FRANK pram 30 watts Ultra-linear Stereo tube amplifier
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 |
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 |
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. |
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 |
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. |
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. |
