What's the purpose of R105 in the Hot Rod Deluxe 2 tone stack?

[freefrog]

I mean: the 220k resistor parallel to the bass pot and which goes to ground, unlike in a typical Bassman tone stack...

In my understanding of self-taught hobbyist, it increases the Q of the mid dip and slightly shifts it down, giving more low mids & high mids... hence these frequencies between 800hz & 2khz which certainly "cut" the mix but don't sound Fenderish to my humble ears...

if it has also any technical utility, I'll be grateful to learn something new. :-)

Thx in advance to confirm, correct or complete my thoughts (I plan to mod the amp, so I don't want to damage it because of something that I wouldn't have understood).

[Jazz P Bass]

Huh.
I never took notice of that resistor.
It would seem that it would make the bass & mid 'interactive'.
http://www.blueguitar.org/new/schem/...deluxe_pre.gif

[freefrog]

Huh.
I never took notice of that resistor.
It would seem that it would make the bass & mid 'interactive'.
http://www.blueguitar.org/new/schem/...deluxe_pre.gif

Thx for your answer.

Whatever is does, it "squeezes" the spectrum and boosts the high mids in an unpleasant way.

Having already soldered a jumper across the mid pot, I plan to clip this 220k resistor, therefore...

Opinions and advices welcome. :-)

[pdf64]

Doesn't it just limit the max bass?
But in that case they could have got the same effect by using a 100k bass control.
I don't see how it does all this other stuff mentioned, especially altering the Q of the mid scoop
Pete

[freefrog]

Doesn't it just limit the max bass?
But in that case they could have got the same effect by using a 100k bass control.
I don't see how it does all this other stuff mentioned, especially altering the Q of the mid scoop
Pete

My first thought was that it limits the bass. Then I've used the .tsc calculator, trying to take in account how this parallel resistor interacts with the serial resistance of the T/B/M pots : I've ended with a curve whose mid dip was narrower, whatever is the reason.

Anyway, I've clipped R105 today then tested the amp with a frequency analyzer: there's slightly less bass now ; the bowed curve between 800 & 2000hz is gone too. And the mid dip looks a touch wider. The amp finally sounds like a Fender...

I've also mounted a pre-gain bright switch on the volume pot, with a 120pf cap: it adds sparkle above 4khz and completes the stock bright switch, which created a peak @ 4khz.

Maybe I'll change C1 the next time but I'm done today. At least, now, I've shared my results with you, folks. :-)

[Pedro Vecino]

When I tried it many years ago (by ear) I understood that this resistor was there basically to keep a certain amount of mid frequencies presents avoiding emphasize the lows independently. This makes sense in a pre-drive equalization circuit to maintain the integrity with that rudimentary overdrive. In other words: although is common to both channels is related to the lead channel response to avoid excessive contrasts between bass and mids.
Not sure if that matches what you have obtained with the analyzer. That was my observation

[52 Bill]

Also note that if you are trying to get back to "typical" sound, the slope resistor R12 is 130K and not the usual 100K.

[freefrog]

When I tried it many years ago (by ear) I understood that this resistor was there basically to keep a certain amount of mid frequencies presents avoiding emphasize the lows independently. This makes sense in a pre-drive equalization circuit to maintain the integrity with that rudimentary overdrive. In other words: although is common to both channels is related to the lead channel response to avoid excessive contrasts between bass and mids.
Not sure if that matches what you have obtained with the analyzer. That was my observation

Thx for this info!

I must admit that I focus on the clean channel: I use drive pedals with it. I keep the onboard drive as a spare only. So, I've not yet tested its response. I've just played the yellow and red modes, and noticed that the sound was correct with moderate settings.

Also note that if you are trying to get back to "typical" sound, the slope resistor R12 is 130K and not the usual 100K.

Thank you to remind me that parm. Actually, I'm reluctant to change it because I'd have to change the tone caps too... Of course, the 22n cap is "off" compared to a typical Fender schematic but according to .tsc, it can give something close to a 47n cap as long as it is used with the 130k resistor.


Footnote: I've just changed C1 for another polarized cap (25µ / 35v). The amp sounds noticeably cleaner. I've to play it, now. :-))

[Leo_Gnardo]

Also note that if you are trying to get back to "typical" sound, the slope resistor R12 is 130K and not the usual 100K.

Uh oh... you found part of my secret tone-fattening formula. For those who complain about "too bright" I reduce the value of R12 first thing, and easy enough to parallel another resistor to R12 without having to tear the whole board out. In fact I aim for 33K to 68K as the value for the composite resistor.

[52 Bill]

Uh oh... you found part of my secret tone-fattening formula.

Back in the '80s when Fender had those blackface amps with push pull switches on all of the pots, they had changed that resistor to something like 150K and then had a switchable parallel resistor to get the value back down to 100K.

[Pedro Vecino]

I do not remember any Fender that use that system. Could you tell what model?
I've seen in Rivera amps (called "notch" if not remember bad). To avoid the high voltage on the switch requires prior isolation of a capacitor.

[52 Bill]

I do not remember any Fender that use that system. Could you tell what model?
I've seen in Rivera amps (called "notch" if not remember bad). To avoid the high voltage on the switch requires prior isolation of a capacitor.

The ones that I am thinking about are the Fender 30 and the 75, probably designed by Rivera when he was with Fender.

[isaac]

That resistor in the tonestack is probably what others have said about it, but you mentioned, after you took it out, it sounds like a fender...

There are other issues with the tonestack that aren't fender styled... SO if you care, I'll point them out. The slope resistor (130k) to be like a BF twin would be a 100k. the mid pot would be a 10kl wire like this: jumper the wiper of the mid pot to the top of the mid pot (like on the bass pot) remove the 200k (like you already did.) and the .022 cap on the mid pot, is wrong, unless you want a super reverb tonestack, because only on the super reverb did they put a .022 cap to the mid pots. The twin and the deluxe etc. , had .047. The .022 will give you a good amount more low-mids than a .047. Try it in the TSC. Hope this helps. Again, the biggest mod out of all these ones I just pointed out, would be the 220k (but you already did that) and the jumper from the wiper and the top of the mid pot, it will get a way more fendery response and tone. Even without the other value changes. Also with a 100k slope resistore vs 130k you will get more bass and mids than before which might be nice, which you can always roll back a little bit more than before anyway.

[isaac]

Uh oh... you found part of my secret tone-fattening formula. For those who complain about "too bright" I reduce the value of R12 first thing, and easy enough to parallel another resistor to R12 without having to tear the whole board out. In fact I aim for 33K to 68K as the value for the composite resistor.

I missed this post the first time through... So anyway, you like 33-68k for the slope resistor? How does that work for you, does it ever get flabby or too much? I've not tried messing with that resistor before although I've tried pretty much everything else. Also how do you think it would work to put separate resistors in there one for bass and one for mids, so I could get a touch more bass, but a good amount more mids? I've also not tried that, you don't even see that on anything.

[Leo_Gnardo]

you like 33-68k for the slope resistor? How does that work for you, does it ever get flabby or too much?

Sort of pushes the EQ curve closer to tweed tonestack. By suppressing the upper harmonics, it allows more fundamentals and lower harmonics to come through. Has a bit to do with psycho-acoustics. If there's too much treble, the ear tends to ignore the lower frequencies. Yes, it can get flabby, depending on where the tone controls are set, what speakers, what guitar pickup etc. Too much bass in any tonestack brings on the flab. Many of my customers are looking for robust tone, even fat (blues guys & gals, also jazzers), but just shy of flabby. (I take flab to mean so much bass that it overloads the power amp, obscuring a clear tone.)

Also how do you think it would work to put separate resistors in there one for bass and one for mids, so I could get a touch more bass, but a good amount more mids? I've also not tried that, you don't even see that on anything.

Haven't tried that, but I have put in switches to select stock or other value "slope" resistors in some mods. Sunn used to include a "contour" pot in some of their tonestacks - you could vary the slope resistor to any reasonable value.