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-   -   UL to triode mode conversion on cathode bias amp-different cathode resistors needed? (

bigwill 29th December 2007 07:08 PM

UL to triode mode conversion on cathode bias amp-different cathode resistors needed?
Quick question - I have a Radford STA15 and I'd like to try it in triode mode. It currently has KT77s in it and biases up fine. Am I right in guessing ultra linear output stages essentially act as triodes at around DC? (The screen goes to B+ through the OPT through a resistance much like connecting the plate and screen together via a resistor)

Will disconnecting the ultra linear tap force me to change the cathode resistance to retain the correct bias?

Amp schematic

tubewade 29th December 2007 07:39 PM


The schematic link did not work for me.

You should not have to change the cathode resistance(s) to switch from UL to triode mode. The voltage at the screens should not change from one configuration to the other and the quiescent current through the valve should be the same either way.

Best regards.

Burnedfingers 29th December 2007 07:39 PM

Your link didn't work for me.

Svein_B 29th December 2007 07:40 PM

I believe that should be quite OK.

A as reference, Allen Wrights PP-1C is designed from the start for triode operation and it also uses 470 ohm cathode resistors for EL34 at around 380V.


PS. I had no problem with the link

HollowState 29th December 2007 07:48 PM

Well it wouldn't work directly for me either. Sometimes you have to go through the home page protocol first.

bigwill 29th December 2007 08:02 PM

Thanks very much everyone, I will perform the mod and see how it sounds! I find the amp to be slightly solid statey - it has a large amount of negative feedback which may contribute to this

hemgjord 30th December 2007 12:37 AM

Hi, be careful with all that feedback.
Triode connecting the outputs will change miller cap. and Rp, this will move around 3dB points and potentially cause instability.

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