Bias pots for the ST120 Monarch Limited Edition FrankenAmp

If yall remember right, I junked the little bias pots on the VTA board for my custom build and replaced those with some el cheepo Chinese 50K pots offa da bay.

It didn't go so well. Smoked one, so I replaced those with some Fender 50K pots. That's what I got in there right now. Good quality, but not a true linear taper. Right around where I need them to be, they get twitchier than a millipede with restless leg syndrome.

That's an even dozen pots so far if anyone's keeping count.

So ... to make a long story longer ... I found some "precision" pots at that auction site that looked like they'd fit the bill. 10:1 ratio, so I figure those should give me lots of range to play with when dialing the bias in. Once again Hong Kong source, but I usually have pretty good luck with their stuff. Took forever to get here from China - blame it on the monsoons. Always figure on delays this time of the year.

Anyway ... they got here today, and I gotta say, they look and feel like some nice pots.






But wait, all you pot people are saying ... those look a LOT like Bourns? You know, the ones Mouser sells for $15 each ... without the knobs?

Uh yup ...



Here's the Mouser link:

http://www.mouser.com/ProductDetail/Bourns/3590S-2-503L/?qs=ka0oSW1bB1JSm13%252bOBM7sg==

Here's the link on da bay:

http://www.ebay.com/itm/290836022433?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

They're fake Bourns, and will most likely develop the same problems - the ones I bought at SEG in ShenZhen did. Some were inconsistent from new, while the rest developed issues with use. I had two that would drift so much they caused a 40% change in bias voltage. No free lunch.

Maybe try PEC pots.  I've used them in my previous ST70 and MkIV builds.

http://www.tubesandmore.com/products/potentiometers_0?filters=Brand%3DPrecision%20Electronics%2CResistance%3D50k

Their audio taper pots make great volume controls too.

Eh. I'll give em a try ... what's the worst that could happen?



If these don't work out, I'll add them to my big ol' pot o' pots and bite the bullet on the real deal ...

Anybody know any other options for precision pots that won't break the bank? I do like the idea of multiple turns ...

I look for AB, Clarostat, etc on eBay or surplus sales. A lot of the ones produced for military come in foil lined sealed bags. Quality stuff can be had for a couple bucks.

Got bored, so I put the VOM to em ...

Rock steady and smooth top to bottom on all five. Stop anywhere and they lock in nicely, and right on target.

No telling what they'll do with power ... I'll have the extinguisher handy ...

No rush ... got another project on the bench right now ... this might take a while ...



Bonus round! Name that receiver!

sKiZo wrote:Got bored, so I put the VOM to em ...

Rock steady and smooth top to bottom on all five. Stop anywhere and they lock in nicely, and right on target.

No telling what they'll do with power ... I'll have the extinguisher handy ...

No rush ... got another project on the bench right now ... this might take a while ...



Bonus round! Name that receiver!

Model RatsNest 001

sKiZo wrote:Eh. I'll give em a try ... what's the worst that could happen?



If these don't work out, I'll add them to my big ol' pot o' pots and bite the bullet on the real deal ...

Anybody know any other options for precision pots that won't break the bank? I do like the idea of multiple turns ...

If the Chinese pots don't hold up, why couldn't you just order the Bourns pots from Mouser and re-use the dials? Unless, of course the 10:1 rotation ratio on the Chinese pots is provided by the dial, not the pot. The Bourns pots provide a 10:1 ratio without the dial.

Best Regards,
Charlie

Luddite wrote:If the Chinese pots don't hold up, why couldn't you just order the Bourns pots from Mouser and re-use the dials? Unless, of course the 10:1 rotation ratio on the Chinese pots is provided by the dial, not the pot.  The Bourns pots provide a 10:1 ratio without the dial.

There's always that ...

Come to find out, the knobs are another $10 each at Mouser. The pots are a true 10:1, so not sure what all those knobs will accomplish in real world usage except to count turns. Seems like a particularly useless feature except for maybe centering the pots before I install them, and I got a VOM for that. As is, I'll probably stick with the knobs I got as they match the ones on the volume controls.

One nice feature of the new knobs is that they DO lock down, but ... the bias controls are pretty well protected from errant oopsies.

MontanaWay wrote:
Model RatsNest 001

Maybe this will help ... made a bit of progress by slapping the front end back together.

Lemme see here ... the amp bone's connected to the ... tuner bone.
The tuner bone's connected to the ... tone bone.
The tone bone's connected to the ... driver bone.
Now here's a pic for the boardddddddd ...



I did have a few parts left over ...   

MarantZ..........What # you know..........hehe...

Can you tell any diff in the sound of you new bias pots..??..
For years i have Used Acoustat an Quad ESLs... B&Ws.....with some Apogee ribbons for fun....
I can tell when i make any change....
like pulling these chep old chokes out of a pr of MK3....best thing i have done to any Dynaco tube amps.. well theres alway using a varic ....to run all stock parts for 20 more years......after 50years of haveing tube amps.....An cant forget the tube GZ34s diode for SS....
But most here say cant hear any diff in any thing.... but how diff tubes sound in there Amps......
Well Have fun...All just one tube heads o-pine


Thanks for the site Bob.....