6L6 Plate and Screen Voltages

[Structo]

I was wondering if you guys that use a choke in your ODS amps have the same voltages on your 6L6 plates as you do on the screens?

When I had a 330R choke resistor instead I had a good 5v less on the screens.

I have 560R 5W screen resistors at this time.

I notice with the choke the plate and screen voltages are pretty much equal.

Is that something to be concerned about regarding tube life or is that the norm?

Do any of you run a resistor in series with the screen supply between the choke and screen resistors?

[David Root]

My #124 build runs Vp and Vs virtually the same at about 430V quiescent. I have not measured it in action, but when you start drawing power Vp starts to drop and Vs follows it. I imagine the rates and ultimate depth of drop also depend on how much plate and screen filtering you have.

I did run a resistor much as you describe in the screen supply line in one build, but IIRC the choke was after the dropping resistor. In retrospect I probably didn't need it as it was running 6550s.

I am looking at a SSS build insinuating itself into my near future, and I am more concerned about running 6L6s at 520Vp and 510Vs, as I have a nice quad of Mesa STR-415s as an alternative to the std HAD-approved 6550s.

Has anyone measured how the plate & screen voltages move with power draw?

[talbany]

A choke is used in place of a series resistor because the choke allows better filtering and less voltage drop. An "ideal" inductor would have zero DC resistance.(Most Fender chokes around 100 ohms) If you just used a larger resistor, you would quickly come to a point where the voltage drop would be too large, and, in addition, the supply "sag" would be too great, because the current difference between full power output and idle can be large, especially in a class AB amplifier.
At idle a couple volts difference is normal..Screens should never be higher..

[Structo]

I should try hooking up a meter to the screens and play fairly loud to see if it lowers from the plate level.

Do you think I should install 1K screen resistors?

[talbany]

6L's have a pretty stout screen supply so unless you play the thing wide open (or close to wide open) for any length of time or find yourself replacing screen resistors.... 470-500 should be fine..leave it..If your amp is set up for both 34's and 6L's I would use the 1K's..34's have a wimpy screen supply

Disclaimer
If either the grid resistors or screen resistors are of inadequate value the effects of blocking distortion or a starved or under regulated screen supply are not subtle anomalies by any means..you WILL!! notice it...Both grid and screen resistors directly relates to the operational characteristics of the amp and should not be considered tone tweaks IMO!!.. Just thought I would mention this..

Tony

[Structo]

Yeah at one time I did experience blocking distortion so I know what that sounds like.
I forget what I had going on at the time I had that as far as values go.

Just seems kind of weird that before I had about 5v less on the screens than I do now.

[ampdork]

Resistors drop more then chokes.

You could run the numbers on screen dissipation just to be sure but I am betting they are fine and dandy.

[guitardude57]

I've had blocking distortion with amps that had a 40-50 volt difference between plates and screens.....but not with a 2 volt difference....strange.

My recent ODS high plate has a 2-3 volt difference and no problems. If I had space, we would have placed a switchable Choke/Resistor. Who knows, may jumper one in (390 ohm or so) to see the difference sometime.

I did find that Sovtek 6L6wxt's don't like the 500 volt plates. Blew the silver right off the top of one of them. FLASH....ooops.

50 year old RCA's sound better anyway.

[Structo]

yeah I never had more than 3-5v difference with the screens being the lower number.

I guess I will refrain from changing anything because from the vast numbers of ODS style amps built here, I haven't noticed anybody dropping the screen voltage.

I was wondering since I am in the planning stages of a 100w amp, does the precision power supply drop the screens more than the old style PS?

[brewdude]

I just replaced the 330R resistor with a choke on my 6V6 D'Lite and now the screens are ~1.5 volts HIGHER than the plates!

Should I be concerned?

I haven't been able to really test it sonically yet.