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    Diode Mod plus TDR

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    WntrMute2

    Posts : 98
    Join date : 2010-11-21

    Diode Mod plus TDR

    Post by WntrMute2 on Sun Apr 22, 2012 10:34 am

    I just finished installing the TDR which seems to be working properly as I have 420 volts at the appropriate pins on V2. The problem is that after the diode mod, I only have 270 volts at pins 4 and 6 on V2. As mentioned the appropriate voltage shows up on thins 5 and 7. Any ideas as to what is going on here?
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    anbitet66

    Posts : 132
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Re: Diode Mod plus TDR

    Post by anbitet66 on Sun Apr 22, 2012 12:14 pm

    Don't check the voltage there, you will be reading pulsing DC. If you use a digital meter, it won't give accurate results.

    Check the output voltage at pin 8. It should have dropped about a volt from previous readings without the diodes.

    Tony

    WntrMute2

    Posts : 98
    Join date : 2010-11-21

    Re: Diode Mod plus TDR

    Post by WntrMute2 on Sun Apr 22, 2012 7:04 pm

    Not to doubt the above answer, but anyone else want to contribute? I'm afraid I didn't check the voltage at pin 8 prior to the mod. maybe I should undo measure and redo.
    Thanks Dave

    Bob Latino
    Admin

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

    Re: Diode Mod plus TDR

    Post by Bob Latino on Sun Apr 22, 2012 7:26 pm

    On an ST-70 pins 4 and pin 6 on the rectifier tube socket to chassis ground should measure maybe 355 to 370 volts AC. The diode mod should have virtually no effect on the AC voltages you measure off those two pins. If your rectifier is in there and your quad cap is connected to pin 8 of the rectifier tube, you should measure about 420 to 440 volts DC off pin # 8 of the rectifier tube to chassis ground. The actual voltages that you do get are predicated on your line voltage. Sometimes even a couple of volt rise on your AC line voltage can cause noticeably higher AC voltages on the power transformer. As a result all your AC and DC voltages on the amp will be higher. Higher than normal line voltages will also shorten tube life especially the rectifier tube. If your amp is going through a few rectifier tubes each year, you should check your line voltage. If your line voltage is more than 121 volts, I would run your amp off a variac set at 118 volts. The amp will sound the same with no noticeable loss of power and your tube life will increase.

    Bob

    WntrMute2

    Posts : 98
    Join date : 2010-11-21

    Re: Diode Mod plus TDR

    Post by WntrMute2 on Sun Apr 22, 2012 8:56 pm

    Thanks Bob, that's what is weird, the voltage at pins 4 and 6 are 270V AC. (after the diodes) but 420 or so when measured before the diodes. I've buttoned it up and it sounds fine but I would like to understand what is going on. My line voltage is 120 and stable.

    GP49

    Posts : 732
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Diode Mod plus TDR

    Post by GP49 on Sun Apr 22, 2012 9:48 pm

    WntrMute2 wrote:Thanks Bob, that's what is weird, the voltage at pins 4 and 6 are 270V AC. (after the diodes) but 420 or so when measured before the diodes. I've buttoned it up and it sounds fine but I would like to understand what is going on. My line voltage is 120 and stable.

    Perhaps your digital voltmeter is not reading TRUE RMS voltage on the AC scale?

    hawaii.ken

    Posts : 157
    Join date : 2012-01-31

    Re: Diode Mod plus TDR

    Post by hawaii.ken on Sun Apr 22, 2012 11:35 pm

    WntrMute2 wrote:Thanks Bob, that's what is weird, the voltage at pins 4 and 6 are 270V AC. (after the diodes) but 420 or so when measured before the diodes. I've buttoned it up and it sounds fine but I would like to understand what is going on. My line voltage is 120 and stable.

    Draw a sine wave with a line in the middle separating the top half from the bottom half. Now erase the bottom half. That's what you're trying to measure.

    1973shovel

    Posts : 23
    Join date : 2009-12-13

    Re: Diode Mod plus TDR

    Post by 1973shovel on Mon Apr 23, 2012 6:22 pm

    Bob Latino wrote:Sometimes even a couple of volt rise on your AC line voltage can cause noticeably higher AC voltages on the power transformer. As a result all your AC and DC voltages on the amp will be higher. Higher than normal line voltages will also shorten tube life especially the rectifier tube. If your amp is going through a few rectifier tubes each year, you should check your line voltage. If your line voltage is more than 121 volts, I would run your amp off a variac set at 118 volts. The amp will sound the same with no noticeable loss of power and your tube life will increase.

    Bob

    This is why I like using a CL-90 inrush current limiter, yes, even when using a 5AR4. I have a CL-90 mounted in an external outlet box, which I plug my various Dynaco amps into, then the outlet box plugs into the wall. As an example, I just checked my wall voltage at 122.6 VAC. I plugged my VTA ST-70 (with a 5AR4 rectifier) into the external CL-90 equipped outlet box and turned it on. The voltage feeding the ST-70 measured 119.7 VAC after the two minute ramp-up time.

    The added benefit, in addition to the AC voltage drop, is that the switch, power transformer and 5AR4 are given a ramp up rather than being hit with full voltage and current. Some claim this will increase their longevity. A Variac is a better solution, but they cost several hundred dollars new. A CL-90 is $2.25 at Dynakit parts!

    Note: I put the CL-90 into an external box because I only had one on hand, and with the external box I could use it with a few different Dynaco amps I own. For practical purposes, installing a CL-90 inside the amp is the way to go. One word of caution: Allow them some ventilation. They DO get hot!


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