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    Triode SDS Cap Board (ST-70) Resister Values

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    jerkbag

    Posts : 3
    Join date : 2012-04-19

    Triode SDS Cap Board (ST-70) Resister Values

    Post by jerkbag on Thu Apr 19, 2012 12:06 pm

    Hi All,
    Based on advice on here I've decided to upgrade to a triode SDS cap board. I have a KTA Hifi Driver board, so my E19/E20 B+ Voltages that I'm looking for are 380v/250v. I've got the board hooked up with jumpers on the bench, and I am finding my R9/R10 resister values (the 6.8k/~22K from quad cap). to go WAY low to get my 380/250. My B+ coming at the first stage settles to ~410, which is a little low (420-450 is normal, right) but I've tried different rectifier tubes so I'm thinking that's just how it is. But even with 2.7K/14K resisters the driver board B+ is still too low.

    My question is -- is there anything to worry about? Or do I just use whatever resister values I need, or even jumper the R9, to get the desired voltages? On forums I am reading people using 6.8/50K resister combos to get the desired voltages of 380/250. Though perhaps this board is setup different then a reg quad cap.

    Anyone have experience with this?

    EDIT: Actually looking further it seems like the voltage is too low, as first quad cap stage should be like 480, not 410, right? Okay so my steps are to confirm rectifier is putting out ~480 at pin 8 disconnected. If that's the case, where should i be looking for my voltage drop? I have the bias pots all set to 0v as I'm testing the board, does that matter? Thanks!

    EDIT #2: Okay I promise this will be my last edit. I am confused because I have seen Bob post a voltage chart like this

    GZ34 – Pin 2 to ground – 420 - 450 volts DC
    Pin 8 to ground – 420 - 450 volts DC
    Pin 4 to ground – 355 - 380 volts AC
    Pin 6 to ground – 355 - 380 volts AC

    Any EL34 – Pin 1 or pin 8 to ground – approx .400 volts DC (depends on bias setting)
    Across pins 2 and 7 – 6.0 – 6.5 volts AC
    Pin 3 to ground – 400 - 435 volts DC
    Pin 4 to ground – 400 - 435 volts DC (usually 1 or 2 volts higher than pin 3)
    Pin 5 to ground – minus 30 to minus 40 volts DC
    Pin 6 to ground – minus 30 to minus 40 volts DC
    Pin 8 to ground – approx .400 volts DC (depends on bias setting)

    Quad cap –-- Section # 1 (square symbol) –------ 420 – 450 volts DC
    Section # 2 (half circle symbol) --- 420 – 445 volts DC
    Section # 3 (NO symbol) ------------- 420 – 445 volts DC
    Section # 4 (triangle symbol) ------ 380 – 420 volts DC

    And yet somewhere else I have seen an identical chart that has ~480 on the PIN GZ34, and about +50v more on each stage of the quad cap. I believe above is for a VTA driver board.

    I guess my question is -- does/can the driver board (in my case the KTA HiFi one) alter all the quad cap voltages? And in that case as long as my disconnected from Cap Pin 8 Rectifier reading is good, should I just adjust resister values as needed to get my desired voltages? It still seems weird that mine are so different from others using the same board.

    Sorry for the long post


    Captain Coconut

    Posts : 213
    Join date : 2009-10-13
    Age : 64
    Location : Great White North

    Re: Triode SDS Cap Board (ST-70) Resister Values

    Post by Captain Coconut on Thu Apr 19, 2012 2:34 pm

    Hi Jerkbag, I'll leave that question to the experts, but while I'm here - are you related to my neighbor Dirtbag?

    danf

    Posts : 58
    Join date : 2009-01-19

    AC voltage at transformer?

    Post by danf on Thu Apr 19, 2012 3:25 pm

    Your output voltage will vary proportionally to the AC voltage at your plug. If your power supply produces 450 VDC with 125 VAC in, you will measure 414 V with 115 VAC in. A range of 115VAC to 125VAC is encountered over the US, and the voltage will usually be time dependent in any one location. You can get these cool "Kill-a-Watt" meters to safely monitor your local conditions. The amplifier output power will vary with these voltage variations. If I am trying to get repeatable measurements, I will use a constant voltage transformer and/or a variac.

    Low power supply voltages can be associated with bad filter capacitors, but you will want to see if your AC voltage is on the low side before looking for other issues.

    jerkbag

    Posts : 3
    Join date : 2012-04-19

    Re: Triode SDS Cap Board (ST-70) Resister Values

    Post by jerkbag on Thu Apr 19, 2012 7:06 pm

    Thanks. Okay it turns out it's something with the rectifier. What to make of this:

    To check I lifted pin 8 wire to the quad cap and measured DC at pin 8 to chassis with rectifier hooked to nothing, and it was LOWER then when it was hooked up to the Quad Cap, around ~330v. I checked pins 4/6 and they were a solid 370 ac. I swapped 5AR4s and same thing. To be extra sure I removed the 1n4007 diodes I had from pin 7/6 and 5/4 on the rectifier tube socket. At this point, it started blowing fuses 5 seconds or so after startup.

    What am I missing here? This can only be the rectifier, but if I've tried different tubes, it is possible this is the rectifier tube socket? It makes no sense to me how the tube socket itself could cause this -- is that possible?

    danf

    Posts : 58
    Join date : 2009-01-19

    You need a capacitor!

    Post by danf on Thu Apr 19, 2012 7:42 pm

    This measurement doesn't prove that the issue is the rectifier.

    If you have no reservoir capacitor, you will not develop the full DC voltage. Remember that the rectifier only produces clipped AC pulses, not true DC. You **must** have a capacitor connected for valid measurements. Did you check your AC input voltage?

    Before proceeding further you might have a look at the power supply tutorials at
    http://www.freewebs.com/valvewizard/index.html

    Please be cautious working around these voltages. It is easy to get distracted when debugging.

    jerkbag

    Posts : 3
    Join date : 2012-04-19

    Re: Triode SDS Cap Board (ST-70) Resister Values

    Post by jerkbag on Fri Apr 20, 2012 8:30 am

    Of course -- thanks Danf! I kept thinking that isolating the rectifier tube would be the way to track down issues. Anyway -- all is good now. What it turned out being was that I had the bias to 0v intending to set it after the quad cap stage was working -- after getting the bias in the ballpark the voltages all came up into the normal range. I guess that's what it was the whole time I didn't realize it affected the other voltages.

    On a side note: anyone know how to properly mount a Triode SDS cap board -- do you remove the OT bolts entirely and replace with the mounting hardware?

    harpy

    Posts : 94
    Join date : 2012-02-04

    Re: Triode SDS Cap Board (ST-70) Resister Values

    Post by harpy on Fri Apr 20, 2012 10:47 pm

    Don't know if this helps (MkIII) : "I have devised a printed circuit board that bolts directly under the output transformer without having to move any parts. It can be mounted in place using longer 8-32 bolts and a couple stand-offs or using the original transformer bolts and 8-32 threaded stand-offs."

    I just bought the blanks boards for the Mk-III and they came with no hardware. I was curious on the length of the stand-offs. I know you got the ST-70 kit, but I would think the chassis depth would be similar.

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