"Cut" AND "Presence" knobs?

[dehughes]

km6xz ....... has the right idea....... but keep in mind that this is not ment as
a "damping" control..... the application is not for "fidelity"....... I've had the
best results with a variable feedback control in amps that do not other wise
have much feedback...... with inverters other than chathode coupled long tail
with the control usually keep to a relatively small but critical range.......

Is that to say then that the 6G6-B wouldn't lend itself well to such a control? I've done something similar to a Deluxe Reverb, and it worked out wonderfully...5k linear pot in series with feedback resistor. I'm thinking that if I halved the NFB resistor to about 25k and then installed a 50k pot, I'd get some play on either side of stock. What do you think of that?

[iknowjohnny]

I'm thinking that if I halved the NFB resistor to about 25k and then installed a 50k pot, I'd get some play on either side of stock. What do you think of that?

thasts exactly how mine is....50k pot and a 22k R. ok, well 22k instead of 25, but virtually the same thing. And it works great. Had a 100k pot at first, but the 50 works better because the 100k plus the 22k R made for too high of a total FB resistance and so the last maybe 1/4 to 1/2 of the rotation was useless. I had someone elsewhere telling me using a variable FB pot could be damaging to the amp, but i don't see why as long as there is 22k R between the loop and the tap.

[iknowjohnny]

Oh, now that's clever....very. Perhaps that'd be a neat option to a typical Cut control. Would that work just as well in a 6G6-B Bassman circuit, do you think? I'd like to be able to get the amp back to "stock", so I'd have to use a 25k pot and, say, a 33k resistor?

I'm not the guy to answer all that. I'm no tech, just a guitar player that knows enough to build an amp, but i don't know much theory at all. All i can tell you is that it sound much like a cut control except the times i implemented a cut in this very amp it lost way more overall volume. I find this also allows me to get a lot more sounds out of the amp when playing with the balance between the NFB control, presence, and the treble and mid controls. It has made it so that they all do more because of the way they work together. seems like the amp went from a one trick pony to about a 4 tricker !


EDIT: sorry....should have made one post instead of two. Can't delete posts tho.

[Andy Le Blanc]

the hard part is to impliment this....... so that it is effective to ear
and so that it is an effective control....IE.... you hear the effect as you turn
the control thru-out the rotation....... and keep the operation of the amp in
a stable range..... I found it to be a better control with some amps than others
a variable feedback is useable in any amp...

[dehughes]

Cool. Thanks guys. MUCH appreciated. I'll meditate on this and get back to it once I get closer to building the thing...

[iknowjohnny]

the hard part is to impliment this....... so that it is effective to ear
and so that it is an effective control....IE.... you hear the effect as you turn
the control thru-out the rotation....... and keep the operation of the amp in
a stable range..... I found it to be a better control with some amps than others
a variable feedback is useable in any amp...

Mine is a variation on a 2204 with more gain and a different PSU. With this amp it's very complimentary. With treble and presence controls in the mix along with the variable NFB control, It really allows me to fine tune it to most any frequency balance i've ever gotten from any amp i've ever owned. I realize thats a extremely bold statement, but it's true with one exception...you of course can't cop the "voice" of some amps, especially those that have very unique voices like a vox for example. But it can go anywhere within the marshall realm and even a bit towards fender. My only concern is having only 22k between the 16 ohm OT tap and the NFB loop when the pot is turned all the way. any idea whether that could cause any issues? It hasn't so far, but then i haven't cranked it up for any length of time.

[km6xz]

km6xz ....... has the right idea....... but keep in mind that this is not ment as
a "damping" control..... the application is not for "fidelity"....... I've had the
best results with a variable feedback control in amps that do not other wise
have much feedback...... with inverters other than chathode coupled long tail
with the control usually keep to a relatively small but critical range.......

Hi Andy
Whether intended or not, damping factor and control of the speaker changes with feedback, the Z ratio of load to source changes, and the ratio of those two factors IS damping factor.
For effective FB influence, the amp has to have a least a modest amount of excess open loop gain, with a reasonably narrow range of phase shift between the loop enclosed stages. Just willie-nilly adding FB paths can cause expected unwanted circuit conditions: oscillaton for one, peaky response is another. The higher the open loop gain the more this potential pitfalls can occur if the phase shift is large, as what happens with reactive elements(in the tone circuit or bypassing) and point of FB injection. If the FB source is the speaker leads of the OP xformer, reversing the leads can often be the cause of oscillations.
Feedback is worthy of a more technical thread if anyone is interested, it is the most important single parameter in determing the transfer function of a circuit, and therefore the sound.

[iknowjohnny]

km6xz,

You are obviously quite knowledgeable, so let me ask you. Is having a loop with a 22k R and a .0033 cap in place of the normal 100k R right after the tap in any way dangerous to the speaker or OT's health? Someone elsewhere seemed to suggest this and i now would like to know for peace of mind. If it IS, what would a minimum resistance for safely be?

[km6xz]

km6xz,

You are obviously quite knowledgeable, so let me ask you. Is having a loop with a 22k R and a .0033 cap in place of the normal 100k R right after the tap in any way dangerous to the speaker or OT's health? Someone elsewhere seemed to suggest this and i now would like to know for peace of mind. If it IS, what would a minimum resistance for safely be?

I am not sure of which amp in particular you are modding but the 22k in parallel with a .0033 uFD cap will lower the gain of lower frequencies and lower it a lot for upper frequencies. How much depends on the Z of the source(output sampling point) and injection points, What are you wishing to accomplish?
There should not be any risk in lowering the gain this way. The gain would increase with higher values of the feedback resistor, meaning there is less degenerative feedback. A direct short is about as low a resistor value you can use and that would be very safe(assuming it is an audio path only, where there is no DC potential across the resistor) but it would also lower the gain a lot. Feedback causes the amp to sound tighter and more controlled, the speaker also will work more precisely for less overshoot. That is great for Jazz such but most Blues players would not like that. A lot of even harmonic distortion, is created by a "loose" sounding amp and that can be warm and "round" sounding, particularly on bass. The sound is thicker, and the main reason people say tube power amps sound louder than solid-state.

The amp will not be high-fidelity but that of no importance in generating tone, but it is vital in reproducing sound. Take your best tone monster amp and put a vocal or CD through it to get that point. It well be awful. Just as plugging a guitar into a precise $30,000 hi-fi rig will sound like crap...thin and without warmth or balls.

Taking the feed back out entirely would be the only thing to caution about unless the circuit is known as to how much open loop gain and phase shif it has. A number of moderan amps with high open loop gain will oscillate like a broadcast station if the feedback is lowered enough or cut completely.

[iknowjohnny]

Actually i already have it like this and it sounds fantastic and doesn't seem to lose a lot of gain. yet the tone is super smooth and rich and compressed as i lower the resistance. but the thing is, i don't always just lower it all the way. It's great because by lowering it to various degrees and playing with the treble and presence controls, i'm able to fine tune the tone like never before with any marshall. So i use it many ways. Then sometimes i just lower it all the way and turn the presence all the way up and adjust the treble for the right balance of overall brightness and the result is an almost rockman like sound. I also have a switch that removes NFB completely and of course sounds worlds different.

By the way, i have bypassed the res cap many times, and to be honest i really don't hear a change in the hi and low frequencies, just a loss of lows. So to me it sounds like that cap ADDS lows, whereas from what you're telling me it sounds like you were saying it LOWERS the low end but that it seems like it adds lows because the highs and mids are lowered even more than the lows. is that what you meant? Because clip leading that cap out makes the lows go away a lot.

So in short, i really just wanted to know it's safe because as far as the usefullness it's a god send. I look at it like this....you know how most amps have a inherent voice that you're pretty much stuck with? Well, this allows me to totally change the amp's inherent voice so that i can not only just set the frequencies to where there is a balance, but i can change the amp's character completely. I can't see myself needing another amp after this because i've been close to feeling like that about certain marshalls which were way less versitile and even not as good as far as tone QUALITY. Unfortunatly my gigging days ended several years ago so thats no longer an issue anyways. But if i was still playing out i honestly think this could be my last amp. And a big part of that is the VNFB.

Oh, and it's a home built thats a variation on a single channel JCM800. It has a good bit more preamp gain tho and the PSU is fairly different.

[CaseyJones]

Late return to this thread but...

My favorite "less is more" amp would have to be the Cornford Harlequin. Not exactly one knob. The volume knob and the way it's wired is the key, I could chisel the other knobs right off and never miss 'em.

[Luthierwnc]

I'm getting in late too but I think of presence as an "offensive" player while the cut plays defense. If you can, hold off on selecting the cut pot and cap before you hear the amp. You may want to always have some attenuation so maybe go with a 250ka/.0022. From very little to a lot, 500ka/.0047.

The most common amps with cuts are Vox/Matchless designs with two differently phased preamp channels going to either side of the inverter. The usual transformer-driven negative feedback presence ties to the bottom of the PI so a cut is the only practical way to clip the highs after the PI entrance. With a pentode front-end, it may be the only tone control at all (like an AC/15).

Cheers, Skip

[d95err]

Perhaps you should try a Hiwatt style presence control. It can both boost and cut treble depending on how you turn the knob. Clever!

http://www.schematicheaven.com/newamps/ ... _dr103.pdf

[dehughes]

Perhaps you should try a Hiwatt style presence control. It can both boost and cut treble depending on how you turn the knob. Clever!

http://www.schematicheaven.com/newamps/ ... _dr103.pdf

Interesting....it is connected to the feedback loop, kinda. I've not used this amp before...how does this control work? What is its effect?