I want to install a pentode/triode switch in the back of my twin. At the same time I am going to set the amp up so that I can run el34s if I want(Since I already have them).
What is everyones opinion on these two mods.
Are there any other mods to do to the power section that are really worth the trouble.
It is a 1973 100 watt silverface. Had a cap job and converted to bf bias and specs.
I am going to drill two holes in the chassis. one for the switch and the other for a pot so I can adjust the bias from outside the amp, but that is about all I want to do to the chassis. Thanks
Pentode/Triode switch question
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dragonbat13
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Re: Pentode/Triode switch question
I have never liked the "pseudo triode" mode setting, particularly on beam power tubes. I think a half-power switch (disable two of four outputs) would serve a similar purpose and would sound better.
EL34s use a *lot* more heater current than 6L6 tubes, so putting four EL34s in there would certainly put a strain on your PT. Many people have gotten away with this in the 2 tube bassman-sized circuits, but I haven't heard much about the 4-bangers. I would suggest trying it at home and making sure your PT doesn't get too hot to touch after running in standby for an 30-60 minutes.
--mark
EL34s use a *lot* more heater current than 6L6 tubes, so putting four EL34s in there would certainly put a strain on your PT. Many people have gotten away with this in the 2 tube bassman-sized circuits, but I haven't heard much about the 4-bangers. I would suggest trying it at home and making sure your PT doesn't get too hot to touch after running in standby for an 30-60 minutes.
--mark
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FUCHSAUDIO
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A suggestion
Replace your AC socket with an 8 pin tube socket (same hole spacing etc.) and use for a remote bias test point. You could even add a second bias pot for EL-34's, and switch the tubes, and test the bias easily without removing the chassis. I like this alot for convenience.
BTW: I also suggest adding resistance to the plate leads going to the triode pentode switch, and raising the screen resistors from 470 to maybe 680 or 800 ohms, even 1-K . The 6L6's can live with higher values (and put out slightly less wattage with longer life). The EL-34's don't like the lower 470 ohm values as much, and will red plate and/or die quicker with too low a value. I use 680 on the tube and 470 on the plate leads, which minimizes the voltage change when switching from triode to pentode (and minimized the switch pop).
The old ground switch location (you do have a 3-prong cord and don't need it do you ?
). I also find some brands of 6L6's sound vastly different when connected as triodes. Not sure why, but it's night and day, depending on who made the tube.
A mini toggle would work well as a half power switch also, btw.
BTW: I also suggest adding resistance to the plate leads going to the triode pentode switch, and raising the screen resistors from 470 to maybe 680 or 800 ohms, even 1-K . The 6L6's can live with higher values (and put out slightly less wattage with longer life). The EL-34's don't like the lower 470 ohm values as much, and will red plate and/or die quicker with too low a value. I use 680 on the tube and 470 on the plate leads, which minimizes the voltage change when switching from triode to pentode (and minimized the switch pop).
The old ground switch location (you do have a 3-prong cord and don't need it do you ?
). I also find some brands of 6L6's sound vastly different when connected as triodes. Not sure why, but it's night and day, depending on who made the tube. A mini toggle would work well as a half power switch also, btw.
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