Changing Tube Type in an Amp

[charvel750]

Guys, if I have EL34 and want to change to say 6550 or 5881, besides the bias, what else has to be changed inside the amp? Like a circuit, or a resistor?

[Cygnus X1]

[Cygnus X1]

[Cygnus X1]

The pinout is slightly different.
Pin #8 is tied into both cathode and screen grid 3 on the 6550.

It looks like theEL34 is a 25 watt tube, and the 6550 is a 50 watt tube.
You would have to compensate by changing the output transformer.
The filament draw on a 6550 might be too much for the power tranny as well.

[StukaJU87]

My 100 watt Marshall 2203 clone was built with EL34's. Dave Hirsch (Dave's Sound Repair, Whippany, NJ) converted it to 6550's for me. He did not change the transformers. Maybe you can call him and ask him what he had to do.

http://www.gti.net/junebug/dave/

[charvel750]

Thanks, I don't believe transformers need to be changed. In fact, I'm thinking only the bias setting is changed. The EL-34 biases with 25% dissipation divided by plate voltage, then times about .7 so that means in my case: (25/470).7=37 But for the 6550 tube, I think you use 35% for dissipation, making it: (35%/470).7=52 so....

[DonP]

If you have one of Aspen Pittman's Tube Amp books, it tells how to do it.

[charvel750]

I'll have to check his book, unless you wouldn't mind referencing it real quick...

[DonP]

Sorry, I'm at work in Nashville and the books are at home in Cinci.

[Spivonious]

Not to hijack, but you work in Nashville and live in Cincinatti? That's a long commute!

[DonP]

It's a fuck long commute! And before this I worked in Cleveland. Shoot me now?

[Spivonious]

I just checked it on Google Maps... 4 hours each way?? Do you ever get home, or do you have to turn around and go back to work?

[j2379]

im not sure how to do it but every marshall or EL34 amp ive owned, ive had converted to 6L6 type power tubes. Not once did i need to have the transformers replaced. IIRC one amp needed new tube sockets but they may have been because they were old crap.

[toejam]

im not sure how to do it but every marshall or EL34 amp ive owned, ive had converted to 6L6 type power tubes. Not once did i need to have the transformers replaced. IIRC one amp needed new tube sockets but they may have been because they were old crap.

The same is true about my Carvin V3 head. It's got EL34s stock, but you can run 5881s/6L6GCs with just a flick of the bias switch. I assume you'd just flip the switch, put the other type of tubes in and you're good to go, but on an amp without the switch, you'd have to have it rebiased.

[DonP]

I just checked it on Google Maps... 4 hours each way?? Do you ever get home, or do you have to turn around and go back to work?

From my home to the apartment in Cleveland was 3 1/2 hours, to Nashville it's 5 1/2 because of traffic. I am home on the weekends.