AC30CC repair help/advice

[HTH]

ok, I've got an AC30CC here thats burnt out a screen resistor (100R, 1W).

I know Vox traditionally used 100R resistors here, but with the hard way EL84s are run in these amps and the poorer quality of EL84s today, would it not make more sense to increase the screen resistors to at least 470R to make the amp more reliable?

[Steve Conner]

It might make the valves last longer, but it'll probably change the tone.

You get Russian military grade EL84-a-likes that are pretty tough.

Whenever I hear of burnt screen resistors, I think bad speaker connection somewhere.

[HTH]

It might make the valves last longer, but it'll probably change the tone.

You get Russian military grade EL84-a-likes that are pretty tough.

Whenever I hear of burnt screen resistors, I think bad speaker connection somewhere.

this one already had those Rusky mil-spec valves in it, but I have plenty of them on hand and am therefore replacing all the EL84s.

the weirder one to track down/explain is the previous GZ34 in this amp literally blew up (smashed the glass envelope), can only assume it was always bad and an accident waiting to happen.

I've went ahead and put 470R screen resistors in the amp and will know tomorrow if its had much affect on the tone. I must admit that I don't hear a difference when I've tried this on various other amps.

[Gtr_tech]

It might make the valves last longer, but it'll probably change the tone.

You get Russian military grade EL84-a-likes that are pretty tough.

Whenever I hear of burnt screen resistors, I think bad speaker connection somewhere.

Or shorted tube. 84s in combos get rattled to the point of internal misalignment and they go poof. Pretty common around here. They can even be misaligned right out of the box. I've seen Mesa branded EL34s with screens that glow dim at idle then go all "Edison" with signal on them. They don't make 'em like they used to....

[Steve Conner]

I've seen Mesa branded EL34s with screens that glow dim at idle then go all "Edison" with signal on them. They don't make 'em like they used to....

EL84s and EL34s are pentodes, so they don't have aligned grids to start with. The "Edison" light show in EL34s just means that the screen grid is getting overdriven. Most EL34 guitar amps run with too much screen voltage and too small a screen resistor, following the example of Jim Marshall's original plexi.

If you see a few screen wires glowing in a beam tetrode, well, that is a bad tube. Either none of them should glow, or if the screen is being overdriven, all of them should. I have a KT66 (sadly one of a pair I got in a trade) whose screen wires glow in a little round spot, with a matching melted spot on the plate next to it. It has a kind of snap action, in that the spot doesn't light up until you start playing loud, but then it stays glowing, and the spot on the plate starts to get red, until you turn the amp off.

[HTH]

Thanks for the advice all. I went ahead and put the 470R screen resistors in and can't notice any negative impact on the tone.

The amp is now working fine and I'm happier knowing that the screens are protected a little bit better now (marginally I know).

No wonder AC30s have a bad rep with reliability running the plates around 350v with the screens not much lower. Was it a case of the designers not expecting the amps to be used overdriven so they considered that the amp would run fine within 'clean' limits ???

[Ted]

the weirder one to track down/explain is the previous GZ34 in this amp literally blew up (smashed the glass envelope), can only assume it was always bad and an accident waiting to happen.

I had exactly the same thing on an AC30CC a while back - there was nothing left of the valve glass so I couldn't even tell where it had been made...