5E3X2 no screen resistors

[lowell]

hello all,
1) I recently built a 5E3X2. I'm thinking of adding some screen resistors as there are none. Anyone know what a good value'd be for inducing a good amount of squish? Also, do I need to put one per tube?
2) Can someone explain why the filament center tap is connected to the top of the cathode resistor/cap on pin 8 of power tubes?

[Enzo]

SOmeone else can recommend screen resistor values.

The cathodes of the power tubes sit at a pretty steady DC voltage, with that bypass cap keeping it steady. That is a cheap and convenient place to steal a positive voltage to bias the tube heaters, instead of making another supply for the purpose. By elevating the heaters to a positive DC level, it prevents electrons from the heater flowing to the cathode. SUch AC curent flow can inject 60Hz hum into the preamp stages. If the heater is +15 or +25 or whatever, then the cathode will always be more negative, and will not attract those electrons.

It therefore reduces hum in the high gain stages.

[Bruce / Mission Amps]

I have never found it necessary to use individual screen resistors on the 6V6 sockets in this amp.
Because the DC resistance of the 35 watt output transformer (actually a modified clone of a silver face Pro Reverb OT)... is a little lower then a tweed Deluxe OT, ...the 2k7 ohm resistor (minimum of 2 watts) in the screen node of the B+ rail is good enough to keep the screen voltage lower then the plate voltage and it will limit the total amount of current anyhow.
Are you building a new one or modding?

[Earl]

My old '62 Princeton has been running without screen resistors for a long time, with no ill effects

MY RCA19 book states that using a power tube in circuits that result in a large change in plate/screen currents should use a screen grid voltage from a point on the B+ rail (properly filtered).

[lowell]

Bruce,
I'm modding my own previous build. I understand that no screen resistor is NECESSARY, however have read about its affect on perceived compression and "squish." You?

[Bruce / Mission Amps]

Originally Posted by lowell

Bruce,
I'm modding my own previous build. I understand that no screen resistor is NECESSARY, however have read about its affect on perceived compression and "squish." You?

You'd need to make the individual screen resistors pretty large to get much from that... maybe 2700 to 3900 ohms each?
If you want a little more power tube squish, maybe you could try to change the 2k7 2w screen node resistor to 5K1 or 7K5 at 2w and change the 22K to something like 15K to 12K to keep the preamp voltages up.
Or if you want a softer preamp distortion and a little power tube compression, change the preamp node dropping resistor from 22K to 39K or 47K and use a 5K1@2 watt screen node resistor.
I haven't tried that but it seems like the lower plate voltages on the preamp tubes would nose over easier.

Bruce

[lowell]

ok cool, thanks!