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Post subject: Replacement Audio Taper Pot For Solid State Princeton 65
Posted: Tue Mar 02, 2010 8:33 am
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Hey gang.

I have a roughly 8 year old solid state Fender Princeton 65 (non-DSP) amp that I'm looking to tweak. This is not the tube reissue of the Princeton, but the solid state model that was later issued as the Frontman 65.

The original volume pot is a linear taper, and as a result, it's a pain to try to dial in the right volume. The slightest touch of the control makes it go from off to !!!THIS LOUD!!! :shock:

An audio taper pot would even the curve out & fix the problem. Does anyone know where I can get the correct physical fit and electrical value audio taper pot for this amp? It's a pretty decent little amp, except for this problem.

Why manufacturers use linear taper pots in applications where an audio taper should be used baffles me. It's bad engineering.

Thanks,
Gib :-)


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Posted: Thu Mar 04, 2010 12:06 pm
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Post subject: Re: Replacement Audio Taper Pot For Solid State Princeton 65
Posted: Sat Mar 20, 2010 9:38 am
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Oddly enough, I'm trying to do exact same thing with the exact same amp. There are a few places online that sell the "pop-in" style pots you are looking for but I'm also wondering what taper to get. A and B are used to refer to linear and audio tapers (can't remember which is which) so I seems like you could just get the opposite of what the amp comes with but there's a snag. The volume pot for the clean channel is 50k and is a marked "15A" but the distorted channel (which also seems to have the same volume "problem") has a pot marked "B." So it seems like both channels have the same problem, yet they are using different tapers so it seems like at least one would work the way we are expecting.

Do you think that both channels have the same issue with volume? I do but to be honest, I've only used the amp a couple times so far and I stuck with the clean channel mostly.

I'm thinking about just replacing the DC input jack that made me open the amp up in the first place and giving up on the volume problem.


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Posted: Thu Mar 25, 2010 3:30 pm
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I've got a Princeton 112 plus from 1995, with the same problem: on volume setting 2 I'm blown off the attick :shock:

The schematic says "50kOhms30C" for both the clean volume and drive volume pot; where C=logarithmic/audio taper.
I've got the impression that Fender put 50k linear pots in both volume positions (the bass/treble pots are 50k linear-type A). I haven't opened my amp yet to check.

Has anybody experience with changing these pots to 50k log?
Does the rotation-direction matter?

Thx, FF


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Posted: Thu Mar 25, 2010 4:25 pm
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fastfolkert wrote:
I've got a Princeton 112 plus from 1995, with the same problem: on volume setting 2 I'm blown off the attick :shock:

The schematic says "50kOhms30C" for both the clean volume and drive volume pot; where C=logarithmic/audio taper.
I've got the impression that Fender put 50k linear pots in both volume positions (the bass/treble pots are 50k linear-type A). I haven't opened my amp yet to check.

Has anybody experience with changing these pots to 50k log?
Does the rotation-direction matter?

Thx, FF


C, huh? That's interesting because a C taper pot isn't log, it's reverse log, which increases resistance quite quickly and then levels out a bit.
http://www.generalguitargadgets.com/ind ... &Itemid=44

The schematic for my Princeton 65 shows a 50k "15A" for the clean volume, a 100k "30C" for the distortion's drive and a "100kB" (I assume B is the taper) for the distortion channel's volume. If A is audio and B is linear, then I think it's odd that both channels have the same volume problem if they have different tapers. You would think one would have the problem and the other wouldn't. I might just buying one of each taper in a 50k and 100k model and see which gives me the best results. We'll see.


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Posted: Fri Mar 26, 2010 12:56 am
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According to this site: http://sound.westhost.com/pots.htm , C means LOG /AUDIO in "older" schematics. I don't know how "old" "older" means, but the schematic of my Princeton is dated December 1992.
In "newer" pots, the "C" does no longer exist.

The clean volume circuit in my Princeton 112 is a non inverting opamp with the potmeter to ground -> a lower value results in higher gain. But there are quite a few caps as well, so I presume there's some tone compensation (?) based on the volume pot setting(?).
The Princeton 112 schematics are on the web, if you're interested.

Do you happen to know which size pots+shafts are in my amp? I'd like to buy the pots beforehand, so I only have to open up my amp once :D

Thx, FF

PS: Can't you just reverse the "end leads" to make a LOG pot from an ANTI-LOG?


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Posted: Fri Mar 26, 2010 6:28 am
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fastfolkert wrote:
According to this site: http://sound.westhost.com/pots.htm , C means LOG /AUDIO in "older" schematics. I don't know how "old" "older" means, but the schematic of my Princeton is dated December 1992.
In "newer" pots, the "C" does no longer exist.

The clean volume circuit in my Princeton 112 is a non inverting opamp with the potmeter to ground -> a lower value results in higher gain. But there are quite a few caps as well, so I presume there's some tone compensation (?) based on the volume pot setting(?).
The Princeton 112 schematics are on the web, if you're interested.

Do you happen to know which size pots+shafts are in my amp? I'd like to buy the pots beforehand, so I only have to open up my amp once :D

Thx, FF

PS: Can't you just reverse the "end leads" to make a LOG pot from an ANTI-LOG?


I'm no expert but I would imagine 1992 would be recent enough to use the modern markings. :) However, apparently there is some disagreement about what those mean. In the link I provided, there was a C (inverse log) as well as a G.

And yes, as the resistance between the wiper and the hot lead increases, the resistance between the wiper and the cold lead decreases accordingly. The total between the two "halves" should always equal the pot's resistance value.

In my amp, the pots are "snap-in" style pots. The 50k 15A pot has a Fender part number of 003-7323-000 but I couldn't say what your amp would have. Maybe you could find a picture of your amp's circuit board? If so, you could probably pick the right style of pot you need. They only use a couple different styles/form factors.


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Posted: Fri Mar 26, 2010 1:32 pm
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I haven't opened up my amp; to be honest, I do not yet know how to remove the knobs :(
I see no screws, you just "pull"?

The PC board drawing shows the pots are soldered on the PC board with their 3 connections, without that "extra" mechanical bracket.
Do you know if the pots are secured on the front panel as well, or only on the PC board?
Unfortunately, I cannot find a photo of the internals of my amp (Princeton 112 plus)

Thx again for the info :D

FF


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Posted: Fri Mar 26, 2010 4:11 pm
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fastfolkert wrote:
I haven't opened up my amp; to be honest, I do not yet know how to remove the knobs :(
I see no screws, you just "pull"?

The PC board drawing shows the pots are soldered on the PC board with their 3 connections, without that "extra" mechanical bracket.
Do you know if the pots are secured on the front panel as well, or only on the PC board?
Unfortunately, I cannot find a photo of the internals of my amp (Princeton 112 plus)


Yeah, the knobs just pull right off. And really easily too, compared to the knobs on a guitar for example. Be careful though, the pots on my amp have plastic shafts. Pretty flimsy. On my amp, the pots aren't secured to the panel, just the PCB.


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