The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all products and all products

    bad bias on st70



    Posts : 6
    Join date : 2011-02-02

    bad bias on st70

    Post by bongoman on Fri Mar 29, 2013 6:36 pm

    Hi; I have installed a VTA driver board in my ST 70. I can't get one tube on one side to bias up. The tube bounces from .400mv to 1.600 mv repeatedly seconds apart. Adjusting the pot only makes the voltages bounce around. The whole PS is brand new including a new PT; new can cap; new bias resistors; new rec tube. I changed that out also just to see. I use the original Dyna .02 three legged cap in the PS. I have done; voltage checks; all good
    Replaced the bad tube socket
    replaced the tube with three different ones
    replaced the wiring to the front sockets
    checked the circuit wiring against the schematic; the photo and the other three tubes that bias up correctly.
    Reflowed all solder joints
    At this point I'm stumped. Any help would be greatly appreciated.


    Posts : 175
    Join date : 2010-03-07
    Age : 79
    Location : Houston Texas

    Re: bad bias on st70

    Post by stewdan on Fri Mar 29, 2013 7:31 pm

    Have you checked that the 10 OHM biasing resistor on the tube that you can't set bias on really reads 10 OHMs.

    (Normally, if you have a bad biasing resistor the biasing voltage reads 50 volts instead of 0.50 volts, but I guess other variations are possible.)

    I recently had this happen on an ST-120, bias was to be set to 0.50 volts, but my bias voltage on that tube read 50.0 volts. I checked the resistor and it was bad. Replaced it and my problem went away.

    Put your meter probe on one end of the resistor at Pin 8 of the socket and the other probe somewhere on the chassis and see if it reads 10 Ohms.

    If it does not, replace the resistor.
    Bob Latino

    Posts : 2639
    Join date : 2008-11-26
    Location : Massachusetts

    Re: bad bias on st70

    Post by Bob Latino on Fri Mar 29, 2013 8:00 pm

    Hi ..

    Stew has given good advice ... make sure that the 10 ohm bias resistor is not blown for that tube. If you get zero ohms across that resistor, you will need another 10 ohm resistor.

    Two other things to try .. IF the resistor measures OK at 10 ohms ...

    1. Make sure that you have a speaker connected when you set the bias. If some transformers don't have a load on them (the speaker) the circuit on that channel can become unstable. Yes - I know that maybe your other transformer may be stable without a load, but the other may not ?

    2. A bad coupling cap for that tube on the driver board can cause the cycling bias that you are experiencing on that tube socket.



    Posts : 6
    Join date : 2011-02-02


    Post by bongoman on Fri Mar 29, 2013 9:33 pm

    Thanks Stewdan and Bob; I pulled the board; reflowed all the coupling caps and checked all the bias test point resistors. Also connected the speakers to both channels.... Whatever worked worked because I'm getting good readings now. I used the help here to good effect; thanks guys.
    Question; I'm told that bypassing the russion oil caps with small poly's helps with the high frequencies. Any chance thats true? Maybe someone has tried it.

    Posts : 157
    Join date : 2012-01-31

    Re: bad bias on st70

    Post by hawaii.ken on Fri Mar 29, 2013 10:09 pm

    bongoman wrote:Question; I'm told that bypassing the russion oil caps with small poly's helps with the high frequencies. Any chance thats true? Maybe someone has tried it.
    Wait until you get about a hundred hours break-in on your oil caps before try the bypass caps. You might like the way it sounds without them.

    I've seen reports on using small polypropylene or teflon bypass caps but everyone's taste is different.

    Posts : 1441
    Join date : 2008-11-30

    Re: bad bias on st70

    Post by tubes4hifi on Sat Mar 30, 2013 12:06 am

    typical problem and typical solution - 99% of ALL the problems Bob and I get are bad solder connections.
    Could be the way you bought the amp (an old one) or could be the way you put a new one together.
    Something to think about - based on my 50 years of experience in electronics.
    You are better off learning (thru practice, of course) good soldering techniques before doing a project.
    There are literally dozens of soldering videos on YouTube - some of them are really good!
    AND you are better off using high heat for 2-3 seconds then low heat for 5-10 seconds.
    After buying a new 25 or 30 watt solder iron every year for about ten years, I bought a 40 watt Weller temperature controlled iron,
    been using it for about ten years now. I run it at nearly full temp (I'd say 11 on a scale of 10, you know, like the movie joke)
    but it's 4.5 on a scale of 5. A good solder joint at high heat never takes more than 2-3 seconds. Practice!
    This is kind of funny but it has stuck with me for 50 years, because my high school electronics teacher was a little crazy.
    He didn't use a typical 30 or 40 watt solder iron. He used a 140 watt solder gun with a nichrome wire installed instead of
    the usual huge copper element (those things were made to solder chassis parts, not PCBs) but he could solder anything in about 1/2 second with that heat.
    Any longer and of course the nichrome wire tip would break, just like a fuse does.
    The key - you have to heat the PCB AND the resistor/cap/whatever at the same exact time, AND use flux or good solder (here's my pitch for Kester 44).
    Put a small amount of solder on a freshly cleaned tip EVERY SINGLE time you solder a connection, and then you'll see.
    If the tip isn't clean, or there is not a tiny amount of solder on the tip, you don't get heat transfer to the board or the parts leads.
    Then it takes 5-10 seconds, meanwhile the board and the part get burnt. Do it hot and fast, not cool and slow !!!!!!!


    Posts : 6
    Join date : 2011-02-02


    Post by bongoman on Sat Mar 30, 2013 12:39 pm

    I agree and I have listened to this advice carefully. In the past. I use a 60 watt weller iron. I also get mostly nice; shiny cones but this time I might have underdone a spot. As I say; It also might have been that only one speaker lead was connected. Who knows? But that is good advice regarding soldering. Like basketball; it's all in the fundamentals. And being real tall.

    Sponsored content

    Re: bad bias on st70

    Post by Sponsored content

      Current date/time is Sun May 27, 2018 8:43 am