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    Low filament voltage on st-70

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    Serge_K

    Posts : 8
    Join date : 2013-11-09

    Low filament voltage on st-70

    Post by Serge_K on Sat Nov 09, 2013 12:25 pm

    Hello, first post here, I'm not a native english speaker so please forgive some mistakes in the text.

    I have a trouble with my dynaco st-70 (triode elec ef86 driver board, the rest is stock) biasing, when I try to set bias on output tubes the weird thing happens - one channel reaches a maximum of 1.3v and the other channel stuck at 1.2-1.3v and doesn't change the bias voltage when I change the bias pot position.

    I've checked all voltages and noted that the filament voltage on all of el-34 tubes is about 3.3v ac - half of it's normal voltage.

    Could it be a wiring error? Transformer issue? What to check next?

    I would appreciate any ideas, thanks!

    The voltage at pins 5,6 of el-34 is about -45v
    The voltage at pins 3,4 is about 445v.


    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Low filament voltage on st-70

    Post by Bob Latino on Sat Nov 09, 2013 12:34 pm

    Hi Serge,

    How are you checking the filament voltage on the EL34's? You should be checking that AC voltage ACROSS pin #2 to pin #7 to check the filament voltages on those output tubes.

    If your amp has had the selenium rectifier changed out for a diode then it is now possible that the RANGE of the bias pots may be slightly off. The solution is to drop the values of the two 10K bias resistors on the 7 lug terminal strip from what they are now to a LOWER value. Try 5K (1/2 watt is fine) instead of 10K on both of those resistors.

    Bob

    Serge_K

    Posts : 8
    Join date : 2013-11-09

    Re: Low filament voltage on st-70

    Post by Serge_K on Sat Nov 09, 2013 12:47 pm

    Oops, my bad. The filament voltage is OK.
    I've replaced that selenium rectifier indeed.
    I'll try to change the resistors value and report back.
    Thank you very much for your help!

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Low filament voltage on st-70

    Post by Bob Latino on Sat Nov 09, 2013 12:56 pm

    Hi Serge,

    Just another note. The 1.56 VDC bias voltage that Dynaco recommended for each pair of EL34 output tubes is considered a little high by today's standards. You may want to shoot for about 1.25 VDC for each pair of tubes instead of 1.56 VDC. This will give about 40 milliamps of bias current for each of the output tubes. The amp will sound the same and your output tubes will last longer.

    Bob

    Serge_K

    Posts : 8
    Join date : 2013-11-09

    Re: Low filament voltage on st-70

    Post by Serge_K on Sat Nov 09, 2013 1:56 pm

    I've changed the resistors to 5k and now both channels stuck at 1.25v with both potentiometers turned fully clockwise. Both channels bias almost doesn't change as I change the position of a corresponding potentiometer.

    The potentiometers are Bourns 3590s-2-103l 10-turn 10k pots.

    I can leave it at this bias but I'm worried that non-changing bias may be a sign of some other troubles i.e. miswiring or something else.
    I didn't try yet to listen to this amp with speakers attached, maybe I should now to test if this amp work as it should.


    sKiZo

    Posts : 1287
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Low filament voltage on st-70

    Post by sKiZo on Sat Nov 09, 2013 3:27 pm

    Any truth to the rumor that you should never ever power up a tube amp without speakers attached?

    Serge_K

    Posts : 8
    Join date : 2013-11-09

    Re: Low filament voltage on st-70

    Post by Serge_K on Sat Nov 09, 2013 3:30 pm

    Well, i've tested the amp in my system and it works OK, no hum, no hiss so far. Both channels have the same volume.

    So the one issue left is the bias, I can't set the bias voltage higher (or lower on one channel) than 1.25v even I want to.


    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Low filament voltage on st-70

    Post by Bob Latino on Sat Nov 09, 2013 3:58 pm

    Pull the rectifier and turn the amp on. Check the negative DC voltage on pins 5 and 6 on one or both channels on the output tubes while you turn the bias pot for that one channel from one end to the other. See what the negative DC voltage changes from what to what. The voltage should go from maybe -24 VDC to about -47 VDC. There also could be a bad cap on the 7 lug terminal strip. You may want to replace those two 50 uF electrolytic caps that go between lug 1 and 3 and lug 1 and 4. Any 50 uF or 100 uF cap 100 volts or higher will be fine.

    Bob

    Serge_K

    Posts : 8
    Join date : 2013-11-09

    Re: Low filament voltage on st-70

    Post by Serge_K on Sat Nov 09, 2013 4:44 pm

    I did the test and on the right channel the voltage stuck at -38v no matter in what position the corresponding pot is.
    On the left channel the voltage changes from -38v to -43v depending on the position of the pot.

    I've checked both bias capacitors and they both seemed OK ~45uF, 1ohm esr.

    So is it some miswiring?

    sKiZo

    Posts : 1287
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Low filament voltage on st-70

    Post by sKiZo on Sat Nov 09, 2013 4:49 pm

    Test the bias pot?

    Reason I ask is I just had an issue with a runaway tube ... went thru the usual then noticed the pot just didn't feel right. Tested bad, replaced it, and all is well ...

    Bob Latino
    Admin

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    Location : Massachusetts

    Re: Low filament voltage on st-70

    Post by Bob Latino on Sat Nov 09, 2013 4:49 pm

    Yes - check the wiring of the two bias pots from our large stock ST-70 pictorial at the link below.

    Stock Dynaco ST-70 pictorial

    Bob

    Serge_K

    Posts : 8
    Join date : 2013-11-09

    Re: Low filament voltage on st-70

    Post by Serge_K on Sat Nov 09, 2013 4:56 pm

    Sorry, I just redone the test and the results are different.
    On the right channel the voltage goes from -38v to -50v
    On the left channel the voltage goes from -38v to -42v.

    Serge_K

    Posts : 8
    Join date : 2013-11-09

    Re: Low filament voltage on st-70

    Post by Serge_K on Sat Nov 09, 2013 5:57 pm

    Well, I've just unsoldered the Bourns pots from the circuit and temporary soldered the old original Dynaco pots. This fixed the issue - both channels bias voltage can be set from -26v to -50v no problem.
    I suspect that the bourns pin numbering scheme is different, and I effectively swapped the connection points on the 7-lug terminal strip. Does it make any sense?
    Or maybe that Bourns pots just broken, I'll have to check it.

    Anyway, thank you very much for your very helpful ideas, you saved my day! )


    Serge_K

    Posts : 8
    Join date : 2013-11-09

    Re: Low filament voltage on st-70

    Post by Serge_K on Sat Nov 09, 2013 6:17 pm

    Yes, the pins on that Bourns pots come in order 2,1,3 not 1,2,3 so i'm an idiot.
    That will be a lesson for me.

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Low filament voltage on st-70

    Post by Bob Latino on Sat Nov 09, 2013 6:22 pm

    Hi Serge,

     That -26 to -50 VDC is just about perfect .. Yes - those Bourns pots in there must have been wired wrong in relation to the original Dynaco 1, 2, 3 terminal markings.

    Bob

    arledgsc

    Posts : 330
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: Low filament voltage on st-70

    Post by arledgsc on Mon Nov 11, 2013 9:38 am

    Glad you found your problem.  Usually makes sense once you focus on the problem area.  

    Any truth to the rumor that you should never ever power up a tube amp without speakers attached?
    Very true.   Countless number of amps have been ruined in short time operating without any speaker load.  Blown power tubes and fried output transformers are the common results.  This should be at the top of the list for Do's and Don'ts for every new tube amp owner.

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