Screen volta6H6ge - JJ 6V6

[pjd3]

Hello,

Hope you might give some insight on this.

On my current amp build (stereo 6V6/plexi pre) with the PT I am using (330-0-330) and with SS rectification I am expecting to see around 460 on the plates, or close to that up or down. If Screen voltage is the usual "few volts lower that the plate" I might expect around 455vdc on the screens.

This is above the 450vdc max as stated in the JJ 6V6 data sheet, so, in the name of respecting data sheets, is it feasible to place a zener diode in series with the screen and screen resistor, and drop the screen voltage by 15 or 20 volts without corrupting the tone I may get without lowering the screen voltage?

This something in my 3 or 4 amp builds that I never experimented so cannot draw any opinions of.

Is this something worth doing or can I take a chance with 460 plate/455 screens on the JJ 6V6's? One thing I can be certain of is that this amp is a strictly pedal platform that will never come near pushing the output tubes, master volume all the way up, preamp volumes at 9:00.

How would you handle this?

Thank you very much for any input.
Best,

Phil

[maxkracht]

I wouldn't worry about it. Voltage drops across the screen resistor when you play, should keep things in check.

[Roe]

On my current amp build (stereo 6V6/plexi pre) with the PT I am using (330-0-330) and with SS rectification I am expecting to see around 460 on the plates, or close to that up or down. If Screen voltage is the usual "few volts lower that the plate" I might expect around 455vdc on the screens.

This is above the 450vdc max as stated in the JJ 6V6 data sheet, so, in the name of respecting data sheets, is it feasible to place a zener diode in series with the screen and screen resistor, and drop the screen voltage by 15 or 20 volts without corrupting the tone I may get without lowering the screen voltage?

what is the reflected primary impedance and the (shared or individual) screen resistance? Often, it is enough to increase screen resistance. Increasing the individual screen resistors changes the reponse more than adding a shared resistor.

[pjd3]

Hi Roe, thanks,

The OT is 6.6K primary impedance into a pp pair of 6V6s. Each 6V6 would have its own 470 ohm resistor.

I imagined that a zener diode in series with the 470 ohm resistor might serve to bring down the "no-signal" voltage by 10-15 or so volts, and the 470 resistor would create the ratio of voltage and the screen as it usually does when signals become present and you get the usual voltage across the 470 ohm resistor.

Thanks,

Phil D

[pjd3]

Thanks max,

I was precariously hoping that might be an option!

Best,
Phil D

[Stevem]

I have used JJs in Highwatt 50 models with 500 volts on them.

[Colossal]

I have used JJs in Highwatt 50 models with 500 volts on them.

Yep. I had a 6V6 Plexi running 460V on the plates and no issues with the tubes. They took a 22W beating in stride.

[Roe]

Hi Roe, thanks,

The OT is 6.6K primary impedance into a pp pair of 6V6s. Each 6V6 would have its own 470 ohm [screen] resistor.
....

Thanks,

Phil D

What is the resistance between the plate filter cap (460v) and screen filter cap (45xv?!)? Is it just a low-resistance choke (e.g. 110 ohms, 3h, marshalll style choke?)
6k6 and 460v might work with JJs but other 6v6s may redplate.

[lonote]

What is the resistance between the plate filter cap (460v) and screen filter cap (45xv?!)? Is it just a low-resistance choke (e.g. 110 ohms, 3h, marshalll style choke?)
6k6 and 460v might work with JJs but other 6v6s may redplate.

If you have just a choke there at the moment, you can add in a dropping resistor in series with the choke to lower the voltage a little. That of course affects the voltage on all the subsequent downstream nodes.

[Roe]

What is the resistance between the plate filter cap (460v) and screen filter cap (45xv?!)? Is it just a low-resistance choke (e.g. 110 ohms, 3h, marshalll style choke?)
6k6 and 460v might work with JJs but other 6v6s may redplate.

If you have just a choke there at the moment, you can add in a dropping resistor in series with the choke to lower the voltage a little. That of course affects the voltage on all the subsequent downstream nodes.

+ 1. this tends to work better than increasing the individual 470r resistors, keeping more of the original response and attack. Kelley seems to have used 1k and 2k on a quad of 6v6s

[pjd3]

Hi, and big thanks all, good info here as usual.

Yes, from first filter cap to screen filter cap it is a typical 5H , around 100-150 ohm choke.

I sure could try fitting in a resistor there and just adjust upstream resistors to maintain preamp tube plate V should I see any trouble brewing. But from what I'm hearing here, it looks like it may be a go with the JJ's. But, this is certainly something I knew to keep and eye on, because most seem to agree that apart from JJ's, 6V6s may not fend so well with these voltages.
One thing for sure, is that I would not want my amp to go "soft" in any way, want to keep transient edge and dynamic so, glad you mentioned that, it sounds like placing a resistor in series with the choke is the way to keep the tone of the amp while lowering the plate and screen some.

Thank you, !

Phil D

[pjd3]

Just to mention,

On one of my last builds, I did install a B+ MOSFET Reducer circuit as seen around here from a few of RG Green's threads on lowering B+.

It worked great, and I do see that as an optional solution but, it is a bit more invasive than installing a resistor somewhere. And with this being "two amps in one chassis/cabinet" its twice the invasion!

Best

Phil D.

[cdemike]

I'm not able to find the specific article at the moment, but I've read the 6V6s ratings are conservative and that they stand up to some pretty brutal operating conditions. That article was specifically referring to plate dissipation, though, but it seems like Colossal's amp also tortured those 6V6s's without too much trouble. In any event, it sounds like your screens voltage is only just over the maximum rating, and as I understand your build your power supply is pretty true to the Marshall formula. So, in addition to Max's point about the voltage drop across the screen resistor, screens voltage should also drop as the power supply sags. Between the anticipated operating drop in screens voltage and the robustness of the 6V6s, I'd be surprised if your amp kills your power tubes.

[pjd3]

Thank you cdemike.

I'm going to give the JJ's a chance with the voltages as they will be. its sounding like there as good chance they will be fine with what I give them.

If chance has it that something happens, then I''ll just install something that relax's the voltages a bit but I'm finally confident to go forward.

Thank you ver ,much!

Best,
Phil D

[solderhead]

IMO screen dissipation is the parameter that you need to keep an eye on. The JJ 6V6 are robust and they handle high voltage well, but their screens will fail under conditions of excessive power dissipation.

I'm not one to just add a screen resistor in series with all of the other dropping resistors in the power supply rail, as that will invalidate previous load line calculations for every subsequent stage. If I have to add a screen resistor I prefer to do it on a branch of the supply rather than the entire supply so that I don't have to re-draw load lines. Of course if you're not doing load line calculations this probably won't matter to you.