where can i find 25k-RA pots for bfvc vibrato intensity?

MoonshineMan

Senior Member
Messages
7,510
i can find 50k's everywhere, but no 25k's. am i nuts? is there a way to use the 50k's?
i tried the search function but no luck, fwiw.
 

J M Fahey

Member
Messages
2,940
IF you can get RA50k's, try to get a stereo one and put both halves in parallel.

Instant high reliability 25kRA.
Browse the usual suspects: Mouser, Digikey, etc. plus maybe one of the specialized Tube amp shops might have somethging.

Dont forget those specialized in pedal stuff like Small Bear, some crazy old vintage pedal might use them.
 

MoonshineMan

Senior Member
Messages
7,510
IF you can get RA50k's, try to get a stereo one and put both halves in parallel.

Instant high reliability 25kRA.
Browse the usual suspects: Mouser, Digikey, etc. plus maybe one of the specialized Tube amp shops might have somethging.

Dont forget those specialized in pedal stuff like Small Bear, some crazy old vintage pedal might use them.
That might work. I'll see what I can find.
Thanks!
 

pdf64

Member
Messages
7,944
Is the issue scratchiness?
Pots get that when there's dc on them, and that circuit puts a tube's full cathode current via the trem intensity wiper.
So even a good new pot may still exhibit some scratchiness when operated.

The RA tapers used for trem intensity tend to be 30% (rather than the more extreme 10% eg standard CTS audio taper), so a linear taper is not that far off really.
It may be that you lose some resolution at the deeper trem settings, but it may be fine for general use.
 
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MoonshineMan

Senior Member
Messages
7,510
i haven't built the amp yet. i'm just trying to source the parts for now. i'm still a beginner at this stuff. i've built a 5f2-a and want to build something a little more marshall flavored, so i'm going to put the preamp of a 1987 onto a bfvc power section. i'm a total trem junkie, so i'd like to keep the vibrato circuit, if i can just find this damn pot. if i really understood how trem circuits work, i'd probably just tweak the cathode resistor a bit and go from there. it seems to me that with a 68k from the cathode to the wiper, and the 25k pot being between ground and the 6v6 screen/driver plate, that it really wouldn't matter if i went with a 50kRA pot, since it can go hard over in either direction, right?? but i've come across a few posts in giggle searches that seem to indicate that it will cause trouble with bias.
i'm getting more and more tempted to scrap the trem on this one. there won't be reverb. i love marshalls and i love trem and this seemed perfect for a real swampy sound at a volume i can use with a small band.
 

MoonshineMan

Senior Member
Messages
7,510
there's one on mouser but it's a 16mm pot, rated at 60mW. most amp pots are rated for 2W, right??
digikey ahs them but i have to buy 25 of them. for $200+.
:D
 

walterw

Platinum Supporting Member
Messages
38,989
i will say that swapping the 50k RA trem pots in both my silverface amps (early '70s DR and pro reverb) with the 50k linear pots that had the switch at "0" for the trem bypass mod, i found the pot sweep to still be usable.
 

RiftAmps

Member
Messages
207
I use 50kRA for BFVC tremolo, works great with no issues.

I think the Mojotone kit uses a 50kRA also....
 

MoonshineMan

Senior Member
Messages
7,510
Yeah! If 50k's will work safely then Problem Solved!!!
It's a new build, so it isn't a repair or anything.
Does anyone know why everyone seems to have gone to 50k?
 

donnyjaguar

Member
Messages
4,194
I'd have to see the schematic, but I'm guessing you could wire a 50kΩ resistor betwixt the middle (rotor/wiper) and one outer and achieve a similar effect.
 

kevmin

Member
Messages
1,440
50KRA pots actually sound better than the stock value. No worries. I use them all the time, because that is my preferred trem circuit. I use it in higher powered blackface style builds too.
 

MoonshineMan

Senior Member
Messages
7,510
Perfect! Thanks Kev!! I might still try it a few different ways, but I'll probably just lead off with the 50kRA and if it's good, I'll leave it as it is, which I increasingly suspect will be the case.
 






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