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Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    ST-70 No Bias Voltage in Front Output Tubes

    tjplatt
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    tjplatt


    Posts : 6
    Join date : 2020-04-08
    • Post n°1

    ST-70 No Bias Voltage in Front Output Tubes Empty ST-70 No Bias Voltage in Front Output Tubes

    Post by tjplatt Fri Apr 17, 2020 11:42 am

    I built my ST-70 Kit ~7 years ago and it's been great up until a week or two ago.  I have the VTA board with PIO Cap upgrades and was running KT66 tubes pretty much ever since.  

    The other day it started to hum intermediately and I went to check the bias voltages (admittedly I have not been good about checking/adjusting these regularly).  The front left tube was not not glowing at all.  I shut off the amp, swapped the output tubes with a set of EL34s I had and turned it back on, it kept blowing fuses when I tried to turn it on.  

    I swapped my ss rectifier for a tube rectifier, it glowed bright, my front left output tube glowed bright and burned out and it looked like I "let the smoke out" of something on the right side of the amp and burned out the fuse.  I swapped the burned out tube and turned it back on to see if I could figure out what was going on.  All the tubes glowed and it did NOT blow the fuse but I had no bias voltage on the front tubes in both channels.  I then replaced the quad cap thinking that's what had run out of smoke but it did the same thing.  I checked voltages on the amp and everything other than the front tube bias voltages seems to be within range.

    Any ideas?

    -Tyler
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    Bob Latino
    Bob Latino
    Bob Latino
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    • Post n°2

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    Post by Bob Latino Fri Apr 17, 2020 6:16 pm

    tjplatt wrote:I built my ST-70 Kit ~7 years ago and it's been great up until a week or two ago.  I have the VTA board with PIO Cap upgrades and was running KT66 tubes pretty much ever since.  

    The other day it started to hum intermediately and I went to check the bias voltages (admittedly I have not been good about checking/adjusting these regularly).  The front left tube was not not glowing at all.  I shut off the amp, swapped the output tubes with a set of EL34s I had and turned it back on, it kept blowing fuses when I tried to turn it on.  

    I swapped my ss rectifier for a tube rectifier, it glowed bright, my front left output tube glowed bright and burned out and it looked like I "let the smoke out" of something on the right side of the amp and burned out the fuse.  I swapped the burned out tube and turned it back on to see if I could figure out what was going on.  All the tubes glowed and it did NOT blow the fuse but I had no bias voltage on the front tubes in both channels.  I then replaced the quad cap thinking that's what had run out of smoke but it did the same thing.  I checked voltages on the amp and everything other than the front tube bias voltages seems to be within range.

    Any ideas?

    -Tyler

    Tyler,

    Check that the 10 ohm 2 watt resistor that goes from pins 1/8 to chassis ground on the front left output tube socket does still measure 10 ohms ? The tube in that socket probably died and took out that 10 ohm 2 watt resistor with it. If it measures something other than 10 ohms, then it is bad and should be replaced. You can get FIVE new ones at the Ebay link below for very little money .... Make sure that you select 10 ohms from the drop down menu.

    10 ohm 2 watt resistors on Ebay

    Bob
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    tjplatt
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    tjplatt


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    Post by tjplatt Sun Apr 26, 2020 2:47 pm

    Hi Bob,

    Thank for the reply.  One of these resistors measured 5.6 ohms but the others all measured 10 ohms.  I went ahead and replaced the suspect resistor as well as the other front output tube resistor just in case.  Unfortunately, it did not fix the problem, still no bias voltage in the front two tubes.

    I checked the resistance on the bias pots, they seem to be fine. Would the coupling capacitors be the next likely suspect?  

    -Tyler
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    Bob Latino
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    Post by Bob Latino Sun Apr 26, 2020 3:17 pm

    Hi Tyler,

    1. Resolder pins 5 and 6 on the two tube sockets that will not bias. Trace the wire on pin # 6 back to the driver board and resolder that connection also.

    Do those two tube sockets bias now ? No ? move on to number two below

    2. Take out all the tubes. Center the 4 bias pots. Turn the amp ON. Next measure the DC voltage to chassis ground on pins 5 and 6 on all output tube sockets. You should get between -38 to -48 volts DC on all measuring points. If you do get the correct voltages, then the bias system is working OK and the problem is on those two tube sockets. If you don't get the voltages, then the problem is on the driver board.

    3. If you do get the correct voltages, then try cleaning and/or retensioning the socket pins. Sometimes dust can get in the socket pins and prevent good contact with the tube pins.

    Bob
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    tjplatt
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    Post by tjplatt Sun Apr 26, 2020 5:01 pm

    Hi Bob,

    1) I re-soldered the the connections, still No Bias in the front tubes.

    2) Centered the bias pots, checked the voltages from pins 5 and 6 to the chassis on all of the output tubes , I got -54 VDC but could adjust the bias pots to get -38 to -48 volts

    3) Cleaned and re-tensioned the tube pins, still no bias in the front tubes.

    The amp was in triode mode so i flipped it into pentode mode, immediately the 10 ohm resister on the front left tube pins 1/8 to the chassis and the pentode/triode switch lit up and smoked followed by the amp blowing.

    The front left tube was the original tube that had gone out before losing bias and the amp was originally in pentode mode at the time.

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    Bob Latino
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    Post by Bob Latino Sun Apr 26, 2020 5:10 pm

    It is possible that one or both the triode pentode switches are bad. Try replacing the two triode/pentode switches. They are not that expensive ..  Link below ..

    Triode/pentode switches for a VTA ST-120

    Bob
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    tjplatt
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    Post by tjplatt Sun Apr 26, 2020 5:21 pm

    Hi Bob,

    Thanks for your all your help. I just ordered the switches and will replace those as well as the 10 ohm resister that lit up on me and see what she does.

    Are you thinking the pentode/triode switches could be the root cause of the issues or just a symptom of something else?

    -Tyler
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    tjplatt
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    Post by tjplatt Sat May 02, 2020 1:49 pm

    Thanks for you help Bob!  

    I installed the new triode/pentode switches and got her to work!  After putting installing I turned it on in triode mode and switched into pentode mode, the front left output tube resistor from pin 1/8 to ground lit up, i replace that tube and turned it back on and it biased perfectly.  

    Still not 100% sure what the root of the problem was but I'm guessing I blew a tube first, then took out the 1/8 pin to ground resistor and/or pentode/triode switch and chased my tail burning out tubes, switches and resistors until we got all items operating properly at the same time.  Either way, she's sounding as good as ever.  Thanks again!

    -Tyler
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    MechEngVic
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    Post by MechEngVic Thu May 14, 2020 12:57 am

    tjplatt wrote:Thanks for you help Bob!  

    I installed the new triode/pentode switches and got her to work!  After putting installing I turned it on in triode mode and switched into pentode mode, the front left output tube resistor from pin 1/8 to ground lit up, i replace that tube and turned it back on and it biased perfectly.  

    Still not 100% sure what the root of the problem was but I'm guessing I blew a tube first, then took out the 1/8 pin to ground resistor and/or pentode/triode switch and chased my tail burning out tubes, switches and resistors until we got all items operating properly at the same time.  Either way, she's sounding as good as ever.  Thanks again!

    -Tyler

    If you used a tube that had already red-plated or arced, that tube was compromised. It may have worked without trouble but would have been susceptible to red-plating/arcing and taking out the bias resistor. Especially at turn-on when voltages spike. An arc or red-plate causes damage to the sensitive windings inside the plate.
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