triode/pentode high/low power switch

[daz]

I just want to make sure this is right. Am i correct that the only thing that has to be done to switch a pair of el34's from pentode to low power triode is to take the screen resistors where they connect together to the B+ at the choke output and seperate them and remove them from that power source and put each one to it's respective tubes plate? And if i am correct, would i run into any issues using the switch with 6v6 or 6l6?

By the way, while i'm here i have another Q....when running 6v6 (JJ's so they can handle it) is there anything like screen R value that really should be changed? Also, I run them on the same tap even tho i know i should 1/2 it to 4 ohm. But it sounds better on 8.

[daz]

Well, i already did this so no need to answer the question about how it's wired. I now see all it is is taking the screen supply from the plates instead of after the choke. What i noticed which seems odd compared to my old silver jubilee which also had this is that it doesn't hum when in triode mode like the jub did, nor is it as dark. It's darker, but nothing like the jubilee. In fact, it's REALLY sweet ! i remember the jubilee not only got a lot darker but it lacked the harmonic complexity of the pentode mode. This amp doesn't. And because it IS a little darker i can run it with no NFB and it's real alive sounding and not too bright as it is with no nfb in pentode. Lots of nice options here. Between a triode/pentode switch and having a variable NFB and a NFB off (which will be courtesy of a no load variable NFB pot) i have a lot of options to dial in the output section on those days where the amp could use a little more of this or a little less of that. Then add in the 6v6/el34/6l6 variable and thats all she wrote. Right now i'm running triode mode with no NFB and it's just beautiful. Good stuff !

[JoeM]

What kind of circuit is the amp based on? I've never been impressed with triode modes on Fenders, just the couple of times I experimented with it.

[daz]

What kind of circuit is the amp based on? I've never been impressed with triode modes on Fenders, just the couple of times I experimented with it.

Based on a marshall JCM, at least the output is. And this isn't the 1st time i tried this. i did it with a marshall years ago tho i couldn't remember much about it which is why i asked. But like my silver jub that one wasn't too good in triode either. This one when in triode requires i up the treble, but once i do it's not lacking in any way compared to pentode and sounds even sweeter.

By the way, anyone wanna suggest what minimum specs should i get in a DPDT switch for the purpose of switching the screen resistors to the plates?

[tubeswell]

Having built a couple of amps using Merlins pentode-triode morph, I thought about doing the same thing in a PP output stage, You could use a dual ganged pot or two different pots it you wanted to experiment a bit.

[daz]

I'm good with the switch. I must say tho, after implimenting the switch so i can A/B instantly, it is a lot darker i didn't realize how much treble i was adding to get the same or similar result.

EDIT: it really doesn't cut well i a mix. How about this.....triode on one tube, pentode on the other? Any reason that wouldn't work to get a sort of in between sound?

[daz]

Funny....In a few days time i found that i like the triode switch and also found i liked 6v6 in this amp. After a week of using both either by itself of combined, i find i like neither as much as the amp in pentode with EL34's ! ain't that the way it goes? It's amazing, truly stunning actually how something that seems like a truly great sound can reveal itself to be so lousy when used withing a music mix. The 6v6's just sounded weak and thin compared to the 34's, and the triode setting lost loads of articulation and that alive quality the amp has with 34's. Oh well, lesson learned.....for the 648th time. LOL! (note to self: be like Frank and just shut up and play your guitar)

[bob p]

sometimes we like a change just for the sake of having a change, but then after a while we found out that we liked what we had all along.

I have one of those mesa mark series amps that has all of the switches on the output section -- things like B+ switching, triode/pentode, class-a/simulclass, and tube type mixing. although they're fun things to play with for tweaking at home, in the big scheme of things i find that most of these switches are novelties that i don't use all that often. i never fell in love with 6V6 in my amp. i tend to use 6L6 in it most of the time, and the class-A/simulclass setting is useful. and mixing 6L6 with El34 is pretty interesting. that's about it for me though. i could live without all of the other switches.

[bob p]

B+ on pots scares me.

[Rusty.sparks]

I’m thinking about building/ having a guitar amp built that is a low watt Marshall plexi style amp. I had been thinking that the only option for low wattage was going to be a single ended EL84 or 6V6, but from what I’ve been reading the punchiness of the plexi type circuit is dependent on a push/pull circuit. Now I’m thinking about using two 6v6s in push/pull with a triode/pentode switch. I’ve heard that 6v6s are very inefficient in triode, which may be an advantage for practice volumes. Opinions? I’ve heard 6v6s aren’t usually run in triode. Do they sound bad ( no dynamics, weird Eq), or do they mostly just get quieter?

[olddawg]

I personally don't like the Pentode/triode switch. I prefer an MV and a speaker attenuator. The last amp I built I put a cheap $12 100 watt mono LPad on the back panel. Works great. A couple of times my stage amp was accidentally switched into Triode mode when someone else set up my amp. I spent the whole first set thinking I lost an output tube. Not a huge difference in volume... just thin, bright, and wimpy. Lol.

[Mick Bailey]

6V6s in triode mode work well in a hi-fi amp, but I don't like triode switching in a guitar amp - to me it isn't a good sound. I've lifted the circuit from a VHT 12/20 to act as a power-scale with a 2x6V6 amp and it works well. Needs a gate-stopper and a heavier-duty MOSFET, but very few parts needed for power amp control that goes down to very low levels.

[bob p]

I concur. Triode mode does NOT sound very good. Even though I have amps that will do triode switching, I don't bother with it. I think pentodes into an attenuator sound a whole lot better.

In the big scheme of things, I think that triode mode became popular because it was cheap to implement, and it offered a cheap way for amp makers to claim that their amps sounded better because they were more flexible. That was before attenutators were common. If you've got an attenuator I wouldn't bother with triode mode.

[Pedro Vecino]

In some (few) amplifiers with a very electric, vibrant and dynamic sound triode mode can work well as a complementary mode. Not as a power reducer by itself, but as a way to soften it. For example in the Supersonic Fender (both versions with 6V6 and 6L6). They are the only ones in which I have considered it really efficient. In many others it is only an option with little musical entity.

[Gingertube]

When switching from pentode mode to triode mode you get 3 things happening:
1) approx. 1/2 power
2) The drive impedance into the output tranny drops dramatically and this reflects to the secondary as lower output impedance and better speaker damping. You usually have to drop back he global feedback level (if any) to loosten up the speaker again.
3) The Miller Capacitance at the output tube grids goes up significantly and the phase splitter has trouble driving that extra capacitance so you loose top end (darker tone). This doe not happen with the very few amps (mostly bass amps) which have a decent phase splitter driver stage.

On my own home brew PP 6V6 amp I do switch between pentode mode and ultralinear mode (not triode mode). Pentode mode better for shreading and loud lead and agressive rock, ultralinear mode for blues, folk and vintage rock. Ultralinear gives about 2/3 power with 6V6s. Also I use 20% UL taps not the more usual 43% the HiFi guys prefer.

Cheers,
Ian