Anyone connect suppressor grids to bias?

[GerryJ]

....instead of cathode? I saw this on

https://web.archive.org/web/20031205141 ... sson11.htm

It was posted on another board about reliable tube amps, & old Traynors came up. I have several, & while the Bassmasters (both YBA1 &1A) wire conventionally (EL34 or 6CA7, pin 1 g3 & pin 8 cathode) , sure enough my YBA3 Custom Special schematic shows pin 1 g3 getting a feed from the bias supply. As you know, these are high voltage amps (~540 v plate, 530 v screens). Beefing up suppressor grids voltage was supposedly thought up by Garnet Giles & Pete Traynor as a way to minimize current traveling back from the plate, & thereby prolong tube life (presumably thru protecting the screen grids?).
I was curious if any of you use this in your mods or builds- thanks.

[imjonwain]

I've connected the suppressor grid to GND in cathode biased Amps before, which is more or less the same idea. Make g3 negative relative to cathode. I believe there are a lot of old threads on here about that. I changed over to a KT88 in that build and don't remember what my conclusion was lol.

[pdf64]

I’m not aware of valve manufactures publishing anode characteristics for g3 referenced to anything other than its cathode; without that, how to draw a loadline?

[wpaulvogel]

I talked to Chris Merren about this, I was discussing how true pentodes squish roundly at clipping vs beam tetrodes which have sharper corners. He suggested playing with the suppression grid voltage to get even more rounding. I haven’t made any attempt yet but I have a custom circuit EL34/6ca7 amp that I could play around with. I don’t think doing it to a Marshall would be clever until confirming some results.

[martin manning]

Here's what happens if G3 is tied to G1. The curves get a bit less kinky at low (most negative) Vg1 and low anode voltages. I think that makes sense given the function of the suppressor grid. Otherwise they are very similar. Note this trace connects G3 to G1, which is not the same as connecting G3 to fixed negative bias voltage. In the kink region, however, it's similar.

[FUCHSAUDIO]

I asked one of my mentors, the late Ted Hammond about this. He basically said: "it's free, and can't hurt anything". Traynor did it and I do it on EL-34 based mods and production amps. Supposedly it makes things "sound better" the curves Martin put up might back that up. Whether it adds reliability, remains to be seen (how would you really know), but I do see very few amps come back with blown power tubes after doing that. You can clearly see the bias current change if you put a pot on the grid and swing it from ground to raw negative bias value. I never tried it in anything else (6L6's and EL-84's have the suppressor tied to their cathodes internally), but with EL-34's I like it !

[thetragichero]

i did it once, mainly because it was convenient in my layout. didn't blow anything up so that was cool

[bepone]

g3 should be connected to +5 to +25V ref to cathode, to increase available power. info you can find in about every better pentode datasheet. per example 2E22 where they provide curves for positive g3

[pdf64]

g3 should be connected to +5 to +25V ref to cathode, to increase available power. info you can find in about every better pentode datasheet. per example 2E22 where they provide curves for positive g3

Might you be making an apples to oranges comparison?
That may be the case for a valve type intended for use as a class C2 oscillator, but for valve types intended for AB1 audio amps, surely the key constraint for max available power output tends to be the anode dissipation.
It’s simple to increase power output if that’s ignored.
So why bother with an additional power supply for g3?
Surely the great benefit of the modern pentode types used pretty much exclusively in guitar amps is that they provide high output with simple circuits? No point / benefit in getting any more complicated.

https://tubedata.altanatubes.com.br/she ... 2/2E22.pdf

[martin manning]

g3 should be connected to +5 to +25V ref to cathode, to increase available power. info you can find in about every better pentode datasheet. per example 2E22 where they provide curves for positive g3

I can't find any EL34 data sheet that has information for Vg3 < or > 0V.

[bepone]

respond to both:
you can see how the top kink is increased, angle too, alowing a little bit more curve to be extracted in PP loadline setted to pass through the kink. so i never seen this for EL34 but why for EL34 or any other pentode would be different effect? can be traced by hand.

[pdf64]

respond to both:
you can see how the top kink is increased, angle too, alowing a little bit more curve to be extracted in PP loadline setted to pass through the kink. so i never seen this for EL34 but why for EL34 or any other pentode would be different effect? can be traced by hand.

But what is the benefit specific to DC elevating g3, ie that can’t be equally well achieved by other means; so why bother?
The limit of anode dissipation still constrains what is achievable in the various operating classes.

[pdf64]

I've connected the suppressor grid to GND in cathode biased Amps before, which is more or less the same idea. Make g3 negative relative to cathode. …

As I see it, in class AB, won’t the shift in cathode voltage with signal level also cause a shift in g3 voltage, thereby exacerbating cathode bias ‘squish’?

[mhuss]

I could have sworn that I read somewhere (but cant find it right now) that making the suppressor grid negative helps limit max plate current in extreme situations, which makes a little sense intuitively (and is supported by Andy's anecdotal evidence). Since the grid's primary purpose is to direct electrons 'bounced off' the plate/anode (due to impact velocity) back to the plate/anode, a more negative suppressor would likely be more effective at this primary purpose as well. Perhaps this also keeps the stray electrons from heating up the screens further too.

Since Traynor mostly used 7027s (which are beam/KT type), he didn't bother with it on most of his amps.

I don't think it has anything to do with improving the curves or increasing the output power.

[martin manning]

The suppressor is there to improve the curves by reducing the tetrode kink. Evidently it works a little better if its potential is below, as opposed to the same as, cathode voltage.