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    Unstable ST70 Bias and PAS-2 components value

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    maninmac771

    Posts: 28
    Join date: 2011-03-07

    Unstable ST70 Bias and PAS-2 components value

    Post by maninmac771 on Fri Jul 15, 2011 1:06 am

    Hi all. Every time I power up my ST70, I have to adjust the bias because when it is measured the bias often is above 1.56, like around 1.6. Adjusted to around 1.56 (it cannot be maintained at 1.56 flat), then after a while it drops to around 1.52 or sometimes 1.48. Is it normal that bias of tube amps has to be adjusted everytime when powered up? Or is there a problem with my ST70? I have replaced the quad capacitor, the selenium rectifier with diode, the complete driver board and its components, all resistors (except the two 15.6ohm) and capacitors; and am using a new set of Chinese Shugang EL34. Thank in advance for any help.
    Also, any one knows where I can find pictures that show the values of all components on the two boards of PAS-2, like those that I've seen in this forum for ST70?
    Thanks again.

    Bob Latino
    Admin

    Posts: 1961
    Join date: 2008-11-26
    Location: Massachusetts

    Re: Unstable ST70 Bias and PAS-2 components value

    Post by Bob Latino on Fri Jul 15, 2011 6:01 am

    What you are experiencing is pretty much normal. It takes a half hour or so for the bias to stabilize until the amp is completely warmed up. It is then that the bias should be adjusted. Also, your bias will go up and down with line voltage changes. The bias on any Dynaco amp does not have to be "perfect". As long as all tubes on your amp are between 1.50 and 1.60 VDC - you are OK. Another thing to consider is the age of your output tubes. Older tubes are usually worn and harder to bias or one or more of the quad may have a "wandering bias". Sometimes an output tube change can cure bias that won't hold.

    RE: Values of the components on the PAS boards. We have a sticky on the forum above that shows the boards and their parts values. It is at the link below ..

    PAS preamp board parts values

    Bob

    Westy56

    Posts: 63
    Join date: 2011-05-09
    Location: Albuquerque

    Re: Unstable ST70 Bias and PAS-2 components value

    Post by Westy56 on Fri Jul 15, 2011 10:28 am

    Bob Latino wrote: As long as all tubes on your amp are between 1.50 and 1.60 VDC - you are OK.
    Bob

    Are you saying to run the output tubes at a min. of 1.50 VDC ?

    Steve

    Bob Latino
    Admin

    Posts: 1961
    Join date: 2008-11-26
    Location: Massachusetts

    Re: Unstable ST70 Bias and PAS-2 components value

    Post by Bob Latino on Fri Jul 15, 2011 10:45 am

    NO - Personally, I feel a reduced bias point is better for the amp. On a couple of other posts on the forum here I mentioned the following. > Dynaco biased their tubes a little on the high side. The 1.56 VDC bias point gave 100 milliamps of idle current per two output tubes or 50 milliamps idle current per each EL34. Conventional wisdom is that you don't need to bias EL34 output tubes that high. You don't really gain anything with this 1.56 VDC bias point except reduced output tube life. Roy and I feel that biasing a STOCK ST-70 at about 1.25 VDC is maybe a better idea. This bias point gives about a 40 milliamp idle current per each output tube and is probably a better compromise for good sound AND good tube life. The stock power transformer will run a little cooler also. NOTE - the bias points mentioned are for STOCK ST-70 amps with STOCK driver boards and are not meant for ST-70 amps with the VTA driver board or any of the VTA kit or wired amps.

    Bob

    Westy56

    Posts: 63
    Join date: 2011-05-09
    Location: Albuquerque

    Re: Unstable ST70 Bias and PAS-2 components value

    Post by Westy56 on Fri Jul 15, 2011 11:46 am

    I couldn't tell the diff between 1.30 and 1.56 so I run them at 1.34 VDC.
    This is with the original driver board setup.


    Steve

    maninmac771

    Posts: 28
    Join date: 2011-03-07

    Re: Unstable ST70 Bias and PAS-2 components value

    Post by maninmac771 on Fri Jul 15, 2011 4:45 pm

    Thanks Bob. Have to bother you with further questions. I'm planning to by-pass the blend switch and the tone controls, but not sure about doing the same to loudness and filter switches. Is there any difference in sonic performance between by-passing these two switches and just leaving them in the off position?

    tubes4hifi
    Admin

    Posts: 998
    Join date: 2008-11-30

    Re: Unstable ST70 Bias and PAS-2 components value

    Post by tubes4hifi on Fri Jul 15, 2011 5:36 pm

    when those switches are off they are not in the circuit, so you can leave as is, or take them out, but no difference

      Current date/time is Fri Oct 31, 2014 11:48 pm