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Magnavox Rebuilds and Fultons (longish)

Posted by JJ Triode (A) on April 5, 2014 at 14:48:08

In Reply to: Magnavox 9302 Rebuild posted by Fitero on April 4, 2014 at 22:18:10:

Hi Fitero,
I also use Fulton loudspeakers (FMI100s topped by the Fulton/RTR electrostatic cubes) and usually drive them with an EL84-class amp that was originally built as a clone of the same Magnavox console amp you have. I'd like to make a few comments and suggestions about your very nice amp rebuild.

First, a couple of things I really like: the fit and finish and construction look really nice, especially the grounding scheme (large copper ground bus) and general layout. I also like that you are using a shared cathode resistor and bypass cap for each PP pair, as opposed to the original Magnavox scheme with a single resistor for all four tubes, or separate bypassed resistors for each individual tube. I tried all three cathode biasing schemes, and shared-pairs sounded best to me.

Second, one difference: I ditched the Magnavox paraphase front end for a direct-coupled concertina scheme, similar to the first two stages of a Williamson amp. I felt this was an improvement, but since you have already finished your build and like the result, I am not suggesting any changes there. I also use an ultralinear screen connection, but that is not an option with the original output transformers.

Third, some suggestions. Are those large can caps electrolytics or oils? If electrolytic, I suggest paralleling them with smaller-value film caps. Also do this with the other electrolytics in the amp, or even replace them completely with films since you seem to have enough space. The blue 35V, 22 µF film caps sold by diyhifisupply.com in Hong Kong should work well for the output bypass caps, since your bias voltage is only about 12V. Two per side should be enough to replace the electrolytics, or just one per side to supplement them.

Anyway, good luck and enjoy your amp!
-JJ

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