Fender PRRI 65 mid resistor

[Tal_175]

I’ll quibble with this statement a little. If you increase the mid resistor and want to play at the same volume level, you’ll need to add some attenuation somewhere else in the signal chain: e.g., turning down the volume control on the amp or on the guitar, plugging into the #2 jack on the amp, turning down the amp’s master volume (if it has one) or using a volume pedal. All those options “filter the signal” a bit, especially the volume pot on the guitar.
My usual preference for playing jazz with Fender amps is to distribute the attenuation about equally between the available options. I plug in jack #2, and set all volume and MV knobs around 6-8 depending on how loud I need to be and what guitar I’m using.

Sure, I meant attenuation in the tone stack. Volume knob, pedals or the 2nd input jack presumably attenuate the signal in all frequencies (they may not do this perfectly though). However first running the signal through a mid resistor, then engaging treble and bass resistors (caps?) only to eliminate the scoop placed by the mid resistor might be more prone to coloring the signal. My understanding is that these filtering resistors/cabs attenuate an approximate range of frequencies. They also overlap in the frequencies they affect. So I though having a mid pot and setting it to allow less resistance overall is a less "processed" way of getting mids. Of course the volume knob in this case will have to be engaged more (to achieve the same gain), but that's presumably a more natural attenuation.
This is my mental model anyway. I'm not an expert in amp design by any means. I doubt it matters much either way but I was curious nevertheless.

PS. This is akin to having a passive EQ pedal and turning all the frequencies down to 0 and turning the volume up vs putting all EQ's to the natural position and achieving to the same volume as before by turning the volume down. Would there be a tone difference?

[pauln]

How much gain/volume loss do you experience with this tube swap?

John

The relative gain is:

12AT7:12AX7 =80/100
gain in dB = 10 x log(80/100) = -0.969 so -1dB, not enough to really hear or worry about.

To put this in perspective, input #2 compared to input #1 is -6dB

Using a 12AU7 is -6dB

Using the 12AU7 and plugging into #2 input is -12dB

To put that into perspective, 85 watts -12dB is 5 watts

[Gitfiddler]

Sure, I meant attenuation in the tone stack. Volume knob, pedals or the 2nd input jack presumably attenuate the signal in all frequencies (they may not do this perfectly though). However first running the signal through a mid resistor, then engaging treble and bass resistors (caps?) only to eliminate the scoop placed by the mid resistor might be more prone to coloring the signal. My understanding is that these filtering resistors/cabs attenuate an approximate range of frequencies. They also overlap in the frequencies they affect. So I though having a mid pot and setting it to allow less resistance overall is a less "processed" way of getting mids. Of course the volume knob in this case will have to be engaged more (to achieve the same gain), but that's presumably a more natural attenuation.
This is my mental model anyway. I'm not an expert in amp design by any means. I doubt it matters much either way but I was curious nevertheless.

PS. This is akin to having a passive EQ pedal and turning all the frequencies down to 0 and turning the volume up vs putting all EQ's to the natural position and achieving to the same volume as before by turning the volume down. Would there be a tone difference?

Using a passive EQ pedal is my preferred method of increasing/decreasing Treble/Mids/Bass frequencies, especially on Fender amps. Even their mid-heavy '80's Princeton Reverb II benefits from a simple EQ pedal.

[KirkP]

PS. This is akin to having a passive EQ pedal and turning all the frequencies down to 0 and turning the volume up vs putting all EQ's to the natural position and achieving to the same volume as before by turning the volume down. Would there be a tone difference?

A passive EQ pedal is much different than a Fender tone stack. An EQ pedal has a flat frequency response with all tone controls set at the mid-point, much like the tone controls on a PA.

The Fender tone stack was designed to make country and rock guitarist players happy, so it’s flattest with the treble and tone controls set near the minimum.

If by “down to zero” you mean the midpoint (twelve oclock), I agree with you.
(I added frequency responses of Baxandall vs. Fender tone stacks to illustrate the difference.)

[John A.]

The relative gain is:

12AT7:12AX7 =80/100
gain in dB = 10 x log(80/100) = -0.969 so -1dB, not enough to really hear or worry about.

To put this in perspective, input #2 compared to input #1 is -6dB

Using a 12AU7 is -6dB

Using the 12AU7 and plugging into #2 input is -12dB

To put that into perspective, 85 watts -12dB is 5 watts

Interesting. Thanks for the explanation.

John

[KirkP]

The relative gain is:

12AT7:12AX7 =80/100
gain in dB = 10 x log(80/100) = -0.969 so -1dB, not enough to really hear or worry about.

To put this in perspective, input #2 compared to input #1 is -6dB

Using a 12AU7 is -6dB

Using the 12AU7 and plugging into #2 input is -12dB

To put that into perspective, 85 watts -12dB is 5 watts

The common substitutes for the 12AX7 differ in more than just gain. I think a circuit design optimized for a 12AU7 would have different component values than one optimized for a 12AX7. When you substitute, the 12AU7 is probably running outside its optical conditions (e.g., the bias might be off spec). A non-optimized circuit might have a non-ideal frequency response, might make clip more easily, etc. A hifi amp designer would probably change the circuit design when changing tube types, and I imagine some guitar amp designers do the same. But since this is a guitar amp not a hifi, we’re more interested in finding pleasing tones than ensuring accuracy, so I wouldn’t worry too much about the gain numbers and just be guided by your ear.

[asedas]

Very interesting thread, I was thinking about this mod for a while. But my idea was to leave the mid resistor and just add another linear pot 10-15k. This way I would have at position 0 the default, non changed value (factory setting).

[KirkP]

Very interesting thread, I was thinking about this mod for a while. But my idea was to leave the mid resistor and just add another linear pot 10-15k. This way I would have at position 0 the default, non changed value (factory setting).

Makes sense to me, unless you’d miss being able to create a ~20dB notch centered around 500 Hz. :-)
If I wanted the ability to a deep midrange notch, I’d also want to be able to control what frequency it is centered at, which the Fender tone stack can’t do. I think the notch frequency could be moved around by switching in various capacitor values but that gets a little complicated.
Seems like Fender’s mid control was just an afterthought. I’ve never felt a need to turn it below around 8.

[stevo58]

Regarding tubes:
The rated amplification factor, or 'mu,' of preamp tubes is theoretical and only under perfect conditions. In the real world, a 12AX7, which is typically rated as '100,' will rarely exceed a gain of 60. So the amplification factor should be seen as a relative measure.
The first slot in your amp has a huge effect on your tone. A 12AT7 (mu 60) is not a particularly "musical" tube. It was designed as a driver tube, to swing current, which is why it is often used as a reverb driver or an output tube in 1-watt mini amps.
A 12AU7 is very low-gain (mu 20). Too low-gain for use in any amp designed for at 12AX7 (in my opinion).
The two tubes to try in the first slot are a 5751 (slightly higher amplification factor than a 12at7 (70 vs 60), but far more 'musical' - and good NOS examples are easily available) - or a 12AY7 (mu 40) which is often used as V1 in tweed amps. NOS 12AY7/6072s are dicey; it seems the good ones are long gone. EH sells a current production 12AY7 which is fine.

Regarding the blackface tone stack:
To summarize, a passive tone stack works by "throwing away" gain - the treble control bleeds highs to ground, the bass control bleeds lows to ground. Simplified, you can look at the tone stack as a voltage divider, where the mid resistor (or pot) forms the bottom half of the divider. The lower this value is, the more signal will be sent to ground, meaning a weaker signal is passed to the next (recovery) stage. In some versions of the stack, when bass, mid, and treble are all turned to 1, all the signal is sent to ground.

Increasing the resistance increases the gain (or, actually, decreases the gain loss) but as the resistance increases, the effect of the tone controls is reduced, until at some point you have no noticeable effect at all. This results in a very large gain boost, and a very mid-dominated tone. Not very pleasant, actually.

I often add a 'raw' control to the Bass-Treble circuit. This is a 500k linear pot parallel to the mid resistor. I use a relay to switch between the two, as a boost switch. Somewhere around 2.1 on the raw control the value is the same as the mid resistor; I put them in parallel rather than in series because sometimes I actually want LESS gain, which this allows. And the raw control really isn't usable above 4 or 5; it's just not a musical sound, so there's plenty of room above.

You can hear the effect in this video -

- sorry that the playing is not really relevant to this forum, or any good, for that matter. I dink around with the raw control at about 3:15

So, I'd go with a 25K linear pot, or if you find a single setting you like, replace the mid resistor with that value. And I'd try a 5751 in V1.

jmho

Steven