Orange Amps Forum

My second experience with an Orange amp and a red-plating EL34

Or in this case an EL34B

I turned the amp on yesterday and crackle crackle - funny noise - and looked inside to see one of the output valves red-plating and turned the amp off immediately

I didnt notice this before so I think it could be a new issue. Though I bought the amp second hand so I cant say with 100% percent certainty.

I've just ordered a matched pair of JJ EL34's and some new 12ax7's and will do a full re-valve as soon as they arrive but dang I didn't think this was gonna happen!?

I thought that the R30 was cathode biased and so it wouldnt be possible for valves to draw too much current? I saw another thread on here that suggests the valve

may be damaged and this is the reason that it is drawing too much current. Anyone have any experience with this ??

B

It might be that there is damage. If so, you would expect a decent tech to be able to diagnose and resolve it. If it's just a single valve red plating then replacing the pair of power valves should fix it. If you replace them yourself, look carefully as they come up to temperature to make sure the pair are even and that neither appear to be wanting to red plate. You could also try just swapping the pair as seeing if the red plating follows the tube...

If you're worried - take it to a tech.

Cathode biasing isn't the 'automatic' biasing many people believe. It does 'self adjust' to an extent giving a certain amount of leeway for slightly different spec valves, but valves which are way off normal spec (for whatever reason) may still lead to problems.
I have a slight memeory at the back of my mind of hearing of EL34Bs having had rather different current draw from normal. Or one may may just have gone faulty.
To be honest I'd get a good amp tech to check things with the new valves. I'd actually recommend having ANY second hand amp properly checked over anyway...


Andy.

a.hun wrote: ↑

Sat Aug 19, 2017 6:54 pm

Cathode biasing isn't the 'automatic' biasing many people believe. It does 'self adjust' to an extent giving a certain amount of leeway for slightly different spec valves, but valves which are way off normal spec (for whatever reason) may still lead to problems.
I have a slight memeory at the back of my mind of hearing of EL34Bs having had rather different current draw from normal. Or one may may just have gone faulty.
To be honest I'd get a good amp tech to check things with the new valves. I'd actually recommend having ANY second hand amp properly checked over anyway...


Andy.

I think you're right about the EL34B. I recall hearing them not even being able to be biased in some adjustable fixed bias amps without the bias circuit being modified.

I got a set of JJ EL34's and some pre-amp tubes and popped them in on Sunday morning

Lets just say it was a good good day....Amp is sounding miles better, no red plating and

tone for days.

Excellent! I love happy (noisy) endings!!!

Andy.