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    selenium rectifier

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    Clik2media

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    selenium rectifier

    Post by Clik2media on Mon Jan 19, 2009 3:06 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
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    Bob Latino
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    RE: Replacing a selenium rectifier with a diode

    Post by Bob Latino on Mon Jan 19, 2009 3:46 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
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    Clik2media

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    Re: selenium rectifier

    Post by Clik2media on Mon Jan 19, 2009 6:34 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
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    Bob Latino
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    Re: diode

    Post by Bob Latino on Mon Jan 19, 2009 6:42 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
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    Clik2media

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    Re: selenium rectifier

    Post by Clik2media on Mon Jan 19, 2009 6:45 pm

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

    Michael
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    Sal

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    Re: selenium rectifier

    Post by Sal on Thu Feb 05, 2009 6:14 pm

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

    Sal

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    Bob Latino
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    Re: selenium rectifier

    Post by Bob Latino on Thu Feb 05, 2009 6:28 pm

    Sal,

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

    Bob
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    Sal

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    Re: selenium rectifier

    Post by Sal on Thu Feb 05, 2009 11:44 pm

    Your Welcome Bob.

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

    Regards,
    Sal
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    Clik2media

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    Re: selenium rectifier

    Post by Clik2media on Sun Mar 08, 2009 2:52 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
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    Bob Latino
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    Re: selenium rectifier

    Post by Bob Latino on Sun Mar 08, 2009 9:34 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
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    Clik2media

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    Re: selenium rectifier

    Post by Clik2media on Sun Mar 08, 2009 11:13 pm

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

    Post by DynakitParts on Sun Mar 08, 2009 11:55 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
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    Clik2media

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    Re: selenium rectifier

    Post by Clik2media on Mon Mar 09, 2009 12:02 am

    Thanks Kevin , you guys are great!

    Mike

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    selenium rectifier

    Post by GP49 on Mon May 04, 2009 10:49 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.
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    Bob Latino
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    Re: selenium rectifier

    Post by Bob Latino on Mon May 04, 2009 11:33 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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    Re: selenium rectifier

    Post by GP49 on Mon May 04, 2009 1:16 pm

    The recommendation of a lower idle current (bias) could be a matter of what tubes are used. The current tubes from China, Russia and Eastern Europe reportedly are not as durable as the Mullards that Dynaco originally supplied, and on which they based their recommendations. Lower idle current would help lengthen their lives.

    Of course that would lengthen the lives of Mullards, too.

    Flyquail
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    Re: selenium rectifier

    Post by Flyquail on Mon May 04, 2009 6:02 pm

    Hi Bob and everyone,

    This conversation reminds me of a comment on Sheldon Stokes' website that I read several years ago:

    For example, if the bias current is reduced on the output tubes to lengthen their lives, the power supply modulates more with the musical signal, and the IMD goes through the roof.

    I have not tried to verify this myself. Any thoughts?

    Best regards,
    Mike
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    Bob Latino
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    Re: selenium rectifier

    Post by Bob Latino on Mon May 04, 2009 6:54 pm

    Hi Mike,

    The power supply will always "modulate" with the musical signal at any bias point. Whether it modulates more at a lower bias point is a question I don't know the answer to. Dynaco ran their tubes hard. In the original Mark II their recommendation of 1.56 volts divided by the 12 ohm bias resistor on the Mark II works out to 130 milliamps per tube PAIR. This is 65 milliamps for each EL34 output tube. This is way high by today's standards of about 40-45 milliamps for an EL34 tube - BUT - they were trying to squeeze 50 watts from the Mark II and they pounded those tubes hard. They must have gotten short life out of those output tubes. When the Mark III replaced the Mark II in 1957 they wisely used KT88 output tubes which can handle a higher plate voltage and a higher bias point than an EL34 so they dropped the bias resistor from a 12 ohm resistor on the Mark II to an 11.2 ohm resistor on the Mark III and advised the same 1.56 volt DC bias point. The same 1.56 volt DC bias recommendation biased the KT88 output tubes at 140 milliamps per tube pair or 70 milliamps per individual tube. This is still a little on the warm side but KT88's are more robust than EL34 output tubes and can handle a higher plate voltage and a higher bias point much better than an EL34. Conventional wisdom today is 55 to 60 milliamps bias current per each KT88 which puts KT88's in an area of operation which gives both good tube life and good sound quality.

    Bob

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    Re: selenium rectifier

    Post by Flyquail56 on Mon May 04, 2009 9:25 pm

    Bob,

    Thanks for your reply; good points! I realize that I may have taken that quote out of context a little bit. Sheldon was referring to the ST-70, and making the point that some of the design of the amp was such that it compensated for the cost-cutting that went into it, i.e. the output tubes were biased more into class A, which made the current demands on the wimpy power supply more constant.

    I would also imagine that since most people increase the B+ filtering when reconditioning these amps (or even upgrade the PT), the reduction in output tube bias is a non-issue when that has been done.

    Best regards,
    Mike
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    WireNut

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    Re: selenium rectifier

    Post by WireNut on Wed Feb 22, 2017 1:16 pm

    Hi, newbie to this forum- but the ST-70 was my first tube amp back in the 70's. I just bought one and it is due to arrive this Fri. Could anyone supply me with a photo of the selenium rectifier in place on the amp before it was modded? I want to address this right away.
    It sounds like it is a killer (like the death diodes on a VFET).
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    j beede

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    Re: selenium rectifier

    Post by j beede on Wed Feb 22, 2017 2:12 pm

    WireNut wrote:Hi, newbie to this forum- but the ST-70 was my first tube amp back in the 70's. I just bought one and it is due to arrive this Fri. Could anyone supply me with a photo of the selenium rectifier in place on the amp before it was modded? I want to address this right away.
    It sounds like it is a killer (like the death diodes on a VFET).

    Check out the ST-70 pictorial diagram that Bob provides to the forum. It shows the location of the Selenium stack. FYI: I've never had a Selenium rectifier fail in any Dynaco unit in my experience--though I always replace them with the smaller, safer, cheaper, lower forward voltage 1N4007.

    The power supply capacitors deserve your attention. Do you know how long it has been since the "70" you bought was powered up?

    ...j
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    WireNut

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    Re: selenium rectifier

    Post by WireNut on Wed Feb 22, 2017 2:48 pm

    That one was redone for me by one of the tech's at the stereo store I worked at "Sound of Music" it was actually two strapped together by an umbilical.
    This one has the SDS mods done, the power and input.
    The guy did not do the rectifier, but now that you mention stack- I know what you mean... thought there may be another one.
    There was a different post on this forum about those caps having a low voltage rating, I will post on that one.
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    WireNut

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    Re: selenium rectifier

    Post by WireNut on Wed Mar 08, 2017 1:58 pm

    I got the amp and I think because of the SDS Labs cards my hook up is a little different.
    Can anyone tell me how I should bypass my selenium with the 4007?

    http://s593.photobucket.com/user/david_finley2/media/Sel%20Rec_zpswfnqorh4.jpg.html?filters[user]=144860216&filters[recent]=1&sort=1&o=0

    I thought if you uploaded a link to a Photobucket shot that the photo would be visible to the forum?

    stewdan

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    Re: selenium rectifier

    Post by stewdan on Wed Mar 08, 2017 6:35 pm

    Hi Wirenut ---- I looked at your photobucket picture and there is NO Selenium Rectifier attached to the Connection Strip located between the two Bias Pots.  Two of the solder terminals are just used to connect wires together.

    The Circuit Board which you identified as the Input MOD is located above the Bias Pots.  It has SDS on it and looks like one of Sheldon Stokes' Boards which were and are still marketed by www.Triodelectronics.com in the Chicago area.  They may have some documentation on-line.  Check to see if your PCB matches their pictures of what they sell.

    Check your Power MOD Circuit Board, it should have a bunch of electrolytic caps and resistors and may have 2 or 4 1N4007 type  voltage regulators on it. It replaces the Quad Cap on top of the Chassis if it is still there, check the bottom of the cap, there should be no wires attached to it on the underside of the chassis.  There is a picture of a 1N4007 mounted on a connection strip (between the Bias Pots) in POST #5 or #6 of this message stream.

    If the Power MOD PCB has 4 1N4007 type regulators on it, then 2 of them are the replacements for the Selenium Rectifier.
    If just two, then I don't know where they put the regulators.

    Please let us know what you find.  More questions, just ask.
    Stew
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    WireNut

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    Re: selenium rectifier

    Post by WireNut on Wed Mar 08, 2017 6:50 pm

    Here it is but I only see one diode?

    http://s593.photobucket.com/user/david_finley2/media/Sel%20Rec_zpswfnqorh4.jpg.html?filters[user]=144860216&filters[recent]=1&sort=1&o=0

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