Very strange problem

[Bob-I]

I've finished up my Princeton chassis Hybrid and I have a very odd problem. When I have the OD channel selected and the "ratio" control on 0, I have some thin distorted tone bleeding through. I disconnected everything from the relays to the tone stack to the PI input and it still bleeds through.

I find it extremley odd that the tone continues to bleed through even with the PI input disconnected, as if the tone is bleeding through to the power tubes somehow.

Any ideas?

[BobW]

Bob, you may have coupling through your wire harness. Assuming that is the case, try these to determine where the problem is:
1. disconnecting the feedback loop if it goes away, it's being coupled into the feedback loop (more likely)or transformer (less likely).
2. if it's still there, try a small value pF cap across the grids to cathodes one at a time.

If still no change, then it could be a component coupling issue. Try temporarily shielding the OT with metal screen, not the AL type, but make sure you don't short out anything wihle testing. hth gotta run for now, BobW

[Bob-I]

Bob, you may have coupling through your wire harness. Assuming that is the case, try these to determine where the problem is:
1. disconnecting the feedback loop if it goes away, it's being coupled into the feedback loop (more likely)or transformer (less likely).

I actually already tried this thinking of the xformer coupling

2. if it's still there, try a small value pF cap across the grids to cathodes one at a time.

Cool

If still no change, then it could be a component coupling issue. Try temporarily shielding the OT with metal screen, not the AL type, but make sure you don't short out anything wihle testing. hth gotta run for now, BobW

I'm not sure I understand this one.

[Bob-I]

Ok..........

I moved the OT secondary connections to outside the chassis, the primaries already are outside entering right between the tube sockets.

I then narrowed this bleed down to OD2. Removing the B+ from this stage removes the bleed.

So I focused on any potential leads that may interfere, shielded the one lead the passes by other ckts.

I double checked for potential for lack isolation at the PS, making sure that there's a PI filter seperating the OD stages from the PI, all components check out.

Overall this amp sounds great, one of my best, except when I turn the ratio control below about 10:00 when the bleed becomes noticable.

I'm out of ideas. Anyone!

[Luthierwnc]

Hi Bob;

Does your ratio pot go to a dead short at "0"? If not, will shorting it with a screwdriver kill the bleed?

Can you let us know if you have a scope and signal generator? The first thing I would do is look for signal where it shouldn't be. I count five ways to ground the signal in this preamp. Even without a scope, plug a CD player in and look for AC after the pots to see if it the next stage is picking up noise.

This is one of those deals where it could be on the bench for hours. The last thing I do before I power one of these up is douche it out with an air hose. Copper strands can hide almost anywhere -- especially in the wax of those old Fender boards.

sh

[Bob-I]

Hi Bob;

Does your ratio pot go to a dead short at "0"? If not, will shorting it with a screwdriver kill the bleed?

I've shorted this out, and disconnected not only the pot but also the PI input.

Can you let us know if you have a scope and signal generator? The first thing I would do is look for signal where it shouldn't be. I count five ways to ground the signal in this preamp. Even without a scope, plug a CD player in and look for AC after the pots to see if it the next stage is picking up noise.

This is how I determined that the signal is coming from OD2, and I see it at the PI. I can't figure out how it's getting there.

This is one of those deals where it could be on the bench for hours. The last thing I do before I power one of these up is douche it out with an air hose. Copper strands can hide almost anywhere -- especially in the wax of those old Fender boards.

sh

Already thought of that. Took the board out and double checked.

Thx, but still no answer.

[tictac]

Have you tried moving your leads around with a chopstick (something non conductive) and monitoring the noise? I've found the output of V2b to be very sensitive to lead dress.

TT

[stevlech]

Maybe the cause is bleed within the dual triode itself. I've had a Twin I couldn't completely shut off until I changed V2. Just a thought.

[glasman]

Is it coming back on the power supply for V2. Hang your scope on the B+ to V2 and see if you dtect the signal. If so, try a .1uf at the supply point for the resistors (V2a PLate and V2b plate) to ground. Just an idea.

Gary

[Bob-I]

Maybe the cause is bleed within the dual triode itself. I've had a Twin I couldn't completely shut off until I changed V2. Just a thought.

I don't think that's it. It's not bleeding from V2A to V2B, it's bleeding from V2B to V3.

[Bob-I]

Is it coming back on the power supply for V2. Hang your scope on the B+ to V2 and see if you dtect the signal. If so, try a .1uf at the supply point for the resistors (V2a PLate and V2b plate) to ground. Just an idea.

Gary

That's makes some since, but wouldn't I be looking for a signal at the V3 node?

[Bob-I]

Have you tried moving your leads around with a chopstick (something non conductive) and monitoring the noise? I've found the output of V2b to be very sensitive to lead dress.

TT

Yes, that was the first thing I tried. I was fully expecting to hear a change, there was none at all.

[BobW]

I then narrowed this bleed down to OD2. Removing the B+ from this stage removes the bleed.

This seems to be either an OD2 component problem or a pwr supply/grounding issue.

Bob, try these:
1. disconnect any snubbers you may have at OD2 and/or
2. replace the coupling cap at OD2, (.0047?) Also,
3. remove the treble bleed ckt if you're using that ckt too (.001 w/250k trimmer)
4. where are the B+4 filters and FET loading resistors grounded?

HTH

[Bob-I]

I then narrowed this bleed down to OD2. Removing the B+ from this stage removes the bleed.

This seems to be either an OD2 component problem or a pwr supply/grounding issue.

Ahhh... grounding on the PS... could be. I'm using a 4x20uF can cap with a common ground. I wonder?????

Bob, try these:
1. disconnect any snubbers you may have at OD2 and/or

I replaced them but not removed them.... easy enough to try.

2. replace the coupling cap at OD2, (.0047?)

IT's .005uF and I've checked it for DC leakage but again, easy enough to try.....

Also,
3. remove the treble bleed ckt if you're using that ckt too (.001 w/250k trimmer)

Nope....

4. where are the B+4 filters and FET loading resistors grounded?

HTH

I removed the FET loading resistor, the B+ 2-3 and 4 are all a common ground on the can cap. This could be an issue. Maybe I should replace that can with discrete caps at least as a test. It's really tight inside that little chassis, honeymoon fit.

Thx Bob.

[glasman]

Is it coming back on the power supply for V2. Hang your scope on the B+ to V2 and see if you dtect the signal. If so, try a .1uf at the supply point for the resistors (V2a PLate and V2b plate) to ground. Just an idea.

Gary

That's makes some since, but wouldn't I be looking for a signal at the V3 node?

I would think the source would be V2.

Several manufacturers install small poly caps across their filter caps to reduce ripple and hi-frequency crap on the power supply.

Gary