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FRANK pram 30 watts Ultra-linear Stereo tube amplifier

chrisW

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Feb 27, 2021
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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



New diode rectifier.
 

chrisW

Active member
Feb 27, 2021
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Yes, from those days before printed circuit boards.
Yes, valve equipment and printed circuit boards are not good bed partners. It's very difficult to design a printed circuit board to keep all the high voltage stuff from injecting interference into the signal path, especially in a fully integrated amp'. Also to stand any chance of getting a quiet amp' you still need a twisted pair for the heater circuit, or go to the extra PCB complication of a DC heater supply.
 
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insider9

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Sep 20, 2016
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This amp looks great but fair warning not careful DIY of a valve and can lead to death. Please take all the precautions and unless you really know what you're doing contact a professional.
 
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chrisW

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Feb 27, 2021
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This amp looks great but fair warning not careful DIY of a valve and can lead to death. Please take all the precautions and unless you really know what you're doing contact a professional.
Thanks for the advice, but I've been playing with this stuff since 1964 when I left school and started earning. Still have all my finger, not too sure about the marbles though :^)
 

chrisW

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Feb 27, 2021
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Here is another one of my pride and joy amps'. This one isn't quite as good as the Frank, but still a great sounding amp'.
This one has been completely rebuilt with all new caps and resistors, bar the wire wound resistors. Both amps have all matched quad & pair Russian military gold grid valve sets. Both amps have been fitted with soft start to protect the power supply caps and valve heaters.


If anyone fancies a read, here is the manual: Lafayette LA-224B Amplifier
 
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chrisW

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Feb 27, 2021
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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.
modified-selenium.jpg
 
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chrisW

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Feb 27, 2021
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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. :rolleyes:

dropper resistors.jpg
 

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