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FRANK pram 30 watts Ultra-linear Stereo tube amplifier

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Post by chrisW Sat Feb 27, 2021 11:32 am

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
FRANK pram 30 watts Ultra-linear Stereo tube amplifier 1
FRANK pram 30 watts Ultra-linear Stereo tube amplifier 2
FRANK pram 30 watts Ultra-linear Stereo tube amplifier 3

New diode rectifier.
FRANK pram 30 watts Ultra-linear Stereo tube amplifier 4

chrisW
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Post by Tom L. Wed Mar 31, 2021 10:11 am

I am not familiar with that amp, but if you get no answers, I would look up the output tube spec sheet and see what they recommend for plate voltage according to the class of operation.
You will have to add a B+ power resistor or increased the value of the original one if there is one already in there.

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Post by AJ2X Wed Mar 31, 2021 11:18 am

That is a beautifully built amplifier, and the Ultra Linear circuit is widely praised. Those facts, and the photos provided, don't help solve your question, however. The schematic would be much more useful, and where exactly you measured the 345 volts. Looking at the data sheet for the EL84 (6BQ5), 345V seems to me like too much plate voltage.

As you say, selenium rectifiers age poorly, but they generally fail by increasing their internal forward resistance and thereby getting hotter and hotter, while the output voltage drops. 295K seems an extremely high value for a forward (conducting direction) resistance -- 295 ohms would be more like it for a failing part. Still, the forward drop under load for most seleniums is in the order of 10-30 volts, while that of silicon diodes is more like 1 volt. Typically it's recommended to used a 10-20 ohm, 1 or 2 watt resistor in series with a silicon diode when replacing a selenium stack. The resistor can be adjusted to keep the DC voltage the same as the original design spec (which you don't have, and alas I don't know either).

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Post by chrisW Wed Mar 31, 2021 11:37 am

The B+ was measured straight off the rectifier + under load volume down.  I stripped out the selenium rectifier and rebuilt it with 4 UF5407 800V 3Amp Soft Recovery Ultrafast Rectifier Diodes. The bias diode was selenium too, I also replaced that with one of the above. I added a series resistor to get the B+ which had risen to 373V back down to 345V again, and reset the bias. The EL84s specify 300V on the plate, but there are many amp designs out there that run them way above that and don't seem to have any trouble. I have a set of Russian reflector military grade EL84s in there, and by the time it gets to the plates, it's down to 340V. The bias is set to -13.5V and it sounds lovely at that. So unless it starts eating EL84s I'll leave it at that. I also fitted soft start to take the startup surge of the heaters and PS caps'.
FRANK pram 30 watts Ultra-linear Stereo tube amplifier Rect-d10FRANK pram 30 watts Ultra-linear Stereo tube amplifier Seleni10FRANK pram 30 watts Ultra-linear Stereo tube amplifier Soft-s11

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Post by AJ2X Wed Mar 31, 2021 4:13 pm

It sounds like you have things well in hand! Nice re-stuffing of the rectifier case (I'm unfamiliar with that kind of enclosure for selenium rectifiers, though I do dimly recall a product notice about that package style, about 40 or 50 years ago).

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Post by chrisW Mon Jun 14, 2021 10:04 am

I found a website that can calculate inflation for any amount from any date to today. In 1961 when it was made my amplifier cost 10,000 Belgium Franks, which as near as I can calculate was £142.00 in 1961. With inflation it says it would cost £3,242.38 to buy today. Nice to know.

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