I have an Ampeg J12T that has an issue. It has a cathode biased EL84 class AB output stage. It distorts at really low volume. Signal is good up to the PI stage. After that at around 1W or so output it starts to distort really badly.
I measured the DC voltage at the 120 ohm cathode bias resistor at 9.9V, as the schematic says. However when I read the current level using an in circuit probe inserted below the tube in an extender socket, I read 13mA in each tube.
.026A x 120 ohm = 3.12V not 9.9V ?
If I use the ohmmeter on the resistor it measures 120 ohms.
How can ohms law be violated? I mean it's the law!
This amp had one side not working for a while, and only 1 tube was conducting. So the OT had an unbalance DC current through it for a while and could have damaged the OT by being way out on B-H curve and going into saturation.
But even that doesn't explain the failure to obey ohms law?
Has anyone seen a 5W power resistor that ohmed out good, but when you put 20mA through it, the resistance increased?
I have new resistors on the way, but I suspect that won't be the issue.
I am going to remove the tubes and see if there is still a voltage on the cathode resistor? There must be some other path for current besides the tube.
j12t by Dennis Kelley, on Flickr
I measured the DC voltage at the 120 ohm cathode bias resistor at 9.9V, as the schematic says. However when I read the current level using an in circuit probe inserted below the tube in an extender socket, I read 13mA in each tube.
.026A x 120 ohm = 3.12V not 9.9V ?
If I use the ohmmeter on the resistor it measures 120 ohms.
How can ohms law be violated? I mean it's the law!
This amp had one side not working for a while, and only 1 tube was conducting. So the OT had an unbalance DC current through it for a while and could have damaged the OT by being way out on B-H curve and going into saturation.
But even that doesn't explain the failure to obey ohms law?
Has anyone seen a 5W power resistor that ohmed out good, but when you put 20mA through it, the resistance increased?
I have new resistors on the way, but I suspect that won't be the issue.
I am going to remove the tubes and see if there is still a voltage on the cathode resistor? There must be some other path for current besides the tube.
j12t by Dennis Kelley, on Flickr
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OK. I re-read my post about this from 2 years ago. The ohms law issue is the fact that the ammeter adds resistance to the reading. So with the ammeter removed I can reply on the voltage reading across the bias resistor. It doesn't tell me how much is on one side vs the other.
I don't think that is the problem.
So i measured the resistance of the primary windings of the OT. One side is 105 ohms, but the other side is 144 ohms. that sounds like a BIG difference. I suspect I have some shorted windings on the 105 ohm side.
There is no short the the ground or from primary to secondary. But that seems like too much imbalance. That was measured between the windings with the rest of the circuit disconnected.
So i guess I am ordering a OT for my J12T.
I don't think that is the problem.
So i measured the resistance of the primary windings of the OT. One side is 105 ohms, but the other side is 144 ohms. that sounds like a BIG difference. I suspect I have some shorted windings on the 105 ohm side.
There is no short the the ground or from primary to secondary. But that seems like too much imbalance. That was measured between the windings with the rest of the circuit disconnected.
So i guess I am ordering a OT for my J12T.
No, it's not. Even 40% diff is normal. The 40 Ohms diff is "nothing" in a 8K load.
They are different because they wind one half primary, secondary, other half primary. For the *same* number of turns each half, the outside winding MUST be longer; and thus higher resistance. But ideally resistance is "small" compared to load, and resistance unbalance is lower.
You could have shorted turns. But I've seen far too many cases where someone spent for new iron and shipping, and it wasn't the OT.
I'd strip the 120r and install two 220r one per cathode. 1/2 Watt resistors won't smoke in an hour, and you just want a quick cross-check on resistor(s) and each tube individually.
I would also have to use two cathode bypass caps. I could just put a 1 ohm resistor in series with the cathode. It may not be wired in a way to allow that to be done.
I can measure the voltage drop across the primary windings. If I know the resistance I can calculate the current.
If it is not the ot then I don’t what else to look at?
I can measure the voltage drop across the primary windings. If I know the resistance I can calculate the current.
If it is not the ot then I don’t what else to look at?
Back on this again. I changed the cathode resistor to lower the bias current just a bit. It was just running a bit too hot. That did nothing to fix the noise issue.
So the amp sounds thin and tinny. And there is a sizzle when I try to turn it up, so there is some high frequency oscillation.
Since I don’t know where the bad sound is coming from I will try to use the scope to isolate it. Other than to run different frequency sine waves I’m not real sure how to do that? So I am open to suggestions on how to do that.
So the amp sounds thin and tinny. And there is a sizzle when I try to turn it up, so there is some high frequency oscillation.
Since I don’t know where the bad sound is coming from I will try to use the scope to isolate it. Other than to run different frequency sine waves I’m not real sure how to do that? So I am open to suggestions on how to do that.
Fixed it!
I decided to get this amp back on the bench, after almost a year of being stumped!
I found the same issue as last time.
I had changed the OT, which had no effect. I also had the cathose bias resistor changed to 240 ohms.
I noticed while watching the output voltage that at highvolume the waveform started to show crossover distortion. So I think the lower current allowed the tube to go intocutoff, before it started flattopping. I went back to the original 120 ohm cathode resistor. It no longer showed the crossover distortion at high volume, but it still sounds horrible.
I found two other issues, that although not right I don’t think were the problem. The tube sockets were corroded. But wiggling the tube did not make any noise.
One of the screen grid resistors was actually 200 ohms instead of 100 ohms. I measured the voltage across them, at 1.2 volts and 0.6 volts. So they had the same current. I can’t see a 0.6 volt difference in the screen voltage to be very significant.
I kept hearing this high pitched ringing fuzzy unpleasant distortion, especially when first hitting a chord. This amp has a 220 pF ceramic cap right across the output of the PI. I added a cap here on a Blues Jr a couple years back that was oscillating at 80kHz. So I thought perhaps that cap was bad, and was there for a reason. I replaced it with a 220pF metal film cap.
So after all these changes I am happy to report the amp sounds great! I also used a different speaker, but not likely an issue. So the hi pitched ringing is gone. The amp is loud as heck now. On strong chords it does flatop, but no crossover distortion, and it still sounds really nice!
I had to gludge up a few resistors, and I will replace the bad sockets. The metal film cap is only rated at 100v, while the ceramic that was there was 400v, but I can’t see the control screen having much voltage on it?
I have been trying to fix this amp since Nov 2016! This thread was my second attempt. It really sounds great, and it is dead quiet!
I may go ahead and put the original OT in it and sell the replacement one. I still think the bias current is a bit high, but perhaps it needs to be there to make sure the tubes don’t go into cutoff too soon.
I decided to get this amp back on the bench, after almost a year of being stumped!
I found the same issue as last time.
I had changed the OT, which had no effect. I also had the cathose bias resistor changed to 240 ohms.
I noticed while watching the output voltage that at highvolume the waveform started to show crossover distortion. So I think the lower current allowed the tube to go intocutoff, before it started flattopping. I went back to the original 120 ohm cathode resistor. It no longer showed the crossover distortion at high volume, but it still sounds horrible.
I found two other issues, that although not right I don’t think were the problem. The tube sockets were corroded. But wiggling the tube did not make any noise.
One of the screen grid resistors was actually 200 ohms instead of 100 ohms. I measured the voltage across them, at 1.2 volts and 0.6 volts. So they had the same current. I can’t see a 0.6 volt difference in the screen voltage to be very significant.
I kept hearing this high pitched ringing fuzzy unpleasant distortion, especially when first hitting a chord. This amp has a 220 pF ceramic cap right across the output of the PI. I added a cap here on a Blues Jr a couple years back that was oscillating at 80kHz. So I thought perhaps that cap was bad, and was there for a reason. I replaced it with a 220pF metal film cap.
So after all these changes I am happy to report the amp sounds great! I also used a different speaker, but not likely an issue. So the hi pitched ringing is gone. The amp is loud as heck now. On strong chords it does flatop, but no crossover distortion, and it still sounds really nice!
I had to gludge up a few resistors, and I will replace the bad sockets. The metal film cap is only rated at 100v, while the ceramic that was there was 400v, but I can’t see the control screen having much voltage on it?
I have been trying to fix this amp since Nov 2016! This thread was my second attempt. It really sounds great, and it is dead quiet!
I may go ahead and put the original OT in it and sell the replacement one. I still think the bias current is a bit high, but perhaps it needs to be there to make sure the tubes don’t go into cutoff too soon.
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