Building a mid-60's JTM45'ish amp with lots of NOS parts - which pots would you use??

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Eric Klinger
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Building a mid-60's JTM45'ish amp with lots of NOS parts - which pots would you use??

Post by Eric Klinger » Fri Sep 11, 2020 2:39 am

Hi Gang.

This forum is truly the only group of peeps that could appreciate my "dilemma" - I hope some of you old heads are still keeping an eye out here!!!!!!

I am building a 1964'ish JTM45 style amp where I have sourced all of the era correct parts - through the signal path... so the circuit board will contain all Piher resistors and Mullard mustard caps. For transformers/choke I choose George's Drake replicas (super excited about these!!!). Otherwise, F&T filtering, higher wattage vitrohm wire wounds (in filtering sections)... not sure about plate load resistors yet, and not sure about potentiometers to use....

I have an opinion these days that good tolerances of the correct parts get you 90% of the way there (or there abouts). Of course I am after that 10%!!! I have a modern 2014 Marshall JTM45 RI where I have changed the caps out to Sozo Next Gen and I have a hard time imagining a better sounding amp (yes, even with the crappy stock Dagnall Trannys). But that is what this great experiment is about. Of course however I have NEVER played the real thing.... sooooooo...

A long time ago I used PEC, but my recollection of that amp, while favorable, left my opinion rather indifferent when it came to most "upgrades" (including Mercury Magnetic trannies - from which I ended up having a tad of buyers remorse over).  So I did not forgo the 15 euros (moved to Belgium about 10 years ago *sigh*) per pot this time (though I spent more per mustard cap).

In recent experiences I think that Alpha standard 24mm pots have left me with little desire to use anything else... though in many of my amp builds I have used CTS, more or less only because "that's what a high quality amp has installed". In my stash, the Alpha pots are very very close to their advertised values - 1M pot usually reads extremely close to 1M, where a CTS pot of the same value will read 850k to 900k.

AFAIK they are all made in Korea... even Bourns (which I also have).

So my immediate contenders are Alpha 24mm, CTS 24mm, Bournes HRT 24mm pots. I have not measured the Bourns yet, but the Alphas are bang on the $$$ with their tolerance.

Are there any ideas I am missing here? Or is there a supply of NOS that I am unaware of??? HEEEEEP PLEASE!! What would you do?
:jimi:

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Eric Klinger
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Re: Building a mid-60's JTM45'ish amp with lots of NOS parts - which pots would you use??

Post by Eric Klinger » Tue Sep 15, 2020 3:17 am

okay. I am going with the Alpha pots... they are far more accurate in their tolerances.

:peace:

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Re: Building a mid-60's JTM45'ish amp with lots of NOS parts - which pots would you use??

Post by danman » Tue Sep 15, 2020 6:48 pm

I love Bournes in my guitars and Alphas in my amps. PEC's are very precise but I have had several that developed a scratchiness that I could not get rid of.

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Eric Klinger
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Re: Building a mid-60's JTM45'ish amp with lots of NOS parts - which pots would you use??

Post by Eric Klinger » Tue Sep 15, 2020 11:01 pm

danman wrote:
Tue Sep 15, 2020 6:48 pm
I love Bournes in my guitars and Alphas in my amps. PEC's are very precise but I have had several that developed a scratchiness that I could not get rid of.
haha same - I often use Bourns in the guits unless it's a "keeping to the original" thing.

Reading through Randall Aikens white papers/q and a's, he says the same of the PEC pots... and for amps in the field... well, they have to go back to the manufacturer for a replacement... bummer. Of course that is not what I am doing...

The last amp I built had CTS and when I changed them to Alphas I liked the amp over all much better - so that's my story... sticking to it!

Thanks!

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Re: Building a mid-60's JTM45'ish amp with lots of NOS parts - which pots would you use??

Post by glpg80 » Thu Oct 15, 2020 10:36 pm

I used CTS in my replacements for my Marshall and don’t really have any concerns...? I only replaced what was needed of course.

What made you like alpha over CTS?

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Re: Building a mid-60's JTM45'ish amp with lots of NOS parts - which pots would you use??

Post by neikeel » Sat Oct 17, 2020 9:27 am

If you can find some original large pots AB/RS type, particularly those that measure a bit high they are the extra 10%, not just my opinion have a search of old threads. Generally I use Alphas as they are smooth and work well. When I have used the Mil spec wire wound PECs I did not really feel/hear any benefit.
Neil

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Re: Building a mid-60's JTM45'ish amp with lots of NOS parts - which pots would you use??

Post by Eric Klinger » Tue Oct 20, 2020 3:28 am

I pondered some AB for a while... I couldn't find all values at that time however... but it would be a good idea to build a set up over how ever much time is needed. I'd love to use some on spec originals!

As to why I personally choose Alpha... for one example, the volume pots are supposed to be 1M. Many of the CTS pots I have read more in the 850k region where a lot of the Alphas are over 1M, though barely over (1.02 for example). But for a volume/gain pot, that's another 150k resistance to ground.... equates to a bit more of everything (gain, bass, treble).

Otherwise, I can't see a reason not to use them, I think they both have the same country of origin. So unless it is something special (nos AB), I feel quite fine with alpha pots... can't say the same for most other components. But even the one time in my life I used PEC in a 1959 build, I just didn't feel they warranted the extra 8x price per pot. In the EU these are over 16 euros a piece now! Mind you, for this amp, price was not an issue when it came to quality parts (where I absolutely knew they'd make a difference) - but you just sort of reach a point where your pissing away money for little to no return. IMO anyway!! :thumbsup:

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