I never took notice of that resistor.
It would seem that it would make the bass & mid 'interactive'.
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).
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
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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. :-)
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
Also note that if you are trying to get back to "typical" sound, the slope resistor R12 is 130K and not the usual 100K.
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.
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.Also note that if you are trying to get back to "typical" sound, the slope resistor R12 is 130K and not the usual 100K.
Footnote: I've just changed C1 for another polarized cap (25µ / 35v). The amp sounds noticeably cleaner. I've to play it, now. :-))
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.
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.
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.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.
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.
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. :-)
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.
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?
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.
Isaac, can you elaborate a bit more on your comments?
Additionally, I'm looking for a way to increase headroom reasonably in the clean channel without having to perform major surgery.
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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