psych effects of pot taper and perceived amplifier power

[pula58]

My old Fenders were very loud with volume=3 (silverface and blackface amplifiers). My new builds seemed less powerful than I had expected. With vol=3 they were not that loud. So, I started looking at various pots, specifically their taper, and found that they are not even close to 20%, and most are below 10%. By contrast, an old 1Meg audio taper pot (taken out of a 1969 Silverface Fender bassman head) measured at 27%.

So, in my amps you have to turn the vol knob up quite a bit further to get the same output watts. In a way this is good, because it makes the volume easier to control. But, to some folks who have tried my amps they perceived the amp as being less powerful than their old Fenders, even though my amps had more actual headroom!

[VacuumVoodoo]

That's what happens when people listen with their eyes.

[pdf64]

Yes, my feeling is that pot taper is a significant aspect of the user interface. BF Fenders seemed to use about 30% taper on vol and treble, 10% on the bass and middle. They just don't feel right with 10% (or 15%) taper pots on the volume and treble, or anything other than 10% on the bass; in fact it can be difficult to get a suitable setting, due to lack of resolution.
Whereas I find that with a 10k mid control, linear is best.
Weber audio pots seem to have a suitable taper ~25% for the vol and treble.
CTS default audio taper is 10%, Alpha seems to be 15%.

Incidentally, I have a hunch that the european standard of 10% audio taper was a significant factor in the development of the Marshall tone. When copying the 5F6A, the use of a low gain tube in V1 (eg 12AY7 or 12AU7), combined with a much less sensitive 15ohm cab, would result in a 10% taper volume control needing to be turned up very high in order to shift some air.
Swap V1 to a 12AX7, and increase the NFB resistor to 100k etc, and the potential for significant overdrive resulted.
If 30% taper pots had been stocked in the RS catalogue, things may have been different.
Pete

[pula58]

I made some lab measurements of 4 pots.

Old Fender/CTS 1Meg volume pot from '68 Bassman head
New Mojo/CTS audio taper 1Meg
New CTS (from CE Dist) 1Meg audio taper
New "precision" (from CE dist) 25K audio taper

See attached plot.

The old CTS (from a '68 Fender Bassman) Pot, at a setting of "5" has 28% output, but, all the three other new pots (Mojo/CTS, CTS from CE, Precision from CE) are closer to 7% !

You would have to turn up the new pots to a setting of somnewhere between 7 and 8 to get the same output as the old pot at asetting of 5!

Let's say you are used to the old Fender amps, and set your volume knob to 4 or so. If you built a new amp, using new pots, you'd have to turn up the volume knob to be roughly "7".

That is a big difference!

[Milkmansound]

I prefer the new pots. Old Fenders were too sensitive - the sweet spot was very small. I like to have range on all of the pots, but volume and reverb mix are the most important to have large sweet spots. The new CTS pots seem right to me.

[pdf64]

Nice work, thanks for posting it.
How does a linear pot come out on the chart?
edit - using that same measurement method.
Pete

[ToneMerc]

Nice work, thanks for posting it.
How does a linear pot come out on the chart?
Pete

For comparison of tapers, see page 5

http://ampgarage.com/forum/viewtopic.php?t=16555

TM

[pula58]

Nice work, thanks for posting it.
How does a linear pot come out on the chart?
edit - using that same measurement method.
Pete

I haven't measured the linear pots. I would assume they are a straight line, but, since I have not measured one yet, I can't say for sure.

[pdf64]

Thanks TM, here's the centra lab chart from that thread
http://ampgarage.com/forum/download/file.php?id=20926
It seems that the actual slope of the old Fender audio pots is basically parallel to the linear slope, which aligns with pula58's chart.
Pete

[pula58]

adding some measurements of old 250K tone pots, also taken out of a '68 Bassman Head.


It seems like the old 1Meg volume pots had 30% , or so, at their halfway setting

Whereas the old 250K tone pots were roughly 20% at halfway.

And new "audio" pots seem to be called 10% but I measure them at ~ 7% or 8% at halfway.

[printer2]

But all of the pots achieve 100% output on 10. So really the pot with the least amount of 'jump' would probably be the most usable.

[Baxtercat]

A party came to look at two near identical SF Princeton Reverbs I had for sale.
Of course they bought the one that appeared louder at 'two' than the other one.
They even made a point of telling everyone in the room that the one was 'better' than the other.

[pula58]

A party came to look at two near identical SF Princeton Reverbs I had for sale.
Of course they bought the one that appeared louder at 'two' than the other one.

Thank's what I'm talking about!

Check out attached plots where I made SPICE models of the new and old pots (from lab measurements) and used LTC spice to simulkate the small signal frequency response. I set treb=5 and bass-=4 on old pots, with fixed midrange resistor of 6.8K. Then, I take the same circuit, replace it with new pots set to treb=5, bass=4 and fixed 6.8K mid range resistor. First plot is comparison with BOTH circuits using the old volume pot (model) with colume=4. The second plot (compare tone stack with old vs new_2) is with the circuit with new pots ALSO having a new volume pot, volume set to 4.

[pula58]

See how much less scooped the circuit with the new pots is if its tone controls are set to habitual settings?

That might be why some people claim that the newly-built replicas of old classic Fender AB763 circuits sound like they have more mid range. It's because, at the old habitual settings, the new circuit has less bass and less treble, which makes the mids seem more apparent.

Plus, obviuously, the newer circuit has to be turned-up a lot more (on the vol knob) to be as loud. Some poeple might perceive that as meaning that the new amp has less output power or less "gain", since theyt have to turn it upo more to get the same volume as before.

[Structo]

Amazing.

I do believe the psychology about where that knob points is huge!

"Dude, my amp is loud as hell, but I only have to set it at 5 with these NOS pots!"