Bias pot valve in Plexi 6V6

[dude]

I have a 50KA bias pot in my 1987 model 6V6 Plexi, same schematic as posted in Sluckey’s 6V6 Plexi in schematics here.
Biasing the amp is a hassle, to get 26mV very hard to get the pot exact or even near without going back and forth several times turning the pot.
 I wondering if using a 25kA would be easier to adjust, l understand l might hive to change the valve of the R off the pot to ground. But am l thinking correctly? Also, would a 25KL be better?

[sluckey]

I have a 50KA bias pot in my 1987 model 6V6 Plexi, same schematic as posted in Sluckey’s 6V6 Plexi in schematics here.
Biasing the amp is a hassle, to get 26mV very hard to get the pot exact or even near without going back and forth several times turning the pot.

My schematic specifies 50K-L. Very easy to adjust. Why are you using an audio taper? If you want very precise adjustment, get one of these 25 turn trim pots...

     https://www.tubesandmore.com/products/trimmer-bourns-3296y-multiturn-top-adjust-pcb-mount

[Latole]

Your problem wouldn't come from the pot being an A or a B but from the construction of the pot.

 And if it's like the pots I've seen in some Marshalls (photo), they don't work very well.
In my opinion, you need a better-quality pot.

[dude]

Thanks for the replies, the pot is on the back panel in the spk extension hole, sorry it is a 50KL. I have an old 3/8” hole pot 50KL that has a locking nut, guy on eBay sells them but don’t adjust like the 25 turn trimmer pot Sluckey suggested. I’ve used the trimmer pots, internally, work great) but like to keep the pot in the spk jack, 3/8” hole.
So, taking this in consideration would an alpha 25kL with correct resisters for 6V6’s make the adjustment easier? 50Kl works but takes a bit of back and forth to get the bias correct. On second thought, if it’s available do they make a 50KL 25 trim trimmer that will fit a 3/8” hole? Could use a plate or washer over hole but most trimmer pots have no nuts, (l have two)

[Latole]

Did you try to clean the pot ?

Show picture

[jjasilli]

Lots of people use 50K pots.  Pots should be linear, or else most the adjustment will be confined to one side of the pot. 


Audio taper is best suited for adjustment of the perceived volume of audio signal to compensate for the way the human ear hears volume.  Our ears hear changes in volume logarithmically; audio taper compensates for that, by making the change in volume appear to be linear to our ears.  Bias voltage is not an audio signal. 


A 10K or 25K pot will make it easier to fine-tune the adjustment of bias voltage.  E.g., consider a 1/10th rotation of a pot: that's a change of 5K for a 50K pot; but only 1K for a 10K pot.  It's easier to get small incremental changes in bias voltage with a smaller value pot.  But as sluckey says, a multi-turn 50K pot is another good solution.  If you use a smaller value pot, then you do need to compensate with the values of surrounding resistors to get back into the proper range of bias voltage.  The downside of a smaller pot is less overall adjustability.  If you later change tubes, using a small value pot, there may not be enough range to bias those tubes.  This means rebuilding the bias circuit around the small value pot, to accommodate those tubes.

[dude]

you do need to compensate with the values of surrounding resistors to get back into the proper range of bias voltage.  The downside of a smaller pot is less overall adjustability.

Thanks, I understand

[dude]

Did you try to clean the pot ? Show picture

Just a pot mounted to spk jack, bias connected to two lugs, and a resister grounding one lug to pot, have to take chassis out for pic.
Pot is clean, works fine but the higher valve makes it more difficult to adjust. I’m going to try an alpha 25kl pot, if l can’t find a 25 turn trimmer l can get to fit the spk jack hole.  I’ll select the best resistor to get in the middle range of neg voltage for the bias pot, I tapped off a leg of the PT for bias supply with 3W dropping resistor, so l’ll try changing the R to ground on pot.
Thanks

[sluckey]

would an alpha 25kL with correct resisters for 6V6’s make the adjustment easier? 50Kl works but takes a bit of back and forth to get the bias correct

You can try a 25K pot to see if you like it better. I'm skeptical. I wonder if you are being a bit critical? I always consider setting the bias to be a back and forth thing, especially when you are trying to hit some magical number exactly. I use milspec RV6 style single turn locking pots and don't have any problems setting bias. You can see these bias pots at the bottom of this page...

     https://sluckeyamps.com/sunn/sunn.htm

[dude]

would an alpha 25kL with correct resisters for 6V6’s make the adjustment easier? 50Kl works but takes a bit of back and forth to get the bias correct

You can try a 25K pot to see if you like it better. I'm skeptical. I wonder if you are being a bit critical? I always consider setting the bias to be a back and forth thing, especially when you are trying to hit some magical number exactly. I use milspec RV6 style single turn locking pots and don't have any problems setting bias. You can see these bias pots at the bottom of this page...

     https://sluckeyamps.com/sunn/sunn.htm

I bought those same pots, you sent me the link of the seller on eBay. Yes, they are nice, locking nut keeps any accidental turning when bias pot mounted open. I get you saying overly critical, maybe. A mV here or there is nothing anyone is going to hear or red plate a tube. I have a lot of American made used 6V6’s and l like to experiment biasing them to match on current draw and tone to my ears. Yeah, might take a bit of back and forth to get a mV or 2 to where you want to be, l agree it’s not that critical. But even those military spec pots take a bit of playing around to get to a number. I have been finding it takes a bit of time getting two tubes off by 3 or 4 mV to get the best in between for both tubes. I’m probably a little anal, too ha.
     
For one thing, not to change the subject, the difference in tone from various used, good American made power tubes matched close isn’t all that it’s cracked up to be. Yes, some pairs have slightly better harmonics, tone, whatever you want to call it but not that much. I can say good used American power tubes are better sounding, IMHO, than the brand new stuff out there now. “For What it’s Worth”.
Cheers, and thank you for all your help in the past, l’m grateful there are people like you who share their knowledge. 
Thank you

[Latole]

If you've followed the schematic correctly and the pot is in good condition (sorry, but I'm not sure), the problem lies elsewher

[dude]

If you've followed the schematic correctly and the pot is in good condition (sorry, but I'm not sure), the problem lies elsewher

Latole, there is no problem. I have several Plexi 6V6's all bias fine, sound great and bias stays steady, just that the higher value of the bias pot's range is wide and takes a little more fiddling to get the set point. Sluckey says I'm a little picky, maybe so. I was asking if a lower valve pot would give me less swing and it would as JJasilli mentioned just need to use the correct biasing R's.
I probably knew the answer to my question before I posted.., but it's confirmed now. Fender used a 10K bias pot in a lot of their amps, but Leo wasn't worried about matching and biasing power tubes, he just put in what was the cheapest and at that time most all tubes were USA made and were excellent.

[Latole]

dude,

I do bias on many Fender amps and the 10 k pot  work very well.

I don't know where you read that Leo didn't worry about bias or matched tubes.

And the cheapest parts at the time (which I knew well) were of better quality than many, if not most, of today's expensive parts.
Back then, there was only one quality in America

[sluckey]

I chose to use a 50K pot mainly because I had a bunch of RV6 50K locking pots on hand. Using a 50K pot and 47K resistor gives a wide adjustment range and will accommodate several different tube types but it is touchier to adjust. Since you have a lot of tubes to play with, I would think you would like this wide adjustment range.

Using a 25K pot and 47K resistor as Marshall did, gives a narrower adjustment range but is easier to adjust. And that is fine too. You will have to also adjust the bias range resistor to provide the desired adjustment range. Sometimes you may even have to change the bias range resistor for different tubes of the same type!

When I'm fine tuning a bias circuit for the first time, I like to set the pot to midrange, then experiment with the bias range resistor until I have my desired goldilocks' bias voltage. This means I will have an equal swing above (too cool) and below (too hot) the goldilocks' point (just right).