Anyone try the "Hazen mod" to G3 supressor grid

[Bas Horneman]

in tubes where g3 is not internally connected to the cathode?

DECWARE - The Hazen Mod

[planet10]

Bas, you beat me to this.

So far it seems to be a positive sonically (i haven't heard it, but Chris did it to his Jolida PP EL34, and Geek is trying it in his/my DynaMutt). Daniel has concerns about the undefined DC operating point and what happens when things go sideways.

I posted a similar inquiry on the JoeList and there has been one interesting reply. A similar concern with DC op point, but also some speculation about what is happening.

A high value R from suppressor to ground (or to a negative bias*) could be the fix for the DC op point.

The suppressor to negative is a technique that has been touted by Bill Perkins for some time, and Eddie Vaughn pointed out that this was used with good results in a Traynor "copy" of a Marshall from the 70s.

dave

[planet10]

Here is Phil's JoeList post

Well it's certainly damned interesting, although it would have been nice if they had revealed the voltage that the suppressor ends up with. It certainly is the case that manipulating the various screens can positively affect the sound; I believe Steve Bench said something about manipulating the "extra" grids to get a better sound in the 6HA5, or maybe 6GK5 (6GU5?) tubes, which are officially triodes, but use extra grids to help their performance. Raising the suppressor of an 803 125 watt pentode to +40 V makes the output ruler flat down to amazingly low plate voltages.

It SOUNDS like they've found a way to move the suppressor to its "natural" operating voltage, the point where the secondary electrons from the plate no longer make the suppressor more negative. However, I would be scared to death that it would simply charge beyond the cap's voltage limit, resulting in a rather big POW! somewhere. It's almost like a grid-leak in its operation, and it sure SEEMS like you need a parallel resistance to keep the voltage from going too far negative. Maybe they have one and just didn't say so? Maybe you could get the same result -- or even a better result -- simply by giving the suppressor a negative voltage adjusted by ear to sound the best? Once you found that point, you could use a parallel resistor of the right value to hold that voltage, or even a zener diode. At high frequencies, the cap looks like a wire to the cathode, so whatever is happening is doing so at DC to low frequencies. VERY COOL, regardless, and shows that we've still got lots to learn!

Phil

I also have some SV83 (6P15P) on order so we can try this on our El Cheapo variant.

dave

[pmillett]

Being the engineer I am, I have to wonder if this "sonic improvement" they are hearing is simply an operating point shift. I would like to see the G3 voltage and plate current, before and after...

Pete

[planet10]

It could well be... the negative bias thing requires a rebias.

dave

[funkytek]

Is it really shifting the operating point?
Does the suppressor grid really have much effect on the DC op point?

The cap is in series with the suppressor grid, which is normally tied to the cathode. This takes the suppressor grid out of the circuit at DC and low frequencies depending on the cap value (which they don't specify).

It seems like it's function (the suppressor and this mod) is a large signal AC phenomenon. The cap tunes in when the suppressor takes effect, in frequency.
You are in effect tuning the maximum energy transfer in the frequency domain.

Kind of kinky

[planet10]

Deckert & gang are using 0.1 uF, Eddie suggested going as large as 1 uF.

dave

[Sch3mat1c]

W...T...F?

This is the audio equivalent of foot binding. It has no practical effect, may be detrimental, and serves only to entertain those silly enough to follow the fad.

[smoking-amp]

Varying g3 voltage on a dual control pentode certainly has some effect. These have a more dense g3 winding than normal pentodes, so the effect should be larger there. Positive voltage on g3 for a 6LE8, for example, squares up the characteristic curves (+12V to +15V on g3 giving very square Mosfet like curves, but go too positive and the g3 sucks up current itself). A negative voltage rounds the corner off the curves.

http://scottbecker.net/tube/sheets/135/6/6LE8.pdf

A positve g3 also lowers screen current since more electrons make it to the plate. A negative g3 will raise it, maybe dangerously. I would suggest not only measuring the g3 voltage with the g3 cap, but also check g2 current.

Don

[GK]

Using a feather is kinky. Using the whole chicken is perverted.

How about just half the chicken?