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    VTA high voltage problem

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    Babgrip

    Posts : 27
    Join date : 2013-01-30
    Age : 55
    Location : Jax FL

    VTA high voltage problem

    Post by Babgrip on Wed Jan 30, 2013 1:18 pm

    Just finished power up on ST-70 rebuild w/ VTA board,CE quad cap,Mesa Rectifer,Winged C Output Tube.The voltages are all High. subed E/H GU4 dropped voltage 14V. Sounds horrible especially low freq, also i have 90ma on the Rt channel input jack with no source ,zero on Lft input.any thoughts
    Thanks
    Rect #2/8 538vdc #4/6 405vac
    Quad cap
    #1 538v #2 535v #3 535v #4 478v
    EL-34 #1 .043-.064vdc # 2/7 12.8-13.76vdc #3 535vac #4 535vac
    #5 -61vdc #6 -61vdc #8 .042-.064vdc


    Last edited by Babgrip on Wed Jan 30, 2013 3:10 pm; edited 5 times in total (Reason for editing : Left out info)

    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: VTA high voltage problem

    Post by tubes4hifi on Wed Jan 30, 2013 3:41 pm

    did you set the bias on the output tubes? seems your bias voltage is really high, which means the tubes are shut off.
    You need to adjust the bias, measuring across the 10 ohm resistor on each output tube, should be close to center on each bias pot.
    With no bias, the output voltage will be way too high, with no current through the output tubes it will sound awful.
    Seems you also have a problem with something on that right channel, should be no voltage or current on the input jacks.

    Babgrip

    Posts : 27
    Join date : 2013-01-30
    Age : 55
    Location : Jax FL

    Re: VTA high voltage problem

    Post by Babgrip on Wed Jan 30, 2013 3:46 pm

    Bias on three tubes is set at the recomended 40v the fourth tube is 61v that is as low as I can get it ,but one tube did fall off my work table and may have been damaged resulting in the higher bias setti g

    tubes4hifi
    Admin

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    Re: VTA high voltage problem

    Post by tubes4hifi on Wed Jan 30, 2013 3:48 pm

    I hope you are talking about 0.40 vdc (400mv) and not 40vdc !!

    Babgrip

    Posts : 27
    Join date : 2013-01-30
    Age : 55
    Location : Jax FL

    Re: VTA high voltage problem

    Post by Babgrip on Wed Jan 30, 2013 5:09 pm

    Yes 0.40 vdc

    Babgrip

    Posts : 27
    Join date : 2013-01-30
    Age : 55
    Location : Jax FL

    Re: VTA high voltage problem

    Post by Babgrip on Wed Jan 30, 2013 5:58 pm

    How does tube bias effect AC voltage of 405 in to rectifier tube and DC voltage of 538 out of rectifier tube. Doesn't tube bias effect B+ voltage .Isn't the output tube voltage further down the path to musical nirvana ?

    Bob Latino
    Admin

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    Location : Massachusetts

    Re: VTA high voltage problem

    Post by Bob Latino on Wed Jan 30, 2013 5:59 pm

    Babgrip wrote:Just finished power up on ST-70 rebuild w/ VTA board,CE quad cap,Mesa Rectifer,Winged C Output Tube.The voltages are all High. subed E/H GU4 dropped voltage 14V. Sounds horrible especially low freq, also i have 90ma on the Rt channel input jack with no source ,zero on Lft input.any thoughts
    Thanks
    Rect #2/8 538vdc #4/6 405vac
    Quad cap
    #1 538v #2 535v #3 535v #4 478v
    EL-34 #1 .043-.064vdc # 2/7 12.8-13.76vdc #3 535vac #4 535vac
    #5 -61vdc #6 -61vdc #8 .042-.064vdc

    Check your incoming line voltage !

    All your voltages are WAY HIGH - The AC voltages on pins 4 and 6 of the rectifier tube to chassis ground should be 360 VAC to maybe 375 VAC. At 405 VAC you are much higher than you should be. I would almost bet that your incoming line voltage is 125+ VAC. Also - If you are using an ORIGINAL Dynaco PA-060 power transformer > that transformer was designed to give 360 VAC off pins 4 and 6 to chassis ground with an input voltage (that was prevalent in the 1950's to the mid 60's) of 115 to 117 VAC. Todays 120 VAC on an original ST-70 will usually give about 375 VAC.

    If your line voltage is way high then you may have to run the amp off a variac set at maybe 118 VAC.

    Bob

    Babgrip

    Posts : 27
    Join date : 2013-01-30
    Age : 55
    Location : Jax FL

    Re: VTA high voltage problem

    Post by Babgrip on Wed Jan 30, 2013 9:40 pm

    Bob
    Line voltage is 121VAC . Transformer is a PA-521, is this thing wired 220vac,2 wires on power switch Red,White/Red. Should I remove one of them . I wonder if it was a factory built unit for export . Was it offered with this transformer in the U.S. No rivets or label but some things had been changed before I acquired it. Tube sockets, with a poor re-wire job, bias caps driver board caps , input jacks & speaker terminals. If bias pot date coding is accurate amp is a 1967 model ,tube cage is gray in color and appears original.
    Thanks Ben

    Bob Latino
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    Re: VTA high voltage problem

    Post by Bob Latino on Wed Jan 30, 2013 10:15 pm

    Hi Ben,

    The original Dynaco PA-521 has BLACK and BLACK/WHITE and VIOLET and VIOLET/WHITE primary wires (4 wires) and could be wired for either 120 VAC or 240 VAC. The fact that you are telling me that the amp has RED and RED/WHITE wires on the power switch tells me that this PA-521 power transformer on your amp does not have the standard power transformer wire colors. On a standard Dynaco PA-521 the VIOLET and VIOLET/WHITE go to the power switch and the BLACK and BLACK/WHITE wires go to the fuse post. On a standard PA-521 the two RED wires go to pins 4 and 6 on the rectifier tube socket.

    See if you can post a photo here on the forum. (Check the sticky above on how to post a photo) By looking at the wiring, one of us here can probably figure out if the amp is wired correctly ...

    Bob

    Babgrip

    Posts : 27
    Join date : 2013-01-30
    Age : 55
    Location : Jax FL

    Re: VTA high voltage problem

    Post by Babgrip on Wed Jan 30, 2013 10:24 pm

    Will try to post pics . Your description of wiring is correct , I thought the colors had faded over time did not realize that violet was a color used. Darn thought I found easy fix

    Babgrip

    Posts : 27
    Join date : 2013-01-30
    Age : 55
    Location : Jax FL

    Re: VTA high voltage problem

    Post by Babgrip on Wed Jan 30, 2013 10:48 pm

    Interesting note I was checking voltages again and the readings were in spec ?. Check line voltage 114vac ? This evening it is a bit chilly and I turned on space heater it is on the same circuit so I reckon I have three choices buy a Variac which I do need , buy a modern Transformer or only use amp in the winter. I will put in back in line and see how she sounds.what should I look for as far the 90ma I have on RT channel RCA

    tubes4hifi
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    Re: VTA high voltage problem

    Post by tubes4hifi on Fri Feb 01, 2013 2:25 pm

    I'd look for a new tube to replace the one that's running too much bias.
    The current through the output tubes affects the B+ voltage, the more current, the less voltage.

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