Wurlitzer/Fisher Amp Musings

[wrenchone]

I am now the proud (?) owner of a pair of Wurlitzer organ amps, number 650172. They must be reasonably recent, as they have 7868s (the novar version of 7591A) for outputs. The tubes are all gone, but that's not a biggie, as I intended all along to gut the amps for iron and other useful parts. The power transformers are truly monstrous, and I would assume that the output transformers are wound for good bass extension. After seeing the guts of these amps, I no longer feel any shame about my point-point construction skills - they look rather like someone ate the parts, then barfed them out into the chassis (well, maybe not that bad, but they'll never win any awards for aesthetics).

Is anyone familiar with these beasts? I can test the transformers on the bench after I clip them out (nice, long leads), but it would be nice if someone out there was already familiar wth the amps and their capabiities. Given the tube set, I'm assuming that the output transformers are 6600 Ohm with 20-30W capability. I was thinking of using the iron to bulild an amp using screen-driven 6JN6s , and 6600 ohms would be just what I need.

On sort of the same note, I have the iron from a Fisher 500B receiver (no, I wasn't the ghoul that gutted it). I was thinking of building an amp around this iron using EH 7591As, but I might also think of using screen-driven sweep tubes instead. Can anyone comment on the sonic pros and cons between the two approaches?

[Eli Duttman]

On sort of the same note, I have the iron from a Fisher 500B receiver (no, I wasn't the ghoul that gutted it). I was thinking of building an amp around this iron using EH 7591As, but I might also think of using screen-driven sweep tubes instead. Can anyone comment on the sonic pros and cons between the two approaches?

I frequently find myself "ranting" about taking advantage of the same fact Fisher and Scott took advantage of. Circuitry that drives "12" W. tubes also drives 7591s.

A 12AT7 based "El Cheapo" style splitter/driver gets the job done well, from both the sonic and electrical perspectives. Greinacher voltage doubling the power trafo's highly competent bias winding should provide all the "headroom" needed in the negative rail. Employing combination bias safely allows the use of 330 KOhm 7591, grid leak resistors. That, in turn, keeps splitter/driver gain high enough to get full power O/P from a CDP source, without an additional gain stage. 330 KOhm 7591 grid leak resistors also allow the use of good sounding, but inexpensive, 100 nF./630 WVDC Panasonic ECQ-P(U) coupling caps. between the small signal circuitry and the "finals".

BTW, a slow starting negative supply for the 'T7s is a good thing. Obviously, several ways of slowing 'T7 B- rise down are available. A 117Z6 would certainly do the job. Under no circumstances, should 7591 bias voltage turn on be delayed.

[wrenchone]

Eli, this all assumes I'll end up using the 7591s in the first place. They aren't hard to drive, and I have my own ideas as to how to do it. Same for the sweep tubes ( harder to drive, but my own way to try), though I'll do some breadboarding first to make sure I'll have the drive voltage that I expect. My real question here regarded the sonic signature of P-P screen driven sweep tubes vs. the 7591As. This is especially a question as the sweep tubes will very likely be running close to Class B in the manner described by SY and Berning.
As for supply voltages, I may use the power iron that came with the amps, but then again, I may bulid yet another switching supply and have my pick of well-regulated B+, bias, and filament voltages. The power we're talkng about here is well within the capabilities of a simple ZVS flyback supply, and I already have a design that works like a charm. The supply can be tucked away neatly out of sight, and makes for a much more manageable package, weighing in total less than one of the Wurlitzer power transformers.