Screen voltages for 4x6v6s

[Dr d]

Can anyone help, please? I have recently been made aware that the screen voltages for a quad of 6v6s may be an issue in my build. How do you decide the most desirable screen voltage?. In my build I have @450v on the plates, and with 3k3 screen resistors @450v on the screens. The valves are in fixed bias at between 17-19ma on the cathodes running the 6v6s at @55% diss. watts. Does this sound about right screen voltage- wise? How do you work out the best values? Any advice would be gratefully recieved.

[ToneMerc]

A general rule is to go by the tube data sheet as a guide. What tubes are you using? If you are using JJ's because of their tolerance of high voltages, the data sheet calls for a max of 450V. In my latest build which used a quad of JJ's, I used only a 500 ohm screen resistor. My B+ was slightly above 405V and since those 6V6's don't pull much screen current, the voltage drop was only about a volt or so.

If I wanted to manage the screen voltages, I would either drop the entire rail or either drop just the screens in the B+ rail.

TM

[wjdunham]

Can anyone help, please? I have recently been made aware that the screen voltages for a quad of 6v6s may be an issue in my build. How do you decide the most desirable screen voltage?. In my build I have @450v on the plates, and with 3k3 screen resistors @450v on the screens. The valves are in fixed bias at between 17-19ma on the cathodes running the 6v6s at @55% diss. watts. Does this sound about right screen voltage- wise? How do you work out the best values? Any advice would be gratefully recieved.

If you do decide you need to drop the voltages down closer to 400V (which I do in 6V6 amps using Twin power transformers that otherwise put 450V on the plates) Mouser sells some nice 50V 50W Zener diodes that you can put in the center tap of the power supply. You only need to dissipate 10W or less, so there's a lot of headroom, and they are less than $20. Put a switch on the center tap and short the zener if you want to keep the high voltage option.
Bill

[Dr d]

Thank you Bill and TM. Yes I am useing JJs, and the Zener sounds like a good option. For now, I am wanting to run the JJs at 450v and just wondered if the 3k3 was dropping the screen voltage enough. BTW, Gary (Glasman) has posted recently that you could drop the B+ a little by adding a 25w 100R resistor in series with the centre tap....may try that in the short term.

[ToneMerc]

The 100-200 ohm 25W alum cased power resistor in the HT center tap was recommended to me by Gary as well id i wanted to drop my voltages further.