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In Reply to: RE: I want to make an EICO HF81 or Rogers clone, sans preamp posted by DAK on August 11, 2017 at 20:35:06
I suspect tone controls are present. EL84s are easy to drive. Look at Fisher's X-100. A 12AX7 voltage amplifier and a 12AU7 "concertina" phase splitter are all that's needed. The 7247/12DW7 shown in the schematic fragment is 1/2 'X7 & 1/2 'U7.
You might try a Mullard style circuitry using 6H30П (6n30p) as both the voltage amplifier and LTP. Unfortunately, it's most unlikely that the power trafo has the requisite heater current capability.
Most definitely, get the gain structure right.
Eli D.
Follow Ups:
I have a pair of Fisher output trans that maybe from an x100 and i will use that schematic for it. cheers.
Please post a photo of the chassis. Perhaps something can be concocted to use all the sockets, without overworking the power trafo's filament winding(s). Also, the OEM tube complement will be of interest.
Simply removing unneeded sockets and using the space freed for filter caps. could be the best bet.
Eli D.
The upcoming Fisher project has already undergone 2 iterations. The first was its original use as a bass amp? for a Hammond organ. I removed most of the components and OPT, and installed the 2 Fisher OPT. Since the original Hammond circuit included taps for fixed bias i used a Scott 299 schematic for building the amp. That sounded really good and i was super pleased with the result, but in a "Homer Simpson" moment i mixed up the 6bl8 and el84 lineup and that kinda toasted the PT. So I got a replacement PT, and decided to follow the Audio Note schematic for their PPEL84 UL amp which is now done, but the amp does not sound as good as the Scott schematic and the gain is too much in the front end causing the amp to be noisy.
So, my future plan is to use the Fisher circuit, after i get this other amp out of the way. thanks for the help.
Good look at the chassis real estate and irons you'll be using will definitely be a nice starting point.
Thanks!
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