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

  1. #1
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    What's the purpose of R105 in the Hot Rod Deluxe 2 tone stack?

    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).

  2. #2
    Supporting Member Jazz P Bass's Avatar
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    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

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    Quote Originally Posted by Jazz P Bass View Post
    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. :-)

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    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

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    Quote Originally Posted by pdf64 View Post
    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. :-)
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  6. #6
    Senior Member Pedro Vecino's Avatar
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    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

  7. #7
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    Also note that if you are trying to get back to "typical" sound, the slope resistor R12 is 130K and not the usual 100K.

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    Quote Originally Posted by Pedro Vecino View Post
    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 (25 / 35v). The amp sounds noticeably cleaner. I've to play it, now. :-))

  9. #9
    Senior Member Leo_Gnardo's Avatar
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    Quote Originally Posted by 52 Bill View Post
    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.

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    Quote Originally Posted by Leo_Gnardo View Post
    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.
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  11. #11
    Senior Member Pedro Vecino's Avatar
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    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.

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    Quote Originally Posted by Pedro Vecino View Post
    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.

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    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.

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    Quote Originally Posted by Leo_Gnardo View Post
    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.

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    Quote Originally Posted by isaac View Post
    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.

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    I've thought about dueling the mid pot to vary the slope resistor in a small range, and use the mid pot like usual. I'd just have to reduce the value of my pots to make both pots reasonable values. But I'm very familiar with reducing pot values with resistors. Oh yea, tweed? That's interesting. Now what if I try this... Think twin reverb preamp, right before the 250pf and the 100k meet, between there and the plate of the tube, i actually in one design have a .047 cap, it was just to keep the plate voltage away from the tonestack, in this one particular design it just needed it, not for the bass though, but what if I put a cap after the volume control to reduce bass a little overall if the slope mod is too much. (still on a twin reverb type preamp.) It's not a fender amp, its a custom something or other i built, but thats the idea.

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    hrdcorrectedtonestackvsfendersmallpic.jpg
    Quote Originally Posted by isaac View Post
    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.
    Thx to you and Leo_Gnardo for your helpful replies!

    I understand what you say and I agree. Simply, the 130k slope resistor + the 22n mid cap appear to give the same curve than a Fender tone stack with a few adjustments.

    As an attached file, I post a picture about it. Green curve= typical Fender tone stack, all controls @ noon. Orange curve = BMT controls @ 6, 3, 6 with the HRD tone stack as I've modified it for the moment (jumper on the mid pot, 220k additional resistor removed, all other parts intact). As you can see, TSC draws a similar spectrum although the settings and components are different. It's "good enough for me" right now.

    I'll jusst have to try the amp in rehearsal, in order to decide if it requires further mods or not to my ears. :-)

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    Ok, you did you do the jumper wire on the mid pot. Big difference huh? If you wanted it to be the most fenderesque you could go crazy and blackface the amp... at least in the clean channel, but i haven't heard of anyone doing that... Anyway, Hope you like it, and make sure you tell us how it has worked out for you.

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    Senior Member Leo_Gnardo's Avatar
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    Quote Originally Posted by freefrog View Post

    Thx to you and Leo_Gnardo for your helpful replies!

    I'll just have to try the amp in rehearsal, in order to decide if it requires further mods or not to my ears.
    Thanks for the mention AND the chart!

    Keep us posted, LG.

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    Senior Member Tone Meister's Avatar
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    Yeah, I'm also interested to know what you think of these changes after playing the amp a while. What mid cap did you end up with?

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    The .022 will give you more mids in general, also a little more low mids than before, as compared to the regular mid increase. I'm tempted to put a second .047 in parallel with my first one so I'd get kinda about .022, but when I do it, put a dual pot on so I can control the mids, but Also control how much that cap is paralleled in so the more I increase the mid pot, the more the mids actually increase. But yes, Tonemeister, he should be getting a very blackface style tone. Should sound very close. Please make a youtube or sound clip where we can hear it. Hopefully you have a clip from before the mods.

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    Senior Member Tone Meister's Avatar
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    Quote Originally Posted by isaac View Post
    The .022 will give you more mids in general, also a little more low mids than before, as compared to the regular mid increase...
    I presume that you mean with the R105 220K resistor removed, because the stock value for the C6 mid cap is 0.022 uF. I've been using an 0.015 here, and recently an 0.02 when I ran out of the 0.015. I'm always interested in hearing what others are doing in these amps and why.

    Isaac, can you elaborate a bit more on your comments?

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    Senior Member Tone Meister's Avatar
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    Additionally, I'm looking for a way to increase headroom reasonably in the clean channel without having to perform major surgery.

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    Quote Originally Posted by isaac View Post
    I'm tempted to put a second .047 in parallel with my first one so I'd get kinda about .022, but when I do it, put a dual pot on so I can control the mids, but Also control how much that cap is paralleled in so the more I increase the mid pot, the more the mids actually increase.
    Check your theory here about paralleled caps.

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    Sorry 52 bill, I meant series caps with a pot to bypass it. And Tonemeister, I forgot that the standard value was .022, but I was talking mostly if you use a BF twin tonestack and then mod from there. Sorry for confusion guys. And Tonemeister, I'll look at the full schematic see what headroom helps I might be able to direct you to... For clean channel headroom... Some tweaks, Change the first cathode cap to a 25uf instead of 47uf. Replace volume pot with 1MA, and you may want to take out the bright circuit on the second cathode cap. And reuse the switch for a regular bright cap on the volume control, you'll probably want it on the clean channel volume. then replace that cathode circuitry with a 820 ohm resistor in parallel with probably around a 5-10uf cap. You could go lower if you think it's too bassy. Also, I've never actually tried one of these amps, but I hear that the drive channel is really dark and unusable. In which case I'll take a small peak at that... try removing the 250pf cap and the 1M resistor that are in series at the grid of the third stage. and change the .047 cap at the master volume in the drive channel for a .022 and then get rid of the 390pf cap on the master. Now again, I've not tried this amp, nor have I worked on one, but I know many people say they have bad drive channels, so these are theoretical mods. Also the schematic you posted above says the PI is standard blackface type so two 1M resistors, a 22k and a 470 resistor right? Does the amp have a 12ax7 or 12aT7. If its a 12ax7, try a 12aT7 that should help, with headroom, and maybe even overall tone. Hope some of this helps.

    Once more, I haven't worked on or tried this amp, its theoretical, although the clean channel is pretty basic and I've worked on so many other ones, but I haven't worked on many drive channels. I've done several but not many, so please forgive me if something doesn't sound good, or it didn't work out.

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