I notice that most of the tube data sheets spec tube screen voltages much lower than the B+ (for example, 6L6 at a B+ of 450V with screens at 350V.) Most guitar amps run the screens pretty close to the plates with perhaps just some unbypassed dropping resistors. I understand that if you increase the resistance of the screen resistors, you'll increase screen sag, but what happens if you run the screens off a fixed lower voltage (i.e. a low impedence power source?) I imagine this would be very good for tube reliability, but how about output power and the overdriven sound?
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Effect of lowered screen voltages on beam power tubes
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a lower screen voltage will reduce power output, and it will usually increase tube life (all other factors being equal). having a stiffer screen supply will result in a stiffer response though. the outputs may end up being more for the stiffer supply as the one using a dropping resistor could drop the screen's voltage lower than the screen supply on the stiffer supply. this depends on the exact circuits and voltages. i think amps running on lower voltages tend to have more power coming from the screens, as opposed to the plates.
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Originally posted by Bruce / Mission Amps View PostSome of the harp players I do work for like the screen voltages set up pretty low (by modern guitar amp standards) ... it seems to give them a sense of a bit more squirt or some sag and a softer-greasier tone.
I'm not sure if that is 100% accurate because I don't play harp.
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Thanks for your responses...I'll be interested to learn the outcome of your experiments , Chrome. I may have a report of my own, but that amp project is a month or so off. I strongly suspect there will be a big difference between lowered screen voltage via a resistive divider versus a stiff lower voltage supply, but who knows? I don't, or I wouldn't be asking here!
Nathan
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This 1986 patent by Mesa Boogie founder Randal Smith tells how he used the screen supply to change the "Playing Characteristics".
http://patft.uspto.gov/netacgi/nph-P...MITH-Randall-cNow Trending: China has found a way to turn stupidity into money!
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In a pentode (EL34) or power beam tube (i.e. 6L6) lower screen voltages lower the tube's gain.This lowers the amps gain in toto, and lessens any linearizing effects that feedback (if used in that amp) may be having. It will overdrive less easily, but be tubier sounding.
This won't affect preamp tube distortion, if any.Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!
Oh, wait! That sounds familiar, somehow.
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Couple questions, for an EL34 amp, about how big of a screen voltage reduction would result in a noticeable change in tone & feel? Also, could a zener diode between the screen cap and the screens be a good way to reduce the screen voltage, unlike a large value resistor between the plate and grid supply nodes?
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Originally posted by R.G. View PostIn a pentode (EL34) or power beam tube (i.e. 6L6) lower screen voltages lower the tube's gain.This lowers the amps gain in toto, and lessens any linearizing effects that feedback (if used in that amp) may be having. It will overdrive less easily, but be tubier sounding.
This won't affect preamp tube distortion, if any.
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Originally posted by guitician View PostThis 1986 patent by Mesa Boogie founder Randal Smith tells how he used the screen supply to change the "Playing Characteristics".
http://patft.uspto.gov/netacgi/nph-P...MITH-Randall-c
I think the patent application has the most unintentionally hilarious description of playing an amplified guitar I have ever read. It's like it was written by an anthropologist from an alien planet.
Nathan
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Originally posted by octal View PostInteresting. I suppose I could have deduced that by looking at the curve charts of preamp pentodes, but I didn't! I don't think I've ever seen curve charts for output tubes at different screen voltages.
http://www.freewebs.com/valvewizard1/se.html
and here:
http://www.freewebs.com/valvewizard1/pp.html
and here for EF86:
http://www.freewebs.com/valvewizard2/EF86.htmlBuilding a better world (one tube amp at a time)
"I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo
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Originally posted by octal View PostI notice that most of the tube data sheets spec tube screen voltages much lower than the B+ (for example, 6L6 at a B+ of 450V with screens at 350V.) Most guitar amps run the screens pretty close to the plates with perhaps just some unbypassed dropping resistors. I understand that if you increase the resistance of the screen resistors, you'll increase screen sag, but what happens if you run the screens off a fixed lower voltage (i.e. a low impedence power source?) I imagine this would be very good for tube reliability, but how about output power and the overdriven sound?
The question was for Beam Power Tubes. What I do know is what I read in the data sheets. For example, I could run an RCA 813 beam power tube with a plate voltage as high as 2 KV, but the screens no higher than 750 volts. For non Beam Power Tubes, I just follow the published data sheets, as a start ; then work my way from there. I've found that power tubes tend to be more forgiving on applied voltages, rather than pushing the plate current spec and the plate dissipation spec.
-g
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I found an interesting example of a vintage amp with a larger than usual spread between the plates & screens. The Gibson GA-86 (see attachment for schematic.)
The 6v6 is triode connected and used as a sort of screen voltage regulator, though it actually does not regulate- the grid is biased by a voltage divider comprised of the 22k and 100K resistor, so the cathode will follow B+ divided by the ratio of the resistors. If the 100K was bypassed by a cap, it would be a classic capacitance multiplier, but that evidently was not what they were going for.
Does anyone care to speculate what they were going for?Attached Files
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Originally posted by octal View PostI notice that most of the tube data sheets spec tube screen voltages much lower than the B+ (for example, 6L6 at a B+ of 450V with screens at 350V.) Most guitar amps run the screens pretty close to the plates with perhaps just some unbypassed dropping resistors. I understand that if you increase the resistance of the screen resistors, you'll increase screen sag, but what happens if you run the screens off a fixed lower voltage (i.e. a low impedence power source?) I imagine this would be very good for tube reliability, but how about output power and the overdriven sound?
Lowering the screen voltage pushes the curves down, so now the load line passes above the knee (unless you ALSO increased the load impedance of course, in which case it would sound much the same). If it passes above the knee, you move back into strong 2nd harmonic territory- very 'triode like'. It's great for hifi, but can sound a bit "boring" for guitar. Kinda 'soft and fluffy' (it's hard to describe!), not really 'defined'.
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