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Vintage threads from the first ten years

BFDR Parasitic Oscillation

12/31/1999 5:17 AM
Matt	BFDR Parasitic Oscillation Hi Guys, Have a '67 BFDR which has a parasitic oscillation problem on the vibrato channel. The output power is real low and sure enough on the scope, there is a large high frequency component on the 1000 Hz signal being injected into the amp. If I place a .001 cap across the wiper and ungrounded side of the tremolo Intensity control, the power is back to normal and the signal quite clean. I don't want to rob the amp of frequency response and I thought it was only silverface amps that suffered from conditions that made them susceptible to oscillation. Has anyone experienced this? Is there a way to improve it without resorting to suppressor caps? I've resoldered joints, tried to move wires away from each other, kept signal wires near chassis, changed tubes, even Deoxit'd the tube sockets. My last amp was a totally different situation with a faulty vibrato roach. This one isn't so easy Thanks a million! Matt

12/31/1999 5:17 AM

Matt

BFDR Parasitic Oscillation

Hi Guys,

Have a '67 BFDR which has a parasitic oscillation problem on the vibrato channel. The output power is real low and sure enough on the scope, there is a large high frequency component on the 1000 Hz signal being injected into the amp. If I place a .001 cap across the wiper and ungrounded side of the tremolo Intensity control, the power is back to normal and the signal quite clean. I don't want to rob the amp of frequency response and I thought it was only silverface amps that suffered from conditions that made them susceptible to oscillation.

Has anyone experienced this? Is there a way to improve it without resorting to suppressor caps? I've resoldered joints, tried to move wires away from each other, kept signal wires near chassis, changed tubes, even Deoxit'd the tube sockets. My last amp was a totally different situation with a faulty vibrato roach. This one isn't so easy :-(

Thanks a million!
Matt

12/31/1999 7:49 PM
SpeedRacer	In no particular order.. 1. Try other 6V6's 2. Place cap plate-to-plate on PI (like Bassman.. start at 100pF and go up until osc stops) 3. Try other PI tube 4. Keep playing with layout - try and get hold of known "good" DR to examine.. also check against Fender layout diagrams from factory. Keep OT plate leads away from PI grid leads or at 90 deg. 5. Use bigger grid stoppers - keep leads as short as possible.. go from 1k5 to 5k6, or even 10k. Don't give up.. this is the tough stuff!

12/31/1999 7:49 PM

SpeedRacer

In no particular order..
1. Try other 6V6's
2. Place cap plate-to-plate on PI (like Bassman.. start at 100pF and go up until osc stops)
3. Try other PI tube
4. Keep playing with layout - try and get hold of known "good" DR to examine.. also check against Fender layout diagrams from factory. Keep OT plate leads away from PI grid leads or at 90 deg.
5. Use bigger grid stoppers - keep leads as short as possible.. go from 1k5 to 5k6, or even 10k.
Don't give up.. this is the tough stuff!

1/1/2000 12:28 AM
Bird Dog	All very good points. I am curious about the placement of your .001 cap-- it appears from the schematic that the only thing attached to the wiper of the intensity pot is one leg of the opto-oscillator (did I get that right?). Does the oscillation cease when you disconnect the wiper (without the cap installed)? I am wondering if the "roach" is introducing the ossilation, as I see nothing else that could be the culprit, at least in the vibrato circuit.

1/1/2000 12:28 AM

Bird Dog

All very good points. I am curious about the placement of your .001 cap-- it appears from the schematic that the only thing attached to the wiper of the intensity pot is one leg of the opto-oscillator (did I get that right?). Does the oscillation cease when you disconnect the wiper (without the cap installed)? I am wondering if the "roach" is introducing the ossilation, as I see nothing else that could be the culprit, at least in the vibrato circuit.

1/2/2000 8:12 AM
Tracy	Matt The parascitic oscillation problem of silverface Deluxe Reverb amps as noted by, " Gerald weber of Kendrick amps" Was supposedly caused by lead dress wire routing. After moving those wires around in my amp without results, I tried shielded wire on the input and volume control leads and the problem was fixed. Shielded wire at these points has become standard on my home brew amps. If you try this only ground the shield at the face of the amp, and tape or shrink wrap the other end. If the shield touches ground at both ends you could create a ground loop and cause more noise. Tracy

1/2/2000 8:12 AM

Tracy

Matt

The parascitic oscillation problem of silverface Deluxe Reverb amps as noted by, " Gerald weber of Kendrick amps" Was supposedly caused by lead dress wire routing. After moving those wires around in my amp without results, I tried shielded wire on the input and volume control leads and the problem was fixed. Shielded wire at these points has become standard on my home brew amps. If you try this only ground the shield at the face of the amp, and tape or shrink wrap the other end. If the shield touches ground at both ends you could create a ground loop and cause more noise.

Tracy

1/3/2000 6:55 AM
Matt	Thanks Tracy and everyone else! I tried shielded wire for the intensity pot since the problem ultimately was localized there(with some oscillation also initially occurring in the preamp that was easily resolved). The real problem was the Intensity pot. I'm going to replace it and it should be fixed. I moved the connection going to the amp temporarily to the wiper and the problem was cleaned up. The pot seems to measure fine, but the problem continues when connected per the schematic. Kinda strange. I have tried so many things to get this to work. The thing that kills me is that these amps are so simple inside! I couldn't imagine what was happening! The amp sounds so awesome I'm glad I don't have to modify the circuit to get it back. I think I've learned something here, but also reinforced that parasitic oscillation can be a real pain to find and resolve! Now I can play the thing! Woo hoo! Thanks! Matt

1/3/2000 6:55 AM

Matt

Thanks Tracy and everyone else!

I tried shielded wire for the intensity pot since the problem ultimately was localized there(with some oscillation also initially occurring in the preamp that was easily resolved).

The real problem was the Intensity pot. I'm going to replace it and it should be fixed. I moved the connection going to the amp temporarily to the wiper and the problem was cleaned up. The pot seems to measure fine, but the problem continues when connected per the schematic. Kinda strange.

I have tried so many things to get this to work. The thing that kills me is that these amps are so simple inside! I couldn't imagine what was happening! The amp sounds so awesome I'm glad I don't have to modify the circuit to get it back.

I think I've learned something here, but also reinforced that parasitic oscillation can be a real pain to find and resolve!

Now I can play the thing! Woo hoo!

Thanks!
Matt

1/3/2000 9:10 AM
Bird Dog	Maybe there's some crazing or cracking on the carbon trace inside the pot. When you think of how many points in an amp that the signal is suceptible to an inexpensive pot and a pressure sensitive contact point, it's a wonder we don't have more headaches! I had a SR where the reverb pot went bad and it was as if the reverb was maxxed out as soon as you flipped the standby switch-- totally unplayable, and it just happened "all of a sudden" when the owner was playing on stage one night! Glad you figured out your problem!

1/3/2000 9:10 AM

Bird Dog

Maybe there's some crazing or cracking on the carbon trace inside the pot. When you think of how many points in an amp that the signal is suceptible to an inexpensive pot and a pressure sensitive contact point, it's a wonder we don't have more headaches! I had a SR where the reverb pot went bad and it was as if the reverb was maxxed out as soon as you flipped the standby switch-- totally unplayable, and it just happened "all of a sudden" when the owner was playing on stage one night! Glad you figured out your problem!

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