Vol/Tone in a 5C5?

[RC99]

So any thoughts as to whether this would be correct for a Fixed Bias version? If so, I'm probably going to build it on another chassis instead of trying to figure out how to switch between the two in the same one.

[loudthud]

If you hunt around on the Weber forum, I think you'll find that some builders have a hum problem with that layout. There may be a fix, but the layout was never updated.

Look at this thread starting about post #30. Link: http://music-electronics-forum.com/t14986/ There is some suggested fixes but no resolution to the problem.

[RC99]

Much thanks, Thud. I'll definitely have a look through. Do they look like the right components in the right place/connection?

[Dave H]

If you hunt around on the Weber forum, I think you'll find that some builders have a hum problem with that layout. There may be a fix, but the layout was never updated.

Look at this thread starting about post #30. Link: http://music-electronics-forum.com/t14986/ There is some suggested fixes but no resolution to the problem.

The pictures in that other thread show the wire from the bias pot wiper to the power tube grid resistors passing very close and in parallel to the wire joining the two 16u filter caps. There will be high ripple current pulses in that wire and the wire from the bias pot is high impedance (no cap to ground) so the hum could be getting in that way. I’d move the bias pot wiper wire away from the 16u filter cap -ve area and add a cap from the power tube grid resistors to the PI ground (between junction of 250k, 6k8 and junction of 2k5, 25u). I’d also connect the PT CT wire directly to the 16u caps and not to the ground plate.

[RC99]

The pictures in that other thread show the wire from the bias pot wiper to the power tube grid resistors passing very close and in parallel to the wire joining the two 16u filter caps. There will be high ripple current pulses in that wire and the wire from the bias pot is high impedance (no cap to ground) so the hum could be getting in that way. I’d move the bias pot wiper wire away from the 16u filter cap -ve area and add a cap from the power tube grid resistors to the PI ground (between junction of 250k, 6k8 and junction of 2k5, 25u). I’d also connect the PT CT wire directly to the 16u caps and not to the ground plate.

That worried me to, just looking at the layout. I was thinking about how to route that (under the board? Shielded wire?), and/or putting the bias pot on the bottom side of the chassis as well. But the 5C5 chassis is longer than a 5E3/5C3P chassis and there's a little more room to the left of the main circuit board, so maybe that might be enough separation if I put it on its own small circuit board.

[RC99]

Here's what I had in mind.....could anyone comment on which adjustable bias arrangement would work better? Thanks.


(click to enlarge)

v.1



v.2

[RC99]

Theoretical version with reverb, based on the single-tube RJ Audio Add-On Reverb....

[RC99]

Here's what I had in mind.....could anyone comment on which adjustable bias arrangement would work better? Thanks.


(click to enlarge)

v.1
[ATTACH=CONFIG]24096[/ATTACH]


v.2
[ATTACH=CONFIG]24097[/ATTACH]

From the sounds of it....

Correct again!
Glad someone is watching.
I use an additional reverse polarity, 22uF@100v cap right at the junction of the low value resistor... good call!

....version 1 seems the better way to go, but is that enough filtering? Thanks.

[Bruce / Mission Amps]

Neither are completely right.
http://music-electronics-forum.com/a...5c5p_v-1a2.jpg
is better but you need at least a, positive grounded, 47uF-100v bias filter cap right after your bias supply diode too.

[RC99]

Excellent advice Bruce, thanks. So add another 50uf + to ground right at the junction of the 1N4007 and 1K resistor?

[RC99]

Also...the Weber layouts on the 5E3P and 5C3P seemingly show the first and middle lugs on the bias pot joined. Which way is better?

[loudthud]

Also...the Weber layouts on the 5E3P and 5C3P seemingly show the first and middle lugs on the bias pot joined. Is this correct? Other examples with adjustable biases like on the Ceriatone tweeds don't have it.

A common failure mode for a pot is for the center terminal to become an open circuit. If the bias adjust pot is connected like a voltage divider, this means the bias will be lost and the output tubes will red plate. If arranged correctly, the bias pot is connected like a rheostat in such a way that if the center terminal becomes open, the bias increases thus saving the tubes.

[RC99]

A common failure mode for a pot is for the center terminal to become an open circuit. If the bias adjust pot is connected like a voltage divider, this means the bias will be lost and the output tubes will red plate. If arranged correctly, the bias pot is connected like a rheostat in such a way that if the center terminal becomes open, the bias increases thus saving the tubes.

Thanks Thud. So as a safeguard (especially with mini pots), it probably should be done, yes? Like this, maybe.....

[loudthud]

That will work.

[RC99]

Huge thanks again, LoudThud.