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In Reply to: RE: Magnavox Rebuilds and Fultons (longish) posted by Michael Samra on April 06, 2014 at 15:40:55
As Mikey said, I would be interested in perfecting what I have built....
However, when you said "a direct-coupled concertina scheme" I looked at that like cows look at trains.....
If it is in schematic form, then I can read it, and possibly build it, otherwise I am lost, and Mike gets badgered with my pestering calls and messages.
Follow Ups:
Fitero,
You can see an example at
http://www.preservationsound.com/wp-content/uploads/2012/01/WilliamsonAmp_1947.jpg
V1 and V2 are two sections of a 6CG7 or 6SN7 twin triode (the 6CG7 will fit the noval sockets in your amp but has a completely different pinout from the 6EU7.) As mentioned in my answer to Mike's post, I used 0.1 µF caps to couple V2 to the output tubes (stage V3/V4 in the schematic deleted). The very small-value coupling caps used in the Magnavox schematic will roll off your bass badly if you reduce or eliminate the feedback.
One difference from the schematic I reference: I replaced the parallel power supply decoupling filters R2C1, R6C2 with a single filter with 3.3K, 22 µF feeding both first and second stage plates. I did not have room for two filters, and the lower power supply voltage in an EL84 amp (and higher current drawn by a 6CG7 relative to a 6EU7) mandates a smaller resistance. There is some room
for variation in the other resistances, but R3, R4, R5 and R7 should all scale by about the same proportion if you want to change them. R5 and R7 need to be high-precision or closely matched.
Good Luck,
JJ
Thank You for posting the link and taking the time to explain the pertinent info and deviations JJ. Food for thought.
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