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    Same tube keeps going bad

    MechEngVic
    MechEngVic


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    Post by MechEngVic Mon Mar 30, 2020 8:54 pm

    I have a Dynaco ST-70 Series II amp. The V5 position keeps blowing tubes. It's not a connection or solder issue, they have been gone over multiple times and tube sockets have been replaced. This has happened before and is happening after a major rebuild. The only thing not new is the transformer.  Voltages are all within spec and so is the bias. Can an old beat up output transformer cause a tube to go bad?

    Same tube keeps going bad 2djae2B
    MechEngVic
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    Post by MechEngVic Mon Mar 30, 2020 9:39 pm

    I wanted to add that the output transformers are A470's
    MechEngVic
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    Post by MechEngVic Mon Mar 30, 2020 10:51 pm

    I also wanted to add that the tube arcs during warm-up, not right away but several seconds to over a minute into it. Arcing during warm-up is the failure that is happening. It happens once, then it doesn't arc for several hours of use and several on and off cycles. Then it will do it again and again with increased frequency. Now that I see the pattern, I don't let it happen more than a couple of times before I replace the tube. Then all is well for many dozens of hours and many dozens of on and off cycles before it does it with the new tube.
    peterh
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    Post by peterh Tue Mar 31, 2020 2:48 am

    MechEngVic wrote:I also wanted to add that the tube arcs during warm-up, not right away but several seconds to over a minute into it. Arcing during warm-up is the failure that is happening. It happens once, then it doesn't arc for several hours of use and several on and off cycles. Then it will do it again and again with increased frequency. Now that I see the pattern, I don't let it happen more than a couple of times before I replace the tube. Then all is well for many dozens of hours and many dozens of on and off cycles before it does it with the new tube.
    Arc-ing is a symptom of something really bad.
    Looking at the schematics , screen stoppers are missing, maybe the grid stoppers are too far from
    the tube to be of value. DO install these ( carbon comp, soldered close to the socket. 100 ohm
    for the screens, 1k for the grids).
    If this don't stop the arcing , a few turns of a wire thru a ferrit bead, this wire soldered
    on the plate connection very close to the tube.

    I you are posting schematics, could you post the whole schematics please ?


    Last edited by peterh on Tue Mar 31, 2020 2:52 am; edited 1 time in total (Reason for editing : spelling)
    MechEngVic
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    Post by MechEngVic Tue Mar 31, 2020 8:08 pm

    peterh wrote:Arc-ing is a symptom of something really bad.
    Looking at the schematics , screen stoppers are missing, maybe the grid stoppers are too far from
    the tube to be of value. DO install these ( carbon comp, soldered close to the socket. 100 ohm
    for the screens, 1k for the grids).
    If this don't stop the arcing , a few turns of a wire thru a ferrit bead, this wire soldered
    on the plate connection very close to the tube.

    I you are posting schematics, could you post the whole schematics please ?

    Thank you peterh for your response. I am including an image of the entire schematic with the screen stoppers drawn in. They are 100ohm resistors.


    Same tube keeps going bad BwPaN3n


    I am also including an image of the underside of my board so you can see the resistors in place and so you can tell me if they are close enough. I used metal film resistors because I read elsewhere it was ok, but I am willing to swap them out for carbon comps if need be.

    Same tube keeps going bad YkoWy5k

    Please understand, V5 is the only position that is having this problem. I have owned the amp since 1992. It gave me several years of trouble free service before tubes wore out. At the time I replaced them one at a time with the cheapest el34 I could buy at the guitar shop. At that point the bias resistors began blowing regularly, and shortly after an unknown resistor blew and I lost the right channel and I pulled the amp from service.

    Last year, I rebuilt the amp, replacing every resistor and capacitor. After several dozen hours of service and many on-off cycles, V5 arced and red-plated. Then I replaced the original tube sockets and added the screen resistors. I replaced the tube with a matched one. While V5 no longer red-plates, it is still arcing, it has done it twice since the first time for a total of 3 times. Each time I have replaced the tube with a new matched one.

    Which grid are you speaking of adding 1k resistors? The suppressor grid, pin 1?
    MechEngVic
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    Post by MechEngVic Tue Mar 31, 2020 9:29 pm

    peterh wrote:Looking at the schematics , screen stoppers are missing, maybe the grid stoppers are too far from
    the tube to be of value. DO install these ( carbon comp, soldered close to the socket. 100 ohm
    for the screens, 1k for the grids).
    If this don't stop the arcing , a few turns of a wire thru a ferrit bead, this wire soldered
    on the plate connection very close to the tube.

    Also, is 1/4 watt ok for both the 100 and 1k resistors?
    peterh
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    Post by peterh Wed Apr 01, 2020 2:22 am

    MechEngVic wrote:
    peterh wrote:Looking at the schematics , screen stoppers are missing, maybe the grid stoppers are too far from
    the tube to be of value. DO install these ( carbon comp, soldered close to the socket. 100 ohm
    for the screens, 1k for the grids).
    If this don't stop the arcing , a few turns of a wire thru a ferrit bead, this wire soldered
    on the plate connection very close to the tube.

    Also, is 1/4 watt ok for both the 100 and 1k resistors?
    1/4w is ok, carbon comp is recomended

    The grid stopper is a resistor to the grid ( pin 5) It should be very close to the tube socket.
    Looking at the picture it seems to be on the overside that the foild should be cut and
    a grid stopper be mounted across the cut.

    What tubes do you use right now, is this brand that has been arcing ?
    MechEngVic
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    Post by MechEngVic Wed Apr 01, 2020 2:50 am

    There already are 1k resistors between the coupling cap and the pin 5 control grid. That's in the schematic. The underside 100ohm resistors are the screen grid stoppers that I added. If I added a ferrite bead to the plate, would that go between the plate and the transformer?

    I am currently using new production Mullard el34, but the same thing happened with Chinese Psvane el34 but always and only in the V5 position.
    peterh
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    Post by peterh Wed Apr 01, 2020 3:02 am

    MechEngVic wrote:There already are 1k resistors between the coupling cap and the pin 5 control grid. That's in the schematic. The underside 100ohm resistors are the screen grid stoppers that I added. If I added a ferrite bead to the plate, would that go between the plate and the transformer?

    I am currently using new production Mullard el34, but the same thing happened with Chinese Psvane el34 but always and only in the V5 position.
    The fact that one position is more prone to problems is an indication that the wiring is at fault.
    The grid stopper might be at an inappropiate position. I suggest that you add another grid
    stopper CLOSE to the tube socket. No need to remove the original , just add one
    CLOSE to the socket.
    Same with ferrite bead , mount CLOSE to the tube socket ( but wait until the effects of
    stoppers is evaluated).

    Tubes that has arc'ed is considered broken. Get new tubes !

    Also read http://www.tronola.com/html/maximize_tube_life.html

    MechEngVic
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    Post by MechEngVic Wed Apr 01, 2020 3:55 am

    Ok, you've given me a fair amount of tasks to tackle. I will check the V5 wiring and add control grid stoppers close to the socket. Thanks again for your help.
    peterh
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    Post by peterh Wed Apr 01, 2020 5:00 am

    A comment about the ser II schematics
    AN issue is the C15 C16 C25 C26 In the schematics they are spec.ed as 400Volt. That is not
    enough !! AT power on the plate of the triode will get close to full B+ and at the same time
    it will have the full bias (-) at it's other side.
    I'd replace those with 630Volt caps.
    And for stoppers; use carbon comp as they are purely resistive !
    MechEngVic
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    Post by MechEngVic Wed Apr 01, 2020 5:16 am

    I found that out when I first rebuilt the amp! I put in 400v caps and they blew within a few days. The caps in there now are 600v. I think you might have helped me with this issue about a year ago! Should I consider 630v? The tube is arcing at warn up when voltages are spiking and out of balance. I also noticed that the left channel (where V5 is located) takes twice as long to warm up as the right channel. Thanks for taking the time to look at the schematic more closely.
    peterh
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    Post by peterh Wed Apr 01, 2020 5:23 am

    MechEngVic wrote:I found that out when I first rebuilt the amp! I put in 400v caps and they blew within a few days. The caps in there now are 600v. I think you might have helped me with this issue about a year ago! Should I consider 630v? The tube is arcing at warn up when voltages are spiking and out of balance. I also noticed that the left channel (where V5 is located) takes twice as long to warm up as the right channel. Thanks for taking the time to look at the schematic more closely.
    630 is a "standard value". Theu also shold be reduced to between 0.1 and 0.22 uF in size , this
    makes it easier to find and also reduces leakage.
    MechEngVic
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    Post by MechEngVic Wed Apr 01, 2020 5:42 am

    Are you saying to reduce the coupling caps from 0.68uf to between 0.1- 0.22 uf? C15 C16 C25 C26? What about C13 & C23 of the driver tubes? They are also 0.68uf.
    peterh
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    Post by peterh Wed Apr 01, 2020 8:00 am

    MechEngVic wrote:Are you saying to reduce the coupling caps from 0.68uf to between 0.1- 0.22 uf? C15 C16 C25 C26? What about C13 & C23 of the driver tubes? They are also 0.68uf.
    Yes. They are needlessly large and may only aggrevate problems.
    C13 and C23 is ok, they are decoupling caps.
    MechEngVic
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    Post by MechEngVic Wed Apr 01, 2020 11:20 am

    Interesting, would there be a change in sound quality or power output? And if you had to pick, 0.1uf or 0.22uf?
    peterh
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    Post by peterh Wed Apr 01, 2020 12:07 pm

    MechEngVic wrote:Interesting, would there be a change in sound quality or power output? And if you had to pick, 0.1uf or 0.22uf?
    No. In fact some improvements since sub-audial disturbances are prevented from saturatimg

    0.68uF and 270k load gives a 3db limit of 0.87hz
    0.22 = 2.68hz
    0.1 = 5.8hz
    Original Dynaco ST-70 used 0.1uF
    VTA-70 uses 0.22

    Using tubes that needs smaller grid resistors ( R42,R43) such as 6550 where those should be
    50k might need 0.68 uF as the 3db limit is 4.68hz

    But EL 34 is perfectly happy with 270k
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    Post by MechEngVic Wed Apr 01, 2020 12:50 pm

    Ok, I've been reading about this since you mentioned it and what you're saying is making sense. I think I'll settle in the middle and try some 0.22uf caps along with the rest of the fixes and see how it goes. You have taught me a lot since joined this forum. I appreciate it.
    MechEngVic
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    Post by MechEngVic Wed Apr 01, 2020 11:51 pm

    peterh wrote:
    MechEngVic wrote:Interesting, would there be a change in sound quality or power output? And if you had to pick, 0.1uf or 0.22uf?
    No. In fact some improvements since sub-audial disturbances are prevented from saturatimg

    0.68uF and 270k load gives a 3db limit of 0.87hz
    0.22   = 2.68hz
    0.1  = 5.8hz
    Original Dynaco ST-70 used 0.1uF
    VTA-70 uses 0.22

    Using tubes that needs smaller grid resistors ( R42,R43) such as 6550 where those should be
    50k might need 0.68 uF as the 3db limit is 4.68hz

    But EL 34 is perfectly happy with 270k

    I found this calculator and I tried the values closest to a 3db limit of 2Hz (this website suggests 2Hz) and it gave me 0.22uf and 0.33uf. Is it better to be just above or just below this 20Hz Optimal low frequency response?

    https://www.v-cap.com/coupling-capacitor-calculator.php

    Same tube keeps going bad Lq1auog
    peterh
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    Post by peterh Thu Apr 02, 2020 3:22 am

    MechEngVic wrote:
    peterh wrote:
    MechEngVic wrote:Interesting, would there be a change in sound quality or power output? And if you had to pick, 0.1uf or 0.22uf?
    No. In fact some improvements since sub-audial disturbances are prevented from saturatimg

    0.68uF and 270k load gives a 3db limit of 0.87hz
    0.22   = 2.68hz
    0.1  = 5.8hz
    Original Dynaco ST-70 used 0.1uF
    VTA-70 uses 0.22

    Using tubes that needs smaller grid resistors ( R42,R43) such as 6550 where those should be
    50k might need 0.68 uF as the 3db limit is 4.68hz

    But EL 34 is perfectly happy with 270k

    I found this calculator and I tried the values closest to a 3db limit of 2Hz (this website suggests 2Hz) and it gave me 0.22uf and 0.33uf. Is it better to be just above or just below this 20Hz Optimal low frequency response?

    https://www.v-cap.com/coupling-capacitor-calculator.php

    Same tube keeps going bad Lq1auog

    5 hz and 0.1 uF is what you need. It's also smallest and with least leakage.

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