deluxe screen voltage

[mirlo02]

hi,sorry for my english,i have a 65 deluxe reverb and the screen voltage is 2 volts more with the plate,the tubes are new and biased at 70% and the filter cap new,i dont like this,but this is normal in the deluxe reverb?thanks

[jjman]

It is normal and I don't like it either. You can increase the screen resistors to 1k. I did this but the voltage is still 2volts higher than the plate. 429 vs 431 for my '71 DR.

I measured the voltage drop across the 1ks and it is only 0.98volts. So my screen current (at idle) is only 1ma. I have not measured these figures with a signal.

My assumption is that the taboo situation is not a problem since so few of the electrons are flowing into the screen (1ma.) It seems that many Fenders of the BF/SF era would be like this and I don't see people saying that it's causes any real problems.

[Structo]

You know, I am running into the same thing on a 6V6 amp I built.

It started as a Deluxe 5E3 then morphed more into a plexi circuit.
But I am using a similar power supply setup as the Deluxe.

I have a 40-20-20 JJ multi cap.

So mine has 40uf which the 6V6 plate voltage comes off of, then a 5 H 95ma choke which the screen supply comes off of.

I even added a 1K resistor from the choke to the screens in an attempt to lower the screen voltage and it is still just a couple volts less than the plates.

The screen resistors are 470R.
So either it is just a characteristic of a 6V6 or the way the power supply is.

I haven't noticed any ill effects from it, but it is strange that we are all having about the same problem.

BTW, my 6V6GT tubes are 1958 Sylvania's so they are not current production tubes.

[Alexo]

I think you can expect this behavior in just about any amp where the screen supply is choke-fed, and you're using beam power tetrodes.

Apply ohm's law:

Let's say you're biased to 25ma per tube, and the primary DC resistance of your output transformer is 200 ohms, this will put your plate voltage at .025 x 200, or 5 volts below the B+.

Now say you have 10 ma flowing through the choke, and the choke has 120 ohms DCR: you're dropping the screen voltage supply by .01 x 120 = 1.2 volts. And 1 ma through the 1K screen resistor for another 1 volt drop.

You've dropped the plate voltage by 5 volts from the initial B+ and you've only dropped the screens by 2.2 volts. It'll happen every time, unless you have a very high DC resistance choke or screen resistor and or a very low resistance OT. OR if you're running true pentodes, like EL34's, that have higher idle screen current, which will drop the screen voltage more across the screen resistor.

In practice though, as many old amps will attest, it's OK. Personally, I think this actually improves the tone a little. YMMV.

[LOUDthud]

If you look at the plate and screen voltages with a scope, things look a little different.

Note: This can burn up your scope probes if you probe at the tube socket because the plate voltage will swing up to nearly twice the B+ voltage under signal conditions. Best to just look at the B+ and screen supply if your probes can handle that voltage.

At idle, the screen supply is nice a steady DC but the B+ will have 20V or so of ripple that goes above and below the screen voltage. The L-C filter for the choke supply really takes the average of the ripple. In a class AB amp, when you play loudly, the ripple increases and the screen supply drops down because of the averaging effect of the L-C filter. As the plate voltage swings down to the clipping region, that's where screen current really increases and you'll see a dramatic drop in screen voltage due to drop across the screen resistor.

A couple of volts higher screen voltage at idle is nothing to be concerned about.

[mirlo02]

thanks very much,your response is very instructive

[tubeswell]

Adding a 1k dropping resistor from the screen filter node to the screens won't do much (as one of the other posters noted - its only about a1V drop). I built a custom 5E3 for 6V6 or 6L6 with a switchable resistor/choke filter and I got good results with a 4k7 resistor with the choke engaged - gave about 20V difference between the screens and the plates. Use a 5W cement resistor.

FWIW here is my schematic

[Structo]

Thanks for that.

Yeah I thought it was weird that it only dropped it a little bit.

I guess I should do more math and less guessing.

On your Deluxe what are you getting on the power tube plates?

I'm still working on getting the bias right on my 6V6 amp.
If I recall I have around 336v on the plates.

[tubeswell]

On your Deluxe what are you getting on the power tube plates?

With the 6V6s and a 5Y3Gt I got around 354VDC at idle (from memory).


My notes with the 6L6s

5E3 with 6L6CG and GZ34 voltage notes

Switchable CLC / CRC filter

2 x 6L6CG with GZ34 running into an 8K load resistance

PT Spec
325-0-325VAC 180mA primary (with a 15V reverse biased zener making ~310VAC?)
6.3V 4A heater CT
5V3A recto

B+ = 412VDC

With CLC filter (150R 4H 50mA choke and 20uF caps)
Plate = 404VDC
Screen = 379VDC
Cathode (255R 10W) = 30VDC

Cathode to plate = 374
OA Tube current = 30/255 = 0.1176A divided by 2 = 58.8mA per tube x 374 = 22W

4k7 screen grid supply (measuring 4640R) drops 25VDC, which divided by 4640 = .00538A = 5.4mA in total for both screens, x 374VDC screen voltage at that point = 2.02W dissipation

With CRC filter (4k7 5W resistor and 20uF caps)
Plate = 408VDC
Screen = 364
Cathode (255R 10W) = 29VDC

Cathode to Plate = 379
OA Tube current = 29/255 = 0.1137A divided by 2 = 56.9mA per tube x 379 = 21.55W

Sounded very nice

[Structo]

Thanks, that does help.

[FunkyE9th]

tubeswell,

I was looking at your schematic and noticed you have the standby on the center tap. What's the advantage of doing that over having the switch between the rectifier and the 1st filter cap? Is it so that the switch does not see high voltage? Also what is the zener for?

Thanks,

-FunkyE9th

[Structo]

Not sure why the standby is there but the Zener there is a trick to lower overall B+ when it is too high.
I did the same thing on my Hammond conversion amp.

I have seen the HT fuse on the center tap but not the standby.

[FunkyE9th]

Since switches are typically rated below B+, I was thinking maybe it was moved there so the rated voltage is not exceeded?

Shouldn't the zener point the other way to drop B+?

[FunkyE9th]

Never mind... My mistake. The zener is pointing the right way.

[tubeswell]

tubeswell,

I was looking at your schematic and noticed you have the standby on the center tap. What's the advantage of doing that over having the switch between the rectifier and the 1st filter cap? Is it so that the switch does not see high voltage? Also what is the zener for?

Thanks,

-FunkyE9th

Yes the stdby does not see a very high voltage there.

And yes the zener takes down the B+ (and it is the correct way around for this). FWIW it is probably better to use a couple of 9V 5W zeners in series here if you have an amp with pushes the zener close to a 5W dissipation rating, but on this amp the zener only see about 2W-3W, so a 5W zener is okay.