Global & Local Negative Feedback Mods for ODS #102?

[Greenback25]

Does anybody have the schematics for the standard global and local negative feedback mods for Robben Ford style Dumbles?

[norburybrook]

Does anybody have the schematics for the standard global and local negative feedback mods for Robben Ford style Dumbles?

look in the files section and there's a schematic and layout for his #102 amp.


M

[Greenback25]

I already saw that schematic. What I'm interested in are the local and global negative feedback mods that can be engaged via switches. Are there any schematics for that?

[pompeiisneaks]

That part is very easy. You need a spst switch you put it inline between the source of the feed for the negative feedback and it's next point, and connect the two leads of the wires to either side of the switch. toggling it either completes the circuit or disables it.

If you're talking the GNFB on any amp, take hte wire coming from the OT tap, into the switch and then from the switch to the normal place it would go.

If you're tlaking the LNFB on some dumble amps, take it from the one side of the coupling cap for the LNFB side before the 20M or two 10M resistors and cut the connection there, and send leads to the switch, I'd think you want those very close to the tube so maybe on the back of the amp.

~Phil

[Greenback25]

That part is very easy. You need a spst switch you put it inline between the source of the feed for the negative feedback and it's next point, and connect the two leads of the wires to either side of the switch. toggling it either completes the circuit or disables it.

If you're talking the GNFB on any amp, take hte wire coming from the OT tap, into the switch and then from the switch to the normal place it would go.

If you're tlaking the LNFB on some dumble amps, take it from the one side of the coupling cap for the LNFB side before the 20M or two 10M resistors and cut the connection there, and send leads to the switch, I'd think you want those very close to the tube so maybe on the back of the amp.

~Phil

The reason I'm asking is that GNFB on my ODS 102 style amp does some strange things. When it's engaged, the amp functions normally and louder than in the disengaged position, but if I turn it off and the amp gets to its unmodded state, I get these weird oscillations, that can be heard in this clip (I'm switching between active/inactive GNFB mod with some input/master adjustments along the way):

https://vocaroo.com/1nfLTLrKekgy

This oscillation only goes away after I engage the GNFB switch, it doesn't matter whether I'm using FET or normal input, overdrive or clean channel, if the guitar is plugged in or not or if the master is high or low. It's there as long as GNFB is disengaged. What part of the circuitry could cause this? Is the OT maybe damaged or are preamp tubes to blame? Maybe a damaged capacitor/resistor?

[pompeiisneaks]

Well I can only think of the opposite being an issue. If the GNFB is engaged and you're hearing odd noises, then you've got the OT leads reversed phase so you're creating PFB instead of NFB.

That can sometimes be a horrid howling noise, others be a more subtle sound like you're getting. Have you confirmed which direction of the switch is 'open' vs 'closed'? .

Usually removing NFB makes things get less problematic, not adding it. I would guess a simple test of this would be to leave the switch in the 'good sounding side' and jumper the two sides of the switch and see if anything changes. If not it's closed and you've got NFB good and the non NFB is problematic.

One theory I guess is if the NFB is disconnected, the lead that goes nowhere now is acting like an antenna? You could try moving the leads around with a chopstick and see if anything changes.

~Phil

[Greenback25]

Well I can only think of the opposite being an issue. If the GNFB is engaged and you're hearing odd noises, then you've got the OT leads reversed phase so you're creating PFB instead of NFB.

That can sometimes be a horrid howling noise, others be a more subtle sound like you're getting. Have you confirmed which direction of the switch is 'open' vs 'closed'? .

Usually removing NFB makes things get less problematic, not adding it. I would guess a simple test of this would be to leave the switch in the 'good sounding side' and jumper the two sides of the switch and see if anything changes. If not it's closed and you've got NFB good and the non NFB is problematic.

One theory I guess is if the NFB is disconnected, the lead that goes nowhere now is acting like an antenna? You could try moving the leads around with a chopstick and see if anything changes.

~Phil

Yes, I'm sure that the direction of the switch is the way I described it. When I engage it, the amp gets louder, which should be the case fo GNFB, and it also stops with this oscillation noise. When I disengage it, the trouble starts.

The NFB tone is good and the non-NFB tone is the problem here. Any recommendations for that sort of problem?

Here's another clip that shows just how detrimental this oscillation is to the overall tone:

https://vocaroo.com/14YCphswPwqO

[Greenback25]

This is how the GNFB switch looks inside the amp:

https://i.imgur.com/oHISFhz.jpg

SPDT function dictates that the upper and middle parts of the switch are activated when the switch faces up. These upper and middle parts are connected to the rest of the amp, whereas the lower part is not, which means that the down position on that SPDT switch eliminates the GNFB mod. Is that correct so far?

As I said before, the problems only occur when the switch faces down, once the switch goes up, the noise disappears...

[pompeiisneaks]

Actually, in every switch I've used, when the switch is up, the toggle part is down and the bottom two are connected. It's the opposite of how it appears. If you've got the white leads connected to the top two terminals, then engaged would be with the switch down.

Here's a picture of what I mean.

https://www.heathcote-electronics.co.uk/switches.html

See the picture about 4 images down that has ACB on the switches, with the switch to the left, C and B are connected with in in the middle (this is an on-off-on switch so it has a middle) neither is connected and with it to the right A and C are connected.

Or did I misunderstand what you're saying?

~Phil

[pompeiisneaks]

Oh also reviewing that switch, it looks like you have a resistor between the top and bottom? What is that for? that means when you're in one position, you have directly jumpered between the two white wires, but in the other direction you connect between the top white wire the resistor and the second (middle) white wire. Is this a dual selected resistance level? or is that the GNFB resistor? In which case you've got GNFB w/o a resistor in one mode and with it in the other.

I threw together a quick schematic of what I think you've got on the top and what it should be on the bottom:


~Phil

[Greenback25]

Well, I thought the switch would be off if it's facing down, at least that's what the amp builder told me. You're telling me now that it's exactly the other way around?

I'm a bit confused to say the least. The picture doesn't lie, so it has to deactivate the mod in the up position, like this picture shows: https://m.littelfuse.com/~/media/images ... .jpg?la=en

That obviously means that the GNFB mod is actually causing this noise. But why? It just a simple wire disconnection mod. Why would it cause the amp to produce that noise? Is it maybe incorrectly connected to the OT?

[pompeiisneaks]

with the switch off and the noise not there, basically my first theory that the OT leads need to be swapped, you either desolder them from the power tubes and swap them over, or you can swap the output leads from the PI to the power tubes.

~Phil

[norburybrook]

with the switch off and the noise not there, basically my first theory that the OT leads need to be swapped, you either desolder them from the power tubes and swap them over, or you can swap the output leads from the PI to the power tubes.

~Phil

^^^^^^^^^^^^^^^^^^this^^^^^^^^^^^^^^^^^^^^^^^^^^^^^



swap your OT primaries.

[PicknStrum]

Also, just make sure you drain the filter caps before you go in - no disrespect meant if you already know this - it just sounded like maybe you were dealing with an amp somebody else built.

[jfs322]

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