PR bias board cap

[dbaum]

Need some quick feedback. I built a Mojotone PR and I’m now adding an adjustable bias pot per Rob Robinettes layout.
The Mojotone kit has a 100uf cap on the bias board.  Rob has a 25uf cap in addition to the 25uf pot. (Are they linear? So 59uf combined?) The original fender schematic calls for a 50uf cap.
I’m a rookie so I’m wondering what I should do. Leave the 100uf, swap per Rob’s suggestion or go 50uf?  Does it matter much?
At the parts store now so looking for some quick advice. Thanks

[acheld]

Not sure if we're looking at the same layout/schematic (I assume you're talking about AA1164 PR). 

The pot is a 25k ohm with a fixed 10k ohm resistor to ground.

The original AA1164 used a 25uF cap in the bias circuit.  I am not sure why Mojo went to 100, or what advantage it might have. 

Best to post your schematic.

[dbaum]

Here's a screen shot of the Mojotone bias board layout.  Not sure if I should swap out the 100uf cap or not?

[jordan86]

Leave the 100uf cap. It stays. And the 100uf value is not a problem. You need to remove the 27K RESISTOR in parallel with it. (That's what is being replaced with the adjustable circuit). I got this diagram from Andy at Mojotone when I built mine. Notice how they spec'd a 47uf bias cap in the diagram. Different than what they sent you. Not a problem though. Again, the cap uf value is not an issue in this context.

Notice the pot value is also different than Rob's in the bias pot. Not a problem. Rob did it for a wider range of bias. If you want to be able to bias hotter to run 6L6's, you can replace the 27K in my attached diagram with a 15K or 22K. But at 15K you may not be able to bias cold enough for 6V6's.  I did a 10K pot and 27K resistor and it works great. Plenty of bias range.

[dbaum]

Fantastic, thank you!  I have a 25k pot and a 10k resistor per Rob's layout.  Sounds like I'll be fine. 

[acheld]

Curious.  Why does the parallel 22k resistor need to go?   As I understand it, all it does is to allow the bias circuit to discharge when power is removed.  It is present in the AA1164.   

Is there a disadvantage having it present?

[2deaf]

Leave the 100uf cap. It stays. And the 100uf value is not a problem. You need to remove the 27K RESISTOR in parallel with it. (That's what is being replaced with the adjustable circuit). I got this diagram from Andy at Mojotone when I built mine. Notice how they spec'd a 47uf bias cap in the diagram. Different than what they sent you. Not a problem though. Again, the cap uf value is not an issue in this context.

Notice the pot value is also different than Rob's in the bias pot. Not a problem. Rob did it for a wider range of bias. If you want to be able to bias hotter to run 6L6's, you can replace the 27K in my attached diagram with a 15K or 22K. But at 15K you may not be able to bias cold enough for 6V6's.  I did a 10K pot and 27K resistor and it works great. Plenty of bias range.

There are two problems I see with Andy's version.  If the wiper fails, your bias voltage at the 6V6 is gone.  You won't be able to turn the tremolo clear off with the knob unless the bias pot is at the coldest setting.

A 100uF capacitor will reduce ripple as compared to 47uF or 25uF.  A 100uF cap also takes longer to charge when the amp is initially powered up, but it will be sufficiently charged by the time the rectifier tube starts to conduct. 

[texwest]

Curious.  Why does the parallel 22k resistor need to go?   As I understand it, all it does is to allow the bias circuit to discharge when power is removed.  It is present in the AA1164.   

Is there a disadvantage having it present?

The 22k is the resistor that sets the bias in the original circuit. If you add a bias pot a lot of people replace the resistor with a 10k instead.  So they have a bias trim pot in series with the 10k.

[DuaneOh]

A 100uF capacitor will reduce ripple as compared to 47uF or 25uF.  A 100uF cap also takes longer to charge when the amp is initially powered up, but it will be sufficiently charged by the time the rectifier tube starts to conduct.

I built an amp that had a larger than necessary bias capacitor and if you had the amp on and warmed up, turned it off and then turned it back on before the tubes had a chance to cool it would blow the HT fuse. In Merlin Blencowe's power supply book he suggests that the charge time should be less 3 seconds.

[2deaf]

I built an amp that had a larger than necessary bias capacitor and if you had the amp on and warmed up, turned it off and then turned it back on before the tubes had a chance to cool it would blow the HT fuse.

Well.  Don't do that.

Even those that are prone to such activities generally learn after ten to twenty fuses.  They learn even faster when they switch the impedance selector during a power chord.

Also, the PR doesn't have a HT fuse.

[gitarzysta]

Need some quick feedback. I built a Mojotone PR and I’m now adding an adjustable bias pot per Rob Robinettes layout.
The Mojotone kit has a 100uf cap on the bias board.  Rob has a 25uf cap in addition to the 25uf pot. (Are they linear? So 59uf combined?) The original fender schematic calls for a 50uf cap.
I’m a rookie so I’m wondering what I should do. Leave the 100uf, swap per Rob’s suggestion or go 50uf?  Does it matter much?
At the parts store now so looking for some quick advice. Thanks

There cannot be a 25uF pot. 25k pot.

Just leave the 100uF cap in place.

[dbaum]

That was a typo