JCM 900 5881 to EL34's

guitarhurricane

Silver Supporting Member
Messages
138
I have a friend who wants to change the tubes in his jcm 900 with 5881's to el34's. It should be as simple as switching out the tubes and rebiasing right? Anybody have any good info on doing just that with this type amp? There really aren't any amp techs without having to drive 2 1/2 to 3 hrs and I am hoping I can help him do the switch. Any info or thoughts/tips would be greatly appreciatted.
 

rockon1

Platinum Supporting Member
Messages
12,980
Originally posted by guitarhurricane
I have a friend who wants to change the tubes in his jcm 900 with 5881's to el34's. It should be as simple as switching out the tubes and rebiasing right? Anybody have any good info on doing just that with this type amp? There really aren't any amp techs without having to drive 2 1/2 to 3 hrs and I am hoping I can help him do the switch. Any info or thoughts/tips would be greatly appreciatted.
Its more complicated than just a rebiasing.I think they use the pins slightly differently.
 

VintageJon

Member
Messages
153
To do this you MUST connect pins 1 and 8 together and raise the screen resistors to 1K MIMIMUM, (I'd go 1.5K or 2K...)

In addition a re-bias is required...

-Jon
 

Wakarusa

Member
Messages
1,459
This change also usually requires a modification to the bias circuit. The range of the bias circuit for 6L6GC (those Sovtek 5881 it came with aren't really 5881's) generally leaves an EL34 biased cold cold cold.
 




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