# KT77 "s in a Marshall JCM 900 DUAL REVERB 4102



## blazer (Feb 12, 2012)

Time for new tubes in my 1996 Marshall JCM 900 which came with 5881"s.Eurotubes say I can drop in KT77 and bias with no problems.Has anyone tried this and what are your thoughts on these tubes??


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## Emohawk (Feb 3, 2006)

Isn't the 5881 more akin to the 6L6? Not saying it won't work...It probably will, but may not be perfect. I'm sure some of the tech-savvy folks here will chime in.

I have KT-77's in my 2204 (EL34). They're more open & round & a little less compressed. If you can drop them into your 900, I expect it will be a little more compressed. Pure speculation on my part. They bias similarly to the EH EL34's I had in there before.

Here's some info I found (from Dr Tube) on changing from 5881's to EL34's in JCM900's. I suspect it would be similar for KT-77's as those are a drop-in replacement for EL34's with only a re-bias required.

_ If you compare the schematics for the 5881 and EL34 versions
you'll see that the differences are minor.

The power tube sockets are already ready for EL34's, and 
don't need to be rewired.

The EL34's need a "less negative" bias voltage than 5881's, 
so the bias circuit has to be changed slightly. I'd suggest 
adding a resistor of 47k parallel to R28 (should be 47k). 
That way the bias current range shifts up (less negative 
control grid voltage).

Furthermore I'd replace the screen grid resistors (R31 & R34 
470R/5W) to 1k/5W ones and not to 2k2/5W ones as suggested 
on the EL34 schematic._


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## WCGill (Mar 27, 2009)

What Emohawk said and there's also the issue of output transformer mismatch which shouldn't be a deal-killer. El34's usually run around 3.4k and 6L6's 4.2k. I'd be more leery of running 6L6's at 3.4k. I've never measured the primary impedance of this amp so I'm not sure exactly what's in there.


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