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    Bias connection on ST70.

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    poconoman

    Posts : 106
    Join date : 2013-01-17

    Bias connection on ST70.

    Post by poconoman on Sat Feb 23, 2013 5:16 pm

    On the PC-3, what location numbers go to the left and right front bias socket pin? Is it location 3 and 18 on PC-3 to pin 5 of the sockets?

    Thanks

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Bias connection on ST70.

    Post by Bob Latino on Sat Feb 23, 2013 6:23 pm

    Yes - In regards to a STOCK Dynaco driver board > On the left channel pin 5 on the front socket goes to eyelet #3 on the driver board. On the right channel pin #5 on the front socket goes to eyelet #18.

    Bob

    poconoman

    Posts : 106
    Join date : 2013-01-17

    Re: Bias connection on ST70.

    Post by poconoman on Sat Feb 23, 2013 6:43 pm

    Bob Latino wrote:Yes - In regards to a STOCK Dynaco driver board > On the left channel pin 5 on the front socket goes to eyelet #3 on the driver board. On the right channel pin #5 on the front socket goes to eyelet #18.

    Bob

    Thanks Bob! And yes, it's a stock board. Now, all the other socket and board connections would have to do with supplying power to a Pas? Therefore, just for bias reading, these connections are not required. Correct?

    On another matter. An SS rec will use less power, therefore that extra power can be distributed for better use, BUT it's warm up to the power tubes is much faster than a tube rec. The question, is there a mod where an SS rec can produce a slower warm up or it's best to use a tube rec because the difference of the the two rec types is marginal?

    Thanks again.

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Bias connection on ST70.

    Post by Bob Latino on Sat Feb 23, 2013 6:49 pm

    Correct - Those connections to pin 5 on the front tube sockets are not needed.

    If you have ANY ST-70 IMHO > You should just use a GZ34 tube rectifier. The Weber WZ34 and WZ68 solid state rectifiers do have a short built in time delay of 3 to 5 seconds. Over the past few years, I have found that this short delay on the Weber rectifiers is enough to delay the application of high voltage until the filaments come on.

    Bob

    poconoman

    Posts : 106
    Join date : 2013-01-17

    Re: Bias connection on ST70.

    Post by poconoman on Sat Feb 23, 2013 7:47 pm

    Bob Latino wrote:Correct - Those connections to pin 5 on the front tube sockets are not needed.

    If you have ANY ST-70 IMHO > You should just use a GZ34 tube rectifier. The Weber WZ34 and WZ68 solid state rectifiers do have a short built in time delay of 3 to 5 seconds. Over the past few years, I have found that this short delay on the Weber rectifiers is enough to delay the application of high voltage until the filaments come on.

    Bob

    Thanks for clarifying everything about the bias connections and I'll stick with the tube rec.

    jgsulliv

    Posts : 3
    Join date : 2013-01-14

    VTA board in Triode SS chassis

    Post by jgsulliv on Fri Mar 08, 2013 5:05 pm

    I'm a newbie to this forum and to tube audio in general. I've replaced caps in a magnavox 9304-20 and recently bought an ST 70 and PAS 3 with the intent to do the same.
    Well, one thing leads to another and I now have new VTA board with PIO caps, new quad cap, RCA inputs, new Triode PA 60S and Triode SS chassis.
    The reason I chose the Triode chassis is that it eliminates the superfluous power take off sockets (PTO) for a "cleaner" look (to my eye, anyway).
    So, here's my question. How do I handle the bias test points? In the VTA instructions, bias left and right go to PTO pins 4 (back tube) and 8 (front tube).
    There are only two holes for bias test points in the Triode chassis. Do I need to drill two more holes for bias test points for the back tubes ? Can I ignore (eliminate) all other wiring to the PTO javascript:emoticonp('Question')

    Thanks for any light you can shed here.
    Jim

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Bias connection on ST70.

    Post by Bob Latino on Fri Mar 08, 2013 5:33 pm

    jgsulliv wrote:I'm a newbie to this forum and to tube audio in general. I've replaced caps in a magnavox 9304-20 and recently bought an ST 70 and PAS 3 with the intent to do the same.
    Well, one thing leads to another and I now have new VTA board with PIO caps, new quad cap, RCA inputs, new Triode PA 60S and Triode SS chassis.
    The reason I chose the Triode chassis is that it eliminates the superfluous power take off sockets (PTO) for a "cleaner" look (to my eye, anyway).
    So, here's my question. How do I handle the bias test points? In the VTA instructions, bias left and right go to PTO pins 4 (back tube) and 8 (front tube).
    There are only two holes for bias test points in the Triode chassis. Do I need to drill two more holes for bias test points for the back tubes ? Can I ignore (eliminate) all other wiring to the PTO javascript:emoticonp('Question')

    Thanks for any light you can shed here.
    Jim

    Jim,

    The thing is - Those front tube sockets are NOT superfluous ... When you use the VTA driver board you need those two front tube sockets because two pins on each socket are used as bias measuring points for the four output tubes. With a Triode chassis you will either have to ...

    1. Drill 2 extra holes in the chassis (as you mentioned) to use as bias measuring points
    2. Open the bottom cover up everytime you want bias the amp and measure the bias voltage directly from pin #8 on each output tube to chassis ground.

    Bob

    jgsulliv

    Posts : 3
    Join date : 2013-01-14

    Re: Bias connection on ST70.

    Post by jgsulliv on Fri Mar 08, 2013 5:47 pm

    Thanks for your immediate reply Bob! I'll figure a way to mount four test points.
    Am I right in assuming that the black wires in the VTA underside illustration (PTO pins 1 and 2 to VTA board and power tube pins 7 and 2) are for PTO power and therefore no longer necessary?
    Thanks,
    Jim

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Bias connection on ST70.

    Post by Bob Latino on Fri Mar 08, 2013 5:50 pm

    Yes - those power take off pins and any wiring to them are not needed and should be eliminated ..

    Bob

    jgsulliv

    Posts : 3
    Join date : 2013-01-14

    Re: Bias connection on ST70.

    Post by jgsulliv on Fri Mar 08, 2013 6:25 pm

    Thanks Bob, you're remarkable in your knowledge and accessibility. I appreciate it!
    Jim

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