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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
tablejockey
3,889 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
michaelangelo
1 posts
 
Beautiful work and a beautiful piece!  Did you experience an increase in voltage when you upgraded from the selenium? I recall my father having what appears to be the same Frank unit and ran into that issue.  It was incredibly rich. I believe the PS caps will also make a difference.  I recently did an upgrade to an MX110 using Arizona's Blue and Green and some NOS Jensen's and the difference was immediately recognizable and well worth the cost. The only issue is space in an MX110.He drove a pair of Tannoy Gold 15's that I have since inherited. So far the only other product I have found with a Loudness circuit that rivals the Frank is in my McIntosh MX 110's. A truly under appreciated feature!  The same design is still used in some Mac preamps today with almost zero change in the circuit since 1961.  Although I love the vintage Mac sound this Frank id a lovely piece of work and a very nice straight design  although I still find it hard to beat the right combo of a properly upgraded Mc225/240 and C20.  Truly delicious! Of course, I'm fortunate enough to have quite a few vintage tube pieces I've picked up or inherited over the last 30 years or so.
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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