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FRANK pram 30 watts Ultra-linear Stereo tube amplifier

I have a FRANK pram 30 watts Ultra-linear Stereo tube amplifier, in which I am replacing the selenium full wave rectifier with a diode full wave rectifier, I am also replacing the selenium bias rectifier with a diode, as well as replacing the electrolytics.

The details of the amp are below.
I have a schematic, but it is missing some vital info. The selenium rectifier had a forward DC resistance of 295K and was pushing out 345V. As selenium rectifiers don’t age well, I don’t know if the 345V being output is too high or too low. As the forward resistance of the diode rectifier is much lower, the output is going to be much higher. What I am trying to find out is what the original design voltage should be. I’m not worried about the bias voltage as it’s for the 4 x EL84 so should be about 11.5V I’m guessing.

Does anyone have any info on this amp?

Made in Belgium by the FRANK factory at the end of the 1950-early’60.
It uses an ULTRA LINEAR circuit, and the famous beige Philips-Mullard mustard capasitors.
Still complete factory original, all cap’s and resistors are still original.
These FRANK amplifiers were all handmade.
This one was made March 1961.
2 x 15 Watt.
The tubes are : 4 x ECC 83 / 2 x ECC 82 / 4 x EL 84.
Works on 110 & 240 volts , speakers 8 - 16 Ohms

Images:
https://cursorium.co.uk/images/1.jpg
https://cursorium.co.uk/images/2.jpg
https://cursorium.co.uk/images/3.jpg
https://cursorium.co.uk/images/4.jpg
https://cursorium.co.uk/images/Rectifier-rebuild.jpg
https://cursorium.co.uk/images/soft-start.jpg
https://cursorium.co.uk/images/Frank_Pram_30-eng.pdf
chris_w_uk

Showing 5 responses by chris_w_uk

chris_w_uk OP
52 posts
 
After much fiddling I have managed to dismantle the selenium rectifier and rebuild it using UF5407 Diode 800V 3Amp Soft Recovery Ultrafast Rectifier Diodes. So it now looks original.
https://cursorium.co.uk/images/modified-selenium.jpg
chris_w_uk OP
52 posts
 
Thanks for the reply.

I didn't realise it could make that much difference, I have some UF5408, I'll swap out the 1N5408 for the UF5408.
chris_w_uk OP
52 posts
 
I'm assuming this would be better than the UF5408 as I don't know about the recovery characteristics of the UF 5408.
https://www.ebay.co.uk/itm/321930598840
chris_w_uk OP
52 posts
 
Two parallel 1K 5W resistors in the B+ gives me 325V at the rectifier and 320V on the plate. I might need to add a 600 Ohm or so resistor into the bias, as with the new diode in place of the selenium the lowest it will adjust to is -13.1V. Not to sure what would be best, about 12.5V at a guess? Full volume with no input the amp is silent  Still waiting for the PS caps from Germany.
https://cursorium.co.uk/images/dropper%20resistors.jpg
chris_w_uk OP
52 posts
 
The B+ went from 345V to 373V with the final (experimented with other diodes) UF5407 rectifier rebuild version. I have no Voltage specs' for the amp, so just took a guess that the selenium rectifier had gone up in resistance a bit since 1961 and added series resistance to bring it down to 355V and set the bias to 13.5V. It seems to be quite happy at that, and sounds lovely. All the PS caps have been changed as have all other electrolytics. The yellow Philips caps are good so they stayed. The cathode bypass resistors in the preamp were also changed. The amp is powering a home built pair of 3 way speakers housing 8" , 6.5" and 4" units in a pair of modified Fisher cabinets.
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