Very informative video. Thank you for sharing this with us. I have had my ST70 for about 25 years and would set the bias at start-up and forget about it. Never again, I will monitor it more regularly.
I had this happen on one of my Heathkit W5-M amplifiers. Had grid leakage on one of the Gold Lion KT-66 tubes that would not show up on a tube tester, besides it took about 3 minutes before the tube would go into runaway. It took the output transformer out, fortunately I had a parts donor amp to rob. The tubes tested fine in my tube tester, of course. Changed output transformer and a new set of tubes, but I kept a meter on the new tubes for several days to monitor the dc balance between the cathode resistors.
Thanks for posting. I'll definitely stay away from JJ tubes. Tubes generally come with a no-return policy, and I certainly wouldn't want to get stuck with some duds.
Terry, could you swap the tubes between the channels to see if the drift follows the tube set. Would that work in determining if the tubes were the issue?
Yes, That is how I found the drifty tube. Moved it to the right and current drifted high. JJ tubes are not my 1st choice when working on vintage amps. I have had many assorted failures with that brand.
After many years of working with tube power amplifiers I’ve arrived at thinking that most circuit designs use grid resistors with too high of a resistance value. For example the Dynaco ST70 Has 270K ohm grid resistors for the EL34 output tubes. Changing the grid resistors to ~100K will reduce the bias drift by a factor of 3. However this lowers the open loop gain and may prevent the driver tube from supplying enough peak AC grid voltage swing to get full rated power/distortion from the amplifier. When the ST70 was introduced in the 1960’s tubes were cheap and sold everywhere. So tube life expectancy was traded off for a simple low-cost circuit design. Also some of today’s current production power tubes aren’t as robust as vintage tubes. In fact the quality of tubes manufactured in the 1960’s was probably better than anything available today. To explain why I think the grid resistors need to be smaller I should say something about the causes of “red plating” and thermal runaway in audio power tubes. First make sure the coupling capacitors are good (no measurable DC leakage at all). This leaves the only possible cause of grid current to be the output tube itself. Ideally the control grid should not emit any electrons at all. However, if the grid gets too hot or has any bits of cathode coating accidently spattered on it during manufacturing, then the grid may actually emit some electrons. A second cause of grid current is the presence of positively charged gas ions inside the tube. The control grid, being the most negatively charged item inside the tube, attracts these positive charged ions. Adding positive charge to the control grid also results in grid current flowing, as electrons must be supplied to balance the positive charge of the gas ions. I suspect that gas inside the tube may actually be the major source of grid current. In conclusion, either grid emission or gas inside the tube will both make the grid more positive. Result: Cathode current increases, making the tube even hotter. This may set up the unstable conditions for “thermal runaway” which causes “red plating.” This just a start for me. Next I want to find ways to stabilize the cathode current without changing the audio performance of the amplifier.
Great video Terry. I've had similar problem with biasing solid state receivers (vintage Marantz). Wasn't sure what do do even after replacing all power supply caps.
Hi Terry I absolutely love your videos and I’m a fan for a long time. Great work and information. I can’t help but say it annoys me the in or unintentional advertising of the meter ( the box is in the middle of the two meters ??? ) Please keep up the good work Trev
@@d-labelectronics you shouldn’t have to buy these, Hioki should gift them to you, after all you have done more advertising for them considering how many viewers have seen your videos with you using and praising their speedy results. I myself would probably buy one also when the time comes that I need a new one because if their good enough for all the work you do then they should be plenty good enough for me. I have had very good results so far with your recommendations, I picked up a Iwatsu, Viz and a Luxo and they are perfect for adding pleasure to my hobby, thank you for all your videos and great advice.
when adjusting the bias do you need to disconnect the inputs and speakers and someone suggested plugging in a RCA ground plug into the inputs...also why did you only use 1.2v❓
You neglect to mention that you “must have a speaker load” hooked up before power on the amp. A load must always be hooked up or you can damage the transformer. Also, not to play music while doing this or turn on your preamp. Leave the preamp off. Do this before you start. With new tubes, turn the bias setting all the way counter clockwise before power up. Then turn on the amp and let the amp settle for 2 minutes or so. Then slowly bring the left channel up to about 1.45 to 1.5. Dynaco says 1.56 but a little under is good for measure. Good for the life of the tubes.
Very informative video. Thank you for sharing this with us. I have had my ST70 for about 25 years and would set the bias at start-up and forget about it. Never again, I will monitor it more regularly.
Good video for ANY tube amp. Too many guitar amp owners just replace the tubes and think that they are "good to go".
Great info Terry, learned something new again..Thanks for sharing.. Ed..UK..😊
I had this happen on one of my Heathkit W5-M amplifiers. Had grid leakage on one of the Gold Lion KT-66 tubes that would not show up on a tube tester, besides it took about 3 minutes before the tube would go into runaway. It took the output transformer out, fortunately I had a parts donor amp to rob. The tubes tested fine in my tube tester, of course.
Changed output transformer and a new set of tubes, but I kept a meter on the new tubes for several days to monitor the dc balance between the cathode resistors.
Thanks for posting.
I'll definitely stay away from JJ tubes. Tubes generally come with a no-return policy, and I certainly wouldn't want to get stuck with some duds.
Thanks D-Lab. Great video as always.
Terry, could you swap the tubes between the channels to see if the drift follows the tube set. Would that work in determining if the tubes were the issue?
Yes, That is how I found the drifty tube. Moved it to the right and current drifted high. JJ tubes are not my 1st choice when working on vintage amps. I have had many assorted failures with that brand.
Thanks Terry.
After many years of working with tube power amplifiers I’ve arrived at thinking that most circuit designs use grid resistors with too high of a resistance value. For example the Dynaco ST70 Has 270K ohm grid resistors for the EL34 output tubes. Changing the grid resistors to ~100K will reduce the bias drift by a factor of 3. However this lowers the open loop gain and may prevent the driver tube from supplying enough peak AC grid voltage swing to get full rated power/distortion from the amplifier. When the ST70 was introduced in the 1960’s tubes were cheap and sold everywhere. So tube life expectancy was traded off for a simple low-cost circuit design. Also some of today’s current production power tubes aren’t as robust as vintage tubes. In fact the quality of tubes manufactured in the 1960’s was probably better than anything available today.
To explain why I think the grid resistors need to be smaller I should say something about the causes of “red plating” and thermal runaway in audio power tubes. First make sure the coupling capacitors are good (no measurable DC leakage at all). This leaves the only possible cause of grid current to be the output tube itself. Ideally the control grid should not emit any electrons at all. However, if the grid gets too hot or has any bits of cathode coating accidently spattered on it during manufacturing, then the grid may actually emit some electrons. A second cause of grid current is the presence of positively charged gas ions inside the tube. The control grid, being the most negatively charged item inside the tube, attracts these positive charged ions. Adding positive charge to the control grid also results in grid current flowing, as electrons must be supplied to balance the positive charge of the gas ions. I suspect that gas inside the tube may actually be the major source of grid current.
In conclusion, either grid emission or gas inside the tube will both make the grid more positive. Result: Cathode current increases, making the tube even hotter. This may set up the unstable conditions for “thermal runaway” which causes “red plating.”
This just a start for me. Next I want to find ways to stabilize the cathode current without changing the audio performance of the amplifier.
Thank's Terry appreciate the video
Great video Terry. I've had similar problem with biasing solid state receivers (vintage Marantz). Wasn't sure what do do even after replacing all power supply caps.
Had the same thing happen in PR clone, tubes tested fine and the screen resistor fried on that tube with the bias set good
Hi Terry
I absolutely love your videos and I’m a fan for a long time.
Great work and information.
I can’t help but say it annoys me the in or unintentional advertising of the meter ( the box is in the middle of the two meters ??? )
Please keep up the good work
Trev
I just purchased the 2nd meter for the test. Wanted two identical models, Not intended for advertisement
@@d-labelectronics
Brilliant!
I apologise if I offended
QAPLA. !
None taken Sir
@@d-labelectronics you shouldn’t have to buy these, Hioki should gift them to you, after all you have done more advertising for them considering how many viewers have seen your videos with you using and praising their speedy results. I myself would probably buy one also when the time comes that I need a new one because if their good enough for all the work you do then they should be plenty good enough for me. I have had very good results so far with your recommendations, I picked up a Iwatsu, Viz and a Luxo and they are perfect for adding pleasure to my hobby, thank you for all your videos and great advice.
when adjusting the bias do you need to disconnect the inputs and speakers and someone suggested plugging in a RCA ground plug into the inputs...also why did you only use 1.2v❓
Could you explain why you set the bias at 1.2 instead of 1.56?
Nice testing sir
You neglect to mention that you “must have a speaker load” hooked up before power on the amp. A load must always be hooked up or you can damage the transformer. Also, not to play music while doing this or turn on your preamp. Leave the preamp off. Do this before you start. With new tubes, turn the bias setting all the way counter clockwise before power up. Then turn on the amp and let the amp settle for 2 minutes or so. Then slowly bring the left channel up to about 1.45 to 1.5. Dynaco says 1.56 but a little under is good for measure. Good for the life of the tubes.
Nice video! Where to put the negative probe?
Thanks /Niklas
To chassis
is your pre amp on while you're doing this test❓I see the inputs are connected to the dynaco amp
Not sure when this was being shot
👍👍
👍
why are you going to
1.2 volts and not 1.56 like it says you should❓and should it matter if the dynaco is 120v american or 230v europe❓
Higher HV due to line voltage. Reducing current thru the tubes to compensate.
The drifting tube was gassy, I suppose?
Thanks! D-Lab 🧯