Possible Mod For VOX AC30CC1

[EL504]

I don't think a pair of EL84's at Vak = 300 V and Vg2k = about 310 V, and with a cathode bias resistor of only 82 Ohm, will survive very long.

I think that's why Roberto suggest changing for a 150R resistor. Dunno for the rest.

Exactly.

OK, maybe you should have written it more clearly, like: "If you want to try running only two EL84's, than change R12 from 82 Ohm to 150 Ohm."

I just wanted to warn TS against destroying a pair of EL84's.

[roberto]

Thanks Robert, you made it even more clear now, that's good!

[EL504]

Vox used to run a quad of EL84's at 50ohms (like on the drawing here selectable between 50 and 82.) The higher the resistance, the cooler they'll run, so why do you say a pair at 82 is going to do worse? Or is it more the implication that if it were a pair it should really be at 100 or 164 ohms at that point?

~Phil

Like what you wrote in your last sentence is what triggered me. On top of that B+ will even be a bit higher than the 322 V indicated in the schematic when running the amp with only 2 EL84's. If I look at the curves for Vg2 = 300 V in the datasheet of the EL84 (again: Vg2 will actualy be higher, like 310 V or even 320 V), I think you will wind up deep in the 'danger zone' (far above the limit of 12 Watt plate dissipation).

But everything is fine now; I just wanted to react on what you posted because I forgot to do that that earlier.

[pompeiisneaks]

Yeah I even got it at the end of my own sentence there... basically it suddenly clicked. On my Trainwreck Liverpool, he has two cathode resistors so you can pull each side, and 1/2 the tubes have one shared cathode resistor and the same with the other (one each per side shares one) you can pull two and not cook tubes

~Phil

[JD0x0]

Calculate your dissipation. I had worked on a similar amp and it was idling at ~130% dissipation on 'hot' and 90% dissipation on 'Warm'
I cooled it down to 105% on 'hot' and ~80% on 'warm'

Also, fit some larger screen grid stoppers. IIRC Vox still uses 100 ohms on many amps. Might've been ok for vintage mullards, but new production tubes will get beat up. Try 1.5k-2.2k

[Roe]

100R is ok, if and only if you have a high-resistance choke (e.g. 420R or higher)

[roberto]

EL84-M, 7189 and 6p14p-ev/er can run hotter than standard EL84s.

[M Fowler]

Yes I run all my builds 4xEL84 at 62R cooler than 50R

I run my 2x EL84 at 150R-160R cooler than 120-130R

EL84's run so damn hot and I watch my plate voltage no more 300-0-300 secondary for me I learn my lessen with to high of voltage on these amps.

260-0-260 works well for most builds.

Mark

[vintagekiki]

https://music-electronics-forum.com/for ... post913631
Modify wiring the cathode connection, so that the amp can work with 2xEL84 (power as AC15) or with 4xEL84 (power as AC30)