Opinions on 6L6 as triode

[stokes]

Going to do some experimenting with wiring 6L6's as a triode.Its a PP with 6L6GB's.I've seen schem where the plate and screen are tied together directly and also seen it with a 1k screen resistor between the plate and screen.I'm thinking the resistors would cause the volts on the screen to be a bit lower than the plate.Any thoughts would be welcome.

[Malcolm Irving]

If the power stage is overdriven, the 1k resistor limits the screen current and helps to prevent screen dissipation exceeding the limit.

[stokes]

Yeah,I can see that,but it seems to me that having a resistor here sorta defeats the purpose to some degree,with the plate and screen at 2 different voltages.

[Malcolm Irving]

That's true, but the screen is closer to the cathode than the plate is. I suppose there would be some kind of 'sweet spot' where the voltage on the screen matches the local electric field (or in other words doesn't disturb the field). I have no idea what that voltage would be, however! But if we did hit a 'sweet spot' screen voltage, perhaps the number of electrons 'terminating' on the screen, relative to the number getting through to the plate, would be in proportion to their cross-sectional areas, which might help to balance the dissipation between the two. I'm sure this idea is not at all important in practice, but it's just interesting to ponder!

[Jazz P Bass]

You do not want to exceed the screen voltage rating limit. (450 Vdc /6L6GC)
In older low power, low B+ voltage amps, this is not much of an issue.

If your B+ is around 450Vdc, you can go lower on that 1K.
I have seen 220 ohm screen resistors on 450 volt amps in triode mode.

As an aside: most players that I have run into that have a Triode/ Pentode switch, do not like the Triode mode.
It gets really 'dark'.
Which would be a benefit if you have an already 'too bright' amp.

[stokes]

This is an experimental platform I have.Its been thru a lot of various incarnations.Right now I have the plates down to about 385v's via a 50 something volt zener on the CT of the PT secondary.I actually have 470 ohm resistors on the screens.I have an adjustable cathode resistor on the power tubes so I tried it with the 6L6's then rebiased for a pair of 6V6's.Seems to be a more pronounced difference with the 6v6's.

[Alan0354]

I never like the sound of triode mode. People like it in hifi amps, not guitar amp.

[mhuss]

I never like the sound of triode mode.

+1. I tried it a couple of times, then stopped.

[Leo_Gnardo]

I never like the sound of triode mode. People like it in hifi amps, not guitar amp.

It's OK to not like triode mode. OTOH I've put lots of normal/triode switches in amps, with good results and happy customers. It does roll off high frequencies - makes for a mellower tone. Also less sensitive, you have to turn up the preamp a notch or two, punch the output section harder. Most of the customers who ask for the mod are trying to "tame" their amps for small-club work. For instance a 15W or so Deluxe Reverb gets down to 7W in triode mode. Perfect for honkin' the blues in a small room. For anybody who needs to go there, it's worth a try, cheap enough to do and reversible.

[g1]

Leo, when you do it in a Deluxe, do you do it with or without screen resistor?

[Loogie]

I think triode strapped is worth a try. Its just another amp sound. I've been experimenting with an EL84 se lately, but I've done pp's as well. I've been switching in a higher value cathode resistor and I'm also switching up to the 16 ohm tap to lower the load resistance. I don't know if that's necessary, but I'm having fun.

One of the things that happens when switching to triode is the screen follows the anode so while choice of screen resistor has an effect (I've switched that too) I think the anode will still follow even if you use a 1k instead of 100 ohm. I could be wrong about this. For EL84 I've tried everything from 100 to 2.2k.

Give it a go. You might like it. Yes it's darker, but you can emphasize the remaining high frequencies by turning down the bass. Besides, sometimes darker is cool.

[Leo_Gnardo]

Leo, when you do it in a Deluxe, do you do it with or without screen resistor?

Screen resistors IN place please, let's not beat those tubes up too badly. I stick with good ol' carbon comp 470R 1W - unless the user has a history of wrecking them, then I go for 5W wirewounds. Doesn't hurt to have tough 6V6's too, modern JJ seem to work fine, old RCA & Sylvania as well. Deluxes - I've even put in 6L6 - of course with bias set accordingly. Not a lot more power but they're not running near or beyond their ratings & tend to last a long long time.

[stokes]

NOS Sylvania 6V6's.Like I said,this is just an experimental "spare chassis" thing,not something I intend to use.I kind of like it,but it'll take a few days to really decide.

[Alan0354]

Screen resistors IN place please, let's not beat those tubes up too badly. I stick with good ol' carbon comp 470R 1W - unless the user has a history of wrecking them, then I go for 5W wirewounds. Doesn't hurt to have tough 6V6's too, modern JJ seem to work fine, old RCA & Sylvania as well. Deluxes - I've even put in 6L6 - of course with bias set accordingly. Not a lot more power but they're not running near or beyond their ratings & tend to last a long long time.

I experimented with JJ 6V6S. I got lower power than two others I have. I did the power test on my amp, I never manage to get above 19W out of it. But just by changing to even the cheap Ruby tubes, I got 22W or more. I never understand why. I used resistor load, driving to clipping to measure the peak voltage. Everything else is exactly the same. The plate voltage is slightly over 400V. So I think I was comparing apple to apple.

I use a 150mA PT and OT of Vibrolux, these are good for at least 30W. So the limiting factor got to be the tube.

[Leo_Gnardo]

I experimented with JJ 6V6S. I got lower power than two others I have. I did the power test on my amp, I never manage to get above 19W out of it. But just by changing to even the cheap Ruby tubes, I got 22W or more. I never understand why. I used resistor load, driving to clipping to measure the peak voltage. Everything else is exactly the same. The plate voltage is slightly over 400V. So I think I was comparing apple to apple.

I use a 150mA PT and OT of Vibrolux, these are good for at least 30W. So the limiting factor got to be the tube.

Agreed, your test method looks fine to me. 19W vs 22W, just a pinch of difference, hardly audible. And I woulnd't fault you for preferring the non-triode circuit. For me, gotta keep the crustomers satisfied. Some want vanilla, others chocolate, and yet others a scoop of each.