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    Solid State Rectifier

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    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Solid State Rectifier

    Post by arledgsc on Sun Jun 30, 2013 11:43 am

    In the ST-120 I have enjoyed the Weber Copper Cap WZ68 as the high voltage rectifier.  The amp is ultra-quiet, detailed, and dynamic.  But I notice Weber sells a version of the Copper Cap without the sag resistor (WS1, diodes + limiter only) and am wondering if anyone has tried it?

    As near as I can determine the WZ68 sag resistor is about 38 ohms (17V drop @ 450mA).  The resistor's purpose is to simulate the tube rectifier voltage drop as current increases.  The would modulate the voltage supply slightly but the ST-120 has so much capacitance that the effect is probably minimized.   But would like to consider elimination of this waste of power and the heat it generates.  If, at idle, the ST-120 high voltage generates about 230mA  current (guesstimate) so the 38 ohm sag resistor dissipates 2W but even more playing music.  

    And going one step further has anyone used solid state diode rectification only (no resistor or thermistor limiter)?  I fully understand the purpose of the thermistor is to limit inrush current.   Removal could be hard on the tubes at start up so just asking.   Thermistors also get warm to hot in operation and another source of wasted power but not as much as the sag resistor.  And yes a relay board or standby switch would be critical to hold off high voltage until the tubes sufficiently warmed.   And with the changes assume the mild bump in high voltage at the first couple of filter caps is OK.  I could live with the WZ68 no problem but the device gets quite warm in operation.

    sKiZo

    Posts : 1308
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Solid State Rectifier

    Post by sKiZo on Sun Jun 30, 2013 2:40 pm

    Hey now ... that's MY question!! tongue 

    Same thing came to mind when I was evaluating things, so I got the word straight from Mr Bob on the subject:

    I don't recommend the Weber WS1 ... Without the slight sag resistor the B+ will come up almost immediately and the B+ voltage will be a little higher than normal. I have never use the WS1 for that reason. It may work OK but I would prefer the 3 to 5 second delay of the Weber WZ68. The higher B+ is only a problem if your line voltage is 122 VAC or above.


    So, apparently no sonic improvement worth speaking of, and you'd have to make some other mods to make it do what the WZ68 already does gracefully. I'd think it'd be ok if you already have a time delay board on the amp though. I've also got a 122+ average VAC here, so if anything, I'm hoping to drop that with the addition of another thermister on the hot side. A CL90 should drop the VAC a couple points, as they never go completely neutral, and get it more in the range of what the amp would like to see. Cheap insurance if it works.

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: Solid State Rectifier

    Post by arledgsc on Sun Jun 30, 2013 7:27 pm

    Thanks for the info Skizo and sorry to cut in line! I have no SQ issue with the WZ68 and why jinx the formula trying something else untested. And I'm running close to $500 in tube cost right now so maybe a little extra resistance isn't so bad if it gets me extra tube life or avoids a costly meltdown.

    sKiZo

    Posts : 1308
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Solid State Rectifier

    Post by sKiZo on Mon Jul 01, 2013 12:44 am

    That's my plan - still building mine, but I hope to keep those bottles healthy as long as possible. A CL90 thermister and the diode mod on the rectifier for starters, and I will be using the WZ68 ... at least at first. I also picked up on a "non-latching" GFCI adapter that will keep bad things from happening if the power blips.

    Also part of the reasoning behind the TungSol KT-120 tubes. Those won't be working hard at all in this chassis. Not sure what they do to the B+ ... maybe Bob can chime in with some measurements as he did some extensive testing using those tubes in his home rig.

    I've got a variac too, but I have concerns about going that route. They're not really built for continuous duty, and mine at least takes forever to stabilize.

    Doesn't mean I don't plan to have some fun ... and this may be a first in the ST-120. Got me a NOS big bottle Phillips 5R4GYS that I have high hopes for. Looks a whole lot like a GZ37, but at half the price. Specs look good except for a higher than "normal" voltage drop which shouldn't be an issue. Llooks right at home amongst the big trees in the power block, don't it?



    I like big bottles and I can not lie
    You other brothers can't deny ...

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: Solid State Rectifier

    Post by arledgsc on Mon Jul 01, 2013 8:45 am

    Nice one! Shocked  Is that chassis your creation?   With KT120s your B+ should be about the same as KT88s if you run 55mA idle current.

    sKiZo

    Posts : 1308
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Solid State Rectifier

    Post by sKiZo on Mon Jul 01, 2013 12:20 pm

    Yeah ... 55ma is what Bob recommended. He'd run his all the way up to 75ma with no issues, but no noticeable improvement in dynamics, so he cranked it back down.

    I've been having fun with an ST-120 here. Got a thread going on it ...

    http://dynacotubeaudio.forumotion.com/t1656-custom-chassis

    Thanx for the reminder - I really should update.

    Short version ... What you see in the pic is a dry fit of the main components just to check the layout. Stripped it down and been working on the wooden base for the last week. Put the last coat of poly on yesterday and that should be set up enough to do a final polish before assembly. I'll practice on a test board first - it's been really humid here, and the drying times are ridiculous. Got most of the small stuff mounted back to the chassis plate last night - I'll hold off on the transformers and meters until I get the case fully assembled - hopefully in the next day or so.

    EDIT > > I just did some digging and it seems I need to let the Watco wipe on poly finish I used cure for at least five days before attempting a final polish. Ah, well ... no rush. That'll give me time to find my orbital polisher.

    PS ... the plate is an original stolen design, roughly based on several other plates I've found on the web. Designed it using Front Panel Express free software, then ordered it from them. Scary part was doing the bend for the front panel.

    Mostly going for more room for those big bottles and larger chokes, not to mention meters and stuff. I also hid the VTA board and relocated the bias pots just for shitzengiggles. You can have all sorts of fun when you go outside the boundaries of the original chassis.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Solid State Rectifier

    Post by Bob Latino on Mon Jul 01, 2013 4:05 pm

    In my own VTA ST-120 amp I was running the KT120's up to 75 milliamps each mainly to test that the AMP could take the extra current flow and slightly higher filament demands of the KT120 tubes. I ran my own amp this way for 5+ hours with no ill effects on my own amp. That was a "test" and I don't recommend that anyone run KT120's at this bias level. Recommendation for biasing the VTA ST-120 is 55 milliamps per tube (.550 VDC per tube) the same as a KT88. I am sure that the KT120 tubes themselves can take a higher B+ and current flow than the VTA ST-120 can provide. In the VTA ST-120 the Tung-Sol KT120 tubes "are used as KT88's" > IOW at KT88 current flows and output levels. They also give a slightly "warmer sound" than KT88's and allow the end user to (if they want to) tailor the sound slightly to the warmer side of neutral. Tests show that the KT120's give very slightly more power (about 8%) than KT88's in the VTA M-125 monoblocks by just a simple tube substitution. I am sure that there would also be a similar slight power increase in the VTA ST-120. Someone Emailed me and asked why the slight increase in power with just a tube substitution? The only thing that I can figure is that the larger plates on the KT120 tubes possibly transfer more electrons than KT88's at the same drive level from drive circuit?

    Bob

    sKiZo

    Posts : 1308
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Solid State Rectifier

    Post by sKiZo on Mon Jul 01, 2013 4:56 pm

    Less restrictive grid maybe?

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