Valveking mh 20 negative feedback question

[Chuck H]

The circuit is complicated because there is a cathode bypass cap dependent on input from and voltage division by the NFB circuit as well as the NFB circuit depending on that cathode circuit for it's voltage division. Further this with the 220n cap being interactive between the circuits as well. I think moving R78 to the location you indicated would increase HF bypass on the cathode by reducing resistance between the cathode circuit and the 220n to ground. Increasing treble, though not technically "presence". Adding 820R to the exiting 220k (moving R78) prior to the 220n circuit does very little to change the voltage division there so I wouldn't expect any notable change to the actual presence affect on HF.

[Rod]

Just one more question,

if i have more negative feedback entering in the preamp(for exemple if i keep the R78 value) i will have less destortion, more bass, more headroom, but what also what will happen when i raise the volume is that it seems that the sound becomes more thinner. Could this thinner sound when raising volume due to more negative feedback enter in the preamp ?

What else could be the problem of the amp becames more thinner when i raise the volume?


I dont know why, but it seems to me that little amps with just only 2 powertubes are very trebly, dont have enough punch and bass.

[Chuck H]

Your description of what happens to the sound with more NFB and higher output is likely perception due to the Fletcher Munson curve (what happens to your ears with changing volume). More NFB does not mean more bass. It means cleaner bass. Another affect of reduced NFB at higher outputs would be more distortions. This (perceptively) can "fill out" the sound and make it seem less thin perhaps.?. But there are other circuit parameters in play (like bias, load impedance and phase errors) that also affect the effect of NFB, especially when an amp is pushed close to or into clipping, and how it is likely to be perceived. Regardless of what is actually happening. With too many circuit specifics interacting it's impossible to apply blanket analogies about what more NFB and less NFB sound like. That is circuit and amplifier specific to some degree. But since most guitar amps follow a similar topology the NFB level can often be recognized in it's contribution to tonal character. Your circuit, however, is not as typical. And I find trying to predict affects of it's adjustment tricky because the NFB and the earlier stage that begins the loops cathode bypass circuit are sort of shared. In fact, R78 is NOT the feedback resistor. R23 is. The shared circuits between the NFB loop, presence circuit and cathode bypass circuit for V3 is comprised of C5, C23 and even C59 and R38, R77 and R78. All six of those components can be said to be in the cathode bypass circuit for V3 and also in the NFB/presence circuit. Since the two circuits affect each other relative to phase and voltage potential, but the cathode bypass circuit for V3 also operates independently for that stages frequency response that "sees" the loop and feeds the loop, well, you can see it's complex. Too complex for me to just look at it and make predictions on circuit changes and too complex to be used as a model for what changes to a NFB loop sounds like.

[J M Fahey]

A couple details:
1) that is a "trick" NFB circuit.
Not only smooths output sound, lowers impedance (increases damping) and lowers dstortion, which is its main purpose, it also equalizes the power amp.
You have a Bass boost and a Hi Mids/Treble one.

2) C59 in series with R61 boosts Bass from 450/500Hz down, stopping at about 80Hz. (R23 in parallel with C59).

3) C23 in series with R38 boosts mid/highs above 800Hz or so.
Increasing R38 will decrease boost as mentioned by Chuck H so "it will have less presence".
Just to have a clear idea of what this does, lift the grounded leg of R38 and add a switch there, so you instantly switch from full to zero Presence boost, leaving all else the same.

4) in general, most complaints about "too bright" , "too much highs/presence/buzzy/thin" sound , also called "non pedal friendly" come from amps being played at low volume, alone, at a home situation, etc. ; once you play with others (specially after getting a few cymbal crashes a few inches away from your ears) nothing is"too bright" any more.
That "excess Presence" is probably what helps that amp cut through the mud soundwall.

Try the "kill presence" switch I suggested above .

Or play alongside a loud drummer

[Steve A.]

Another point... this is an amp head so the speaker used is another factor. I am a big fan of Vox-style Cut controls to match the output to the speaker used. It is usually fairly easy to add to practically any push pull amp because you can always trace the circuitry to the output tube grids. I like to use a pot with a switch so that I can remove it from the circuit if desired.

Steve A.