What exactly are they talking about?[/quote]"There is also a much rarer oscillation suppression method Fender had, it involved a .02 cap and a 220k resistor going from one leg of the power section then held 100 ohms above ground via the feedback loop."
SF Twin Oscillation after Blackface
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SF Twin Oscillation after Blackface
I've modded an old Twin Reverb Silverface to Blackface specs and redid the filter caps. I'm getting a little oscillation now. I've poked around with a chopstick a little but didn't fond anything. In looking around for solutions I ran across this in a newsgroup:
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Stevem
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- Joined: Fri Jan 24, 2014 3:01 pm
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Re: SF Twin Oscillation after Blackface
Is that amp a master volume model?
What is ther serial number stamped in the chassis on the rear above the preamp tube section, also many times on the inside of the chassis stamped in black ink you will find the circuit number stamed.
Many times this is near the power transformer.
In general the mod you did increased the gain of the driver / PI section and this to keep explanations simple will effect the grid wires and how they pick up signal levels that they should not within the amp.
The first and easy think to do is in the preamp section raise up all the grid wires from pins 2 and 7 up off of the other wires on each socket as much as possible, also ohm check all of the preamp components to ground connections and confirm zero resistance in regards to what your meter reads shirted and reflow all of the connections on the power supply filters.
These connections on the filters are very important as besides removing ripple hum they stop signals from stage to stage from possibly making the issue that you have.
If this year amp is still one of the models that has a unsheilded wire off of the input jack to grid on the vibrato channel then I would replace this with a sheilded cable that has a foil sheild besides a braided shield, and this cable should be grounded at the input Jack end only.
At the tube end be sure to tape off or better yet heat shrink tube off the ground so that no shorts can take place.
A high resistance / poor connection on the preamp tube pins can make for this issue also. If you rock around all of the preamp tubes a bit is there any noise or signal cutting out taking place?
You can run a cut off round wound D string into each pin socket to clean up any crap and tarnish, but be sure to drain the power supply first.
Do this by unplugging the amp from the wall and turning the standby switch on as if you where gonna use the amp , wait 2 minutes and the power supply will be drained down to less than 10 volts.
One other change you should make would be on the screen supply / 3rd power filter.
This cap may be grounded at the front of the amp along with all the preamp grounds and it should not, extend that ground wire and then ground it over by the power transformer where there should be two grounding stacks already.
What is ther serial number stamped in the chassis on the rear above the preamp tube section, also many times on the inside of the chassis stamped in black ink you will find the circuit number stamed.
Many times this is near the power transformer.
In general the mod you did increased the gain of the driver / PI section and this to keep explanations simple will effect the grid wires and how they pick up signal levels that they should not within the amp.
The first and easy think to do is in the preamp section raise up all the grid wires from pins 2 and 7 up off of the other wires on each socket as much as possible, also ohm check all of the preamp components to ground connections and confirm zero resistance in regards to what your meter reads shirted and reflow all of the connections on the power supply filters.
These connections on the filters are very important as besides removing ripple hum they stop signals from stage to stage from possibly making the issue that you have.
If this year amp is still one of the models that has a unsheilded wire off of the input jack to grid on the vibrato channel then I would replace this with a sheilded cable that has a foil sheild besides a braided shield, and this cable should be grounded at the input Jack end only.
At the tube end be sure to tape off or better yet heat shrink tube off the ground so that no shorts can take place.
A high resistance / poor connection on the preamp tube pins can make for this issue also. If you rock around all of the preamp tubes a bit is there any noise or signal cutting out taking place?
You can run a cut off round wound D string into each pin socket to clean up any crap and tarnish, but be sure to drain the power supply first.
Do this by unplugging the amp from the wall and turning the standby switch on as if you where gonna use the amp , wait 2 minutes and the power supply will be drained down to less than 10 volts.
One other change you should make would be on the screen supply / 3rd power filter.
This cap may be grounded at the front of the amp along with all the preamp grounds and it should not, extend that ground wire and then ground it over by the power transformer where there should be two grounding stacks already.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: SF Twin Oscillation after Blackface
All great information and thanks so much for all the help. It's a late '70's master volume and will check the grid wires, as all the preamp wires are laying pretty close together. Will check everything else as you suggested also.Is that amp a master volume model?
What is ther serial number stamped in the chassis on the rear above the preamp tube section, also many times on the inside of the chassis stamped in black ink you will find the circuit number stamed.
Many times this is near the power transformer.
In general the mod you did increased the gain of the driver / PI section and this to keep explanations simple will effect the grid wires and how they pick up signal levels that they should not within the amp.
The first and easy think to do is in the preamp section raise up all the grid wires from pins 2 and 7 up off of the other wires on each socket as much as possible, also ohm check all of the preamp components to ground connections and confirm zero resistance in regards to what your meter reads shirted and reflow all of the connections on the power supply filters.
These connections on the filters are very important as besides removing ripple hum they stop signals from stage to stage from possibly making the issue that you have.
If this year amp is still one of the models that has a unsheilded wire off of the input jack to grid on the vibrato channel then I would replace this with a sheilded cable that has a foil sheild besides a braided shield, and this cable should be grounded at the input Jack end only.
At the tube end be sure to tape off or better yet heat shrink tube off the ground so that no shorts can take place.
A high resistance / poor connection on the preamp tube pins can make for this issue also. If you rock around all of the preamp tubes a bit is there any noise or signal cutting out taking place?
You can run a cut off round wound D string into each pin socket to clean up any crap and tarnish, but be sure to drain the power supply first.
Do this by unplugging the amp from the wall and turning the standby switch on as if you where gonna use the amp , wait 2 minutes and the power supply will be drained down to less than 10 volts.
One other change you should make would be on the screen supply / 3rd power filter.
This cap may be grounded at the front of the amp along with all the preamp grounds and it should not, extend that ground wire and then ground it over by the power transformer where there should be two grounding stacks already.
What I would still like to know is:
What is this and what do they mean by "...going from one leg of the power section then held 100 ohms above ground via the feedback loop."?"There is also a much rarer oscillation suppression method Fender had, it involved a .02 cap and a 220k resistor going from one leg of the power section then held 100 ohms above ground via the feedback loop."
I'd never seen this before and was curious as to what it is and where it goes.
Re: SF Twin Oscillation after Blackface
This is probably what is being referred to. This is from a TR with MV/push-pull switch, maybe even the same amp you started with."There is also a much rarer oscillation suppression method Fender had, it involved a .02 cap and a 220k resistor going from one leg of the power section then held 100 ohms above ground via the feedback loop."
This is just a band aid fix and is not part of any black face circuit that I recall.
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Re: SF Twin Oscillation after Blackface
Ah thanks sluckey that answers my question. 
Yes it is a Twin Reverb with the push-pull master. No, I know it's not standard and is just a bandaid and I'd rather it be right than just patched. I'd just never heard of that before and couldn't visualize what it was exactly.
Yes it is a Twin Reverb with the push-pull master. No, I know it's not standard and is just a bandaid and I'd rather it be right than just patched. I'd just never heard of that before and couldn't visualize what it was exactly.Re: SF Twin Oscillation after Blackface
I hesitate to suggest something that Gerald Weber has proposed (but I was doing before he published). Grid wires feeding the output tubes can get pretty long in a Twin. If you flip the PI components on the board upside down so the grid leads come off the "bottom" you can save 2 or 3 inches of antenna.
Re: SF Twin Oscillation after Blackface
You know as a matter of fact you're right. Never thought about it but it makes so much sense you just have to wonder why it wasn't done that way to begin with!Firestorm wrote:I hesitate to suggest something that Gerald Weber has proposed (but I was doing before he published). Grid wires feeding the output tubes can get pretty long in a Twin. If you flip the PI components on the board upside down so the grid leads come off the "bottom" you can save 2 or 3 inches of antenna.
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: SF Twin Oscillation after Blackface
I belive Gerald was saying this in reference to tweed amps which are more tightly packed, also one the signal gets out of the mixer (V4) it's at a high level that would only effect / impart oscillations back into a section before / up stream from V4, which in a long twin chassis is pretty far away!
If you want a better sounding amp and kill the likely sourse of the issue too
then do the Gerald mod of installing all of the tone stack caps and resistors on the pots and that removes 8 inches of grid wire!
One other thing you can do is prob aorund to find a grid wire that ( and a grid wire is also anything down stream after the coupling cap) makes the sound and or pitch of the oscillation change.
Unsolder this wire and extend it a lot so you can pull it far up out of the chassis.
Chances are the issue will stop and if it does then take this wire and lay it back down and in while folding it shorter to find a route that will not make the issue fire up.
Yes, sometimes implementing the shortest grid leads will not do the trick, and a long route will work.
Also sometimes just spinning a coupling cap around the other way will be all that's needed due to the built in sheilding of how it's made .
If you want a better sounding amp and kill the likely sourse of the issue too
then do the Gerald mod of installing all of the tone stack caps and resistors on the pots and that removes 8 inches of grid wire!
One other thing you can do is prob aorund to find a grid wire that ( and a grid wire is also anything down stream after the coupling cap) makes the sound and or pitch of the oscillation change.
Unsolder this wire and extend it a lot so you can pull it far up out of the chassis.
Chances are the issue will stop and if it does then take this wire and lay it back down and in while folding it shorter to find a route that will not make the issue fire up.
Yes, sometimes implementing the shortest grid leads will not do the trick, and a long route will work.
Also sometimes just spinning a coupling cap around the other way will be all that's needed due to the built in sheilding of how it's made .
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: SF Twin Oscillation after Blackface
I can't remember what Gerald was mentioning specifically, but flipping the PI shortens the output grid leads. The tone stack wiring Steve mentions is pretty significant. The leads on the "vibrato" channel are long in BF models and got worse in the Silverfaces. They also run right along the B+ bundle, some of which have a lot of ripple. What I've done, instead of mounting caps on the pots (which works, BTW) is replace the tone leads with a 3-conductor shielded wire, grounded with the input. Less effort. Works, too.
Re: SF Twin Oscillation after Blackface
Ok well, found the source of the oscillation. After getting the scope out and working through the preamp to narrow down the area it might be coming from, I noticed I hadn't jumped the last .1 cap to the 220k in the PI! 
On another note, the last cap off of the 1Meg that was supposed to be a .1uf was a .01. Made those changes and it sounds amazing!
On another note, the last cap off of the 1Meg that was supposed to be a .1uf was a .01. Made those changes and it sounds amazing!
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: SF Twin Oscillation after Blackface
Yup, the step by step back track will 90% of the time turn up your over site !
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: SF Twin Oscillation after Blackface
[/quote]wattsup wrote: In looking around for solutions I ran across this in a newsgroup:
What exactly are they talking about?"There is also a much rarer oscillation suppression method Fender had, it involved a .02 cap and a 220k resistor going from one leg of the power section then held 100 ohms above ground via the feedback loop."
The SF Bassman 100 and several varieties of SF Twin Reverb 100's had that .01 cap and a 220K resistor between the 100 ohm resistor in the NFB loop and the inverted output from the PI. They seem to have dropped this when they went to the 135's.