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Siemens El34's and Plate Voltage maxes

ericb

Gold Supporting Member
Messages
9,641
Hi guys, I have a large variety of amps and tubes and I've had quite the mixed results with Siemens in high plate voltage amps. Sometimes they seem to seem to handle the 485-500 volts just fine, and with some of my Siemens they don't at all, but then they'll work fine in lower voltage amps.

My question is do any of you know originally what they were designed to be able to handle? I really am not asking what is a safe range for them , as I've experienced that for myself , but just wondered if they were designed or spec'd only up to a certain plate voltage by design. THANKS MUCH for any info!

Eric
 

Blue Strat

Member
Messages
30,202
Interesting question. I'm not sure if anyone under 80 and who had worked for RFT (the actual manufacturer) would know this.
 

ericb

Gold Supporting Member
Messages
9,641
Interesting question. I'm not sure if anyone under 80 and who had worked for RFT (the actual manufacturer) would know this.

Hey I was aiming it at the 85 year olds out there! :D. No seriously , I've never seen their original specs and was wondering as I have a good deal of them and they vary a lot in what they can handle! .

Eric
 

Mark Kane

Platinum Supporting Member
Messages
3,783
Eric, I don't know for sure but back in the '80's tube guys were saying not to run the screens over 450 volts. I'm guessing that was based on specs but it could have been experience.
 

ericb

Gold Supporting Member
Messages
9,641
Eric, I don't know for sure but back in the '80's tube guys were saying not to run the screens over 450 volts. I'm guessing that was based on specs but it could have been experience.

Thanks Mark , I figured they wouldn't be spec'd /designed for the high 400's-low 500's, as I'd read about people recommending against it ,and also 'experienced' totally inconsistent results with them at those voltages, but was just curious as to the real design of them.

SO , now I'm waiting for you to chime in on the Ultimate Attenuator 4:1 mismatch thread :D They're going to lose loads of TGP business , unless Ho or the UA company chime in with their point of view and explain things.

Eric
 

Mark Kane

Platinum Supporting Member
Messages
3,783
Thanks Mark , I figured they wouldn't be spec'd /designed for the high 400's-low 500's, as I'd read about people recommending against it ,and also 'experienced' totally inconsistent results with them at those voltages, but was just curious as to the real design of them.

SO , now I'm waiting for you to chime in on the Ultimate Attenuator 4:1 mismatch thread :D They're going to lose loads of TGP business , unless Ho or the UA company chime in with their point of view and explain things.

Eric

I dig the Richter thing as much as the UA. I've got lots of old amps that should have blown up a long time ago and can't explain why they didn't but I've also blown up a lot of them when I knew better. I would never tell anyone it's "safe" to use any attenuator on a pegged amp. We're all just lucky most of these trannies are built over spec.
 

Blue Strat

Member
Messages
30,202
Eric, I don't know for sure but back in the '80's tube guys were saying not to run the screens over 450 volts. I'm guessing that was based on specs but it could have been experience.
...and you'll note that in 99% of guitar amps, the screen voltage is tightly linked to the plate voltage (1-4 volts difference at max) so quoting plate voltage is virtually quoting screen voltage.

EricB: When they do fail, what do they do? I would expect that you'd get arc'ing...indicating too tight of a space between two conductors/electrodes inside the tube. Generally that's what max voltage ratings are about.
 

ericb

Gold Supporting Member
Messages
9,641
...and you'll note that in 99% of guitar amps, the screen voltage is tightly linked to the plate voltage (1-4 volts difference at max) so quoting plate voltage is virtually quoting screen voltage.

EricB: When they do fail, what do they do? I would expect that you'd get arc'ing...indicating too tight of a space between two conductors/electrodes inside the tube. Generally that's what max voltage ratings are about.
They seem to have red-plated and stopped being able to be biased. (does that make sense) i.e. the current will just start rising and rising. I quickly will pull them at that point! I don't have any in high-voltage amps anymore, BUT I just got a Bob Bradshaw (Martin Golub) modded SUperlead from 1995.. It's got 1 of the Bradshaw/CAE Crunch mods and absolutely works great. The guy that sold it to me had Siemens in it. When I got it , they were biased WAY low(i.e way too hot) and there was about 70ma plus going thru them.. .not just 1 of them , ALL 4 of them.. I was shocked.. It's about 485 volts on the plates also . SO , once I measured that and figured that out, I re-biased at about 30-36ma (the 4 aren't matched at this point) and it sounds just fine , BUT I don't even want to use them in there, as I won't trust them I don't think based on all I've read. ANyway , I was VERY SHOCKED at 70ma and being they're Siemens at 485volts they were working and not arcing , not blowing up ,etc. 1 was rather red-plating though

Eric
 

Blue Strat

Member
Messages
30,202
Sounds like you just may have just had a few bad tubes. It happens. It's difficult to draw conclusions unless you try 50 pairs of tubes in 2 amps (one high voltage, the other low) and collect data. Anything less than this would generate sketchy, at best, "stats".
 

ericb

Gold Supporting Member
Messages
9,641
Sounds like you just may have just had a few bad tubes. It happens. It's difficult to draw conclusions unless you try 50 pairs of tubes in 2 amps (one high voltage, the other low) and collect data. Anything less than this would generate sketchy, at best, "stats".

Probably so .. I probably have about 16 Siemens ,and the ones that 'ran away ' with things at really high plate voltages seemed to hold the bias and work fine with 400volt amps.. Weird, but I didn't know if that's something to do with their specs, or they were just weak or whatever. . Regardless, no biggie, I was just curious

Eric
 




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