Hi, can I clarify some of the suggestions for the EL34 conversion based on what has been written so far.
I have a nicely working 18W Classic with a big fat PT and a large OT (1900 Prim but swapping the Output taps to reflect around 4K with an 8ohm load)
1/ All seem to concur installing >1k screen resistors. The Lite 11b shows this mod as removing the 1k B+ rail resistor and installing the 2x 1k on the screens of the (EL84's in this case).
This wouldn't work as such on a Classic as the 1.5K then goes on to feed the PI, so those values would need to be altered as well ???
2/ ZP and others suggest the PI changes as R11 to 470 and R17 to 12k. The Phat Ass 26 schemo shows 470/15k so pretty close and the author seemed to like it tho' only quoting 18W @ 45ma.
Is this generally agreed as adequate adjustments to the PI to drive the EL34's?
3/ Cathode resistor 130-330 @ 10W and a 'big cap' around the 1000 mic mark?
I'm hoping to get around 30+ Watts and retain elements of the Classic tone.
And yes- I acknowledge the importance of messing with the OT in particular, but so far I have to say, with the EL34's running, it still sounds pretty close to the original 18W Heyboer OT
Thanks in advance
Vino
EL34 Summary
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Re: EL34 Summary
For the screens, you want to keep the dropping resistor in the B+ rail then run individual 1k/5W resistors from the screen supply cap to the screen of each tube. The IIb keeps the dropping resistor, so I'm not sure what schematic you're referring to. You definitely want a separate screen supply cap.
That sounds about right for the PI.
For the EL34 cathode resistor, I suggest individual resistors for each tube. If your B+ is low (around 350V), 270 Ohms would be good. If it's higher, I would go up around 350 Ohms. 5W should be fine if you're using individual resistors.
Personally I can't stand the feel of the giant cathode cap, but a lot of people like it. If you're using individual resistors, you'll also need individual caps. You can use half the value you would with a shared resistor since it's the product of the resistance and capacitance that affects the time constant.
That sounds about right for the PI.
For the EL34 cathode resistor, I suggest individual resistors for each tube. If your B+ is low (around 350V), 270 Ohms would be good. If it's higher, I would go up around 350 Ohms. 5W should be fine if you're using individual resistors.
Personally I can't stand the feel of the giant cathode cap, but a lot of people like it. If you're using individual resistors, you'll also need individual caps. You can use half the value you would with a shared resistor since it's the product of the resistance and capacitance that affects the time constant.
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