Your opinion on 813?

[Jax]

I have 5 NOS Philips 813 in my closet. They all have a small getter so I don't have to run them cherry red hot but will probably have to freshen them up a bit first.

I was thinking of doing something with them. So I'm asking y'all what could be done with them in audio.

Triode connected? If so, connect the beam formers to plate or ground?

Operating point? Load? Drive?

[EC8010]

813 makes a very good triode, and is more robust than 845, although hungrier on heater current. Yes, connect g2 to a. 7N7 made a very successful triode-strapped 813 amplifier which has been mentioned previously on this forum.

[dhaen]

The 813 has been discussed several times on this forum. Try doing a search for 813.
This thread gives some triode curves.

[Brett]

I love them. Awesome triodes.

Try this for some inspiration, though my final preference was for a pentode driver (sorry no schems).

They also work quite well at lower B+ around 600 too.

[Fuling]

I too have some 813 that I´ll put to use when I get the time and money.
One pair of STC´s with graphite plates and three pairs of Philips, to be exact.

Im going to run them at 850V, 110ma and 5k which gives 29W (!!) if the grid is pushed to +20V.

Tube setup from input to output:

EC/PC86 - DC-coupling - 6C19 - Lundahl IT - 813 - Hammond OPT.

HV rectifiers will be 6AU4GTA.

[Jax]

I have pondered some over this and from now it's officially a project.

I'm going to try around 850V 90mA fixed bias as a starter. This would be around -80V bias. Setting -10V as maximum grid voltage (the tube seems to behave odd above -10) the required input would be around 50V RMS for max swing. Rather easy tube to drive.

A load of 6.5 kohm seems good to me. I have sent an email to Sowter asking what they think about it.

With a sensitivity of 2V RMS for maximum power out the gain of the driver should be 25 without feedback.

I have seen a few designs on the net (there are not many), most seem to use transformer drive or RC with a high Gm pentode. My own idea is to use a suitable triode as input followed by a CF with cathode resistor connected to a -200V supply. Adjusting the CF bias to set the 813 bias as well. CF capacitor coupled to the input triode.

I do have a bunch of NOS C3m available but C3m triode connected has too low gain. 6SL7? Other ideas?

CF: I'm tempted to try EL360 triode connected as CF. Why CF? 813 spec says no more than around 30 kohm grid leak.

What do y'all think? I would like to avoid interstage transformers at this time.