voltage divider for screen grids

[Andy Le Blanc]

thank s for the posted scheme, that series regulator looks wonderfully simple.

[tubeswell]

Here are some others examples of G2 regulation. Those ones use an EL34 and half ecc83 to do the job.

http://mundomusicalmeria.com/mundomusic ... n_2500.jpg

http://mundomusicalmeria.com/mundomusic ... 000_vr.jpg

Wot Andy said.

[pula58]

The amps great, very responsive, it warmed up and got more "tube" like.
I played around with all sorts of divider arrangements etc... my only goal was get the difference between the plate a screen to 0 or -, the tone quality was a surprise. I did notice that you can shut off the tube with the screen grid, if you keep going with it. It was a surprise realization that the screen where positive at idle, you look at the data sheet and you NEVER see that.
End up making assumptions for years, glad to get a handle on a measurement that reveled it. I've measured it in all the fenders that come across my bench for service too, so far it seems very consistent.

Hi Andy, are you saying that on all the old Fenders you've had on the bench that (at idle) the screen voltage (at the tubes screen pin) was actually higher than the plate voltage seen at the output tubes?

This must mean that the OT primary series resistance is pretty high, because how else could the plate voltages be lower than the screens? That is, the screen supply can't be higher than the amps B+ ( to which the OT CT is connected) because it is "downstream" of it. So, since the screen supply voltage must be at least a little bit lower than the B+, the plates must be dropping volts across the OT's series primary resistance for the plate voltages to get below the screens.

[ChrisM]

The amps great, very responsive, it warmed up and got more "tube" like.
I played around with all sorts of divider arrangements etc... my only goal was get the difference between the plate a screen to 0 or -, the tone quality was a surprise. I did notice that you can shut off the tube with the screen grid, if you keep going with it. It was a surprise realization that the screen where positive at idle, you look at the data sheet and you NEVER see that.
End up making assumptions for years, glad to get a handle on a measurement that reveled it. I've measured it in all the fenders that come across my bench for service too, so far it seems very consistent.

Hi Andy, are you saying that on all the old Fenders you've had on the bench that (at idle) the screen voltage (at the tubes screen pin) was actually higher than the plate voltage seen at the output tubes?

This must mean that the OT primary series resistance is pretty high, because how else could the plate voltages be lower than the screens? That is, the screen supply can't be higher than the amps B+ ( to which the OT CT is connected) because it is "downstream" of it. So, since the screen supply voltage must be at least a little bit lower than the B+, the plates must be dropping volts across the OT's series primary resistance for the plate voltages to get below the screens.

That is most likely the case. I've noticed this with Deluxe Reverbs.

In some Fender the OT primary impedance is 8K which can lead to a "fairly" large DC resistance.

This coupled with the fact that at idle the screens are really not drawing any current it makes sense that the screen voltage could be a couple volts higher than the plate voltage. The screen dropping resistor or choke sees very little voltage across it (if any) at idle.

[Andy Le Blanc]

nearly every fender across the bench since first noticed it.
And when you apply signal the condition flip flops making a fizz when you push it into distortion.
Keeping the screens slightly negative cleans up and warms up the amp,
I suspect it alters ratios of even/odd harmonic distortion in the push pull circuit.

measure the difference between the plate and screen and the socket.

[ChrisM]

EDIT: See my response below

Tried this in a 6V6 18W today.

The amp has a choke and resistor in series leading to the screen supply. With this the screens were already a bit lower than the plates.

6V6's had 470R 6.5W screen resistors. Added 220K from screen to ground. Used a SPST switch to break the ground connection for both resistors. This allowed me to A/B the voltage divider mod.


Voltage divider dropped about 10V, cool.
Tone wise there was more compression, bit less gain and bit less top end with the screens lowered. Cool little mod (very subtle) but not what I was going for in this amp.

In the big Fenders (who kill the tube's with HV) and are a tad bright I think this would be a great mod.
For my 18W amp I was already using 6V6's, not a super high B+ and a tube recto so the tonal affects of the mod didn't do it for me.

[Colossal]

Tried this in a 6V6 18W today.

Voltage divider dropped about 10V, cool.
Tone wise there was more compression, bit less gain and bit less top end with the screens lowered. Cool little mod (very subtle) but not what I was going for in this amp.

Cool report Chris. I'm using 1k/220k in a 4xEL84 amp with a TW power supply. I like the tonal effects of it and I think the highs are pretty well balanced in the amp. I do notice the subtle but audible increase in compression. Seems like added insurance where you want to enjoy the tonal benefits of very low screen resistance but need to ensure a braking system for those moments where we must, diverge from linearity

[Andy Le Blanc]

hey alright, I'm glad your checking it out.

I usually end up after the simple mod with the screens around the neighborhood of 5v less than the plates, I do like the tone best with the screens just a few hundred mv less.
there is a range in there to be explored, so if the screens are a tad hot you can tune it in, once there is a bleeder you can get after the value of either
the screen grid resistor or the bleeder and be as critical as you want.

[ChrisM]

UPDATE:

I actually discovered last night I had my screens connected to the PI supply As a result my screens were way lower than they should have been (about 80V too low but the tone sounded great). I have sinced fixed this issue; voltage wise the screens and plates are about 1V apart with the screens connected to the actual screen supply. Tone wise the amp is kinda stiff, bright and harsh.

Cathode wise the voltage has changed now that the screens have been corrected. When the screens were connected to the PI supply the voltage was 18.5V now with the screens connected to the actual screen supply the cathode voltage is 24.5V.

Now regarding the somewhat stiff, bright, harsh tone I tried the voltage divider screen mod (seems like a good time). This mod totally fixed things up! Now the screens are 6V below the plates (344V and 350V respectively). The tone is smoother and livelier. After doing the voltage divider mod the cathode voltage came down a bit to 23.3V.

Next up I am going to add a 25V Zener in parallel to my cathode resistor and cap. This is part of the Paul Ruby fizz trick. I did the mod before with a 20V Zener when my cathode voltage was 18.5V. Though I have not experienced any fizz I think having the Zener mod will be good to have. I imagine with my ES-335 and the amp cranked this mod could be good insurance.

My 6V6's cathode is 271R with a 560uF (measured) bypass cap. Pretty stiff (fixed bias) cathode combo. The 25V Zener is 5W btw thought the cathode resistor is only dissipating ~2W.

I also have my plate dissipation hovering right around 100% so right where I want it. Calculated it: 107%

[ampdoc1]

I just glommed onto this discussion tonight. I described the problems I had with a Vox NT15 in another thread. Is there any likelihood that adding this to the amp would further diminish the harshness/dying note distortion? Adding larger snubbers on most of the tubes reduced it a lot.

Thanks,
ampdoc1

EDIT I wasn't thinking this thru. Would this be applicable to a cathode bias?

[jelle]

Ampdoc, yes it is. jelle

[iknowjohnny]

Tried this on a JCM 800 type amp and it seemed like it DID add clarity, but at the expense of smoothness. Tone seemed a bit harder. Just added 220k's to ground from each screen on the socket leaving the 1k screen resistors the same. For me at least I preferred it back to the the way it was.

[Structo]

Hate to beat a dead horse but this still intrigues me.

Yesterday I was working on my 100w ODS clone and measured the difference between the screen and plate.

6L6GC, 500 ohm screen resistors.

Power supply has a choke between B+ and Screen node.

It was difficult to measure because the value seemed to jump all over the place, but approximately -450mv.

I believe I had the black probe on the screen and red on plate.

So basically 1/2 volt.

Is this correct? Black on screen, red on plate?

Should that give a negative value?

Any ideas why the value would jump around?

[martin manning]

Tom, that would say that the screen is higher than the plate. It's easier to measure plate and screen voltages to ground. Try that and see if they are stable.

[Structo]

Yes, when I have measured the voltages to ground the plate and screen are about the same.

Andy had suggested measuring them at the socket to see the difference in potential between them.