Vox AC30 grid stopper burn

[Ice cream man]

Hi forum,
I am looking for some help with a Vox AC30, the grid stopper resistors on the two right hand side have burn marks on them.



Would it be the case that pair is over worked or else under worked and heading towards class A, so the cathode is becoming more negative than the grid and current is back feeding to ground via the two 220K resistors R53 and R50 or am I heading the wrong direction in assuming that.

I am measuring 10VDC on all 4 cathodes



Thank you in advance for any help.

[Steve Conner]

I think the most likely explanation is that one of the tubes in those sockets failed and shorted between grid and screen. It could be an intermittent short. I'd replace the tubes, replace the burnt resistors, and check the screen resistors.

[Ice cream man]

Ok thanks, one valve did fail some time ago and I have a new set of output valves to install.

With the old set of EL84's at Idle, I am reading 80mA on one pair and 102mA on the other pair using the current shunt method and a reasonably good multimeter, does that seem within reason?

All of the resistors in the power section are all good to go according to my multimeter.

Also is it normal to read 10VDC on the cathodes of a VOX AC30 with no signal applied as I am used to Marshalls and Fenders that don't use cathode biasing?

[pdf64]

What is the el84 plate voltage, either side of the OT primary?
Depending on that, 10V is reasonable; however the shared cathode bias resistor means that if one tube has reduced / no conduction, the other 3 run hot.
Pete

[Ice cream man]

Hi,
I am reading 330V on one side and 332V on the other side and with the amp switched off, 59 ohms and 52 ohms from the center tap of the output transformer to each side.

[pdf64]

Put the new el84 valves in and see what the voltages are then.

[Ice cream man]

Right ok, I have replaced the valves with new ones.

The voltage readings are the same as I would expect but the current has changed to approx 91/92 mA per pair which tells me that the valves are running at around 100% of their maximum rating at idle (15W)

Is this normal for a Vox AC 30 with cathode bias for the power valves? and does the 70% rule not apply here as seen in the fixed bias amps?

[Enzo]

It is normal for class A amps, which the vast majority of push pull amps are not.

[Ice cream man]

Ah, ok. I was under the impression that the Vox's were class AB1 and not A as the output transformer is center tapped so it works in conjunction with the two pairs of valves in a push pull manner. Plus there is a phase inverter.

[Steve Conner]

I think it's normal for the AC30 which was known for running its tubes stinking hot. Some owners were rumoured to fry eggs on the faceplate.

[Ice cream man]

So is a case that both pairs of output valves are biased in a class A fashion with the shared bias resistors and bypass capacitor, but operate in the push pull manner (class AB) with the phase inverter and center tapped transformer?

Thanks for all help as I am new to the Vox AC30.

[Steve Conner]

No, it's quite possible to have a push-pull Class-A amp. It just implies that both tubes stay conducting even at full output. If one of them turns off at any time, you're in Class-AB.

I like to make the distinction between design class and operating class: The AC30 was designed as a Class-A amp, but it can be driven into Class-AB at high volume. Many AC30 owners increase the value of the cathode resistor somewhat for longer tube life. Brian May doesn't, he just roasts his way through set after set of NOS Mullards.

[Ice cream man]

Thanks Steve, all info on the AC 30 is much appreciated and yes one valve did go on this amp some time ago which would give a reasonable explanation for the pair of grid stoppers to have burn marks on them.

[Enzo]

Read up a little on class of operation. It is a separate issue from circuit format. A push pull amp can be designed for class A, class AB, class B, or even class C. Class C is for guys who play in Morse code though. You could design a single ended amp in something other than class A, but it would wind up missing part of the waveform. SO about any single ended - think Champ - amp is going to be class A. But the single ended-ness is not what makes it so. Just as the push-pull-ness of a circuit is not what makes it class AB or not.