Twin Reverb - Amp circa 2005 Trem issues

[Chuck H]

Well I guess that pot is fine as far as it goes. If you want the original arrangement/circuit and you don't mind the high ticket I think I would tolerate the shaft sticking out. Though I might cut it off and Dremel a new slot too. All this would be fine except that we're still discussing a part that has nothing to do with thread content.

[Chuck H]

2.2m sets up a proportional bias so if the -50V is more positive in the circuit, the proportion is going to be to close to the output stage current curve. Giving it more negative bias will increase that proportion ratio.

I don't know if this is over my head or doesn't make sense. Can anyone explain whats intended here?

[Helmholtz]

I don't know if this is over my head or doesn't make sense. Can anyone explain whats intended here?

Doesn't make sense to me. See my earlier posts. The bias voltage (value) has zero effect on the tremolo.

[Chuck H]

Thank you. And it was posted after it was clearly demonstrated that this was not a bias modulation tremolo. But even if it were the wording and terminology are so nebulous that I questioned myself instead of the phrasings. Like a Jedi mind trick.

[Helmholtz]

Right. But it was posted after it was clearly demonstrated that this was not a bias modulation tremolo..

There are posts I don't even bother to comment.
I mean it's all obvious from the schematic:
When the LFO is running (footswitch shorting to ground) it's disconnected from the bias supply.
A bias modulating tremolo doesn't need/use an opto.
Just see where the LDR connects.

And even if it were the wording and terminology are so nebulous that I questioned myself instead of the phrasings. Like a Jedi mind trick.

Not familiar with Jedi, but I wondered if the poster has a language barrier.

[nosaj]

Not familiar with Jedi, but I wondered if the poster has a language barrier.

Now that's Hilarious!!
nosaj

[DrGonz78]

Has there ever been a Fender Twin with a bias wiggler circuit? Then you think hmmm maybe something new. But looking at the schematic it explains everything you need to know. Only had a few tremolo amps with bias wiggler. Best sound was from 1959 Tremolox with Jensen speaker. Seems like we just need to clear up the misunderstanding and I have to wonder what the OP was saying. Any additional information is appreciated and I think that might help explain something’s.

[Chuck H]

Has there ever been a Fender Twin with a bias wiggler circuit? Then you think hmmm maybe something new. But looking at the schematic it explains everything you need to know. Only had a few tremolo amps with bias wiggler. Best sound was from 1959 Tremolox with Jensen speaker. Seems like we just need to clear up the misunderstanding and I have to wonder what the OP was saying. Any additional information is appreciated and I think that might help explain something’s.

I think Helmholtz had it right. The bug is basically worn and the neon bulb (or the respose of the LDR as per Tom) just isn't doing the job anymore. I hope a suitable replacement can be found. Parts being such as they are now. And...

I did do a two big bottle bias modulated tremolo amp that puts high-ish voltage on the power tubes. I still have to work out some things and dial it in some years later and just haven't gotten to it. Not recommended. I think bias modulated trems are easier to implement and work better with nominal plate voltages. Maybe I'll get this amp just right some day and change my mind but for now I'd say DON'T DO THIS.

[The Dude]

I don't know if this is over my head or doesn't make sense. Can anyone explain whats intended here?

It cannot be explained. It is not a correct analysis of the circuit.

[sparkies]

It cannot be explained. It is not a correct analysis of the circuit.

Over the years there are two things that have changed. The bias circuit and the gain of the triode that lights the LDR. If the output section bias is correct, then I would play with the gain on the tube. 56K is what was used as the cathode resistor instead of a 100K in this schematic.

As far as the tapped potentiometer, its really not a good circuit and I would set up the bias circuit the older way that is a standard circuit. That is why I was hesitant to show the source of one. Because its nothing special to the circuit and its actually bad practice to design a circuit that current flows through the wiper.. Even though its kind of avoidable, I wouldn't use the wiper on the control in series with the voltage divider like the tapped potentiometer design.

[Chuck H]

Over the years there are two things that have changed. The bias circuit and the gain of the triode that lights the LDR. If the output section bias is correct, then I would play with the gain on the tube. 56K is what was used as the cathode resistor instead of a 100K in this schematic.

I haven't seen this. Perusing all the Fender LDR tremolo circuits new and old I can't find any that are using a 56k cathode resistor for the actuator triode. Can you post a link?

[sparkies]

I haven't seen this. Perusing all the Fender LDR tremolo circuits new and old I can't find any that are using a 56k cathode resistor for the actuator triode. Can you post a link?

That would have been one of the hand drawn ones...

AA673 Which has the standard bias circuit. But if you still want balance, I would substitute the 220K feed resistors with a 500K potentiometer.

[Helmholtz]

Different value bias feed/grid leak resistors will not change power tube bias, as these resistor don't carry a DC current, so don't drop voltage.
Again bad advice.

[Chuck H]

You beat me to it Helmholtz All that can do alter the grid load for either side of what should be a balanced circuit.

And I'm not sure what's meant by "one of the hand drawn ones" WRT the schematic. Aren't they all hand drawn? Is this just more nebulous wording to divert the issue?

[sparkies]

Different value bias feed/grid leak resistors will not change power tube bias, as these resistor don't carry a DC current, so don't drop voltage.
Again bad advice.

What are you really referring to?