Traynor eating output tubes

[drewl]

So I repaired this amp last month, outout tube socket had arced, I replaced it, tuned it up and all was well.
Amp came back with a fried EL34.
The owner put in a new pair of Mesa EL34's which, going by the remaining good one, were biased a little low, the one that died has the plate pin three shorted to pin 2.

This sucka' is putting out 120w into 8 ohms!
Even strumming a guitar into it, it's pulling close to 200ma per tube when cranked!
Yikes!
I added surge protectors on the plates, and was going to add a couple screen grid resistors on pin 4, might help tame it.

[Gtr_tech]

Screen resistors. Current production tubes won't take the abuse old ones would. 1k 5w

[Gtr_tech]

Also if there's no control grid stoppers on the output tubes, add those too. Std Mar$hall 5.6k values work fine.

[drewl]

Well, it's got 1.5k grid stoppers, I could always bump them up too.

That's what I figured about new vs old tubes.

I have some great old 6CA7's from an old Traynor of mine, but I'm not giving them up!

[audiopete]

The 1.5K grid stoppers are fine. They are to stop RF interference and if you have no problems then no need to change them. Definitely get on the screen resistors. You can use new Winged C EL34s. They sound great and have higher voltage ratings than others. 800V plate and 500V screen.

I scored some 6CA7s in an old Traynor and they are actually 70s UK Mullard XF3 EL34s test new. You have to check. They were a lot of re-labelled tubes back in the day and Canadian amp makers used a lot of British parts for obvious reasons.

[Chuck H]

And... If it's the same tube failing each time, you may want to check for a fault in the bias connections for that tube.

[pdf64]

'The 1.5K grid stoppers are fine. They are to stop RF interference and if you have no problems then no need to change them'
Grid stoppers (on both the sreen and control grids) may also act to mitigate oscillation.
It may be that certain load and signal conditions set off a damaging ultrasonic oscillation, so increasing their value, within reasonable limits, can be beneficial.
Also consider a bad / intermittant contact somewhere in the output transformer secondary circuit, eg speaker lead, speaker socket, I've even had a Fane that went open circuit at high levels.
Pete.

[Gtr_tech]

You'll find that some amps will oscillate at low audio freq's too...motorboat. With Traynors that didn't have the grid stoppers fitted from the factory I've seen this happen to first hand. Old stock tubes may be OK with it, but new mfgr tubes seem to require them.