Effect of lowered screen voltages on beam power tubes

[octal]

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?

[black_labb]

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.

[Bruce / Mission Amps]

Some 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.

[black_labb]

Some 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.

im wondering if the softer tone is due to the extra resistance dropping the voltage, or due to the lower voltage itself? have any insight?

[Chrome Dinette]

I have wondered about the same thing. Eric Barbour advocates lower screen voltages with a bypass cap to ground in a VTV article about EL34's. I started a similar thread in the "tubes" section. I might have time next week to try this out and report back.

[octal]

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

[guitician]

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-c

[R.G.]

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.

[Russ]

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?

[octal]

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.

Interesting. 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.

[octal]

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-c

Looking at the schematic provided, he's basically designed a bypassed, but deliberately saggy screen supply with a specific time constant.

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

[tubeswell]

Interesting. 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.

Some info here:

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.html

[mooreamps]

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?

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

[octal]

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?

[Merlinb]

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?

Assuming all other parameters remain the same, a guitar amp is normally set up so the load line passes somewhat below the knee of the grid curves. This results in quite a bit of 3rd and 5th harmonic, and makes for a 'hard', 'textured' and 'driven' sound- that's what most people want after all. You've got to be careful with the screen though, 'cause it does have a tendency to burnt out under this condition, unless you limit the current.

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'.