Running EL34's (push pull) with 6K6 OT primary

[Silvio55]

Hi guys! I have an OT I got some time ago for a TW Express project, but I never used it, now I want to build another amp (JTM50 based) with this OT. I will use 2xEL34's in the output (just like the Express), this tranny has a primary impedance of 6K6, wich is higher than the normal 3-4K recomended for EL34's. My PT is 300-0-300 and I will use diodes for rectification. Is it safe to use it? should I get another tranny, or can I use it but with mismatched load (8 ohm speaker with the 16 ohm tap)?
Thanks!

[Dave H]

You could try cathode bias. There's an example in the EL34 data sheet where they are running at 450V with a 6k5 a-a OT and individual 465R cathode resistors.

[Gregg]

Is it safe to use it? should I get another tranny, or can I use it but with mismatched load (8 ohm speaker with the 16 ohm tap)?

You can use it with mismatched load.

[pdf64]

Nick's interactive data sheet indicates that EL34 have a hard time at 450V and 3k3 load (assume ~430V on the screen grids).
Plate dissipation well over 30 watts; have a play Interactive Valve Data Sheets
It confirms the stuff Aiken puts in his document Idle Current Biasing - Why 70 percent?
6k6 seems a bit higher than ideal for max power output, but it will keep plate dissipation down, so hopefully a good tube life.
Higher value loads tend to increase screen grid dissipation under high signal conditions though.
About 5k5 seems to be a good compromise.

[Silvio55]

Thanks! So, thereīs no problem using the OT "as is", I'm not looking for max power output, and if it is easier on the tubes the better! I have one more question: if I use a mismatched load, the primary impedance would be 3K3, but haw about the current? Does it get doubled? Would the tranny or the tubes handle it with no problem?

[pdf64]

As to whether the transformers will handle the additional current from operating at 3k3, you'll have to report on their specs?
The increased plate dissipation I noted in Post #4 is likely to reduce their working life, but it may not be any worse than a Marshall etc.

[Silvio55]

I donīt have the specs right now but I can ask the manufacturer as soon as he aswers the mail I sent him yesterday asking for a new choke for this project. But anaway, I think I'll use it as is and leave it there, a slight reduction in power output is actually better for me, I canīt crank my 50W jmp50 plexi clone in most gigs I do (I think I only did it once!).
This project would be a JTM45 with EL34's (KT66 are way too expensive for me, my tube dealer has them at 4 times the price of the EL34's, all JJ's!!!!) and solid state rectification, some people say the high impendace of the OT is part of the JTM45 sound, 6K6 in the earlier ones and 8K later, wich I read is high (for guitar amp standards) even for KT66.