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Old 4th December 2007, 02:59 AM   #1
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Default dynakit pam selenium rectifier replacement help

hello all I'm stuck on a little project. I have 3 of these dynakit pam1 mono preamps one of them isnt in very good shape lettering is all gone and I'm using it to practice on before I start working on the other 2. What Im having problems with is the selenium stack. I have a tutorial on how to replace the stacks in a dyanco pas preamp the stack in it has 4 terminals and it shows you how to put in the new diodes on a terminal strip to run the dc filament and negative bias supply. The selenium stacks in these pams have 3 terminals and this is where I'm stuck I have a picture here http://members.cox.net/cablefreak1/IMAG0103.JPG I also have the power supply that powers 2 of these units it is making a little over 315 volts on the B+ I am not sure on what the ac voltage should be that feeds the dc filament supply. when I test the dc thats coming out of the selenium stack its all over the place starts at 70v then falls to 30 and spikes back up so I am not for sure if its working properly at all. I went back and tested the ac for the filament supply and Its reading alil over 6v ac going into the selenium stack . Its uses 12ax7's so I thought I would find like 12 to 17 volts here I checked all the resistors in the power supply theres only 4 or 5 in there and all are well within spec. Any Ideas on where I should start? I have already started replaceing all the caps in the tone section matching values as closely as possible. All the preamps power on and pass signal but they dont sound to hot. Still searching for a manual with schematic I would imagine the power supply ps-1 has its own manual as well. Any help would be great!!! I forgot to add that I have some 1amp 1000v silicon diodes would these work ?
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Old 4th December 2007, 12:04 PM   #2
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I can only speak for the PAS - but that used AC on the the filament of the rectifier tube. As for the DC for the signal tubes, they used a voltage doubler circuit.
full PAS schematic is here: http://www.curcioaudio.com/pas_mnl.pdf

what I've done is remove the entire selenium stack and the old electrolytics. Replace with a terminal strip, two new (and smaller modern) caps and two run of the mill diodes to rebuild the entire supply. You should get ~11-12 volts.

Dynaco PAM-1 schematic is here : http://www.triodeel.com/pam1.gif - looks like they took the AC from an amplifier and converted it to DC using a voltage doubler.
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Old 4th December 2007, 04:04 PM   #3
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The PAM-1 doubler ran off of the 6.3V filament supply of the amplifier powering it. The topology is a conventional full wave doubler.

Note that selenium rectifiers are both hazardous and hazardous material. You do not want to operate your PAM with the rectifier in place, the magic smoke from these things is toxic.

The device is considered toxic waste in some communities and at the minimum should be tightly bagged before disposal - you might want to contact your dpw for disposal guidelines.
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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Old 4th December 2007, 07:40 PM   #4
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I have read about how toxic they are and thats why Im trying to proceed with caution here this is the 1st time I have tryed to replace one of these and Im at a loss on how it all wires together with the hum balance pot. I have been trying to find examples on the internet of these being used in heating filaments and this guide that I found by curcio http://www.enter.net/~cae/PAS2-3X%20...eplacement.pdf you will prolly haft to copy and paste the link. That is the the best example I have come across but as I said before I am really confused as to how the hum balance pot is wired with with the diodes.
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Old 7th December 2007, 01:15 AM   #5
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never mind I figured it out!!!!!!!!!
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