VOX AC30 QUESTION?

[zozoe]

Hi all & hope everyone's well!! My question is can 2 of the 4 EL84's be safely pulled much like I have done on a Fender Twin? Some may say, why not just get an AC15,,, but I've owned many Voxes thru the decades, AC50, AC10-Twin(sigh), and several 30's, & there's nothing like an AC30!! Now all gone, but I'm eyeing one, & was just curious? And if yes, what precautions or mids needs to be made?
Thanks all!

🕉

[martin manning]

You can, but you would have to double the speaker impedance (or miss-match impedance selector down one click), and double the cathode resistor value from 50 to 100Ω.

[wpaulvogel]

The power section of an AC30 is cathode biased. They use resistors bypassed with capacitors on the cathodes of the power tubes to establish bias voltage. A modification could be performed on the amp to reduce the output section to only two tubes but it would require a switch and more cathode resistors. It would reduce it to the output of an AC15.
On the other amps you speak of with just removing tubes, they are fixed bias and their cathodes are grounded. A negative voltage supply is fed to the grids of the output tubes. On these amps you can basically remove one tube from each side and you should switch the impedance selector to match the new impedance.

[zozoe]

So if there's an 8 ohm speaker in it, turn the selector to 4 ohms?

[thetragichero]

that's the least of the things to do
find an attenuator if you need to crank it without disturbing the neighbors
this pulling tubes business is a lot of hassle and risk for very little (if any) benefit

[martin manning]

So if there's an 8 ohm speaker in it, turn the selector to 4 ohms?

Correct, and that should always be done when running with half the output tubes removed, but in this case if you don't increase the cathode resistor value the output tubes will be running at about twice their rated power dissipation (idling, with no signal) and they will burn up very quickly, most likely in seconds. A switchable cathode resistor is an internal modification that would require some skill to implement. As for any amp, when half of the power tubes are removed the voltages throughout will increase a bit due to the reduction in current draw.

As thetragichero alludes to above, pulling two tubes reduces output current by half, and along with doubling the primary impedance that results in a 3 dB reduction in power at the speaker. Noticeable, but not a dramatic difference.

[pdf64]

...can 2 of the 4 EL84's be safely pulled much like I have done on a Fender Twin? Some may say, why not just get an AC15,,, but I've owned many Voxes thru the decades, AC50, AC10-Twin(sigh), and several 30's, & there's nothing like an AC30...

Even with all the changes mentioned, an AC30 with 2 tubes pulled will be a rather different beast, so the exercise seems pointless; as with the reduced current draw the HT voltage will rise and become much stiffer.
I suggest to change the rectifier to a 5Y3 or 5R4, or insert a suitable dropper resistor between rectifier and the reservoir cap and rest of the amp, eg 220 ohm 10W.

[Roe]

don't do it. the tubes wont last long. install a ppimv instead

[martin manning]

Pointless? The point might be to satisfy one's curiosity!

A 5Y3 rectifier is a good idea to lower voltages when running half the output tubes, though, as long as it's a NOS or JJ to get the expected voltage drop. 5R4 is an odd duck, only available NOS or used.

A slightly more than 2x Rk value could be used to run two tubes a bit cooler too.

PPIMV would be a good option, still requires an internal mod, and finding a good place to put the dual pot.