Primary impedance for OTs - output and tone

[Fischerman]

3800 is not correct for 2 EL84s, it is kinda low even for one..
so speaker 8 ohm must be inserted to 4 ohm tap from the amp, then the load line will be ok (7600 ohms aa)

It sounds better that way too imo.

[Colossal]

FWIW, I ended up using the extra node for the screens but used a 10k dropper instead of 22k. That put the screen node at about 290vdc with 350vdc plates at idle. About 10.2vdc on the 120ohm cathode resistor.

I’m now building a 15W Liverpool in a sister chassis using a custom Fender Deluxe OT that is 6k6 at 2/4/8 ohm secondaries but my intention is to run it 4/8/16 ohm. This kind of puts it at the other end of the spectrum wrt OT primary impedance for 2xEL84s.

Thanks for your as-built values! Your cathode voltage seems spot on. I will do some math As an experiment, many years back now, I ran an 18W Lite IIb derivative at half the impedance to simulate what the Matchless would be doing and liked it, sonically. It was much more thumpy. That amp had a 2k2 screen dropping resistor and individual 1k screen grid resistors so the voltages were reasonably low enough for the test. That amp had 345V on the plates and 320V on the screens and (I think) 11.2V on the cathode. But the Matchless amps utilize huge OTs and I had a small A-frame Radiospares in the 18W so it wasn't an ideal comparison. But the low primary impedance did have a big low end, as I recall.