 1970 or later. Boards look original.

I own an AC30 from that year but mine is an Italian version and has the GZ34 rectifier placed behind the power tubes on the same chassis part.

Yours might correspond to the Dallas version, see schematic at bottom of this page: https://www.voxac30.org.uk/vox_ac30_..._diagrams.html

BTW, vintage AC30s didn't have standby switch (just saw Jon already mentioned that).

You should try to find the 2 rectifier diodes. Look below the PT.

Do you have a bulb limiter?

If there's a cap connected to the fuse holder, it should be removed. Might even be shorted.

[Mick Bailey]

The green electrolytic is week 21, 1974.

[Jon Snell]

My tired eyes didn't see the date codes on the capacitors.
Well spotted Helmholtz.

Digging a little deeper, the answer lies in the rectifier, if it hasn't been fiddled with.
If it has a full wave bridge, it is a Dallas from the mid 70s. If it has two diodes it is from 1978.
As I wrote earlier, take a look under the mains transformer.
The sticky label suggests the 7th week of 1974. Another red capacitor states December 1970 as date of manufacture.
It would be a good idea as the amplifier is out of the box, to fit the mains lead correctly and safely. There must be a cable restraint and there isn't one at the moment.
Pull on the mains lead hard and it could be very dangerous, if the earth or neutral lead pulls off!

Edit; More information added ...

It is probably the AC30/6 Dallas Arbitter model. The only model to have Treb and Bass controls with just two rectifier diodes. If that lines up, then that will confirm it.
Nice amplifiers but check all of the solid wires that connect to the valve holders; they can break off at the solder, causing all sorts of intermittent faults.

[pdf64]

Just to add to above, it’s asking for trouble to operate output stages that use shared cathode bias unless a full set of output valves is fitted and they’re all working. eg ensure none have a blown screen grid resistor.
All valves have to be fitted and conducting for proper bias voltage to be established.
The AC30 and 5E3 tweed deluxe are famous examples of amps using shared cathode bias.

all I can do is remove the power tubes one by one

So that isn’t a good idea for such amps, because the output valves that are working will dissipate heavily. Unless perhaps a light bulb limiter with a suitable bulb is used, and the test is momentary.

[Greg Robinson]

VOX has never fitted a "Death Cap" because they are dangerous

If this was a model exported to the US, during that time period it would have been fitted with a death cap, as sockets with protective earth were not yet ubiquitous.
Certainly appears to have one.

[Mick Bailey]

It has a tubular cap that looks to connect between the fuseholder and chassis. Maybe this is shown on an export model schematic.

[rockman627]

Yes I am using a light bulb limiter.
Model is AC30. That's all that is on the sticker and back plate.
Death cap doesn't make sense on a 3 prong plug, which is why I asked. What is that caps purpose? I'm taking it out.
Then I will look for that rectifier and before I turn the power back on, the tubes are going back.

[rockman627]

Okay, got the back off. I see 2 diodes under those green caps on the left. I guess I'll order By133 diodes. I removed the "death cap”. The light bulb limiter is still bright.

[Helmholtz]

Don't just replace parts. At least test the diodes before changing them.

While both diodes are out/disconnected see if the bulb limiter stays dim now. If not the PT is suspect.

[rockman627]

I removed the diodes, bulb is dim. Alrighty then!
Using my multimeter, one diode had a small voltage drop one way, nothing the other. The other diode had a small voltage drop both ways. Also one green cap was a little scorched.

[rockman627]

The amp turns on and makes sound. However, there is no vibrato. There is also some hum but the filter caps seem original so I'll replace them. If I can figure it out I would like to lower the self bias , those tubes get really hot. That schematic mentioned earlier is close but not exact. Maybe the boss will spring for new el84s too.

[Helmholtz]

To lower power tube dissipation you could use a 68R cathode resistor. What is B+ and cathode voltage?

The mustard caps are very reliable (and sought after by vintage guys). I haven't seen one that had failed or drifted.
Replacing those without reason might lower the value of the amp.

[Jon Snell]

The amp turns on and makes sound. However, there is no vibrato. There is also some hum but the filter caps seem original so I'll replace them. If I can figure it out I would like to lower the self bias , those tubes get really hot. That schematic mentioned earlier is close but not exact. Maybe the boss will spring for new el84s too.

1N4007 are ideal in this amplifier and most others for the HT rectifiers. Back when we made AC30s, 1N4007 was not around, we had a choice of Mullard BY127/BY133 or RS REC53A diodes. We didn't fit capacitors across the diodes as it serves no purpose in this design.

No foot switch = no vibrato.

Check the wires on the oscillator valve base for breaks if the vibrato is switched on and nothing happens. Check the anode of the oscillator for voltage swing when vibrato switch in on.

Ref bias; many thousands of AC30s run with 50R as the bias resistor for many years. Why change it?
The EL84s run in or nearly in class A for that VOX sound.
VOX and other amplifiers of this vintage used to hum slightly, (that is how the stage engineer new it was on as the pilot lamps used to blow in transit ).