Screens.....

[C Moore]

Correct me if I am wrong..... but it seems like I have seen a few schems of guitar amps that do not use screen resistors.
Would there be any problem then if I were to use just One Screen Resistor to feed a pair of EL84.?
I have real estate problems, and will have a hard time making two resistors fit.
Thank You

[rdjones]

Correct me if I am wrong..... but it seems like I have seen a few schems of guitar amps that do not use screen resistors.
Would there be any problem then if I were to use just One Screen Resistor to feed a pair of EL84.?
I have real estate problems, and will have a hard time making two resistors fit.
Thank You

Yes, this is one variation that can be used for screens.
I believe Traynor used this arrangement in at least one design.

Just remember that the screen buffers can be viewed as being in parallel, so a single 470 Ohm would be roughly equivalent to a pair of 1Ks.
Double the wattage rating (or at least the calculation).

rd

[C Moore]

OK.
Thanks for the info.

[rdjones]

OK, it's the YBA1 that has the screens connected together and a single 470 ohm 10W.

rd

[es345]

Screen resistors reduce the voltage at the screen especially at medium full power as a type of safeguard for the screen.

Most of the time the power stage tubes are working in class B. In fact most of the time the current is flowing only through one side of the PP stage while the other side is switched off. That means only one of the both resistors is basically in use.

What is the consequence?

Having only one common resistor for both sides with the double value of the two separate ones will double the voltage drop at the screens. As a result the behaviour of the powerstage will change. If you don't want to change the behaviour is better to stay with the same value as for the two separate ones. For sure the common resistor must be able to handle the total power of both separate ones.

[tubeswell]

Just because a few old amps didn't have screen grid resistors doesn't mean that the resistors isn't a good idea. 470R at each screen doesn't really have much tonal impact, but is better in terms of enabling tube longevity. 470R on a 6L6/6V6 is all you need to eat up that excessive screen current when the plate voltage hits the bottom of the cycle

[jdw3]

I am having the same problem.

I have a '68 Traynor BassMate. I modded it slightly for better guitar frequencies and slightly more gain, nothing major. As a result it sounds killer and better when cranked high.

I am into my 3rd set of el84s. I started popping fuses after.

From what I am finally learning, I probably need screen resistors. I have read in Kevin O'Connors TUT3 that el84s need 2.2K ohm/5w minimum.

Does this sound like it will cure my tube frying problem?

[Phil_S]

Cut the leads on the R short and put it on the tube pin. You don't need much real estate to do that and you can leave one end flying.