Pentode/Triode switch implementation with an on/off/on DPDT?

[aiyiadam]

I am planning to add a pentode/triode switch to a Mesa Mark III Blue Stripe (Simul-Class). Am I right to assume that I can use a DPDT switch with a disconnected middle position? Is it really safe to let the screen grids of an EL34 float?

[Enzo]

Float? No. Typically it is done by using the switch to connect the screen to either the screen supply node or to the plate.

[aiyiadam]

Thanks for the reply.. That is the usual implementation with a typical on/on DPDT switch. I have only an on/off/on DPDT switch, which I can use at the moment. And I assume that the switch will occasionally be in the middle position where the screen grids will float.

[pdf64]

I think it would be ok, floating screens will just bias the output valves fairly deep into cut off. It would sound awful if at all, but no harm done.
Some amps implement standby in a similar manner.
Report back if you ever end up actually using triode mode. I recall some people find it useful, but I suspect they’re a tiny minority.

[Chuck H]

I think it would be ok, floating screens will just bias the output valves fairly deep into cut off. It would sound awful if at all, but no harm done.
Some amps implement standby in a similar manner.
Report back if you ever end up actually using triode mode. I recall some people find it useful, but I suspect they’re a tiny minority.

I thought the same thing regarding the alternative "standby" mode. This could surely simplify the switch location. If the standby current mode were disabled in favor of this alternative the triode/pentode switch could be located where the current standby switch is rather than drilling a hole somewhere else for it. The middle position of the switch is then "standby" and the upper and lower positions would be triode or pentode respectively.

[pdf64]

Save panel space - combine 2 useless switches in one

[aiyiadam]

Personally I don't really like the triode operation of pentodes.. However, in this special case the pentodes are hardwired as triodes by Mesa and the amp owner wants to have the pentode option. This amp seems to have a relatively small power tranny and it might become to hot in the pentode mode. I don't know. We'll see I guess..

The other subject regarding saving space.. I am using progressive DPST switches from Carlin, which let me combine the stand-by and power switches...Mine are rated for AC, but I didn't have any problems until now..

[Leo_Gnardo]

Report back if you ever end up actually using triode mode. I recall some people find it useful, but I suspect they're a tiny minority.

FWIW I've install lots of triode/pentode switches, probably about a hundred by now. Triode mode does 3 things: reduce gain about 6 dB, roll off high frequencies, limit power. I ALWAYS advise customers before advancing with this mod. I do not recall anyone asking to have it removed except in a couple of cases (two) where they wanted the amp "returned to stock" so they could sell it with no mods which might reduce its value.

The reduction in power, that's expected, that's why this mod is done. No problem. Triode mode power typically is 30-35% of power in normal operation, a noticeable cut.

Decreases sensitivity? Now you have to dial up your volume to get back to the same gain - which most often gets you into a nice overdrive spot. Although - some might object to resetting off their "favorite numbers." Oh well...

High frequency rolloff? Some might claim triode makes their amp sound mushy. Here again, a touch to the treble and/or presence knob can put some sparkle back, but that might not satisfy some, especially those who have their "favorite numbers" and anything outside that zone is unacceptable. Overall though, I'd say the reduction in high frequency response makes for a more ear-friendly overdrive tone.

One of the best reviews I got was from an elderly fellow @ 70, who played guitar in a local blues band. "They're about to kick me out! They say my Deluxe Reverb is just too brash for what they're trying to do." Triode mod, then "My band's impressed! And I love the tone. You saved my career!" Well, a career in a hobby band, but that's still something. Who needs that blow to the ego at 70, to get kicked out of a band.

Some amps have triode mode built in, but don't call it that. Marshall Silver Jubilee amps for instance. That 50/25 or 100/50 switch, that's exactly what they do.

Right triode isn't for everybody. So know what you're getting into before making the jump. And be prepared to back out of it if you think it stinks.

What I'm curious about - in a Simulclass Mesa amp, if you choose so-called "class A" which is definitely power-limited, then bring on triode mode, now you can expect likely 5 or so watts out of the behemoth amp at clip. But who knows? The tone might be "to die for." So . . . I"ll be scanning to see how it turns out for you aiyiadam.

[Chuck H]

I had a triode/pentode switch on a little MB Subway amp. I like it, BUT... I have it on a 3-pos rotary switch that has pentode-triode-triode with a change to cathode bias. The change to triode with cathode bias makes a significant and useful change to the tone and feel of the amp. I was not much inspired by the triode mode with fixed bias option and never used it.

Another concern besides the power transformers rating for the amp in question might be the OT primary specs. Since the amp is currently hardwired for triode mode for two tubes it's possible (probable) Mesa adjusted the OT primary impedance for more ideal operation with this circuit (this could be tested). Ideally a triode operated pentode would use a significantly lower primary impedance. Though this is never a consideration with typical triode/pentode switch mod I think Mesa would have been aware of it. So it seems possible that the existing OT primary impedance could be lower than ideal for all tubes operated in pentode. This could further exacerbate current draw through the PT which was already a concern.

I honestly don't think either transformer will be problematic with the changes proposed. Just putting it out there for consideration.

[g1]

Interesting that this model uses different bias voltages depending whether domestic or export version.

[aiyiadam]

This is an interesting amp indeed..

I installed the triode switch.. The pentode mode is a bit louder and it sounds better to me. It is nice to have this option on the amp. In the middle position of the toggle switch the tubes are turned off.

The power transformer seems to be a bit struggling even in the triode Simul-Class mode. I don’t know the details, but it is probably not the original tranny. And yes the primary impedance will also change.. I wouldn’t push that amp very hard..

Thank you for the replies and let me also add a few photos..

[pdf64]

I don’t think that the OT primary impedance will change, whatever the output valve configuration / arrangement. Rather that is set by the speaker and the turns ratio. Actually, if it’s the original OT, it would be interesting to know the turns ratios.
What does change is the output impedance, which might be expressed as speaker damping, ie the ratio between the load impedance and the output impedance.
Output impedance is unrelated to the intended load impedance.
I suspect that it may be the reduction in output impedance in triode mode that I dislike. That will tend to counteract the amp - speaker interaction, which might for me be something I like.

[catalin gramada]

Pentode/triode mod= swap screen grids to plates / reconfigure the power supply to benefit by clc filtering/ bias changing/ beefing the driver stage to proper drive the power tubes/ nfb change. I don't think all of these may be done by a switch. If you don't adapt the circuit to triode tube configuration you get nothing but poor results at half power output.
It is nothing wrong with converted pentode tube to triode connection. Wrong is to drop a triode into a pentode designed circuit.But we are talking about a guitar amp meant unexpected effects, although predictable, may work for playing interest.

[Helmholtz]

Yes, triode mode increases speaker damping.

Another essential difference is that triode ouput has no compression like pentodes/tetrodes in typical circuits.
So triodes will sound/feel stiffer. Similar with UL operation.

[kg]

i've used an on-off-on switch to toggle between triode and UL mode for 20+ years in the BAGA.

i don't linger in the middle position, but have never had any issues switching while 500+ V are on it.

jm2c
ken