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Tubes - when to replace?

4/19/1998 9:25 PM
dave bush
Tubes - when to replace?
i just bought a used amp Acoustic brand (anyone heard of these?), it has four EL 34s in it. It sound okay, though maybe the tubes need replacing. I have tried swapping some of the 12 ax7 preamp tubes around, and the reverb improved with certain combinations, which may indicate that some are more worn than others or that everything could do with replacing. Do preamp tubes wear out much? Its a little hard to tell at the moment, as the amp is quite complex, it seems to have 5 pre amp tubes for a single channel amp (it is very flexible). I spose one is the phase spitter. Anyway, I guess the big question is when do I replace the power tubes? Are there any visible signs? The amp sounds okay to me except some of the tone controls arent that responsive (though then again it also has a graphic eq, so very unsubtle tome modifications can be made).  
Any advice is welcome!
4/20/1998 7:58 AM
Steve M.

I've had two different models of Acoustic amps in the past, the 164 and the G100T. Both used 6L6s in the power section, so if your amp is either of these, you might want to change the power tubes back to these type. I don't know if the filament winding on the PT can handle the extra current.  
There are some Acoustic amp schematics in Pitman's book.  
FWIW, I owned both of these amp before I became interested in tubes, but a tech I used to take these to told me to get rid of the G100T as it was unreliable, and in some cases very difficult to fix if some of the circuitry went bad. This was very unfortunate as I felt the amp sounded really good. I didn't really like the 164 very well, however.  
Hope this helps you out.  

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