HELP needed from tube amp gurus! b15n distortion and fade problem

[whitelines]

Hey tube amp experts (@agedhorse and friends...), I could use some advice.

My early 60's Ampeg b15n usually sounds great, but occasionally starts to hum and the output gets softer and distorted. It usually does not do this, but this time, I caught it on video.
When it gets in this mode, toggling the standby switch from on to off, then to off again brings back the normal glorious b15 sound. I believe that if I were to then use the amp for a while, it would go back into the distorted and soft state, though I did not catch that in this video.

I've taken it to my local tech twice, and he is really good, but he has not been able to recreate the symptom. This problem does not recreate on demand, if it did, I'm sure he would have solved it. I did not have this video before, and thought I'd get a pre-diagnosis before I took it back in.

Do any of you tube amp experts out there have an idea what might be the issue or definitive steps my local tech should do. I suspect this is such a distinctive issue that someone will know right away what it wrong and needs to be replaced.

Thanks for any help that you guys can send my way!

 

[whitelines]

p.s., the tapping/clicking sound in the video is not the amp, its a little shi-tzu walking on the wood floor.

p.p.s, all the tubes and tube rectifier have been tested at the tech shop with his old-school tube tester. no-problems.

p.p.p.s - see the end of the video where I toggle the standby on and off several times. The sound of the amp as it transitions to standby goes through a tone/volume area that sounds similar to the issue at hand. It's like the amp gets stuck in a zombie zone between standby and on when it happens.

 

[Wasnex]

Hey tube amp experts (@agedhorse and friends...), I could use some advice.

My early 60's Ampeg b15n usually sounds great, but occasionally starts to hum and the output gets softer and distorted. It usually does not do this, but this time, I caught it on video.
When it gets in this mode, toggling the standby switch from on to off, then to off again brings back the normal glorious b15 sound. I believe that if I were to then use the amp for a while, it would go back into the distorted and soft state, though I did not catch that in this video.

I've taken it to my local tech twice, and he is really good, but he has not been able to recreate the symptom. This problem does not recreate on demand, if it did, I'm sure he would have solved it. I did not have this video before, and thought I'd get a pre-diagnosis before I took it back in.

Do any of you tube amp experts out there have an idea what might be the issue or definitive steps my local tech should do. I suspect this is such a distinctive issue that someone will know right away what it wrong and needs to be replaced.

Thanks for any help that you guys can send my way!

Here's a possible hint. That sort of a dull roar (hum) is typical of a mismatch of current in the output tubes.

The output section is pushpull. You have two tubes connected to opposite ends of a center-tapped transformer. The (positive charge) or voltage flows from the center tap (A red wire) outward towards each tube (blue and brown wires). I put a green rectangle to draw your eye to the center-tapped primary of the transformer.

The hum cancels when each tube pulls approximately the same amount of currently.

So my guess is either one of the tubes is changing state and conducting really hard, or practically turning off.

If it's conducting really hard you may see evidence because the plate will turn a cherry red. Actually I think I see this in the video. Check the tube to the right of the Plexi Ampeg logo. It had a red glow when the amp was humming. After you flip the standby off an on, the glow is gone. My guess is one of the 270K bias resistors is intermittent or the tube socket needs retensioned or cleaned.

If the tube has been allowed to redplate for long it may not last for much longer.

Good luck!

 

[StephenM]

when was the last cap job done on the amp ?

 

[agedhorse]

I would also suspect an intermittent power tube.

I would avoid "cap jobs" unless it can be shown without a doubt that a bad cap is causing the symptoms. Cap jobs are just another term for hand job unless it can be shown that a cap is failing. My guess is that 95% of all replaced caps are perfectly fine.

 

[whitelines]

Here's a possible hint. That sort of a dull roar (hum) is typical of a mismatch of current in the output tubes.

If it's conducting really hard you may see evidence because the plate will turn a cherry red. Actually I think I see this in the video. Check the tube to the right of the Plexi Ampeg logo. It had a red glow when the amp was humming. After you flip the standby off an on, the glow is gone. My guess is one of the 270K bias resistors is intermittent or the tube socket needs retensioned or cleaned.

Good luck!

@Wasnex, you may be correct sir! To check, I turned the combo on with the tube cage off. Neither power tube is anything but a little orange at the top and bottom, the heaters I believe. I'll clean and tension the sockets as well as look for suspicious cracking, burning around the resistors as well as test them with my meter while trying to wiggle them. Looking at the schematic, I think I should be able to measure directly across the 270k then the 1k attached to the grid without having to disconnect them.

thanks

 

[whitelines]

@StephenM @agedhorse , I had the caps replaced 5 or so years ago with freshies from fliptop.net .

 

[whitelines]

If I don't find anything obvious with the resistors, (if the fail is internal to the resistor, I assume there is not an easy way to test for intermittency) I'll just tidy up the sockets and try it for a while. If it happens again, I'll swap in some new power tubes and see what happens.

 

[Wasnex]

@Wasnex, you may be correct sir! To check, I turned the combo on with the tube cage off. Neither power tube is anything but a little orange at the top and bottom, the heaters I believe. I'll clean and tension the sockets as well as look for suspicious cracking, burning around the resistors as well as test them with my meter while trying to wiggle them. Looking at the schematic, I think I should be able to measure directly across the 270k then the 1k attached to the grid without having to disconnect them.

thanks

As @agedhorse mentioned, it could easily be an intermittent tube as well. You could swap the tube and see if the problem stays with the socket or follows the tube.

Sounds like you may have enough of a background to handle this yourself .

 

[whitelines]

If it happens with new power tubes, I'll replace the 270k 's and maybe the 1k's if its easy.
w

 

[whitelines]

As @agedhorse mentioned, it could easily be an intermittent tube as well. You could swap the tube and see if the problem stays with the socket or follows the tube.

Sounds like you may have enough of a background to handle this yourself .

I write firmware for a living and live the the 3.3V schematic world, so debugging tube amps is a stretch. Rest assured, I'll make sure to unplug the amps discharge the caps before I do anything so I don't let the smoke out of the amp or me.

 

[Wasnex]

I write firmware for a living and live the the 3.3V schematic world, so debugging tube amps is a stretch. Rest assured, I'll make sure to unplug the amps discharge the caps before I do anything so I don't let the smoke out of the amp or me.

That part of the circuit is pretty simple. You might want to read up on how fixed bias works.

If you don't know, rather than simply shorting the caps, it's better to use a shorting jig with a resistor. I use some insulated test leads and 33K, 3W resistor I swapped out of one of my amps. It takes a little longer to fully discharge a capacitor, but no worries with leads sticking to the terminals due to arcing. A lower value resistor would be better, but the 33K was handy .

Confirm the caps are discharged with your multimeter.

 

[whitelines]

That part of the circuit is pretty simple. You might want to read up on how fixed bias works.

If you don't know, rather than simply shorting the caps, it's better to use a shorting jig with a resistor. I use some insulated test leads and 33K, 3W resistor I swapped out of one of my amps. It takes a little longer to fully discharge a capacitor, but no worries with leads sticking to the terminals due to arcing. A lower value resistor would be better, but the 33K was handy .

Confirm the caps are discharged with your multimeter.

 

[whitelines]

Yeah, I’ll bleed them with a power resistor from work
W

 

[Deleted member 165050]

Cap jobs are just another term for hand job unless it can be shown that a cap is failing.

 

[Microbass]

This one looks rather warm compared to the others.. @Wasnex has a good eye!

 

[beans-on-toast]

If it’s an intermittent issue with a power tube, it may not reveal itself with a tube tester. Tapping the power tubes while the issue is occurring might help reveal which tube it is.

Another possibility is a bad standby switch. In this amp, the standby runs through the high voltage center tap to ground. It stresses the switch and eventually they fail. Even when in the switch on position.

Another possibility is a problem with the rectifier tube. If one of the two diodes is intermittent, hum will come and go. At the very least, check the socket tension. If you have a replacement tube or plugin solid state module, swap it in and test.

 

[bumperbass]

I worked on one about a year ago. The solder joints all looked fine, even under high magnification, yet reflowing a few fixed it.
Tap around with a small wooden stick to see if you can get it to mess up.

 

[markjazzbassist]

but a whole new set of tubes (you should have a backup set regardless) and try swapping them. could be the rectifier tube.

 

[Wasnex]

If you can get the amp to repeat the problem, I suggest looking at both output tubes. Pretty sure it's only the tube on the right that is red plating, but I could be wrong. Also FYI my ears are really what keyed me to the problem. (hint, hint) That sort of dull hum typically means the current in the output transformer is badly out of balance.

If it's just the one tube, I think the best thing to do is simply swap the tubes (with one another) and see if the problem follows the tube or not. Could be a tube, could be a resistor, could be a flaky solder joint. If the problem follows the tube, you know you need a new set of tubes and a setup.

I don't recommend just throwing a new set of tubes in the amp. If new tubes are required, it's a good idea to check the current passing through tubes to make sure the amp is biased to the proper range. This requires bias probes or the installation of bias sense resistors off the cathodes of the tubes.

AFAIK, the amp does not have variable bias, so there also a good chance it will require changing out a resistor in the bias supply if the tube current does not fall in the proper range. This is probably work for a more experienced tech.

If you just throw a set of new tubes in and they are biased to hot, your basically just throwing away good money and risking a major melt down that could take out the transformers.

While it's possible to measure or extrapolate the tube current off the plate and transformer, I don't recommend these procedures for a self-professed novice, due to the lethality of the voltages. I don't even use these procedures myself. Not that I can't use the procedures, I just prefer to use other methods.