kt88's in a marshall 4210?

[davebolden44]

was just wondering if somebody could guide me along with a potential mod, i was thinking about putting some kt88s in my 82 jcm800 4210. i work on my own stuff and have built a couple tube amps now but I'm not much of an engineer more like a paint by numbers guy. i read somewhere that the drake 784-139 transformer will handle the current. i was just hoping somebody would give me the quick and short of whether i need to change the bias supply or grid resistors or if this is unwise. or if a simple swap out and re bias is all that is necessary.

[martin manning]

What plate voltage and OT primary impedance do you have now?

[davebolden44]

i have not measured anything inside this amplifier yet. unfortunately i wont have time to do so until the weekend. all i know is that its bone stock, no mods aside from a replacement speaker. its getting a cap job hopefully this weekend at which time i will take some readings and post them.

[martin manning]

It would be good to get some actual measurements. Is yours a US model with 6550's? If so I believe the changes will be minimal, if any, since 6550 are close to KT88.

[davebolden44]

no its a british/el34 model.

[martin manning]

no its a british/el34 model.

So now you know what needs to be modified- two resistors in the bias circuit, reduce power tube grid leaks to 150k, and add 1k screen resistors.

[davebolden44]

heck i just noticed the us values for 6550s right on the schematic. hadnt realized it before. thanks for the guidance