I started tinkering around with my little 4-4-0 (express preamp -> 6sn7 power amp) and bumped into something that I think I need to take care of. It seems I have an oscillation that kicks in (in spurts) as the power tubes start to clip.
The signal is clean after every pre-amp stage. But starts to get fuzzy on just parts of the wave after the phase inverter. I am using the Ruby zener trick on the power tube grid leak resistors. And I thought that may be causing it. But it did the same thing with they were removed (so I put 'em back in).
Below are some screen shots of my scope. Channel one is connected to the speaker out (which is driving a weber mass) and channel two is connected after the coupling cap on the 100k side of the phase inverter. In each shot I am increasing the signal from just before the power tubes clip to well into clipping.
The grid stoppers for the power tubes are mounted on the board. Moving them to the sockets is one idea I have. But I'm not certain that is what is going on. Anyone else seen this? Or have an idea on what may be causing it?
Mike
Oscillation action shots!
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Oscillation action shots!
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tele_player
- Posts: 311
- Joined: Sat Sep 29, 2007 3:27 am
Re: Oscillation action shots!
I've never tried a weber MASS, but I'd be curious to see if the same behavior appears with a pure resistive load.
Robert
Robert
Re: Oscillation action shots!
I had the same oscillation as seen in pic 1. I tried so many changes I lost track. The oscillation finally disappeared after increased screen resistors, a tube swap, and increasing the output stage feedback. I was so glad to finally be done with it, I boxed it up and gave it back without more work to see which, or what combination solved the issue.
Tube junkie that aspires to become a tri-state bidirectional buss driver.
Re: Oscillation action shots!
Yes, grid stoppers mounted at the power tube socket terminals, and consider twisting the wires carrying the phase splitter outputs to them.
Review the potential for coupling between the power amp plate / OT primary / secondary and the phase splitter circuit.
The issue seems to be worse on one side of the p-p circuit than the other, so bear that in mind.
Yes, the oscillation may not occur with a resistive load, but it still needs to be stable with a speaker type inductive load.
Review the potential for coupling between the power amp plate / OT primary / secondary and the phase splitter circuit.
The issue seems to be worse on one side of the p-p circuit than the other, so bear that in mind.
Yes, the oscillation may not occur with a resistive load, but it still needs to be stable with a speaker type inductive load.
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fperron_kt88
- Posts: 72
- Joined: Wed Oct 10, 2007 2:19 pm
- Location: Montreal
Re: Oscillation action shots!
Hello Romberg,
I had a very similar problem a few years back in a Marshall Major. The relationship between the phase inverter and the power tubes was reversed. The crests out of the PIs push pull action were cutoff-cutoff instead of saturation-saturation (if I remember correctly).
The sharp edges sent parasitic harmonics all over the place at just the wrong moment...
The solution was to swap the PI outputs relative to the grids and correspondingly swap the output transformer leads to make the feedback happy again.
See the video: https://www.youtube.com/watch?v=TRfByCL74rM
And the thread: http://ampgarage.com/forum/viewtopic.ph ... c&start=15
Perhaps you have the same issue?
Good luck!
I had a very similar problem a few years back in a Marshall Major. The relationship between the phase inverter and the power tubes was reversed. The crests out of the PIs push pull action were cutoff-cutoff instead of saturation-saturation (if I remember correctly).
The sharp edges sent parasitic harmonics all over the place at just the wrong moment...
The solution was to swap the PI outputs relative to the grids and correspondingly swap the output transformer leads to make the feedback happy again.
See the video: https://www.youtube.com/watch?v=TRfByCL74rM
And the thread: http://ampgarage.com/forum/viewtopic.ph ... c&start=15
Perhaps you have the same issue?
Good luck!
...
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frankleslie
- Posts: 6
- Joined: Mon Dec 08, 2014 11:36 am
Re: Oscillation action shots!
Hi, I had a very similar oscillation with the output stage of a 57' De Luxe reissue. The tubes was (just installed) NOS RCA 6V6s. I changed one tube to another NOS RCA 6V6 and the problem went away.
First time I built an amp with 4 EL84s I foolishly put the grid stoppers on the turret board and discovered I had built a decent long wave transmitter. I decided to stop this oscillation nonsense and put 4 100k gridstoppers right on grid pins and not only did it cure the problem it made the amp really sweet when driven into distortion. It sounded great! Since then I use as large grid stoppers as I can. No more 1k5s for this boy!
First time I built an amp with 4 EL84s I foolishly put the grid stoppers on the turret board and discovered I had built a decent long wave transmitter. I decided to stop this oscillation nonsense and put 4 100k gridstoppers right on grid pins and not only did it cure the problem it made the amp really sweet when driven into distortion. It sounded great! Since then I use as large grid stoppers as I can. No more 1k5s for this boy!
Re: Oscillation action shots!
Thanks for all the tips and ideas/areas to check out. Long story short. I fixed it (I think).
I did sorta remember this amp coming to life with a howl and having to swap the primary OT leads to fix that or all it would do is oscillate out of control. But it has been a while since I built it and I now have a scope. I checked the phase of the OT tap the negative feedback is derived from. It is in phase with the first grid of the phase inverter. Merlin's book says this is what it is supposed to be (since the signal is injected on the second grid). So, that was good to eliminate.
I then moved around a few wires on the PI and power tube. But that dang spike of noise was still there. It just suddenly appeared right when the power tubes started to clip. And the signal was also on the power tube grids and PI coupling caps. But no where else.
I was about to call it a day and planed to move the grid stoppers tomorrow when I thought to try throwing another 100pf across the PI plates. The original 4-4-0 does not use one and for some reason I installed a 50pf (prolly to match the kelly schematic). And that 150pf across the PI plates made all the pain go away.
I'm not real sure exactly what this cap does. I've heard it called a fizz cap before. I'll have to do a little more reading about it. It is possible that my fix is just a band aid and that a better one needs to be done. In particular I might try a different power tube as suggested above. I only have the one 6sn7 at the moment. The oscillation might still be there and increasing this cap just hides it?
Mike
I did sorta remember this amp coming to life with a howl and having to swap the primary OT leads to fix that or all it would do is oscillate out of control. But it has been a while since I built it and I now have a scope. I checked the phase of the OT tap the negative feedback is derived from. It is in phase with the first grid of the phase inverter. Merlin's book says this is what it is supposed to be (since the signal is injected on the second grid). So, that was good to eliminate.
I then moved around a few wires on the PI and power tube. But that dang spike of noise was still there. It just suddenly appeared right when the power tubes started to clip. And the signal was also on the power tube grids and PI coupling caps. But no where else.
I was about to call it a day and planed to move the grid stoppers tomorrow when I thought to try throwing another 100pf across the PI plates. The original 4-4-0 does not use one and for some reason I installed a 50pf (prolly to match the kelly schematic). And that 150pf across the PI plates made all the pain go away.
I'm not real sure exactly what this cap does. I've heard it called a fizz cap before. I'll have to do a little more reading about it. It is possible that my fix is just a band aid and that a better one needs to be done. In particular I might try a different power tube as suggested above. I only have the one 6sn7 at the moment. The oscillation might still be there and increasing this cap just hides it?
Mike