Layout 5th Generation 183

[VacuumVoodoo]

Not raw at all. The 250K OD pots take care of that, combined with the EL34 tubes. 250K pots add a considerable amount of miller effect.

Scott, could you please explain how this pot contributes to the Miller effect?

[dogears]

Well I suppose I could be mistaken, however I thought that the pot size would have an effect on the source impedence. Aiken kind of corroborates this, no?

Miller capacitance can kill the frequency response rather easily, so there are a few things to consider when designing guitar amplifiers. If you desire to minimize the effect of the Miller capacitance on frequency response, you can do the following things:
Reduce the output impedance of the previous stage. This can be accomplished by lowering the value of the plate load resistor, using a tube with a lower internal plate resistance, or lowering the value of any series or shunt attenuation resistors. Obviously, all of these things will affect the gain of the stages, so this must be taken into account as well.
Reduce the gain of the stage. The Miller capacitance is proportional to the gain of the amplifying stage, so using a lower stage gain will reduce the Miller capacitance, thereby increasing the frequency response.

Not raw at all. The 250K OD pots take care of that, combined with the EL34 tubes. 250K pots add a considerable amount of miller effect.

Scott, could you please explain how this pot contributes to the Miller effect?

[talbany]

Good read..

http://www.aikenamps.com/MillerCapacitance.html

[Mac Daddy]

[IMG:799:773]http://i15.photobucket.com/albums/a373/bduds/DDual.jpg[/img]

[VacuumVoodoo]

Scott, you're on the right track. Well, you know me, I'm picky regarding technical correctness.
The pot is part of AC plate load i.e. pot resistance is parallel with plate resistor of the tube driving it. This, through action of Miller effect affects the input impedance of that driver stage. This input impedance in turn is part of load on the preceding stage. Ergo: the pot resistance value affects not only the triode driving it but also any and all gain stages preceding it. Of course the effect is weakens as you move further away from the pot towards input. As the say, everything's connected.
Here, for your reading pleasure, is a copy of Miller's original paper from 1920. http://web.mit.edu/klund/www/papers/jmiller.pdf

Have fun!

[Chad]

[IMG:799:773]http://i15.photobucket.com/albums/a373/bduds/DDual.jpg[/img]

[makrisp]

[IMG:799:773]http://i15.photobucket.com/albums/a373/bduds/DDual.jpg[/img]

Very Nice!

[chris_sanford]

Scott, you're on the right track. Well, you know me, I'm picky regarding technical correctness.
The pot is part of AC plate load i.e. pot resistance is parallel with plate resistor of the tube driving it. This, through action of Miller effect affects the input impedance of that driver stage. This input impedance in turn is part of load on the preceding stage. Ergo: the pot resistance value affects not only the triode driving it but also any and all gain stages preceding it. Of course the effect is weakens as you move further away from the pot towards input. As the say, everything's connected.
Here, for your reading pleasure, is a copy of Miller's original paper from 1920. http://web.mit.edu/klund/www/papers/jmiller.pdf

Have fun!

Hi Alex,

I don't really see anything wrong with Scott's answer per se. In terms of the circuit in question, the largest effect in terms hi-cut would be from the stage *following* the 250k pot, would you not agree?

Your point that the miller effect is more widespread is well taken though.

Cheers!

chris

[Chad]

[IMG:799:773]http://i15.photobucket.com/albums/a373/bduds/DDual.jpg[/img]

Sorry Mac, Looked a little closer,now I understand.

[VacuumVoodoo]

Scott, you're on the right track. Well, you know me, I'm picky regarding technical correctness.
The pot is part of AC plate load i.e. pot resistance is parallel with plate resistor of the tube driving it. This, through action of Miller effect affects the input impedance of that driver stage. This input impedance in turn is part of load on the preceding stage. Ergo: the pot resistance value affects not only the triode driving it but also any and all gain stages preceding it. Of course the effect is weakens as you move further away from the pot towards input. As the say, everything's connected.
Here, for your reading pleasure, is a copy of Miller's original paper from 1920. http://web.mit.edu/klund/www/papers/jmiller.pdf

Have fun!

Hi Alex,

I don't really see anything wrong with Scott's answer per se. In terms of the circuit in question, the largest effect in terms hi-cut would be from the stage *following* the 250k pot, would you not agree?

Your point that the miller effect is more widespread is well taken though.

Cheers!

chris

Chris, when Miller is mentioned it almost always is in connection with grid-plate capacitance limiting high frequency bandwidth as result of said effect. That's why I reacted to Scotts post where he puts the 250k pot into Miller effect context.
As to the net effect in this particular case I would say that the two 25pF caps have an order of magnitude stronger effect than the pot. Also, assuming cable routing in the layout drawing is true then I would say that plate wire running close to and parallel with 180k grid stopper resistor will limit high-end bandwidth pretty strongly. This cable routing introduces about 3-5pF of stray capacitance between grid and plate which under influence of Miller effect becomes a virtual150-250pF between grid and ground, this together with 180k grid stopper, well, you do the math f=1/(2*Pi*R*C)
You tell me, which has stronger influence on tone: the pot resistance or routing of that plate wire?
Just trying to put things in perspective of influence magnitudes. There's more to it than meets a dogs naked ear, but I'm not in the least interested in building a clone
Have more fun, gentlemen.

[Structo]

[IMG:799:773]http://i15.photobucket.com/albums/a373/bduds/DDual.jpg[/img]

Sorry Mac, Looked a little closer,now I understand.

183-Level, Volume

TKT-Gain, Level?

[dogears]

Thanks Alex...

Fwiw, the layout is not the way the amp is. The plate and grid wires are quite apart.

Also, the 25pf snubbers have little effect. The pot size effects things much more in my experience.... Check the math.

Thanks!

As to the net effect in this particular case I would say that the two 25pF caps have an order of magnitude stronger effect than the pot. Also, assuming cable routing in the layout drawing is true then I would say that plate wire running close to and parallel with 180k grid stopper resistor will limit high-end bandwidth pretty strongly. This cable routing introduces about 3-5pF of stray capacitance between grid and plate which under influence of Miller effect becomes a virtual150-250pF between grid and ground, this together with 180k grid stopper, well, you do the math f=1/(2*Pi*R*C)
You tell me, which has stronger influence on tone: the pot resistance or routing of that plate wire?
Just trying to put things in perspective of influence magnitudes. There's more to it than meets a dogs naked ear, but I'm not in the least interested in building a clone
Have more fun, gentlemen.

[Bob-I]

Thanks Alex...

Fwiw, the layout is not the way the amp is. The plate and grid wires are quite apart.

Scott, aren't the cathode and plate wires also close together? You pointed that out to me awhile ago and I heard a noticeable difference when I redressed the leads.

[talbany]

Since it does effect tone I changed the layout like 124 with the plate cathodes run together.. I even made them the same color as 183..Rev 6 up..

Thanks to all those who looked over the layout..

Tony

[chris_sanford]

Since it does effect tone I changed the layout like 124 with the plate cathodes run together.. I even made them the same color as 183..Rev 6 up..

Thanks to all those who looked over the layout..

Tony

Looks great Tony! Some minor things: the presence cap still says .1uf though (should be 1uf), the bass lead from the Jazz/Rock switch should go to the top of the bass pot, not the wiper, and the yellow leads from the treble pot to the PAB relay and back don't quite connect.

Thank you!

chris