ECC85-EL84 change to Ultra-linear

[pc1093]

HI All,
I want to DIY A EL84 valve amplifier.I got a ECC85-EL84 diagram. I find there ultra linear output transformer more choice. (this is one of my choice https://www.edcorusa.com/cxpp10-8k

In these diagram, Could I change each pin 9 of EL84 to the G1,G2 of the UL output transformer? any other component need change or add?
Thank you.

[sluckey]

In these diagram, Could I change each pin 9 of EL84 to the G1,G2 of the UL output transformer?

Yes. That's all you need to do.

[pdf64]

In these diagram, Could I change each pin 9 of EL84 to the G1,G2 of the UL output transformer?

Yes. That's all you need to do.

As UL operation is less stressful to the screen grids than regular pentode mode, I think that would probably be ok for ‘hifi’ use. But with an instrument amp (ie that may get pushed hard / overdriven), without wishing to over complicate things, it would be good practice to add screen grid resistors. eg 470 ohm 1/2 watt between the primary tap and screen grid terminal.

[sluckey]

I have a Sunn Sceptre (2x6550) and a Magnatone M10-A (2x7189). Both are UL. Neither have screen resistors. The only UL amps I've seen with screen resistors have been the really big amps like Fender 135W Twin Reverb, Sunn 2000 or 1200 and Marshall Major. I would not bother with resistors in an EL84 amp.

[pc1093]

thank you all

[pdf64]

A couple of points to consider -
Screen grid current shoots up at high signal levels, especially around and beyond clipping; smaller amps such as the 2xEL84 here seem more likely to get overdriven.
Pentodes such as EL84 tend to inherently have higher ratio of screen grid current to plate current, than beam tetrodes.

The DrZ Route 66 is a rare example of a modern UL guitar amp, built with power amp overdrive in mind; that seems to use screen grid resistors https://www.flickr.com/photos/49108115@N04/4501477895

[pc1093]

thank you all,
If I put a resistor between screen grids and G1/G2, how many ohms should I use?
are there any formula to calculate or any rules?

[pdf64]

Merlin explains the process for pentode amps http://www.valvewizard.co.uk/se.html
In UL the conditions for the screen grid won't be so arduous, as when the plate is being pulled down to 0V, the screen grid voltage, instead of being up near the HT, is 43% down from that.
I guess that a suitable value will end up being in the range 100 - 470 ohms; anything (however small) being better than nothing, if the amp is to be driven hard.

[PRR]

100r does little. 2K can be significant. That's why "470" comes up a lot.