Fattening the clean side of a non-HRM

[martin manning]

... it seems that THE MORE I READ THE LESS I KNOW

I find this is true in general.

My 124-ish amp has a great clean sound and uses the following:

Treble 250k Linear
Middle 250k 10% Audio
Bass 500k 10% Audio

The treble pot could just as well be a J taper (~30% Audio) like Fender used. The only difference in the tone stack wrt the #124 schematic is the treble cap is a 370p.

[ijedouglas]

My 124-ish amp has a great clean sound..

Totally agree although I think HAD did find pots that read low. I think 183 was measured at 311K and 102 at around 400K. I found around 350-400K works great in these amps and gives a good amount of low end without becoming too boomy.

[Today]

Thank you, M.M.! That is exaclty what I shall use: Treble 250kB (linear); Mid 250kA (don't know if I shall get exactly 10% taper, but I shall try); Bass 500kA. THANK YOU, AGAIN!

[Charlie Wilson]

You may want a 250kA for bass pot with a 100k slope and not a 250kA on the low side. Try and keep it 250k or higher. Keep in mind all of the previous posts were talking about a Skyliner not a Classic. Also, the coupler coming out of the clean in all of the Low Plate Classics I have seen is a .01, not .047.
CW

[martin manning]

Since it's wired as a variable resistor in the Skyline tone stack (Rock mode), the bass pot value mostly affects the bass range. A 500k set at 250k or less shouldn't make any difference (that's about 8/10 on a 10% log pot), and the same is true for the loading effect on the preceding stage. Taper is only about the position of the knob, but in some cases the wrong taper can make the control hard to use. A 100k vs. 150k slope resistor will shift the mid dip to the right a few hundred Hz, so that will change the treble/bass balance of the mid's.

[Charlie Wilson]

Bass cap to top terminal of bass pot, wiper out, bottom terminal to 1k tail and ground. Not sure how you can get the same series resistance from top terminal and wiper, and wiper to 1k tail with a 500kA pot.
CW

[martin manning]

Here's what I mean. This is 124 Skyliner schematic snip. In Rock mode the bass wiper is connected to the top, so it's just a 0-500k variable resistance. Not so in Jazz mode, of course.

[Today]

Greetings,

For this CLEAN-ONLY amp (#124 variation) I have in mind (already being built, actually!), what would be better for clean ROCK guitar sound (never distorted), a 100k or 150k slope resistor? Does anyone know what is the (approx.) center frequency of mid dip for each of these two options? Thanks.

[martin manning]

For this CLEAN-ONLY amp (#124 variation) I have in mind (already being built, actually!), what would be better for clean ROCK guitar sound (never distorted), a 100k or 150k slope resistor? Does anyone know what is the (approx.) center frequency of mid dip for each of these two options?

The controls are interactive, and the frequency at the bottom of the dip varies with the settings of all three, from ~300 Hz to over 1 kHz. A 100k vs. a 150k slope will move the dip up ~150 Hz. I think you're going to want to hear the effect of changing the slope resistor value and decide for yourself. It's easy enough to put in a 150k, and parallel a 270k with it to hear what a 100k (actually 96.4k) would sound like. I think the essence of the clean tone is more about the harmonics present as determined by the operating point and the load line (plate and cathode resistor values). The EQ can be adjusted to taste with the cathode bypass caps and the TMB controls. A reduced slope resistor value will put more bass into the mids, which could be what you describe as "fatter." Larger cathode bypass caps will also add bass, but 10u seems to be the standard.

[Today]

Thank you, M.M.! Or would it be ¨m&m¨ ha, ha, ha... I thought so, but it is nice to read this generous, clever confirmation and advice. That's exactly what I shall do: test both 150k and 150k with 270k in parallel and try it out. Nice to know that it is about 150 Hz move on the frequency scale, I did not know that... now I know. Thanks a lot. Sincerely, Sven R. (here: Today)

[Today]

Greetings,

I am building a clean preamp only version of #124. I am not about to have Presence pot (2k linear), but instead it would suit me fine to have a switch that changes between two settings: 1. no resistance at all (like if pot would be at minimum) 2. full resistance of 2k Ohma (like pot would be at maximum). I figure that it would not be neccessary to have in-between resistance, rather, either do not have it at all or max. value.

Is there a schematic that shows that? Also, would the ¨feedback¨ reistor of 4k7 need to be 2W, and would it be tied to 8 Ohm wire of the Output transformer (why all schematics show a tie to 4 Ohm output on OT?).

I am fairly within grasp of these questions, but it is always nice to get a ¨second opinion¨, if possible, of couse... its been too many years that I have been away from tube amps... Thanks in advance. Drawings, schematics of these details are welcome, as well as comments on these issues ... thanks in advance.

[Charlie Wilson]

Are you using a classic tone stack or skyline?
CW

[martin manning]

I am not about to have Presence pot (2k linear), but instead it would suit me fine to have a switch that changes between two settings: 1. no resistance at all (like if pot would be at minimum) 2. full resistance of 2k Ohma (like pot would be at maximum). I figure that it would not be neccessary to have in-between resistance, rather, either do not have it at all or max. value.

This is like the "Accent" switch on some earlier ODS. In that case the accent is a cap switched across the PI tail resistor. What I would do is connect a 2k resistor in series with the presence cap (presence full off), and use an SPST switch to short the resistor for presence full on. Having the 2k there gives you another variable to play with, and you could even use a three-position switch to get an intermediate setting.

would the ¨feedback¨ reistor of 4k7 need to be 2W, and would it be tied to 8 Ohm wire of the Output transformer (why all schematics show a tie to 4 Ohm output on OT?).

I think 3W is overkill. A half-watt is large enough. The values of the resistors in the feedback divider and the output tap used determine the feedback voltage. The values shown in the 124 go with a 100W power amp and the 4Ω tap. If you use a different tap, you need to adjust the feedback resistor value to restore the voltage ratio. For example, if you use the 8Ω tap, the FB resistor should be increased 1.4x to 6k8.

[Charlie Wilson]

Martin, does the tail resistor need to be adjusted or just the negative feedback resistor?
CW

[martin manning]

does the tail resistor need to be adjusted or just the negative feedback resistor?

There are three things going on: PI cathode resistance, FB voltage division, and presence frequency response.

The tail resistor is a small part of the PI cathode resistance, but we'd like it to stay the same.

For the voltage division, Rf can be changed as required. In this 124 example Rf is 4k7 and Rt is 390, making the divider ratio 390/(4k7+390) = 0.077. To move to the 8Ω tap, we need to reduce the voltage ratio by a factor of 0.707 so we increase the Rf by 1.41, and get 6627. Using a standard 6k8, and the original 390Ω Rt, the voltage ratio is 0.054, and 0.054/0.077 = 0.70. Pretty close, within 1%.

The presence full-on frequency response knee goes like 1/(2π(Rf//Rt)). Since the feedback resistor is 12 times the tail resistor, Rf//Rt is dominated by Rt, and you can just change Rf. The original Rf//Rt was 360, and the new value (using 6k8) is 369, +2.5%.