ORANGE OR 120M - SCREEN GRID RESISTOR FAIL

[wpaulvogel]

Not an el34 but EHX 6ca7 I’ve been subbing for EL34’s have been working good in 2 amps I have with 450 volts on the plates and screens. They have a different sound that I enjoy. Tighter full bottom end and a touch less mids. I only said the winged “C” because I had an issue with some tung-sol EL34’s in a 100 watt Marshall. The TS were new and within 30 minutes one tube died red plating that pair. It had 470+ volts. I swapped in the winged “c” quad from TAD and no more problems. $50 per tube though. I don’t know what your B+ is in the Orange.

[Xander8280]

Sounds like a short inside one of those tubes; screen to suppressor since the screen drawing a ton of current but no B+ fuse blow.

Could multimeter your good EL34 pair between the G2 and G3 and compare to the 'bad' EL34 pair, maybe one of those will give some wack reading on your meter.

Really sucks to buy a dud

[jujudubluz]

Hey !

I already ordered a replacement EL34 from TAD !
I had them lying around for 2 years so maybe one of them got hit ...

Anyway - will let you know !

The plate voltages were around 520v ...

Cheers ! Thank you !

[jujudubluz]

ho well -

got my tube today - not matched to the rest even though i left a note ...

[jujudubluz]

Received my EL34 from TAD !
Installed it - biased on the low side with plates @ 510 !

Everything is ok!

How do you guys bias these amps ?

Cheers

JUJU

[Leo_Gnardo]

How do you guys bias these amps ?

25 mA per tube should be well enough, definitely not past 30 mA.

And I can agree that 6CA7 are a good sub for EL34 in high voltage situations, also agree that there is a tone change. But that's the price of circuit survival. FWIW I've also used JJ KT77 and had them work well.

Another "survival" technique is to increase the value of the screen grid resistors. At times I've sneaked 'em up to 1K5, 2K, 2K2
and power spec'd at 10 watts. Old Oranges sure put a beating on the output tubes, otoh nothing quite sounds like them.

[didit]

Hello --

Yes to KT77, both JJ & New Sensor .. albeit just limited experience, and these too do have different sonic shape however to my ears with critical well balanced mids.

Best .. Ian

[jujudubluz]

I stay on the cold side for the moment and the TAD el34 seem to hold well ...

We will see