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    Rectifier question for Bob

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    anbitet66

    Posts : 131
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Rectifier question for Bob

    Post by anbitet66 on Wed Sep 29, 2010 8:37 pm

    Bob, I will soon continue the assembly of an ST70 I pieced together from parts around the internet (sorry, I collected the parts about 4-5 years ago and didn't know about you at the time). I somehow found in my parts box a 1N2389 rectifier. I am curious to try it. Are you familliar with this device? It seems to be a pair of diodes in a metal can, the size of a small metal tube and wired to replace the usual types like 5U4, 5AR4, etc., and similar to a Weber Copper Cap without the time delay feature. Looking on google came up with specs of 600mA and 1600PIV. So I'd guess it could handle being in a stock ST70?

    Would it be appropriate to use it instead of a new production 5AR4? I have a couple of new 5AR4's, but I keep reading of short life spans and fireworks when they go.


    Thank you,

    Tony

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Rectifier question for Bob

    Post by Bob Latino on Thu Sep 30, 2010 5:54 am

    Hi Tony,

    You could *probably* use that rectifier in an ST-70 with no problems but ...

    1. The B+ voltage will undoubtedly be higher with the 1N2389 rectifier. How much higher the voltage will be AND whether the somewhat higher voltage would cause any problems are questions that are not easily answered.

    2. There would be no slow warm up like on a GZ34 tube rectifier. The B+ would be available instantly when you turn the amp on unless you wired in a B+ delay switch. Some say that this "no B+ delay" can cause "cathode stripping" on the output tubes but I personally don't feel that this is a major issue. Dynaco use solid state rectifiers with no B+ delay in the ST-35 and SCA-35 amps. Thousands of these amps are still working fine after 40+ years.

    Bob

    anbitet66

    Posts : 131
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Re: Rectifier question for Bob

    Post by anbitet66 on Thu Sep 30, 2010 3:51 pm

    I agree that "cathode stripping" is overblown. I have other tube equipment and it seems to me a bigger issue is coupling capacitors that become leaky over time will destroy an output tube much faster. I have replaced coupling caps and output tubes (6BM8 and 6BQ5) that were nearly dead on my B&K747b. And since I have an 80,40,30,20 quad cap rated at 525Vdc I will use the chinese 5AR4 I have and report what happens.

    Thank you,

    Tony

    dynacojoe

    Posts : 8
    Join date : 2012-08-22
    Age : 59

    ST-70 Rectifier and Fusing Choices

    Post by dynacojoe on Wed Aug 22, 2012 8:56 pm

    I used solid state rectifiers (with no delay) for a few weeks in my ST-70. I found 7199 triode plate voltage to exceed 400 volts immediately following power on during the few seconds before plate current started flowing. I realize that in many tube amps that are factory equipped with solid state rectifiers this situation has been going on for decades with little trouble. But when one of my 7199s shorted plate to grid in the triode section I went back to a GZ34 to take advantage of the sequencing it offers. It is worth noting that 7199's are rated for 330 max plate volts.

    Also I will offer my .02 on "hot switching" or "short cycling". Yes this will eat a GZ34 on occasion. But what to do about those brief power line interruptions over which we have no control? This may not be for everyone but it has worked fine for me for several months. I changed out the recommended 3 amp fuse with a 1.5 amp fast blow fuse. No problems for several months now and this is a daily driver. Here is why this works: The amp draws about 1.3 amps most of the time so the 1.5 amp fuse is happy. If you crank it loud the current through the fuse will get up to 1.8 amps on the peaks, however a typical 1.5 amp fuse needs at least 3 amps flowing for about 5 seconds to blow it. I have had absolutely no nuisance fuse pops. I did however get up the nerve to test this idea (on a low cost GZ34) by deliberately "hot switching". The result? A blown fuse. I replaced the fuse and the GZ34 was fine. At the greatly elevated rectifier current flow following a "hot switch" the lower rated fuse will open in as little as 10 milliseconds per the fuse charts and it will be able to save the tube. A 3 amp fuse will not be able to open in time, or at all, to save the tube. The 3 amp fuse will save your iron, never your tubes. In addition, the 1.5 amp fuse will protect the EL34s. I have learned these lessons the expensive way. I encourage anyone reading this to check out a fuse time/current chart and take a close look at a complete GZ34 data sheet. It will be an eye opener!

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Rectifier question for Bob

    Post by Bob Latino on Thu Aug 23, 2012 1:09 pm

    A couple of years ago we had a another post on the dangers of "short cycling" Dynaco tube amps. A momentary and quick ON-OFF-ON from your local power company CAN take out a rectifier. There are a couple of ways around this.

    1. Get a portable GFCI (Ground Fault Circuit Interrupter) that requires a MANUAL RESET. If the power goes out even for a split second then the power will STAY OFF and will require you to manually reset the GFCI device back to being ON. Plug this portable GFCI into the wall outlet for your amp and then the amp into the GFCI. There are a number of such devices on the market. Below is a link to one such device ...

    Portable GFCI on Amazon.com

    2. Add the VTA TDR (Time Delay Relay) to your Dynaco tube amp. The VTA TDR ($35 + $5 shipping) delays the application of high voltage to your amp for 17 seconds every time you turn the amp on. If there is a power interruption the TDR will reset itself when the power returns and no high voltage will be applied again for another 17 seconds. The delay is also user adjustable (more or less than 17 seconds) by changing out one resistor on the board. More info on the VTA TDR is at the link below ...

    VTA TDR

    Bob


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