Jump to content
The Klipsch Audio Community
MechEngVic

Same tube keeps going bad

Recommended Posts

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?

 

1566068668_Annotation2020-03-30175004.jpg.bc8ea17dd8d07a45d0e9ab3461c0f118.jpg

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites
10 hours ago, MechEngVic said:

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?

 

1566068668_Annotation2020-03-30175004.jpg.bc8ea17dd8d07a45d0e9ab3461c0f118.jpg

 

 

supersportsteve
on Thu May 12, 2016 9:28 pm
by supersportsteve
This addition will eliminate tube arc and resistors blowing, two pin 4's were feeding capacitors so jumpers were added and theirs traces interrupted. I am told the addition of the more modern power supply and extra capacitance can cause havoc, this should help the tubes last longer.
 
 
 
Dynaco stereo 70 series 2 20160422_150009_zpsqnfyb54t[/img

Share this post


Link to post
Share on other sites
17 hours ago, tube fanatic said:

Did you replace C25?  
 

 

Maynard

Yes sir, C25 is a new 600v Russian PIO cap. If I get desperate, I might replace them all again, maybe 630v Solen film and foil caps. A set of the Solens cost about the same as the matched pair of tubes this problem has eaten already.

Share this post


Link to post
Share on other sites
16 hours ago, dirtmudd said:

This addition will eliminate tube arc and resistors blowing, two pin 4's were feeding capacitors so jumpers were added and theirs traces interrupted. I am told the addition of the more modern power supply and extra capacitance can cause havoc, this should help the tubes last longer.

 
 

Thanks dirtmudd, I actually have screen stoppers installed, I installed them after the first time V5 blew a tube (100ohm). 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.

Share this post


Link to post
Share on other sites
6 hours ago, MechEngVic said:

Thanks dirtmudd, I actually have screen stoppers installed, I installed them after the first time V5 blew a tube (100ohm). 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.

 

 

As you know .. I have had the same problem.. Had a tube red plate..

 

But I pulled the amp out.. And put it away..

 

And the link I that added.. Is my actual

amp...

 

That i purchased from a forum member... But I'm uncertain how many hands it has passed threw..

 

 

If I recall correctly .. After talking with the forum member..

 

He bought the amp.. From the op in that link...

 

And at the time the tube red plated..

I pm you.. For a source for el34's..

 

https://reverb.com/shop/craigslistings 

 

Once you have figured out.. What is the cause.. Or the solution to the problem at hand...

 

I will proceed with a service for my amp..

 

Any forum members here recommend.. A qualified tech ?

 

I don't trust the nitwits by me..

 

  • Like 1

Share this post


Link to post
Share on other sites

is the bias the same on both output tubes on each side?

Share this post


Link to post
Share on other sites
1 hour ago, boom3 said:

is the bias the same on both output tubes on each side?

it's led bias control

Share this post


Link to post
Share on other sites
5 hours ago, dirtmudd said:

 

 

As you know .. I have had the same problem.. Had a tube red plate..

 

But I pulled the amp out.. And put it away..

 

And the link I that added.. Is my actual

amp...

 

That i purchased from a forum member... But I'm uncertain how many hands it has passed threw..

 

 

If I recall correctly .. After talking with the forum member..

 

He bought the amp.. From the op in that link...

 

And at the time the tube red plated..

I pm you.. For a source for el34's..

 

https://reverb.com/shop/craigslistings 

 

Once you have figured out.. What is the cause.. Or the solution to the problem at hand...

 

I will proceed with a service for my amp..

 

Any forum members here recommend.. A qualified tech ?

 

I don't trust the nitwits by me..

 

Then it is you I have to thank for exposing me to the idea of screen stoppers. If you check out the thread I started at the Dynaco forum by the same name you can read the advice I'm getting which is sounding pretty good so far. 

  • Like 1

Share this post


Link to post
Share on other sites
4 hours ago, boom3 said:

is the bias the same on both output tubes on each side?

Like dirtmudd mentioned, the bias is balanced by adjusting a trim pot so that two led's are of equal brightness, one set of 2 led's per channel. It's pretty reliable. I haven't measured with a meter recently but I did when I first rebuilt the amp and all voltages were good. 

  • Like 1

Share this post


Link to post
Share on other sites
16 minutes ago, MechEngVic said:

Then it is you I have to thank for exposing me to the idea of screen stoppers. If you check out the thread I started at the Dynaco forum by the same name you can read the advice I'm getting which is sounding pretty good so far. 

What's the link to dynaco thread ?

 

thanks Mike

Share this post


Link to post
Share on other sites
7 hours ago, dirtmudd said:

What's the link to dynaco thread ?

 

thanks Mike

Please note in the thread where we talk about using 600v coupling caps instead of the 400v written in the schematic. If you keep blowing tubes your coupling caps may need upgrading to 600v.

  • Like 1

Share this post


Link to post
Share on other sites
On 4/2/2020 at 2:51 AM, MechEngVic said:

Please note in the thread where we talk about using 600v coupling caps instead of the 400v written in the schematic. If you keep blowing tubes your coupling caps may need upgrading to 600v.

Hello,

I am no Dynaco servicing expert, but I can see that most advice you are getting is rather voluntaristic. There is absolutely no need for 600V rated coupling caps, but they cannot hurt. Also, the size (capacity) will not hurt anything but the sound - and smaller caps tend to sound better if large enough not to cut off the deep bass.

 

The problem you are experiencing is definitely related to the g1 pin of tube V5 - something erratic happens with the bias voltage. Red plating obviously means too much current but arcing is a sign of overvoltage.

You should definitely check the biasing circuit and it’s caps (C19 if I am correct). Even if the cap was replaced there’s no guarantee it was good...

 

Also, since the two tubes are basically tied together, one will draw more current if the other doesn’t draw enough... led lights are cool but you should check the bias voltage on the pin during startup, on both tubes.

 

Tubes that have arced are not necessarily good only for the bin and you should reuse those until solving the key problem, possibly using it in the other socket.

 

Last but not least, tube sockets can cause these issues and replacing with new ones is a cure if the new ones are good - which they easily might not be if current production. I guess there’s no need to check the tube pins because you are using current production tubes and not NOS (Nasty Old Stuff that has been rusting for decades in some lonely warehouse)... ?

 

Keep in mind that the other tube in the pair could be causing the problem to V5 by intermittently loosing connection with anode or cathode or... 

 

Please recheck along these lines. The pictures and schematics posted are fragments and your attention is being directed at things that either did not exist in the original and working amp. The problem is in something that was there from the start and has felt the bite of time passing by...

Share this post


Link to post
Share on other sites
1 hour ago, Alex Kitic said:

Hello,

I am no Dynaco servicing expert, but I can see that most advice you are getting is rather voluntaristic. There is absolutely no need for 600V rated coupling caps, but they cannot hurt. Also, the size (capacity) will not hurt anything but the sound - and smaller caps tend to sound better if large enough not to cut off the deep bass.

Alex Kitic,

Thank you for taking the time to comment. I'd like to respond to the various items you mention. First, I bought this amp new in 1993. It has been the source of sonic joy for many years, and it has also given me its share of challenges. When I first bought it, it started blowing tubes after a few hundred hours of use, taking the bias resistors with it. So for several years I lived with this amp blowing tubes and resistors. Eventually it blew other resistors on the board and I pulled it from service. Just over a year ago I rebuilt it, replacing it with all new resistors and capacitors. 400v coupling capacitors. After about 100 hours of running I started getting loud popping and arcing, red-plating and blown resistors. This is when I began my first foray of serious research into this amp. There is next to no info out there because so few of them were sold. But I can tell you one thing without the shadow of a doubt: This amp absolutely needs 600v coupling caps. The original caps were Sprague 400v, and although tough, even they only lasted a few hundred hours. This is the number 1 design error of this amp. I replaced the new but failing 400v caps with 600v caps and in several hundred hours I am no longer blowing tubes and bias resistors, except for the trouble with V5, which I don't see as the same problem. I will be replacing the coupling caps with lower value caps (still 600v), for the reasons you mention and to further reduce leakage.

1 hour ago, Alex Kitic said:

The problem you are experiencing is definitely related to the g1 pin of tube V5 - something erratic happens with the bias voltage. Red plating obviously means too much current but arcing is a sign of overvoltage.

You should definitely check the biasing circuit and it’s caps (C19 if I am correct). Even if the cap was replaced there’s no guarantee it was good...

 

Also, since the two tubes are basically tied together, one will draw more current if the other doesn’t draw enough... led lights are cool but you should check the bias voltage on the pin during startup, on both tubes.

What you're saying here is where most of the good advice I've received has been leading me. Your explanation is very good and I'm gonna give your idea of c19 a serious look. In fact I am gonna go over the bias circuit very carefully, including careful measurement of bias voltages.

1 hour ago, Alex Kitic said:

Tubes that have arced are not necessarily good only for the bin and you should reuse those until solving the key problem, possibly using it in the other socket.

  Agreed.

1 hour ago, Alex Kitic said:

Last but not least, tube sockets can cause these issues and replacing with new ones is a cure if the new ones are good - which they easily might not be if current production. I guess there’s no need to check the tube pins because you are using current production tubes and not NOS (Nasty Old Stuff that has been rusting for decades in some lonely warehouse)... ?

 

Keep in mind that the other tube in the pair could be causing the problem to V5 by intermittently loosing connection with anode or cathode or... 

 

Please recheck along these lines. The pictures and schematics posted are fragments and your attention is being directed at things that either did not exist in the original and working amp. The problem is in something that was there from the start and has felt the bite of time passing by...

I had the original tube sockets in when the first V5 event took place (always checked for connections and corrosion when installing a new tube). I replaced all six tube sockets after the first event and keep checking tightness of connections. It has happened to V5 two times since replacing sockets with 2 different brands of new prod tubes. I had spare matched tubes but am down to the last one so I'd like to get this figured out before I put the last one in. As this had been happening, I have been moving tubes around and checking connections at all positions so I really think it's in the circuit somewhere.

 

Again, thank you for the advice. I am including a PDF of the whole schematic and would appreciate you commenting on anything else you see.

 

ST-70wScreenResistors.pdf

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...