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Lower gain preamp tube for 'better' breakup?

10/12/2000 11:42 AM
Chris Lower gain preamp tube for 'better' breakup?
I was thinking of subing my 1st position 12AX7 for a lower gain tube (12AU7 or 12AY7) in my 2204 head. What would this get me? I currently crank the amp via a powerbrake (-3 or -6db cut)but would appreciate 'smoother' OD with more emphasis on power tube breakup. Also, does anyone know makes "Edicron" branded tubes?  
Many thanks  
10/12/2000 4:11 PM

I think there is a tube called 5751 that is similar to the 12ax7 but it has lower gain.  
Just to give you another alternative.  
10/14/2000 2:09 AM
Chris B

You might also try a 12AT7 or a 12AZ7. These have a amplification factor of 60 per section as opposed to the 100 that 12AX7's have per section.  
Chris B
10/12/2000 4:18 PM
Ray Ivers

I would suggest a 5751 or (at most) a 12AY7 sub for a 12AX7 Marshall 2204 first gain stage (2.7K cathode resistor, 100K plate resistor) if you're looking for lowering the gain. If you just plug in a 12AU7 in this amp's first tube position without changing any circuit components, the 12AU7 will be overbiased and will draw very little idle current, and the output swing will be asymmetrical and possibly distorted with high-output pickups (on the other hand, nothing will be damaged by plugging it in - it just won't sound like a correctly-biased 12AU7 would). Replacing all the 12AX7's with 5751's will give you a noticeable drop in overall preamp/driver gain, and might be more what you're looking for, with no circuit changes needed.  
I'm not sure about Edicron tubes - European/British seems to come to mind, but I'm not at all sure.  
Ray Ivers
10/13/2000 1:42 AM
Ian Anderson

I've only ever seen Edicron tubes sold by Maplins if that help any - see their catalogue.  
... Ian
10/13/2000 2:26 AM
Peter S

I agree with Ray. I use 5751 tubes ALOT. You will notice the differnce in gain for sure and the distortion sound is smoother IMO. IMO, 12AU7's are more suitable for duty in the PI position. The same caveat applies would need to change circuit component to get the tune operating correctly. A 12 AU7 in your PI would be the opposite of what you are trying to would give you reduced output tube drive. I would try changing just the first gain stage to a 5751 and see how you like it.  
10/13/2000 7:18 AM
Paul Stansfield
Edicron tubes
I read somewhere that these are re-branded EI tubes. For the money Maplin charge, I'd look elsewhere!  
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