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    Biasing ST-70 & ST-120

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    merlot

    Posts : 22
    Join date : 2008-12-22

    Biasing ST-70 & ST-120

    Post by merlot on Mon Mar 28, 2011 10:49 am

    I have a clarification question with respect to biasing the ST-70 and ST-120. Do the inputs have to be shorted during biasing? I realize the speakers must be connected.

    Can they be open, i.e., nothing connected? Does it matter?

    Thanks!

    GP49

    Posts : 733
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Biasing ST-70 & ST-120

    Post by GP49 on Mon Mar 28, 2011 11:18 am

    Best to have absolutely no signal in the amp at all while setting bias. Shorting plugs in the inputs help with this.

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: Biasing ST-70 & ST-120

    Post by Bob Latino on Mon Mar 28, 2011 12:34 pm

    GP49 is correct. You always measure the bias of any Dynaco tube amp at "idle" which means no signal running through the amp. You always connect the speakers and it is also best to have the inputs either shorted or connected to your preamp. The preamp may be on or off but the volume control should be turned all the way down if the preamp is on. You can really bias the amp with the RCA input jacks open but if they are open the amp will probably show a little hum. On a VTA ST-70/ST-120 amps with triode/pentode switches, the amp should set in the pentode ultralinear mode with the switches facing the power transformer.

    Bob

    merlot

    Posts : 22
    Join date : 2008-12-22

    Re: Biasing ST-70 & ST-120

    Post by merlot on Mon Mar 28, 2011 8:55 pm

    Thanks for the quick response. I've been using a GZ33 and last night went to swap it for a 5AR4 that I know works. I was unable to bias the amp - bias pots were fully counterclockwise on each output tube (Valve Art 6550)and I couldn't get to 0.55v on any of them. I then swapped the 5AR4 for a Weber Coppercap. Same thing. I'll have to take a closer look tomorrow night.

    Thanks.

    Bob Latino
    Admin

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    Re: Biasing ST-70 & ST-120

    Post by Bob Latino on Mon Mar 28, 2011 9:37 pm

    If this is an ST-70 that you are trying to bias with 6550 tubes then you will probably have to drop the value of R39 the main bias resistor. This is normlly a 10K resistor but on some amps with some brands of tubes you need a LOWER value than 10K to properly bias 6550 or KT88 tubes in an ST-70 with a VTA driver board. If you have a stock power transformer IMHO you should not run 6550 or KT88 tubes in an ST-70. 6550 and KT88 tubes will draw too much current from an already undersized original PA-060 power transformer

    Bob

    j beede

    Posts : 328
    Join date : 2011-02-07
    Location : California

    Re: Biasing ST-70 & ST-120

    Post by j beede on Mon Mar 28, 2011 9:44 pm

    FYI--During my month-long Mark III debug and restoration effort I set the bias with inputs open, inputs shorted and with music playing. Any difference in voltage across the 11.2 Ohm resistor for any of those conditions was far smaller than day-to-day drift, location-to-location drift, or the variations seen in trying to fine tune the overly sensitive bias pot.

    merlot

    Posts : 22
    Join date : 2008-12-22

    Re: Biasing ST-70 & ST-120

    Post by merlot on Tue Mar 29, 2011 7:13 pm

    Bob,

    It is actually one of your ST-120s. It was fine and now it won't bias. I've yet to try swapping the output tubes or take any voltage readings. I'll likely save that for later in the week.

    Bob Latino
    Admin

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    Location : Massachusetts

    Re: Biasing ST-70 & ST-120

    Post by Bob Latino on Tue Mar 29, 2011 7:33 pm

    Set all the bias pots to their mid point. Remove just the rectifier tube and open the amp up and turn it upside down on a soft surface. Turn the amp on and see if you can measure about -45 to -65 volts DC on pins 5 and 6 of each output tube. If you get the proper voltages then the bias system is working OK and the problem is with your high voltage power supply (rectifier, quad cap, SCM). Double check also that your meter is set to DC to measure bias at the bias test points.

    If you don't get the proper voltages on pins 5 and 6 then you have to trace the bias system starting with the diode. Check the AC voltage where the red/black wire connects to the driver board. See if you get about 50 to 55 volts AC to chassis ground. If that checks out then check the negative DC voltage on the other side of the diode and see if you get -70 to -75 VDC on the other side of the diode.

    As a last resort > Do a full voltage check as outlined in the ST-120 manual.

    Bob

    merlot

    Posts : 22
    Join date : 2008-12-22

    Re: Biasing ST-70 & ST-120

    Post by merlot on Tue Mar 29, 2011 7:48 pm

    I have to hand it to you and Roy. You guys do an awesome job....

    This is one of my favorite sites...

    EVGuy

    Posts : 5
    Join date : 2011-03-02
    Location : Earlville, IL

    Re: Biasing ST-70 & ST-120

    Post by EVGuy on Tue Mar 29, 2011 11:08 pm

    Bob and Roy were a great help for me recently while I rebuilt my ST70. I had some problems which turned out to be a broken lead on a resistor on the pcb that I must have knocked into while installing it. It's all fixed and sounding great now. I'm using Genalex KT77 tubes, so do they still get biased at .400 the same as EL34s?
    Bernie

    Bob Latino
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    Re: Biasing ST-70 & ST-120

    Post by Bob Latino on Wed Mar 30, 2011 7:29 am

    You can try biasing the KT77's a little higher at .450 in that amp. KT66's sometimes also benefit from a slightly higher bias point in the VTA ST-70.

    Bob

    EVGuy

    Posts : 5
    Join date : 2011-03-02
    Location : Earlville, IL

    Re: Biasing ST-70 & ST-120

    Post by EVGuy on Wed Mar 30, 2011 11:54 am

    Thanks Bob, I'm glad I asked. I got the triode/pentode switches installed with no problems. Well worth it too. The whole thing was well worth it. It's a great sounding amp now!

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