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5881 to EL34 interchangablity in Marshalls

5/24/2000 5:15 AM
Adam Kirk
5881 to EL34 interchangablity in Marshalls
Does anyone know if you can interchange 5881's with EL34's in Marshall Amps (specifically JCM 900).  
I placed a bid on one and I found out that it has 5881's; it must have been an amp sold in the UK. I really need the British EL34 sound (who wants 5881's in a british amp anyway?-sorry if I offend someone). I would really appreciate the info.
5/24/2000 8:56 AM
Ian Anderson

Yup, this is no big deal. I have the mods floating around somewhere that I got at the time when Marshall changed over to 5881's.  
I got the mods to change from EL34's to 5881's, but simply reversing what it says to do would give the same results (no?)  
I believe Aspen Pittman's tube amp book covers this as well if memory serves me right.  
I'm sure someone here will guide you on your path (getting too deep). If you get stuck, email me and I'll have a dig around for the specifics of that mod.  
... Ian
5/24/2000 3:39 PM
Peter S

The mod is very simple and actually involves wiring pin 1 and pin 8 together on the output tubes, and then changing a resistor in the bias circuit so that your bias voltage is in the proper range for EL 34's. You need to check your screen resistors and make sure that they are 1K 5watt wirewound. That's all there is to it.
5/27/2000 11:14 PM

What about the additional filament current? 5881 @ .9 and EL34 @ 1.5! For 2 tubes that's an additional 1.2Amps of current with EL34's.
5/28/2000 1:27 AM
Peter S

The additional current isnt a problem in these amps.
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