Screen grid resistors for Pentode/Triode switched EL84's...

[Steve A.]

I am adding a Pentode/Triode switch to the cathode biased EL84's in one of my Crate V18's which comes stock with 470R 5W screen grid resistors. My notes indicate that a 1k 5W screen grid resistor would be recommended here if a Pentode/Triode switch were used.

However, I was just looking at the schematic for the 2007 Vox AC15HW (hand-wired) and they are using a 100 ohm screen grid resistor and the amp does have a Pentode/Triode switch.

Is there any reason for me to not use 1k 5W resistors as I had originally planned?

Thanks

Steve Ahola

EDIT I just found a great AMPAGE thread I saved from 2001 but it does not specifically address the triode mode:

http://www.blueguitar.org/new/text/scr_res.pdf

One value that Randall Aiken recommended was 2k7... I think that I may go with that instead of 1K.

[ETR]

I am adding a Pentode/Triode switch to the cathode biased EL84's in one of my Crate V18's which comes stock with 470R 5W screen grid resistors. My notes indicate that a 1k 5W screen grid resistor would be recommended here if a Pentode/Triode switch were used.

However, I was just looking at the schematic for the 2007 Vox AC15HW (hand-wired) and they are using a 100 ohm screen grid resistor and the amp does have a Pentode/Triode switch.

Is there any reason for me to not use 1k 5W resistors as I had originally planned?

Thanks

Steve Ahola



EDIT I just found a great AMPAGE thread I saved from 2001 but it does not specifically address the triode mode:

http://www.blueguitar.org/new/text/scr_res.pdf

One value that Randall Aiken recommended was 2k7... I think that I may go with that instead of 1K.

You'll get less power out with a 2.7k but it'll be a lot easier on your tubes. Part of the reason for triode is less power anyway so might as well.

[Steve A.]

Hey, thanks! I went with 1k2 (since I only had a single 1k 2W resister). Perhaps I misunderstood your reply but I assumed that the 2k7 resistor would reduce the power in both pentode and triode modes... is that correct?

Steve Ahola

[ETR]

Hey, thanks! I went with 1k2 (since I only had a single 1k 2W resister). Perhaps I misunderstood your reply but I assumed that the 2k7 resistor would reduce the power in both pentode and triode modes... is that correct?

Steve Ahola

It depends how you switch it into triode mode. You could use the 2.7K for triode and stick with the 100 for pentode for more power. Watch your bias tho. If you use 2.7K for pentode too you'll get slightly less power but mostly a lot more sag.