Cathode bias 6L6s, Voltage rise(50v) on one power tube when treble is turned up????

[billyboy]

Rickenbaker Supersonic M16, 2-6L6 cathode bias, 4 -12ax,u7, . I replaced e'lytics and checked resisters, grounded power cord. Was checking voltages against a B16 schematic and noticed one 6l6 had 525v and the other 480. Rechecking i found the 525v dropped to 480v with treble down and it stays with the socket when switching tubes. Could a cap in the bass/treble controls be doing this?

On the PI, V4, sec a plate is 130v and connected to sec b grid, grid is .007v, cathode is 5.8v, sec b plate is 270v, grid is 130v, cathode is 137v. Should the sec b cathode be higher than it's grid? The sec a cathode cap,50@50, and resister go to ground 12" across the amp by the ac input but thru a .005 cap to ground which is shown on schematic with a bright switch but I have no switch in the amp.

The amp is running much quieter and seems good and strong. Also found an M16 schematic which seems very close but no voltage listed.

Any and all thougths would be appreciated.

[Chuck H]

It sounds to me like the amp is oscillating when power or treble is applied, but only on one tube?!? Odd. Perhaps a bad supression cap or a bad grid resistor. It also occures to me that if the bias resistor has failed short that there may be too much bias voltage on that socket. Also, if that socket screen resistor has failed that would stop current and cause the high reading as well, but it wouldn't seem that it would only happen at high volume or treble settings?!? Any other symptoms?

[kg]

the PLATE voltage on ONE output tube in a push pull pair goes up when you turn the treble up?

when it does so, does it exceed the center tap voltage, or no?

[billyboy]

Evidently my Fluke couldn't handle the volt overage so I got out the B&K and I'm getting different voltage now. I'm getting 480v on the OT center tap. On the left 6l6 if I turn the treble past 1/2 it will read as high as 490 to an occasional 500 on the plate. On the right 6l6 I have to turn the volume up a ways before the treble will make it react and I am getting the same voltages. There is a spot on the treble, 2/3-3/4 up, where there is a pulsing noise and higher up it will squeal sort of sometimes and if I dime the treble it sometimes drops the voltage below normal of approximately 460v.

I had matched the resisters and caps on the driver section when i did the filter caps. I have now added screen grid resisters, 470/1 watt, and suppression resisters, 1500/ 1/2 watt. I went back and recleaned,retension sockets, grounds, inputs,,,,

The mains on/off is in the treble control, could there be something going on there?

I'm in the novice category regards this stuff, maybe it's time for a pro to look it over.
Thanks for the replies. Very much appreciated.

[pdf64]

'There is a spot on the treble, 2/3-3/4 up, where there is a pulsing noise and higher up it will squeal sort of sometimes and if I dime the treble it sometimes drops the voltage below normal of approximately 460v'
Those are pretty strong indications that the amp is oscillating - if you move the meter lead well away from the pre-amp then the effect may be reduced.

[kg]

billy: one thing you can try is to short out the signal at the output tube grids.

a way to do this is to take a large value non-polar cap, say 1uF, and short the output tube grids together. this will keep the DC levels the same (preserving bias) and remove any AC signal. then try the test again, and observe the results.

if it does NOT behave the same way, then move that cap back one stage--ie, the input to the phase inverter/splitter. then retest. to short a single ended stage's signal, just put the other side of the cap on that stage's cathode.

this way you can determine where the instability is starting.

[Steve Conner]

There is a spot on the treble, 2/3-3/4 up, where there is a pulsing noise and higher up it will squeal sort of sometimes and if I dime the treble it sometimes drops the voltage below normal of approximately 460v.

Does it also show these symptoms WITHOUT the meter connected to a 6L6 plate?

It's quite likely that the meter lead is causing the instability, especially if you have it draped over the preamp section, or near your guitar, etc. If the amp works fine when the meter isn't connected, then there's no problem.

[kg]

Does it also show these symptoms WITHOUT the meter connected to a 6L6 plate?

It's quite likely that the meter lead is causing the instability, especially if you have it draped over the preamp section, or near your guitar, etc. If the amp works fine when the meter isn't connected, then there's no problem.

good point. meter lead = antenna!

[Chuck H]

Just a note from something I remember reading... There was a post some time back where a guy had his power tubes VERY close together. Something about that caused interaction that resulted in Vp rise above the rect output at certain amp settings. I'm sorry I can't remember what the technical discussion eventually speculated for the cause of the problem but spacing the tubes further apart did fix the problem.

[billyboy]

billy: one thing you can try is to short out the signal at the output tube grids.

a way to do this is to take a large value non-polar cap, say 1uF, and short the output tube grids together. this will keep the DC levels the same (preserving bias) and remove any AC signal. then try the test again, and observe the results.

if it does NOT behave the same way, then move that cap back one stage--ie, the input to the phase inverter/splitter. then retest. to short a single ended stage's signal, just put the other side of the cap on that stage's cathode.

this way you can determine where the instability is starting.

KG... I tried your suggestion and it starts on the 3rd preamp tube just after the volume controls and before the bass/treble controls. Unfortunately it still leaves me scratching my head as to What/ why. Thanks for the tip.

Chuck, The tubes are about 1/2 inch apart which is way closer than most. Maybe thats part of it.

Steve,, there is definitely something going on with the test leads and where i place my arm/ hand. The 6L6 closest to preamp has the voltage problem.

With the volumes up a bit and bass as well, the treble still adds noise and distortion when you turn it up past the 3/4 point.

Thanks for the help.

[billyboy]

Rickenbacker M16 schematic

Can anyone see something in this schematic that might contribute to oscilations/ distortion at higher treble settings?

[Enzo]

Oscillation is just another name for feedback. It means somethhing in the circuit is picked up and fed into an earlier stage, and reamplified. The schematic shows us nothing about the layout, tells us nothig if what is close to what. If your volt meter probe is on the plate of a tubes, the schematic won;t tell us where the rest of the wire lead sits.

Got a scope? The shielded scope leads might be better than unshielded meter leads.

[Cosworth]

I'm wondering if there is DC at V3A grid drive. It is this stage where compensation is boosted or attenuated. The gate drive has no load at all so any leakage in any of the small value tone capacitors will pull the grid positive and potentially out of operating range. With nearly only capacitive coupling at this grid, near field effects can be expected. I would suspect and change out the 100pf cap. that is hanging on the treble wiper. For grins I might also try the addition of something like 100K (200K+?) from V3A grid to ground for a little stabilization helping to keep the grid centered at 0V. A grounded shield can on a tube suspected of crosstalk should be very revealing if in fact there is crosstalk due to them being too closely placed together.

[km6xz]

Pet peeve about internet forums:

The point that was most telling was that the amp was "quieter and seems good and strong" which ruled out an oscillation in normal usage. Almost any oscillation will have audio range artifacts from beat notes or sound tone changes. This seems like a case of not knowing what a measurement really represents, and the interaction with the observing process. Growing up dealing with RF instead of audio I learned early that observation influences are a significant area of concern.
What concerns me is the degree in which experienced guitar amp tech jump to conclusions that are not consistent with the symptoms and observations, and the suggestion of replacing parts without evidence of the parts are bad. How can anyone guarantee their work if a 1:1 causal relationship is not found between parts changed and the cause of the problem?
When an amp that has been working by the thousands for decades has an unidentified problem it is a low odds probability that the problem is a design or layout problem.
I see a lot of questionable advice given on all the forums that are causing a lot of otherwise good units to be compromised by careless tinkering when the real problem has not been addressed, or in the case of DIY tinkers, over addressed.
Overall, I suggest that if there is not enough information, usually there isn't, to ask for more specific details before making a repair procedure recommendation. If the user can't make the measurement, in fairness to him and the gear, it is best to refer him to a competent tech who would save him a lot of money and time.

I am sure we would all be less than happy if we took our car into a shop and had the mechanic move the engine mounts over 3 inches because there is a knocking sound, or replacing the expensive engine management computer as the first step in solving a windshield problem. Although the prices are lower, the same sort of shotgunning is happening by techs on forums. In most forums, the very first advice is echoed over and over, "change all the tubes", no matter what the symptoms are. $100 later he DIY'er still has a bad amp but now also has a number of unknowns added to the problem.

The frequent directions to restore an amp before repairing it just add too many variables and assumes the user will install the new parts as well and skillfully as the assembly line worker who made it. Fix the amp first. Then when it is no longer exhibiting the symptoms, then they can decide on their own whether they want to restore it. Two different operations and goals. Anyone can....and often does.....repeat the constant mantra "replace all the tubes and all the capacitors" that poster after poster repeats while ignoring the symptoms. What kind of tech would do that? That is right, a non-tech whose only workbench is his keyboard and should not be listened to. I do not know Enzo, but he seems to be one of the few techs on forums who does listen to the symptoms and give advice that is consistent with those observations.

[Steve Conner]

Pet peeve about internet forums:

The point that was most telling was that the amp was "quieter and seems good and strong" which ruled out an oscillation in normal usage.

Yes, which is why I (and at least two other people in this thread) came to the conclusion that the oscillation was CAUSED by having meter leads clipped to things inside the amp. The original poster didn't seem to understand this concept, or really be cooperating with the troubleshooting process, so I gave up. There will be another half dozen newbies along tomorrow.

"replace all the tubes and all the capacitors" that poster after poster repeats while ignoring the symptoms. What kind of tech would do that? That is right, a non-tech whose only workbench is his keyboard and should not be listened to.

Maybe in some chicken shack like The Gear Page but you won't find any of those here. We have two kinds of members: Experienced techs and engineers who know what they're doing, and kids with broken amps who sign up looking for a quick fix, even though they know nothing about electronics. (Some of the latter are probably techs in small music shops. )

The former kind already know everything you said above, having learnt it through experience. The latter don't have the reading skills to wade through your posts, so will ignore them.

Therefore, I think I proved that the Internet doesn't exist, or something.