pentode/triode switch for el84

[wallyk]

I was reading mlannoo's post on his half power switch.I too do not hear much difference between half and full.I was thinking of converting my amp over to a triode/pentode switch.Has anyone tried this and is there a schematic somewhere on a good way to do this?

[markh]

I haven't tried it with EL84s, but I have with EL34s, and I did not like the result at at all. You'd have to be doubly careful with EL84s, as high screen voltage could result in very short tube life.

Has anyone tried the "single tube pot" push-pull to class A control as implemented on Mark Durhams Vajra amp?

--mark

[MaxMix]

I was reading mlannoo's post on his half power switch.I too do not hear much difference between half and full.I was thinking of converting my amp over to a triode/pentode switch.Has anyone tried this and is there a schematic somewhere on a good way to do this?

Hi wallyk . . Just joined this forum.. I have performed the modification you mention.. EL84 in either pentode or triode mode at the throw of a switch.

You need a decent double pole switch, one half to strap the screen grid either to HT or the anode and the other half to switch in an extra 120ohm resistor in series with the cathode . . A cathode bias resistor value of 270ohms is recommended for a single ended EL84 in triode mode. Assuming the existing cathode resistor is 150ohms the extra 120ohms makes this 270ohms. You might need to tweek the value depending on the existing value.

Hope this helps . .

You should get about 2W out . .

[rjgtr]

I built a Vajra so I've tried the Vajra PP/SE control. It is a cool addition and it sounds pretty good too. It results in a good difference in drive and different volumes. It's also really easy to implement; you could even just use a switch.

I have had amps with a Triode/Pentode switch and I'm not a big fan either. It's not that the amp doesn't sound good in the triode mode, but it sounds so much better in pentode mode that I never used triode mode. Triode mode just lacks the extra harmonics.

Another apporach is to use different B+ voltages. When in the range of 50V you can go from a harder bright tone to a browner tone. This could also be done on a switch with a little ingenuity.

Another approach is to add a switchable fixed/cathode bias option, again like the Vajra. This can also give a more useful range of tones.

[wallyk]

Do not have a schematic of the Varja amp.Can I find it here or does someone have a copy?

[CurtissRobin]

It's on the downloads page. Click the Circuit variations link and it'll be at the lower right.

KennyO

[wallyk]

Thanks Kenny found the schematic.Made conversion Monday night.Drastic improvement over high/low switch.Fixed bias on a el84 has a sound all its own.Did not have the guts to try the pp/se pot YET! Has anyone tried that ?