2. tech-talk
  3. 164929-158760

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
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
oldhvymec
5,101 posts
 
There has been a couple of updates "Super hexfeds" are around too, I haven't tried them.. They sure make a difference, and tighten up the upper bass section.. Very inexpensive but what a nuget for sure..

Respect..
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
tablejockey
3,941 posts
 
60+ year old quality. Impressive clean layout. Makes the vintage Macs look an amateur project.

Balance, tone and rumble filter-all sacrilege now, but I'd sure appreciate them. Nice piece.

'm into the aesthetic of vintage stuff. If that were mine I'd match a restored Garrard, Scott FM tuner and some nice vintage Quads(if they will work) all on a nice furniture grade stand.
Maybe throw in a cool R2R deck also.

Good luck on the refresh. 
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
More to discover