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    selenium rectifier

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    Clik2media

    Posts: 40
    Join date: 2009-01-18

    selenium rectifier

    Post by Clik2media on Mon Jan 19, 2009 1:06 pm

    Hello group, newbie here and owner of a number of Dynaco peices, 70's Mk111'S FM's etc. I am starting a remod on one of my ST-70's and I am getitng confusing info on selenium rectifiers, what is the best route to replace this seemingly obvious replacement part.

    thnx

    Bob Latino
    Admin

    Posts: 1958
    Join date: 2008-11-26
    Location: Massachusetts

    RE: Replacing a selenium rectifier with a diode

    Post by Bob Latino on Mon Jan 19, 2009 1:46 pm

    The best thing to do is to remove the selenium rectifier and replace it with a UF007 diode. What you do is take out the selenium rectifier and replace it with a two lug terminal strip. Run the RED/BLACK wire to one lug on the terminal strip. Run the wire from lug 4 of the 7 lug terminal strip to the OTHER lug on your new 2 lug terminal strip. Place the diode with the BANDED end of the diode on the lug where the RED/BLACK wire is connected and the other end of the diode on the other lug of the 2 lug terminal strip and then solder the two connections.

    Sometimes after a change from the selenium rectifier to the diode SOME amps will require a change in the resistors on the 7 lug terminal strip in order to bias the amp properly. Usually the easiest thing to do is to replace the two 10 K resistors between lugs 1 and 2 and lugs 3 and 4 with two 5 K resistors. This resistor change should get your two bias potentiometers into the proper range. Resistors of a lower value may be needed due to the difference in forward current flow of a diode vs. the selenium rectifier.

    Another note - You can get the proper diode, resistors and a terminal strip at any local Radio Shack.

    Bob

    Clik2media

    Posts: 40
    Join date: 2009-01-18

    Re: selenium rectifier

    Post by Clik2media on Mon Jan 19, 2009 4:34 pm

    Bob, thanks very much, I will make sure to follow the instructions to a T, here-in lies my point of confusion however. UF007 or 1N4004, or perhaps other diodes? Which is the best to use for an ST-70 with mostly stock but upgraded boards , parts etc.

    thanks again

    Michael

    Bob Latino
    Admin

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    Location: Massachusetts

    Re: diode

    Post by Bob Latino on Mon Jan 19, 2009 4:42 pm

    Michael,

    I use the UF4007 because I have a bunch of them here but you can use almost any diode for this application. You are only dealing with about 50 or 60 volts at this point in the circuit. The 1N4004 you mention is good for 400 volts which is well beyond what voltage the diode will be looking at. Just make sure that band on the diode faces the RED/BLACK wire from the power transformer.

    Bob

    Clik2media

    Posts: 40
    Join date: 2009-01-18

    Re: selenium rectifier

    Post by Clik2media on Mon Jan 19, 2009 4:45 pm

    Ok now it clear, thanks very much Bob, I appreciate it.

    Michael

    Sal

    Posts: 222
    Join date: 2009-02-05
    Location: Central New Jersey Dynaco-ST70.com

    Re: selenium rectifier

    Post by Sal on Thu Feb 05, 2009 4:14 pm

    Here is a photo of what I did. A picture is worth a 1000 words.

    Sal


    Bob Latino
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    Re: selenium rectifier

    Post by Bob Latino on Thu Feb 05, 2009 4:28 pm

    Sal,

    Many thanks for the photo. It does make it easier to understand ...

    Bob

    Sal

    Posts: 222
    Join date: 2009-02-05
    Location: Central New Jersey Dynaco-ST70.com

    Re: selenium rectifier

    Post by Sal on Thu Feb 05, 2009 9:44 pm

    Your Welcome Bob.

    PS: Great forum you have here, I'll put a link on my website.

    Regards,
    Sal

    Clik2media

    Posts: 40
    Join date: 2009-01-18

    Re: selenium rectifier

    Post by Clik2media on Sun Mar 08, 2009 1:52 pm

    Bob Latino wrote:
    Sometimes after a change from the selenium rectifier to the diode SOME amps will require a change in the resistors on the 7 lug terminal strip in order to bias the amp properly. Usually the easiest thing to do is to replace the two 10 K resistors between lugs 1 and 2 and lugs 3 and 4 with two 5 K resistors. This resistor change should get your two bias potentiometers into the proper range. Resistors of a lower value may be needed due to the difference in forward current flow of a diode vs. the selenium rectifier.
    Bob

    Bob, I am double checking on the 5k replacement resistors. If the pots are out of range [high] I am a bit confused by lowering the resistance, should the resistors be 15k instead of 5k to bring the potentometers back into range? On that same note can the pots be changed to higher range pots say 1 meg to do the same?

    Mike

    Bob Latino
    Admin

    Posts: 1958
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    Re: selenium rectifier

    Post by Bob Latino on Sun Mar 08, 2009 8:34 pm

    Mike,

    On the 7 lug terminal strip you want to lower the value of BOTH 10K resistors. How much you lower it can vary a little from amp to amp. You can do this by adding another resistor in parallel to the ones that are there OR pulling both and trying lower value resistors. What you might try is a 15K in parallel with the resistor between lugs 3 and 4 (6K equivalent) and a 10K in parallel with the 10 K resistor between lugs 1 and 2 (5 K equivalent). That should get you into the correct range. If not try a 27K in parallel with the resistor between lugs 3 and 4 (7.3 K equivalent). I honestly haven't fooled around with the stock bias system in a few years but I do remember that you have to LOWER the resistance on those two resistors.

    I have never replaced those 10K bias pots with anything other than another 10K bias pot. I don't think a 1 meg pot is the answer.

    Bob

    Clik2media

    Posts: 40
    Join date: 2009-01-18

    Re: selenium rectifier

    Post by Clik2media on Sun Mar 08, 2009 10:13 pm

    Bob thanks for clearing that up. one last question, what watt resistor is for that area. Cannot find it in the Dynaco manual

    DynakitParts
    Admin

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    resistor Value

    Post by DynakitParts on Sun Mar 08, 2009 10:55 pm

    Mike,
    Use 1 watt values here..metal film, carbon comp, metal oxide
    or whatever you have available..2 watters are OK to.

    Kevin @ Dynakit

    Clik2media

    Posts: 40
    Join date: 2009-01-18

    Re: selenium rectifier

    Post by Clik2media on Sun Mar 08, 2009 11:02 pm

    Thanks Kevin , you guys are great!

    Mike

    GP49

    Posts: 499
    Join date: 2009-04-30

    selenium rectifier

    Post by GP49 on Mon May 04, 2009 9:49 am

    A good "while you're in there" is to replace the electrolytic capacitors in the bias circuit. The originals are HOW old? I solved a pesky hum problem on a Dyna by replacing those capacitors. You won't know they've gone bad by voltage measurements, either.

    You can increase their value; current capacitors are smaller for their capacitance, so you can get even better filtering of hum if you wish.

    Bob Latino
    Admin

    Posts: 1958
    Join date: 2008-11-26
    Location: Massachusetts

    Re: selenium rectifier

    Post by Bob Latino on Mon May 04, 2009 10:33 am

    GP49 wrote:A good "while you're in there" is to replace the electrolytic capacitors in the bias circuit. The originals are HOW old? I solved a pesky hum problem on a Dyna by replacing those capacitors. You won't know they've gone bad by voltage measurements, either.

    You can increase their value; current capacitors are smaller for their capacitance, so you can get even better filtering of hum if you wish.

    This is a good idea .. You can replace those 50 uF capacitors with up to 100 uF caps with no problems ... Just make sure that the caps are rated for at least 100 volts because those caps will be looking at about -65 volts DC (negative 65 volts DC)

    One thing that bothers some Dynaco ethusiasts when they replace those two electrolytic capacitors is that these two electrolytic capacitors seem to be put in backwards. They trace the "+" or POSITIVE side of both capacitors from pin # 1 on the 7 lug terminal strip to pin # 2 and then to the main grounding lugs next to the quad cap. The positive side of the caps are gounded?? Yes they are. You have to remember that the bias control circuit on the ST-70 is running a NEGATIVE DC voltage so everything is reversed. Next time you have your amp open measure the DC voltage from pin 5 or pin 6 to ground by placing your black probe on the chassis and the red probe on either pin 5 or 6 on then and look at your DMM. It will throw a "-" (negative sign) in front of the measured DC voltage. The measured voltage should be between -22 VDC and -48 VDC and usually around -34 VDC if your bias has been set to 1.56 VDC as Dynaco recommended for each PAIR of EL34 output tubes. This will give you 100 milliamps of idle current for each PAIR of tubes or 50 milliamps for each output tube. While we are on the subject > One way to get longer tube life out of your stock ST-70 with no loss of sound quality is to bias your tubes a little lower than Dynaco recommended - about 1.25 volts DC instead of the 1.56 volts. This will give you about 80 milliamps per tube pair or 40 milliamps per tube. Although I honestly don't have and figures or tests to prove it, the 20% drop in bias current may allow you to get up to 50% more life out of your output tubes

    Bob

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