Has anyone tried the tube power attenuation method of tying the plate and screen grid together? Supposedly this converts a pentode to triode operation and reduces power by ~ 1/2. I am wondering if this is indeed effective, and how it affects tone; also if it is damaging to the tube in any way.
Everyone since Adam 😀 Mostly because it creates much less distortion.
Put "triode strapped pentode" into your favourite search engine and get back to us.
The triode mafia have been upsetting the audiophiles since the late eighties by removing the GNFB from their ST-70s and triode strapping the pentodes.
Here's a guitar amp example (complete with sound clips) from only a decade ago - Duncan Amps Blues 112
Only if it's an 807 with 500V on the plate - check your maximum screen voltage. Decent (e.g. STC) tube data sheets will have suggested operating points for triode mode.
There's a couple of other techniques to keep your screen dissipation under control outside the official operating areas - e.g. using some zeners to drop the voltage (see this joelist posting)
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Remember to keep in mind that reducing the power by half only reduces the volume by 3dB which is just enough to hear a difference. You would need to reduce the power by a factor of 10 to reduce loudness by half. Volume as related to power is a logarithmic function so reducing 100 W down to 10 W will reduce volume by half. Many people feel this type of half power switch is more effective at changing tone than reducing volume as it seems to take some of the liveliness out of the higher power sound. I built a 'Wreck Express clone and with the 6V6s in, it still gets too loud for most clubs when cranked. Of course the 6V6s are ~1/2 the power of the EL34s it also takes.
Triode wiring a guitar amp WILL affect the tone, but how, and how much, depends on a lot of things, the most important is the speaker system, and how it sounds when driven by a much lower source impedance.
A pentode has a rather high plate resistance, and attempts to drive the speaker with current. The damping factor is low and there isn't much control over cone movement and overshoot. This can be improved with some negative feedback, which is often somewhat adjustable via the "presence" control.
A triode has a much lower plate resistance and attempts to drive the speaker as a voltage source. The damping factor is higher and there will be more cone control. This is beneficial to most HiFi amps, but can remove some "life" from a guitar amp.
You don't often see triode connected guitar amps because they can sound somewhat like a solid state amp, at least until cranked.
Want less power, look into a VVR. This lowers the voltage on the tubes, cutting power. Most designs use a VVR to reduce the voltage to the entire amp. There can be some benefit to wiring the VVR just to the screen grids of the output tubes, just to the PI, or both.
In any case the only way to see if any of these concepts suit your speakers, playing style, and volume levels are to try them. What works great for someone, sucks canal water for the next guy!
I would be really interested to hear the difference between an amplifier using VVR and full power. I don't really have a good idea on how speaker distortion affects the final output. Are there any good A/B recordings to listen to?
Is there a limit to how low the voltage can go on a pentode output stage?
Is there a limit to how low the voltage can go on a pentode output stage?
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What speaker(s) were you driving with the 2 x 6V6s? Were they putting out ~22 watts, like a Deluxe Reverb does?
Surprising that it was too loud for a club!
That was with Celestion Vintage 30's in a 2-12" cab. Remember, those 22 watts is what it's rated at for mostly clean power. Since that amp is non-master and relies largely on power tube distortion for it's unique sound, it puts out a higher amount of power when overdriving those 6V6s. But you make a good point since a player could use a less efficient speaker or a 1-12"cab for reduced volume. I haven't found a 1-12" that gives me the sound I like but am looking for one now. Sensitivity is one spec many players forget about. Most guitar speakers are rated for ~98 dB @ 1 watt @ 1 meter. So even with only 1 watt applied, a dB meter would measure 98dB at a distance of 1 meter.
This link gives examples to reference how loud that is: Comparitive Examples of Noise Levels | Industrial Noise Control
Read post #4, middle paragraph of this thread. He uses 6V6s and still adds a power brake sometimes for clubs: https://www.jbonamassa.com/forum/viewtopic.php?id=9915
I guess the point is that 1/2 power switches don't reduce overall volume as much as you might think. And clubs don't want the band to be as loud as they used to.
This link gives examples to reference how loud that is: Comparitive Examples of Noise Levels | Industrial Noise Control
Read post #4, middle paragraph of this thread. He uses 6V6s and still adds a power brake sometimes for clubs: https://www.jbonamassa.com/forum/viewtopic.php?id=9915
I guess the point is that 1/2 power switches don't reduce overall volume as much as you might think. And clubs don't want the band to be as loud as they used to.
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