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    New VTA ST-120 Build! Need Help! SOS

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    Analog Man

    Posts: 57
    Join date: 2013-09-01
    Age: 52
    Location: St. Louis, MO - On The Hill

    New VTA ST-120 Build! Need Help! SOS

    Post by Analog Man on Sat Sep 14, 2013 9:06 pm

    I have just completed my VTA ST-120 and have run into problem with V6, "Front right output tube". I have built many tube amps in the past and pretty much know what I am doing, at least I think so.
    My problem is that the tube gets red hot and I am not getting any bias voltage readings, all 0v. I have checked all of my wiring and all of the voltage measurments as listed in the assembly manual. I have checked all connections and looked for ground short from the driver board to the chassis. I cant seem to find the problem, I'm perplexed!
    Can anyone out there give me something to go on? I am so frustrated with this that I had to call it quits for the night. I hope I didn't damage the output tube, Very $$$.
    Running new KT-120 Tung-Sols. I even did a tube swap to see if it was a bad tube. even after swapping tubes, no difference. I am leaning towards a transformer problem but I don't know for sure.
    Any help you can give would be greatly appreciated!Crying or Very sad 

    hawaii.ken

    Posts: 157
    Join date: 2012-01-31

    Re: New VTA ST-120 Build! Need Help! SOS

    Post by hawaii.ken on Sat Sep 14, 2013 11:36 pm

    Analog Man wrote:My problem is that the tube gets red hot and I am not getting any bias voltage readings, all 0v.
    With no tubes installed and power off, check the bias diode to see if it's still good and verify proper orientation. Also verify proper orientation of the bias caps.

    With no tubes installed, apply power and check to see if you have negative bias voltage at all power tubes at pin-5

    Analog Man

    Posts: 57
    Join date: 2013-09-01
    Age: 52
    Location: St. Louis, MO - On The Hill

    Re: New VTA ST-120 Build! Need Help! SOS

    Post by Analog Man on Sun Sep 15, 2013 5:53 am

    Hi and thank you. I did as you suggested and measured for neg. bias voltage at pin5 on all output tube sockets, I'm getting around neg. 62 volts.

    Analog Man

    Posts: 57
    Join date: 2013-09-01
    Age: 52
    Location: St. Louis, MO - On The Hill

    Re: New VTA ST-120 Build! Need Help! SOS

    Post by Analog Man on Sun Sep 15, 2013 6:55 am

    Here are some pics.
    http://s756.photobucket.com/user/hilldwellerstl/library/?sort=3&page=1

    Bob Latino
    Admin

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    Location: Massachusetts

    Re: New VTA ST-120 Build! Need Help! SOS

    Post by Bob Latino on Sun Sep 15, 2013 8:01 am

    Hi James,

    With the amp OFF > can you measure about 10 ohms at all bias measuring points?

    With the amp ON > What is the DC voltage from pin 8 of the rectifier socket to chassis ground ?

    A "no bias voltage" situation is usually a bad rectifier tube (or a Weber SS rectifier in your case). It could also be that the TDR is not working properly and not passing the secondary power transformer voltage on to the rectifier tube socket. Try bypassing the TDR

    If you did a full voltage check, what readings did you get?

    You had the chassis "hammer coated" .. Sometimes this can interfere with a ground inside the chassis.

    Bob

    Analog Man

    Posts: 57
    Join date: 2013-09-01
    Age: 52
    Location: St. Louis, MO - On The Hill

    Re: New VTA ST-120 Build! Need Help! SOS

    Post by Analog Man on Sun Sep 15, 2013 8:35 am

    Bob Latino wrote:Hi James,

    With the amp OFF > can you measure about 10 ohms at all bias measuring points?

    With the amp ON > What is the DC voltage from pin 8 of the rectifier socket to chassis ground ?

    A "no bias voltage" situation is usually a bad rectifier tube (or a Weber SS rectifier in your case). It could also be that the TDR is not working properly and not passing the secondary power transformer voltage on to the rectifier tube socket. Try bypassing the TDR

    If you did a full voltage check, what readings did you get?

    You had the chassis "hammer coated" .. Sometimes this can interfere with a ground inside the chassis.

    Bob
    Getting 10 ohms at all bias points
    Rectifier: pin 8 = 583vdc
    pin 2 = 583vdc
    pin 4 = 420vac
    pin 6 = 420vac

    Quad cap: square = 581 vdc
    half circle = 581 vdc
    circle = 581vdc
    triangle = 568vdc

    V2: pins 5 & 6 = -62.2vdc
    V3: pins 5 & 6 = -61.3vdc
    V6: pins 5 & 6 = -54.4vdc
    V7: pins 5 & 6 = -60vdc
    I can get all output tubes to bias except for V6. With tube in circuit I get no bias reading at any point. I swapped tubes around and still have the same problem. No ground faults.





    Bob Latino
    Admin

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    Re: New VTA ST-120 Build! Need Help! SOS

    Post by Bob Latino on Sun Sep 15, 2013 9:42 am

    James,

     Your pin 8 rectifier DC voltage is way high ... That 583 VDC that you see on pin 8 of the rectiifier socket should be maybe 515 - 520 with a Weber SS rectifier. Your line voltage must be noticeably higher than 120 volts. Check your line voltage! Try using a 5U4 rectifier tube rectifier instead of the Weber WZ68. This will drop the rectifier pin 8 DC voltage maybe 40 volts. If your line voltage is anything over 122 VAC, you should run the amp off a variac set to about 117 to 118 VAC.

    If any tube will not bias in V6 then there is an issue on the V6 tube socket OR that part of the driver circuit that feeds V6. Check the coupling cap that feeds V6 and all surrounding parts. The triangle section of the quad cap is way high. The triangle section of the quad cap should be maybe 380 to 420 volts. Did you have the driver tubes in when you made this measurement ?

    Bob

    Analog Man

    Posts: 57
    Join date: 2013-09-01
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    Re: New VTA ST-120 Build! Need Help! SOS

    Post by Analog Man on Sun Sep 15, 2013 10:06 am

    No tubes we in the circuit when I made the voltage measurements but the weber was in the circuit.

    Analog Man

    Posts: 57
    Join date: 2013-09-01
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    Re: New VTA ST-120 Build! Need Help! SOS

    Post by Analog Man on Sun Sep 15, 2013 10:17 am

    Line voltage is 119vac unloaded. V6 socket is wired correctly, everything on the driver board looks ok.

    Bob Latino
    Admin

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    Re: New VTA ST-120 Build! Need Help! SOS

    Post by Bob Latino on Sun Sep 15, 2013 10:17 am

    Analog Man wrote:No tubes we in the circuit when I made the voltage measurements but the weber was in the circuit.
    DON'T EVER have JUST the rectifier in the amp with no other tubes when the amp is ON! If you ever have the RECTIFIER in the amp ALWAYS have the other tubes in there. Without the other tubes plugged in, there will be no LOAD on the high voltage system and the high voltage system WILL SHOW WAY HIGH DC voltages. Do another voltage check (especially the quad cap voltages) with all the tubes plugged in. The voltages should come down near normal levels with all the tubes plugged in.

    Also - You will also not be able to measure bias voltage at the four bias measuring point without the output tubes plugged in.

    Bob

    Analog Man

    Posts: 57
    Join date: 2013-09-01
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    Re: New VTA ST-120 Build! Need Help! SOS

    Post by Analog Man on Sun Sep 15, 2013 10:35 am

    With all tubes plugged in, I get no bias voltage at V6. I want to test voltage levels with all of the tubes plugged in but fear tube damage during the process.

    Analog Man

    Posts: 57
    Join date: 2013-09-01
    Age: 52
    Location: St. Louis, MO - On The Hill

    Re: New VTA ST-120 Build! Need Help! SOS

    Post by Analog Man on Sun Sep 15, 2013 10:50 am

    I am too frustrated to go any further today. Will try tomorrow. THANKS FOR ALL OF YOUR HELP!

    arledgsc

    Posts: 220
    Join date: 2012-11-30
    Location: SF Bay CA

    Re: New VTA ST-120 Build! Need Help! SOS

    Post by arledgsc on Sun Sep 15, 2013 11:06 am

    Check to verify the 10 ohm resistors are connected to both pin 1 and pin 8.   The resistor appears to be not connected to pin 1 on V7 (right rear).  The photos are a bit grainy but it also appears V7 pin 8 is connected to pin 7 but could be optical illusion.  Also verify on V6 just to be sure.  The triode/ultralinear switch connections look OK.

    After some more looking recheck the V7 pin 3 black wire back to the triode/ ultralinear 100 ohm resistor.  Again probably optical effects but it appears the pin 3 wire connects to the quad cap and the triode/UL 100 ohm resistor to the output transformer.  I would also suggest rerouting V7 pin 3 wire away from the high voltage stuff.


    Last edited by arledgsc on Sun Sep 15, 2013 12:32 pm; edited 1 time in total

    Analog Man

    Posts: 57
    Join date: 2013-09-01
    Age: 52
    Location: St. Louis, MO - On The Hill

    Re: New VTA ST-120 Build! Need Help! SOS

    Post by Analog Man on Sun Sep 15, 2013 12:00 pm

    10 ohm resistors are connected correctly! Only problem is with V6 and it can only be an issue with the driver board. I have eliminated all other possibilities. I will focus more closely with the driver board and see what I can find there.

    sailor

    Posts: 239
    Join date: 2011-04-04

    Re: New VTA ST-120 Build! Need Help! SOS

    Post by sailor on Sun Sep 15, 2013 1:39 pm

    If your wiring is correct and the correct parts in the correct place then Check all of the electrolytic caps and diode for correct polarity direction. If you are not getting any bios voltage I would think one of the caps in the bios is in backwards or the diode. If so it may now be blown and need to be replaced. I would pull all tubes and rectifier and then check the voltage of the bios. If it is still 0 then you have something wrong in the bios circuit.

      Current date/time is Sat Aug 23, 2014 10:19 am