Happened to take a look at the layout:
http://www.ampwares.com/ffg/schem/champ_aa764_layout.gif
Note that the center of the volume pot is wired to the board, and there is a dotted line to the connection to the tube grid. Companies sometimes leave out a detail, to deter those who might copy their designs, especially early on when they do the documentation. It is very likely that the production models had a grid stopper resistor there.
Pete B.
http://www.ampwares.com/ffg/schem/champ_aa764_layout.gif
Note that the center of the volume pot is wired to the board, and there is a dotted line to the connection to the tube grid. Companies sometimes leave out a detail, to deter those who might copy their designs, especially early on when they do the documentation. It is very likely that the production models had a grid stopper resistor there.
Pete B.
Back Again
OK, I replaced the multi section cap, No Change,
I added a 1.5K grid resistor tested again, No Change, Added a grid resistor to the 6V6, No Change,
Moved the earth directly to chassis next to iec jack No change,
Added a second contact point of the ground bus to chassis at the end near the input, No Change.
Flipped between my known perfect tubes once again at each modification. No Change.
This has to be the freakiest amp ever. If you heard the quality of the tone and the great volume for a single tube amp you honestly would be surprised, Yet, The problem still exisits!
I have played the sister to this amp several times today also. Not even a hint of these problems. It was true to my original layout,
BTW, it is possibly one of if not THE quietest guitar amps I have ever seen, Its easy to forget its even on.
So, I guess this ones got me beat for the time being.
OK, I replaced the multi section cap, No Change,
I added a 1.5K grid resistor tested again, No Change, Added a grid resistor to the 6V6, No Change,
Moved the earth directly to chassis next to iec jack No change,
Added a second contact point of the ground bus to chassis at the end near the input, No Change.
Flipped between my known perfect tubes once again at each modification. No Change.
This has to be the freakiest amp ever. If you heard the quality of the tone and the great volume for a single tube amp you honestly would be surprised, Yet, The problem still exisits!
I have played the sister to this amp several times today also. Not even a hint of these problems. It was true to my original layout,
BTW, it is possibly one of if not THE quietest guitar amps I have ever seen, Its easy to forget its even on.
So, I guess this ones got me beat for the time being.
I know how frustrating it is when you build something and it just refuses to work the way you expect it to. If it was just dead it would be a lot easier to fix. I forgot to ask earlier, does the amp cut off if the volume on the guitar is turned way down such that the amp is not loud, or is this a large signal only problem?
I know that you have been all over this thing trying to fix the oscillation with NO CHANGE! If this is an oscillation problem I would have expected your total rebuild of the amp to have changed something. It may not have fixed the problem, but the oscillation parameters (and therefore the symptoms) should have at least changed. I would have bet on an oscillation issue, but since you have tried just about everybodies best oscillation cures with no luck, maybe the problem lies elsewhere. First, make another attempt to test for oscillation. If this amp is oscillating bad enough to cut off the sound, it should be radiating RF energy that can be detected.
I would try the second best oscillating amp detector that I know of (scope is #1), your TV set. A TV will detect a signal from 54MHz to 806MHz. This works best if you have cable since there is a watchable picture on most channels. This trick will not work on a satellite receiver since it works at frequencies well above the reach of conventional vacuum tubes. Bring the amp into close proximity to a working TV set. With the amp operating normally (guitar hooked up), rotate the volume and treble controls through the point of amp cutoff. Try the TV on all of the channels. If the amp is oscillating you should see (or hear) some interference when the cutoff point is passed. Don't see anything? Loosen the cable where it hooks up to the TV (or cable box, or VCR) so that only the center pin is connected. This should give snowy pictures that are more succeptable to interference. If you see no interference there is a high probability that the amp is not oscillating. What else could be wrong? Well you have changed just about all of the parts. I would attempt the following tests.
Attach a voltmeter using clip leads to various points in the amp. Then turn the amp on and play it with the guitar turned down to avoid changes due to overdrive. Turn it up until cutoff occurs, does any voltage make an abrupt change? I would start with the black voltmeter lead on your ground bus, ant the red lead on the plate of the second 12AX7 section. If there is no change, try probing other points in the amp, one at a time. Something has to be changing when the cut off happens. I am guessing that it has to be the second 12AX7 section, or the 6V6. Since you have changed every thing else, take a careful look at the tube socket for the 12AX7, some crud could cause a leakage path and upset the bias. When you turn up the volume and the treble, you are increasing the resistance from grid to ground.
Good luck!
George
I know that you have been all over this thing trying to fix the oscillation with NO CHANGE! If this is an oscillation problem I would have expected your total rebuild of the amp to have changed something. It may not have fixed the problem, but the oscillation parameters (and therefore the symptoms) should have at least changed. I would have bet on an oscillation issue, but since you have tried just about everybodies best oscillation cures with no luck, maybe the problem lies elsewhere. First, make another attempt to test for oscillation. If this amp is oscillating bad enough to cut off the sound, it should be radiating RF energy that can be detected.
I would try the second best oscillating amp detector that I know of (scope is #1), your TV set. A TV will detect a signal from 54MHz to 806MHz. This works best if you have cable since there is a watchable picture on most channels. This trick will not work on a satellite receiver since it works at frequencies well above the reach of conventional vacuum tubes. Bring the amp into close proximity to a working TV set. With the amp operating normally (guitar hooked up), rotate the volume and treble controls through the point of amp cutoff. Try the TV on all of the channels. If the amp is oscillating you should see (or hear) some interference when the cutoff point is passed. Don't see anything? Loosen the cable where it hooks up to the TV (or cable box, or VCR) so that only the center pin is connected. This should give snowy pictures that are more succeptable to interference. If you see no interference there is a high probability that the amp is not oscillating. What else could be wrong? Well you have changed just about all of the parts. I would attempt the following tests.
Attach a voltmeter using clip leads to various points in the amp. Then turn the amp on and play it with the guitar turned down to avoid changes due to overdrive. Turn it up until cutoff occurs, does any voltage make an abrupt change? I would start with the black voltmeter lead on your ground bus, ant the red lead on the plate of the second 12AX7 section. If there is no change, try probing other points in the amp, one at a time. Something has to be changing when the cut off happens. I am guessing that it has to be the second 12AX7 section, or the 6V6. Since you have changed every thing else, take a careful look at the tube socket for the 12AX7, some crud could cause a leakage path and upset the bias. When you turn up the volume and the treble, you are increasing the resistance from grid to ground.
Good luck!
George
PB2 said:
Tried The 6.3 thing, This trans has a CT on that section. On the working unit, Its grounded to chassis and works correctly of course.
On the non-working unit, I have tried both chassis to CT, and 6.3 to chassis like the original schematic shows. Symptom remained the same on both set-ups.
I am up to 500K on the stoppers. Cant kill it.
Can The PT be leaking? or defective?
Or, Is it possible that the OPT color codes are reversed on the primary? Would that have an effect? Right now and ever since I started this nightmare the opt has been installed based on Triodes original specs sheet.
Another thing of interest,
Since I switched to the bus style ground which attaches to a single point on the chassis at the transformer stud. I am getting considerable buzz or perhaps RF & a very mild hum. Previously I only got a general guitar cord/Guitar hum due to poor instrument/pick-up shielding and close proximity. ( Fender Trademark! )
When the input plug is removed, The input jack grounds via a 1 meg resistor on the input jack/switch.
All my current amps I built are dead silent while guitar is unplugged regardless of volume or tone settings.
On this amp, I am still getting the buzz, The Hum disappears instrument unplugged.
As I raise the volume, the buzz gets very noticeable until the oscillation occurs. Then of course If I continue to raise the volume, It ultimately cuts-off.
I'll tell ya, this ones been a real nightmare. I think we can basically rule out the majority of know cures. I am pretty much ready to concede defeat, Gut it and make a nixie clock on the chassis LOL
Oh hey,, just off topic a bit, Can I make something fun out of a couple 5719 triode, 6111 twin triode & or 5840 pentodes & 5702 pentodes?
The reason I ask, My honey came home from a garage sale yesterday with an old wooden cigar box stuffed full of mostly 5702WA's and a few other mini's mixed in.
Possibly my valve dog whistle project? haha
gene
Hi Gene,
This amp might be oscillating all the time. This would increase the standing current, you might hear it as a hum. When the treble control is increased, it gets worse.
Don't feel bad though. The amp I am fixing just turned into an oscillator, both channels. I hope I'm not on the same frequency as you!
-Chris
This amp might be oscillating all the time. This would increase the standing current, you might hear it as a hum. When the treble control is increased, it gets worse.
Don't feel bad though. The amp I am fixing just turned into an oscillator, both channels. I hope I'm not on the same frequency as you!
-Chris
anatech said:
Chris,
I hope you have a better time then I just have had. Honestly, I thought when I first posted about this, I would have had some silly mistake or wrong connection and it would have long ago been cured.
Every guy that posted possibilites of a fix really in my opinion had great ideas and troubleshooting skills. I learned a conciderable amount from each of them.
I plan on getting more voltage reads on it both standing & at cut-off just for curiosities sake.
I have an old scope, I think It sort of works, But no manual.
I have to find more info and learn how to use it correctly, If memory serves it is an old BK tube model. Been setting under my bench about a year.
I may later try a different PT, I am pretty burned out on this project now. Maybe a day or 365 away from this one mighht be the answer lol
gene
Hi Gene,
This one has me puzzled too! I am dying to find out what the problem is!
Play with the 'scope and get used to it. HF oscillation will turn the normal thinner trace to a thick trace. You do not need to see the individual wave forms. You can look in the working one to see what's normal.
Turn the sensitivity control to the least sensitive (higher V/div) so you don't damage it, AC coupling. Scale down once the leads are hooked up. Do this for each measurement.
The amp turned out to be unstable at lower voltages, I'm using a variac for the first power up. Still in process making sure the bias is stable. (SS amp)
-Chris
This one has me puzzled too! I am dying to find out what the problem is!
Play with the 'scope and get used to it. HF oscillation will turn the normal thinner trace to a thick trace. You do not need to see the individual wave forms. You can look in the working one to see what's normal.
Turn the sensitivity control to the least sensitive (higher V/div) so you don't damage it, AC coupling. Scale down once the leads are hooked up. Do this for each measurement.
The amp turned out to be unstable at lower voltages, I'm using a variac for the first power up. Still in process making sure the bias is stable. (SS amp)
-Chris
Hi Gene,
It is surprising that the grid stoppers do not work and I tend to wonder if there is something fundamentally wrong with the amp.
I believe your supply voltages are low, they should be read with the tubes in circuit.
It's difficult to guess at this without being there with a scope to look at things.
I don't believe that buss bar ground, as implemented, improved anything.
I prefer a star ground with the supply cap as the center of the star. This cap is the source of current which must return to the ground of the cap. Keep the output ground (higher current) away from the input grounds so there is no stray coupling.
Then also if you ground the chassis at one point, usually the input, it just acts as a shield and cannot have any ground currents flowing in it.
Pete B.
It is surprising that the grid stoppers do not work and I tend to wonder if there is something fundamentally wrong with the amp.
I believe your supply voltages are low, they should be read with the tubes in circuit.
It's difficult to guess at this without being there with a scope to look at things.
I don't believe that buss bar ground, as implemented, improved anything.
I prefer a star ground with the supply cap as the center of the star. This cap is the source of current which must return to the ground of the cap. Keep the output ground (higher current) away from the input grounds so there is no stray coupling.
Then also if you ground the chassis at one point, usually the input, it just acts as a shield and cannot have any ground currents flowing in it.
Pete B.
Hi Gene,
Had another thought. It seems to me that the HV sags a lot when you put the tubes in, indicating that the PS has a high source resistance. It also seems that the supply voltage is low with the tubes in even with no signal.
It's possible that the amp draws just enough current with it cranked up to further lower the supply voltage so that the tubes run into cutoff, then the HV comes back as it charges the supply cap, tubes go back to linear operation drag the supply down again, and this oscillation repeats.
Did you compare working amp voltages to this one, with the tubes in?
Pete B.
Had another thought. It seems to me that the HV sags a lot when you put the tubes in, indicating that the PS has a high source resistance. It also seems that the supply voltage is low with the tubes in even with no signal.
It's possible that the amp draws just enough current with it cranked up to further lower the supply voltage so that the tubes run into cutoff, then the HV comes back as it charges the supply cap, tubes go back to linear operation drag the supply down again, and this oscillation repeats.
Did you compare working amp voltages to this one, with the tubes in?
Pete B.
This is a very puzzling set of symptoms to be sure.. I did not see a grid stopper resistor in the last pix of the pre-amplifier stage. I am assuming that you added one - anything from 1K - 20K might work if mounted right at the socket. Add one on both grids, note that 220 ohm resistors right at the plates of the 12AX7 may also help.
12AX7 make wonderful VHF oscillators if grid stoppers are not used, and incidentally shielded wire on the grid without a stopper usually makes it worse not better.
You should have a grid stopper of 1K or so on the grid of any output tube in the amplifier.
Build looked pretty tidy compared to most guitar amplifiers I have seen. I have seen ones with lousy ground practice and poor layout that worked fine - I don't think this is related to your issue.
A scope would make this a lot easier to understand and diagnose.
By now you know shot gunning (wholesale component replacement) is not the answer.
The supply voltages are not that critical and I would not obsess over them as long as they are within 5 - 10% of expected values.
Kevin
12AX7 make wonderful VHF oscillators if grid stoppers are not used, and incidentally shielded wire on the grid without a stopper usually makes it worse not better.
You should have a grid stopper of 1K or so on the grid of any output tube in the amplifier.
Build looked pretty tidy compared to most guitar amplifiers I have seen. I have seen ones with lousy ground practice and poor layout that worked fine - I don't think this is related to your issue.
A scope would make this a lot easier to understand and diagnose.
By now you know shot gunning (wholesale component replacement) is not the answer.
The supply voltages are not that critical and I would not obsess over them as long as they are within 5 - 10% of expected values.
Kevin
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