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    Poor man's solid state replacement rectifier for Dynaco ST-70, Mark II, Mark III or Mark IV

    Bob Latino
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    Post by Bob Latino Thu Jan 28, 2010 9:03 am

    OK - maybe you have wanted to try a solid state rectifier in your Dynaco ST-70, Mark II, Mark III or Mark IV. You can buy a commercially made solid state rectifier that plugs right into your rectifier tube socket. Two that comes to mind are the Weber Copper Cap (model WZ34 or WZ68 - either will work fine - $22 + shipping) or the T-SSR01 solid state rectifier from Antique Electronic Supply ($10 + shipping)

    You CAN also make your own solid state rectifier. Two ways to do it ... First thing to do is get two 1N4007 diodes. Diodes are very cheap. Radio Shack sells a 30 packet of them for $2.79.

    Option 1 > Get an old octal tube that doesn't work anymore and carefully break the glass of the tube. Wear eye protection and gloves and wrap the tube in an old towel and smack it with a hammer. Get rid of all the glass and all the internal tube parts. Use a soldering iron and heat up the pins and pull out all the old wiring to the pins. Run one diode from pin 4 to pin 8. Make sure that the banded end of the diode faces pin 8. Solder both ends into the pins on 4 and 8. Run a second diode from pin 6 to the metal on the banded end of the first diode. Solder both connections. Both banded ends of both diodes should face pin 8. Hey - you've just made a suitable solid state rectifier for your amp.

    Option 2 > You can also just remove your rectifier tube from its socket and hard wire the two diodes on the inside of the amp to the pins mentioned above. One diode goes from pin 4 to 8 and the other goes from pin 6 to 8. Again, the banded end of both diodes is towards pin 8.

    When you turn the amp on the bias will be higher than normal because diodes have a lower forward voltage drop than a tube rectifier. Your 1.56 volts DC bias voltage with the tube rectifier may be at 1.90 volts DC with your solid state rectifier. Maybe you should turn both bias controls DOWN a little even before you turn the amp on. Let it warm up a little and rebias both channels. Solid state rectifiers will supply more high voltage at higher volume levels. The result of this fact is that you will usually notice tighter and better bass at higher volume levels by using a solid state rectifier. At normal listening levels you won't really notice any difference between a tube and solid state rectifier.

    Now the thing is when you turn the amp on the high voltage will be there immediately but you will get no bias at your bias measuring points until the output tubes heat up. Some say that this is bad for your output tubes and can cause "cathode stripping" of the output tubes. The use of a GZ34 rectifier tube does allow the high voltage to come on slower. Some say you should put in a manual "high voltage delay" switch in your amp if you use a solid state rectifier. If you want you can easily put in a SPST switch in series between pin 8 of the rectifier tube and the quad cap. What you would do in a case like this is turn your amp on, wait about 30 seconds and then turn your delay switch on to apply the high voltage. Personally, I question whether a switch like this is really needed. Keep in mind that both the Dynaco ST-35 power amp and the Dynaco SCA-35 integrated amps had solid state rectifiers with NO HIGH VOLTAGE DELAY. The fact that thousands of these ST-35 and SCA-35 amps are still working fine after 40+ years leads me to believe that the need for a high voltage delay switch may be somewhat overstated.

    Bob

    Poor man's solid state rectifier schematic below ...

    Poor man's solid state replacement rectifier for Dynaco ST-70, Mark II, Mark III or Mark IV Solidstatereplacementdiodes

    Commercial solid state replacement rectifiers. T-SSR01 on left and a Weber Copper cap on the right

    Poor man's solid state replacement rectifier for Dynaco ST-70, Mark II, Mark III or Mark IV Solidstaterectifiers

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    Sal
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    Post by Sal Thu Jan 28, 2010 10:01 am

    Bob is right, these are very easy to make. I made one last year with a series resistor to simulate the voltage drop of a 5AR4 tube. Here is a photo of it.

    Regards,
    Sal

    Poor man's solid state replacement rectifier for Dynaco ST-70, Mark II, Mark III or Mark IV Ss5ar4
    Bob Latino
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    Post by Bob Latino Thu Jan 28, 2010 10:08 am

    Hi Sal,

    Thank you for the photo ... Whether you need the drop down resistor or not would depend on the high voltage that you measure off pin 8 of the rectifier socket when using the SS rectifier. If you approach 450 volts DC on an ST-70 then you should probably use the drop down resistor.

    Sal do you remember what resistor value you used in there and maybe what was the amount of voltage drop in your home made SS rectifier ?

    Bob
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    Post by Sal Thu Jan 28, 2010 10:30 am

    Bob,
    I don't remember off hand but when I get home from work tonight I will take a look.

    The value of the resistor is dependent of the load.

    Regards,
    Sal
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    Post by Sal Thu Jan 28, 2010 11:53 pm

    Bob,
    I put a 5 watt 100 ohm resistor in my SS rectifier.

    Regards,
    Sal
    Sfguitarworks
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    Post by Sfguitarworks Mon Jul 05, 2021 5:28 pm

    Hi bob, could I use an 822 1N4001 diode in this application? Thanks! Geoff
    Bob Latino
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    Post by Bob Latino Mon Jul 05, 2021 6:17 pm

    Sfguitarworks wrote:Hi bob, could I use an 822 1N4001 diode in this application? Thanks! Geoff

    A 1N4001 diode is only rated at 50 volts and 35 volts reverse voltage. I don't think that a 1N4001 is the proper diode to use in that situation. You could probably use a 1N4007 which is 1000 volt rated and is 700 reverse volt rated.

                                                                        Bob
    Sfguitarworks
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    Post by Sfguitarworks Mon Jul 05, 2021 7:09 pm

    Thanks Bob. I just ordered the premade. With that in mind, any thoughts on converting the stereo/mono switch to a standby? Thought it would be useful with the ss rectifier.
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    Eshkenazi


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    Post by Eshkenazi Fri Aug 26, 2022 4:56 am

    Hi,
    I would like to build a Mark IV by using a selenium rectifier. Could someone help me with the levels of the voltage and the currunt ( Ex. E60C200 ) ? As I found a cheep place to order the power traf, could you please help me as well what are the output voltages and currents ?
    Thank you in advance. Smile
    lorne
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    Post by lorne Mon Sep 05, 2022 6:34 pm

    Many years ago, I used an ST-70 on a daily basis with two 1N4007's soldered on the octal pins for SS rectification — just as in the above drawing. Otherwise it was a stock unit that ran on the same tubes the whole time.

    After many idle years, I'm currently rebuilding this amp using parts I collected over time — before I ever heard of Tubes4hifi / VTA. It will run a Sovtek 5AR4, the yellow sheet mod, and thermistors to control inrush. I would return to SS rectification, but I'm trying to protect the 450 Volt rated caps on a Triode Electronics cap board. If I were doing this again, I'd use a new quad cap. Back in the days, new quads were hard to find, and some suppliers were not shipping to Japan. But for the larger power X-former, the rest off the amp will be in stock architecture using NOS GE 7199's and Svetlana EL-34's.


    Last edited by lorne on Mon Sep 05, 2022 6:43 pm; edited 1 time in total (Reason for editing : Company name correction)
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    Isstatu1


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    Post by Isstatu1 Mon Mar 20, 2023 10:00 pm

    Bob,

    I've acquired an ST70 that I believe has this mod, but I think it's uninatalled incorrectly. Could you please confirm? I get no bias readings.

    Poor man's solid state replacement rectifier for Dynaco ST-70, Mark II, Mark III or Mark IV 20230310
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    zebu


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    Post by zebu Wed Mar 29, 2023 2:40 am

    Hello. I'll be honest I joined up to ask a question.

    I had been researching a DIY solid state rec for my JTM45, and came across Bob Latino's "poor man's...etc" thread. I'm happy to solder the diodes directly
    to the existing socket. However having replaced a blown solid state bridge rectifier with four loose diodes on a Sovtek Mig50, I do wonder why we are only using
    two diodes with this method and not the usual four. In the case of the Mig, I replaced the encapsulated rectifier with four 4007s twisted together.

    Any insights? Thank you
    peterh
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    Post by peterh Wed Mar 29, 2023 10:49 am

    zebu wrote:Hello. I'll be honest I joined up to ask a question.

    I had been researching a DIY solid state rec for my JTM45, and came across Bob Latino's "poor man's...etc" thread. I'm happy to solder the diodes directly
    to the existing socket. However having replaced a blown solid state bridge rectifier with four loose diodes on a Sovtek Mig50, I do wonder why we are only using
    two diodes with this method and not the usual four. In the case of the Mig, I replaced the encapsulated rectifier with four 4007s twisted together.

    Any insights? Thank you
    Dynaco uses middle tapped HV transformer thus only 2 diodes are needed.

    You _could_ use an old octal socket ( from a broken tube) as mounting "plate" and
    plug this in the rectifier socket, thus no changes in the amp.

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    Tom Pickett
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    Post by Tom Pickett Tue May 16, 2023 3:49 pm

    I made a post today about something similar to this... I've been having trouble with my brand new Ruby 5U4GC rectifier tubes red plating after 15 to 20 seconds.

    I tried to install these two diods but the problem is this brings the B+ up to about 810 volts and I was afraid to leave it on for too long that it might short out a lower voltage capacitor somewhere...

    May need to install a resistor to bring the voltage down.. ?
    peterh
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    Post by peterh Tue May 16, 2023 4:48 pm

    You have a problem . No way 810Volt is possible with just 2 diodes, either your voltmeter
    is uncalibrated or you have connected the power transformer wrong.
    The B+ windings has a midpoint that is grounded and the two "outside" connects to
    the two plates in the GZ34 ( pin 4 and 6 )

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    Tom Pickett
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    Post by Tom Pickett Tue May 16, 2023 5:48 pm

    peterh wrote:You have a problem . No way 810Volt is possible with just 2 diodes, either your voltmeter
    is uncalibrated or you have  connected the power transformer wrong.
    The B+ windings has a midpoint that is grounded and the two "outside" connects to
    the two plates in the GZ34 ( pin 4 and 6 )

    Thats my bad.... I was reading it as AC... The meter should be set to DC for which I'm now reading 410 V.... Sorry... My bad... :-)
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    Post by stewdan Fri May 19, 2023 12:27 pm

    Bob or Sal ---

    One end of the 5 Watt 100 Ohm Resistor in Sal's Picture goes to Pin 8

    How to connect the other end of the Resistor?

    Diode Anodes to Pins 4 & 6, leave diodes upright, twist the Cathode ends together and join to the other end of the resistor.

    ELECTRICALLY THIS SHOULD WORK, AM I CORRECT??

    Thanks

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