6V6 p-p amp screen voltage help?

[Dingleberry]

Hi.

Still working on my never ending tweed deluxe hybrid project...

Plates are around 346V, but screens are 351V.
So my goal is get the screens lower.

Power supply is: First cap (totem polled 2x150uf w.180k bleeders)-
choke (dc-resistance 275 ohms)-second cap 56uf...

Screen resistors are individual 1k/5W.
Bias resistor is 330ohms/15W.

I tried to add a resistor (2k2/4W) in series with the choke to lower the screen voltage, but when I turned the amp on after a while I got whining sound (from inside the amp chassis, not from the speaker), like camera flash loading so I immediately turned it off.


Does it matter if you put the resistor in series with the choke before or after the choke?

Mark Huss plexi 6v6 has 5k/5W in series before the choke.
Should I try or should I just ditch the choke use only resistor?

Any advice welcome.

-T

[JamesHealey]

try some bigger dropping resistors on the tubes, will help drop voltage without screwing up any layout etc.

Try using the 7W Welwyn types something like this for instance:
http://uk.farnell.com/welwyn/w22-4k7-ji ... tt=9505008

maybe not 4k7 straight away though..

[tubeswell]

Plates are around 346V, but screens are 351V...

Power supply is: First cap (totem polled 2x150uf w.180k bleeders)-
choke (dc-resistance 275 ohms)-second cap 56uf...

Screen resistors are individual 1k/5W.
Bias resistor is 330ohms/15W.

This seems to happen when you use a CLC filter between the plate and screen supply nodes in a cathode biased output stage (whereas in fixed bias output stages, there is no cathode resistor in series with the tube, so the plates tend to sit at around the same voltage as the screens). - At least the same thing happened to me the first time I put a CLC filter into a 5E3.

So you can put a resistor in series with the choke, or for a funky sounding amp, put a 2k2-4k7 5W resistor in series with the screen and the screen grid resistors. (You can use 1 x shared 4k7 5W going from the screen supply node to both screens, and if it is unbypassed/not decoupled, you will get some nice screen compression, and it should put the screens at around 20V lower than the plates).

[Dingleberry]

Thank you guys.
That really seems to be the case with cathode biased amps. Choke is not enough, unless it's a really big choke. Plates draw so much more current that the voltage drop across the OT primaries is way mopre than across the choke. Should have take a closer look to the cathode bias vs. fixed bias schematics. But you learn every time you mamke mistakes.

I think that I'll try 4k7 in series with the choke. That's about 5k which is the 5E3 deluxe value.

-T