VOX AC 30/6TP Bias Questions

[opacheco]

Hi

I am in the repair process for a VOX AC30/6TP guitar amp and I would like to be clear in some questions:

1-Is this amp a CLASS A tube amp or Push-Pull Amp??,
If this amps is a Push-Pull amp;
2-What is the best Bias for performance and Rock Music for each tube??

Thanks a lot anticipated,
Opacheco.

[Enzo]

Hi.

First, it isn't Class A - OR - push-pull, an amp is either Class A or Class AB or some other class. Amps are either push-pull or single ended. The two categories are not exclusive, an amp can be push-pull class A. A single ended amp COULD be made in some class other than class A, but we wouldn;t do it in audio. But it isn't the single endedness or push-pullness that determines class of operation.

[Jazz P Bass]

The output section of the Vox AC30/6 TB is push pull, cathode biased.
Vox claims it is biased into Class A.

You cannot set the bias, per tube.
The cathode resistor sets the overall bias of the four tubes.

There is a note on the schematic that states '12.5 Vdc across the cathode resistor, when the amp is outputting 30 watts into a suitable load.
Furthermore the voltage across the cathode resistor should be 10Vdc at idle.

[opacheco]

Ok Enzo, how about of the Bias??.....

Thansk
Opacheco.

[opacheco]

Jazz P Bass,
Thanks, that is the info that I need to know!!...Then this amp is a "cathode bias amplifiers" this amp don't require any adjustments and will work with a wide range of tube plate bias, as the circuit is "self adjusting", right??; Then, How I guess the appropriate tubes (one quad tubes)? or Is that unimportant issue? because ANY Matched quad tubes will work fine.

I am thinking for the "EL34 TUBE AMP DOCTOR MATCHED QUAD"

Thanks
Opacheco

[Jazz P Bass]

A matched 1quad will never hurt an amp.

Unless the tubes are way off the nominal of a 'typical' EL84 tube, you should be alright.

Still, you should check the two measurements.

[opacheco]

Yes Sir!!...I will do the check voltages.

What could happend if I buy tubes for a especific bias Current i.e. 25mili-amperes??, I think nothing rare, Am I right?

Thanks again
Opacheco

[Leo_Gnardo]

FWIW I always order EL84 matched quads with low emissivity AKA Pc (typically 16 to 22) just because there are so many self-biased amps that are set up to run the output tubes at excessive bias currents. I like to see a plate power dissipation at idle equal or less than the tube's spec of 12W. There's not much leeway - below 10.5W you start to hear gravelly sounding crossover distortion & see it on your scope. Find a vendor that will sell you what you want, not just matched sets, but low Pc EL84's for Vox & similar self-biased amps. EL84's with higher Pc can be used in amps that are fixed-bias with a bias adjustment pot.

[Enzo]

I am thinking for the "EL34 TUBE AMP DOCTOR MATCHED QUAD"

PLEASE note that your amp uses EL84, not EL34.

[Gingertube]

I just finished a restoration on one of these. The one change I made to the circuit was to throw out the 50 ohm Cathode bais resistor and the 250uF bypass cap on the four output tubes. Instead each EL84 got its own 235 Ohm cathode bias resistor (2 off 470 Ohm 2 Watt in parallel) with a 47uF / 50V bypass cap. That biased the EL84 a little colder (less current) making it easier on the output tubes and made the amp more tolerant of unmatched output tubes.
That moves the bias point a little further away from Class A but not much. The AC30 is not and never has been Class A.
The Class A claim is marketing B.S. (that is, it is a lie). It is deep Class AB. At power levels up to about half maximum power it is in Class A. At higher powers than this the power tubes on each the push pull sides definitely reach cutoff and that makes it un-argueably Class AB.

That modification was not noticable sonically but inproved the reliabilty of the amp and the life of the output tubes. I recommend it.

For your unmodified amp, a matched quad of EL84 is essential.

Cheers,
Ian

[Leo_Gnardo]

each EL84 got its own 235 Ohm cathode bias resistor (2 off 470 Ohm 2 Watt in parallel) with a 47uF / 50V bypass cap. That biased the EL84 a little colder (less current) making it easier on the output tubes and made the amp more tolerant of unmatched output tubes.

How radical! But - ha! - I've done likewise to my 60's Vox AC100 and AC50 with EL34's.

The AC30 is not and never has been Class A.
The Class A claim is marketing B.S. (that is, it is a lie). It is deep Class AB. At power levels up to about half maximum power it is in Class A. At higher powers than this the power tubes on each the push pull sides definitely reach cutoff and that makes it un-argueably Class AB.

True - Vox & lots of other manuf's depend on the buying public equating self-bias with class A.

That modification was not noticable sonically but inproved the reliabilty of the amp and the life of the output tubes. I recommend it. For your unmodified amp, a matched quad of EL84 is essential. Cheers, Ian

+1 and I'd recommend matched tube sets even with separate cathode circuits for each tube. Don't worry, in time the tubes will diverge and the set won't be matched any more. But it IS good to start out matched,

[pdf64]

'each EL84 got its own 235 Ohm cathode bias resistor (2 off 470 Ohm 2 Watt in parallel) with a 47uF / 50V bypass cap'

Ian, I'm a big fan of individual cathode bias resistors too but I tend to use the original bypass cap value for each.
This is because my understanding is that the cathode source impedance, which is in parallel to the cathode resistor, is the more significant of the two (ie quite a lot lower), and as signal level increases such that the tubes begin to enter cut off for part of the wave, there's no opposite polarity signal cancellation caused by shared cathodes for part of the wave.
How did you come to select 47uF?

'I'd recommend matched tube sets even with separate cathode circuits for each tube'

Leo, my finding is that whilst tube sets matched for cathode current are beneficial when there's a shared cathode resistor (for the reason you describe), with individual cathode resistors it doesn't seem to matter much, as even fairly dissimilar tubes seem to settle at similar dissipation levels.
This may be because the self regulating mechanism of cathode bias can operate effectively, whereas it tends to be somewhat negated when other tubes share their cathode circuit.
I take your point about the benefits of lower emissivity levels.
Thanks - Pete

[Gingertube]

Pete,
Good Point.
I had not taken the impedance looking into the cathode into account so lets do that.

The impedance looking into the cathode of a Pentode or Beam Power Tetrode is 1/gm and for an EL84 (gm=11.3 mA/V on the dat sheet I looked at) that is about 88 Ohms.

With 4 cathodes impedances of 88 Ohms + the original 50 Ohm bias resistor all in parallel the effective impedance to 0V is 22 Ohms parallel 50 Ohms = 15 Ohms

With 235 Ohms + a single cathode impedance of 88 Ohms the effective impedance to 0V is 64 Ohms

That means that the original 250uF could be reduced to 250 x 15/64 = 58 uF on each tube.

I did'nt notice any sonic change with 47uF but perhaps a 68uF would have been better.

Cheers,
Ian