Topic review

Thanks Bob!!!

I have very efficient speakers that can do with only 10w. Also I have been designing speakers for 30 years & would be Bi or t
Triamping with a triode strapped EL84 amp.

Anyone else if you do this tell us your results.

Tazsmonn,

What you have said is correct. On an EL84 based amp the plate lead is pin #7 and the ultralinear screen tap is pin #9. On Dynaco amps the plate leads are blue or blue/white and the ultralinear leads are green or green/white.

Your explanation of what to do is also correct on hard wiring the ST-35 or SCA-35 and also the DPDT swich wiring is correct the way you have mentioned it.

Two things on the ST-35. It you want to use the switches you will have to drill two 1/4 inch holes somewhere in the front face of the amp to allow switching. If you put them on the top surface you will have to remove the tube cage to make the switch each time. On the SCA-35 you have more room on the back to drill two holes. A couple of caveats. Anytime you drill into an original Dynaco amp you usually devalue the amp somewhat even if your drilling results in an "improvement" in the amp's function or features. The other thing is that neither of these amps has much power to begin with and your 17.5 WPC in pentode will become about 10 watts in triode. If you have a smaller room and fairly efficient speakers though, this may not be an issue for you.

Your issue with the Mark IV seems strange in that a 100 ohm resistor didn't work between pins 3 and 4 (pin 3 is the plate lead and pin 4 is the ultralinear screen tap on Dynaco amps with octal sockets) but a 220 ohm resistor did? I have always used a 100 ohm 1 watt carbon film resistor here and never had an issue. Strange that a 100 ohm resistor wouldn't work in the Mark IV ??

Bob

For an El84 would you remove the green or green white wire from pin #9, insulate.

Then put 100 ohm resistor from pin #9 to pin #7?

If this is correct then to add a ultralinear to triode switch you would.

Remove green or green white wire from pin#9 of each tube and solder to separate poles of DPDT switch on the same end.

Solder wire from pin #9 to center pole of DPDT switch.

Solder 100 ohm from pin #7 to DPDT switch opposite end from green or green white wire.

All wiring to and from switch side must be to same tube.

Also I once had a MKIV which I triode, did not work. Was wired correct. Asked a guy about it, he thought it may be osculating at radio frequency, suggested 220 ohms. It worked fine. Is this common?