# YBA-3 Custom Special "Mystery Pot"



## JC103 (Oct 6, 2007)

Hey guys, so my second 1969 YBA-3 is in the shop for a cap job and new pots, plus new tubes and biasing. All went well with the install, but it has developed a high pitch squeal. My tech thinks it is in the presence circuit. Since there were some "get it to the next gig" type of MacGyver mods that had to be reversed... he asked me about a strange pot inside the chassis and whether or not I thought it was stock (I have another 1969 YBA-3 that he's worked on). I found a blog that noticed the same thing: machine log










Does anyone know what this pot is for? 

Also, if anyone has feedback on pot value choices for the amp that would be great too. We replaced all six pots with the values from the original schematic.

Bass: 1M Linear
Treble: 1M linear
Bass Expander: 250K Linear
Treble Expander: 250K linear
Presence: 250K Linear
Volume: 1M Log

*Although the first iteration apparently used a 1M Linear for the Volume. What's the dif?


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## JC103 (Oct 6, 2007)

Schematic: http://www.thetubestore.com/lib/the...ynor-Custom-Special-YBA3-Manual-Schematic.pdf

*non-master volume versions


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## epis (Feb 25, 2012)

That's phase inverter balance pot, if your tech doesn't have signal generator and oscilloscope, tell him to leave it in center position.


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## jb welder (Sep 14, 2010)

And to further add, perfect balance setting (symmetry) will not necessarily give the best sound, as far as harmonics and distortion.


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## Frenchy99 (Oct 15, 2016)

Wow ! did not know this existed...

Does this exist only on the non master version ?

Why would it be on some models and not all ?

Any added bénéfice with this ?


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## dtsaudio (Apr 15, 2009)

I've been inside dozens of traynors and have never seen a pi balance pot. While it looks original, i wonder if it is an old mod, or something they tried at the factory and implemented in only a few amps.


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## JC103 (Oct 6, 2007)

Thanks for the quick info everyone! The pot appears to be original and if you do a google image search you can find quite a few pics of other non-master volume Custom Specials from the 1968-69 era with the same pot.


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## dtsaudio (Apr 15, 2009)

Interesting. Must be part of the "custom special" circuitry. 
I see this adjustment quite often in hifi amps, but not in guitar amps. The differences during adjustment are minimal and probably makes little difference in a guitar amp.


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## nonreverb (Sep 19, 2006)

dtsaudio said:


> I've been inside dozens of traynors and have never seen a pi balance pot. While it looks original, i wonder if it is an old mod, or something they tried at the factory and implemented in only a few amps.


Indeed, I've been inside all kinds of Traynors and this is new to me. I suspect it's not original otherwise there "should" be some version of the schematic with it included.
Not necessarily a valuable addition based on typical guitar amp design....I suppose if you want it to sound a little raunchy one could mess with it although probably best left alone.


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## JC103 (Oct 6, 2007)

My hunch is that 1969 was a transitional year for Traynor and that this is one of those "looks good on paper, lets try it" ideas. I think Pete was always innovating, just look at the YBA-3A for example. The Custom Specials are interesting amps because they evolved quickly over a short time span of approximately 3 years. There is the first version with choke and no master volume running 6CA7's. From what I can tell these arrived on the seen in late 1966 or early 1967, the original schematic is dated 1967. Then there was the move to EL34's (7027?) in around 1969 when they dropped the choke transformer from the design, and then of course the buggy bumper master volume iteration that arrived in 1970 (never seen a choke on those). I have a collection of gut shots from various 1968/69 Custom Specials that were either for sale or getting cap jobs and you can see the pot is there (or was there in some cases) in most of them from that time period. Now I am pretty sure it was not there in my other 1969 Custom Special (#1), so it's hard to say how long they were doing this. More than likely it was a short lived factory mod.


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## Brett Pearson (Apr 26, 2016)

I have a yba-3 with the same balance pot. I have researched it as I was confused when I first found it in my amp, but there is enough mention of it in other forums that I suspect as others here do, that it was a factory mod that was tried and then eventually discarded.


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