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Old 11th July 2018, 07:01 PM   #251
rmyauck is offline rmyauck  Canada
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The first capacitor right after the rectifier tube can't be too high if that's what you mean? It should be about 30uf at the most and there is a very popular SS diode mod that can help the tube rectifier last much longer in big amps.

You simply add UF4007 (best ) or IF4007 Diodes in series with the PS trans secondary leads to pins 4 & 6 of the GZ-34(5AR4) rect. tube. You can use unused pins on the tube socket to mount the diodes if you wish, otherwise you can use heat shrink tubing on the diodes to protect from shorts.

Sonics are unaffected!

tube rectifier diode mod

Last edited by rmyauck; 11th July 2018 at 07:19 PM.
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Old 11th July 2018, 08:47 PM   #252
rmyauck is offline rmyauck  Canada
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I should add this info:

For the SS diodes mod , the banded end goes to the tube socket and they don't replace the tube.
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Old 12th July 2018, 12:09 AM   #253
TheGimp is offline TheGimp  United States
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The diode mod helps with the PIV rating of the tube, However it does not address inrush current.

The maximum capacitor value for the first capacitor after the rectifier is 60uF. Many current production tubes don't like that value and are better off at 40-47uF (which normally run on the high side and may actually be 60uF).

https://drtube.com/datasheets/5ar4-amperex1958.pdf

The second filter cap can be much larger as the choke or resistor acts as a current limiter so the tube does not see the full capacitance.
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Old 12th July 2018, 01:51 AM   #254
rmyauck is offline rmyauck  Canada
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I said 30 uF as dynaco spec'd 20 on the ST-70 and 30 on the Mark III. The SS diode mod should help the tube rect. with those 40-47 uF caps that run high also, but likely the added capacitance wouldn't be noticed sonically so maybe keep it low. One can experiment.
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Old 12th July 2018, 04:46 PM   #255
wanders is offline wanders  United States
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Thanks for the comments. My original question was whether the 100uf 500v cap I installed in the final position could be responsible for my rectifier tube issues. No one seemed to think that was likely. Instead there is concern that the first cap, 47uf 500v, might be too much. Of course that was the spec in the designer's p/s schematic (post 202). I measured the cap in my power supply and it read 44.9 uf; so it is not out of spec. Apparently, the question is whether it would be worthwhile to try a 30uf cap in that spot. I'm not sure what the overall impact would be, however.
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Old 12th July 2018, 05:01 PM   #256
rmyauck is offline rmyauck  Canada
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I do believe George has added that Diode "Mod" to the PS on his PCB's some years ago and it should help greatly for the tube failure. If George spec'd that value for the first cap it should be fine and it measures under 60 uF. Many new tubes aren't as rugged as the ones made back in the day.

Last edited by rmyauck; 12th July 2018 at 05:07 PM.
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Old 12th July 2018, 10:55 PM   #257
colnago55 is offline colnago55  United States
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The power supply for the monoblock version of the UDB is based on the SSE schematic. While the schematic doesn't show the diodes going to pins 4 and 6 per the suggested mod above, the later SSE boards have it.

So it looks like a good idea to me and one I am assuming that George has blessed.
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Old 14th July 2018, 11:26 PM   #258
Sprags is offline Sprags  United States
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Old 14th July 2018, 11:54 PM   #259
wanders is offline wanders  United States
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I did the diode mod. so far nothing has blown up. Seriously, it’s an easy fix and start-up is a little slower and smoother. Thanks for the tip.
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Old 17th July 2018, 01:53 AM   #260
dch53 is offline dch53  Australia
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Default V2 setup and PP signal swing

Quote:
Originally Posted by Tubelab_com View Post
R4 and R13 do adjust the current through the tubes, however it's easiest to set them by adjusting for plate voltage.

Power down, install V1 only, then power back up. Allow for some warm up time and probe the plate voltages. These are easiest to probe on the right hand ends (viewed from the top with silkscreen writing correctly oriented) of R6 and R7. I set these with R4 for about 100 - 110 volts with a 450 volt supply, around 120 volts for 500 volts, so I'm guessing you are looking at 140 to 140 volts. Readjust R10 so that both voltages are equal.

Power down, install V2, power up, wait for warm up, then adjust R13 for about 450 volts on the plates of V2. (ends of coupling caps closest to V2). Readjust R10 for equal voltages on V2's plates.
I'm puzzled by the 450V recommendation for the plates of V2. Even assuming a 500V supply this leaves very little room for signal swing.

Worst case you'd want the MOSFET sources to swing 150Vpp. This would mean the same for the V2 anodes.

It seems to me that 350V on the V2 anodes would be more appropriate.

Am I missing something here?

Last edited by dch53; 17th July 2018 at 01:54 AM. Reason: typo
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