Layout Question: 2K2 Screen Grid Resistors / 0.22uF PT caps

[TheRedDevil]

Hello guys.

After upgrade of my JTM45 reissue (http://forum.metroamp.com/viewtopic.php ... highlight=) I just want to do the same with my '92 1987x reissue amp.

I looked on VelvetGeorge's amp layout and also on Ceriatone layout

http://forum.metroamp.com/viewtopic.php?t=14850

http://www.ceriatone.com/images/layoutP ... iatone.jpg

I found two differences:

a) In the Ceriatone layout I found two 0.22uF caps on boared connected to 325V connectors of PT. I can't find them on VelvetGeorge's layout. Do you know why?

b) I have 2K2 5W screen grid resistors in my '92 1987x reissue amp. In VelvetGeorge's amp layout and also on Ceriatone layout they use 1K 5W screen grid resistors. Do I really have to change them to 1K 5W, if I want a maximum of security for life of my OT?

Hope you can help me.

Regards,
TheRedDevil

[toner]

In the Ceriatone layout I found two 0.22uF caps on boared connected to 325V connectors of PT. I can't find them on VelvetGeorge's layout. Do you know why?

Those are "snubber caps". They reduce rectifier diode switching noise. Modern diodes don't really need these caps. Search for "snubber" for more info.

I'm not sure about the screen grid resistors. 2k2 is probably overkill but I'm not sure how they would affect tone. I think they may roll off a little treble, compared to 1k resistors. ?

[Billy Batz]

According to Larry you have to raise those grid resistors to over 20k before you can notice any treble roll off. I put 10k on and I really didnt notice anything. It may be that its inaudible or so small that the time to change was too long to keep a perfect comparison for the change.

George uses those snubbers on the stock 100W 69 amp. Was it even on original 50W boards around 69 and later? I dont know to be honest but Nik at Ceria doesnt exactly go all out for authenticity with things like this.

[SDM]

According to Larry you have to raise those grid resistors to over 20k before you can notice any treble roll off.......

Good stuff there but that's for grid resistors, think he's asking about the screen resistors here. 2.2K (2K2) 5watt resistors actually afford more protection for the tubes and likely the only reason why Marshall used them for a while. 1K's are the typical/standard Marshall value and help keep the amp sounding like it was meant to. With decent tubes these days and typical 1987 voltages I wouldn't worry about going with the traditional 1K 5watters on the screens.

[Billy Batz]

Jeezus Im staying out of the 50W page. Toner will know what I mean. I seem to be disleczik in here.

[novosibir]

... think he's asking about the screen resistors here. 2.2K (2K2) 5watt resistors actually afford more protection for the tubes and likely the only reason why Marshall used them for a while.

... and make the power stage's sound a bit lifeless, when used with EL34's.

And yes, like Steve already said - Marshall has used this value in the early 90', as power tube quality was terribly bad, so that the amps at least survived the 6 months warranty at that time

At that time, the Russian 5881 was the only reliable power tube available, so some time later in the early 90' they switched to this tube.

Larry