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previous: Bob K. 1) Can somebody help (educate) me o... -- 12/11/1998 10:34 AM View Thread

Re: Ampeg help/opinions needed...Please.

12/11/1998 1:04 PM
Graywater
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Re: Ampeg help/opinions needed...Please.
Bob:  
 
1) I don't have the Gem I schematic handy but looking at the II, and other Ampeg designs with 7199s, your assessment of the PI is accurate. A better way of stating it might be "a split load PI with a pentode voltage amplifier feeding it." The split-load PI is a "unity gain" design (in reality less than unity) that provides no amplification, it only splits the signal into two paths of opposite phase - the stage preceding it, either triode or pentode, provides the voltage swing that the output stage needs (unless there is a driver stage for each phase following the PI).  
 
2) You may be asking the impossible. While the merits of each PI design has been discussed on this forum ad nauseum (check the archives, lotsa goodies there) the nutshell is that each PI has a characteristic sound and you are unlikely to find another design that sounds like the split-load. While it is possible to find another design the pleases you it probably won't sound like the original - if this is your goal.  
 
I don't know what the voltage gain of the pentode stage of the 7199 is but you might be able to get a similar vg from one half of a 12AX7 and use the other half as the PI - at a glance, it appears that the output tubes don't need much of a current source.  
 
Another approach, discussed here a few weeks back, is to replace the 7199 with any of the very available, and cheap, pentode/triode combinations that were commonly used in televisions - the 6GH8 specifically was discussed. There are scores of 6xx8s out there and I am sure that one would meet your needs of low cost and being "stock." I recently journeyed to visit Richie Hall and he "blessed" me with a truck load of tv tubes including a few hundred 6xx8s (and other voltage variants). If you find one that you like, let me know and I'll make you a great deal on lifetime supply of them.  
 
GW  
 
PS: One of the best sounding "conversions" of another amp into a guitar amp (commercial phono amp) uses a 6BR8, with really low plate voltages on the pentode, to drive a UL 6BQ5. This little sucker puts out about 8w stock and really screams - my favorite "home-built" SE.

 
Replies:
Dave Stork The 6AN8 is another good candidate.... -- 12/11/1998 2:31 PM
Bob K. Graywater: Ampeg help/opinions needed...Please. -- 12/11/1998 9:59 PM