Marshall Amp Forum - View Single Post - Preamp Tubes?

MartyStrat54 · 07-04-2009, 04:22 PM

Joe, I found that info about RFT EL34's and it turned out to be on the forum. However, while it did scare me for a moment, I read the rest of the thread and the real truth was posted by a reputable person.

You have to be very careful using a variac because of those heater voltages, I would highly recommend using a separate filament transformer that is run off the 120V line. You don't want to change the filament supply by more than 10% either way so if you have anywhere between 5.7V and 6.9V you should be alright but if you are out of this range you will drastically reduce the life of your tubes and most likely your tone will suffer. Those RFT's should be able to handle 500V on the plate no problem, the EL34 is rated at 800V on the plate, while alot of new tubes can't really handle this the older ones will and I've had good luck with the new production JJ's which I have run at over 600V without any problems. The problem with going to high on the voltage is not that the tube is run harder its that it will internally short between elements due to the high positive potential. As long as your bias is set correctly a tube run at 500V won't be worked any harder than one at 400V.

-Matt at Triodeelectronics.com

So the bottom line is what I have previously posted and that is a NOS EL34 can handle 800 volts on the plate.

Marty

07-04-2009, 04:22 PM	#551 (permalink)
MartyStrat54 Senior Member Join Date: Feb 2009 Location: Formerly Phoenix, now north of KC Posts: 2,583	Re: Preamp Tubes? - Who, What, When, Where, Why, and How? Joe, I found that info about RFT EL34's and it turned out to be on the forum. However, while it did scare me for a moment, I read the rest of the thread and the real truth was posted by a reputable person. You have to be very careful using a variac because of those heater voltages, I would highly recommend using a separate filament transformer that is run off the 120V line. You don't want to change the filament supply by more than 10% either way so if you have anywhere between 5.7V and 6.9V you should be alright but if you are out of this range you will drastically reduce the life of your tubes and most likely your tone will suffer. Those RFT's should be able to handle 500V on the plate no problem, the EL34 is rated at 800V on the plate, while alot of new tubes can't really handle this the older ones will and I've had good luck with the new production JJ's which I have run at over 600V without any problems. The problem with going to high on the voltage is not that the tube is run harder its that it will internally short between elements due to the high positive potential. As long as your bias is set correctly a tube run at 500V won't be worked any harder than one at 400V. -Matt at Triodeelectronics.com So the bottom line is what I have previously posted and that is a NOS EL34 can handle 800 volts on the plate. Marty __________________ MARTY 2009 Gibson Explorer-Hell Raising Machine 1989 Fender Strat Plus USA 2008 Fender Tele USA 2004 Schecter Elite w/DiMarzio's TSL100 with custom 4x12 cab. TSL122 with Man 'O War Speakers. TSL602 #1 with JBL D123 Speakers and 602 #2 with Altec 417 Speakers. "Oh so sweet my friends." DSL401 #1 with Red Fang Speaker and 401 #2 with JBL D123 Speaker. Tubes: Let's just say they are NOS that kick ass.