Solo boost: lifting tone stack vs lifting NFB

[Hardtailed]

I want to implement a solo boost on my new amp, but I don't want just a volume boost.
I read about lifting the tone stack's ground to gain both a level and mid boost, sounds like fun.
I figured I could put a pot between the tone stack and ground and bypass it with a relay: relay open = boost, relay closed = no boost

Simple enough

But now I'm debating maybe lifting the NFB instead. Put a pot in serie with the NFB loop and bypass it with a relay, same concept.

Anyone tried both approaches and could comment?

The NFB approach would provide "some" mid boost in the way of deactivating the presence and depth controls, and I like the lead tone of non-NFB amps,
Also, one of my channels will have a James tone stack which doesn't seem to behave the say way as a classic BMT (did some simulation in the Duncan TS simulator) when lifting the ground, which means I need to have separate pots lifting the ground of 2 different tone stacks as I might need different levels to get a similar boost (I do need a boost on both channels)
The NFB approach saves a pot and makes my front panel much more concise.

[tubeswell]

I like the raw effect of not having GFB on screaming single solo notes. I've never tried lifting the tone stack - I guess it depends on what sort of tone stack you are lifting as to how much boost you would get. (A one-knob stack is typically less lossy, so the boost wouldn't be as much as if it were a TMB stack)

[hasserl]

I've experimented with both, and I prefer the tone stack bypass. For one thing, I got a pop when switching the nfb that I was not able to cure. There may be some way to get rid of it, but it's not worth the effort IMO.

[Hardtailed]

I like the raw effect of not having GFB on screaming single solo notes. I've never tried lifting the tone stack - I guess it depends on what sort of tone stack you are lifting as to how much boost you would get. (A one-knob stack is typically less lossy, so the boost wouldn't be as much as if it were a TMB stack)

Channel 2 has a typical Marshallesque BMT TS
Channel 3 has a James (Bandaxall) TS
Both have more than 10dB of loss in the mid-range, so plenty of potential (I usually like a light 3-4dB boost, it's the increase in mids that makes it cut better)

Through some simulation, it seems that lifting the gruond on a James TS raises the volume more than it boost mids like it does on a BMT

Happy to know I'm not the only one who likes non-NFB'ed leads!

[Hardtailed]

I've experimented with both, and I prefer the tone stack bypass. For one thing, I got a pop when switching the nfb that I was not able to cure. There may be some way to get rid of it, but it's not worth the effort IMO.

Interesting
Were you completely opening the NFB loop, or did you have a resistor accross the switch (therefore just reducing it a bit)
Also, I'd have a DC blocking cap in there which might prevent popping... might

I have an unused switch on my current main amp (Marshall clone) that's just beside the depth control, I'll try maybe rigging it to reduce NFB and see if I like the tone. The amp has no relays so I can't try it as a solo boost while playing.

[hasserl]

Interesting
Were you completely opening the NFB loop, or did you have a resistor accross the switch (therefore just reducing it a bit)
Also, I'd have a DC blocking cap in there which might prevent popping... might

I have an unused switch on my current main amp (Marshall clone) that's just beside the depth control, I'll try maybe rigging it to reduce NFB and see if I like the tone. The amp has no relays so I can't try it as a solo boost while playing.

I set it up with a 100k pot (wired as a variable resistor) in series with the stock 10k nfb resistor, activated by a relay. With the relay non-activated just the 10k resistor was in circuit. Activating the relay added the pot in series with it. This allowed the user to set the pot to taste and activate it with a footswitch.

[jrfrond]

If it's gain boost you want, bypassing the tonestack is the way to get it, because a lot of gain, usually upwards of 20dB, is thrown away, which is why a recovery stage is often needed.

The best way to implement this without the switching noise is to lift the ground at the bottom of the stack and insert a high-value resistor, then place the switch or relay (normally-closed) across that to create the bypass for normal operation. To figure out just what resistance you need, it's best to hook up a pot (let's say 250K) as a variable resistor, hook it in, and dial in the resistance you want while playing, then remove it, measure it and add a fix resistor of the same value.

Remember that when you bypass the tone stack, you are giving up the tone shaping/scooping that occurs, yielding a more midrangey, tweed-like response.

If your tone stack has a midrange control at the bottom like Fender or Marshall et al, you can just switch to a higher-value pot if you'd like, and make it continuously-variable. The fixed resistor method does allow you switching capabilities though.

[flythair1]

Hi MEF Community,

I have built four of the AX84 designs, a P1eX, two SEL's and an October and incorporated a tone stack ground lift in all of them to very good effect. It is easy to put in a switch and with a shorting jack, you can also use a footswitch to over ride the panel switch.

All I have done is break the ground on the mid pot and you pick up a lot of gain. It's a very simple method to get a quasi lead channel that you can put on a footswitch.

Scott

[Gaz]

I've experimented with both, and I prefer the tone stack bypass. For one thing, I got a pop when switching the nfb that I was not able to cure. There may be some way to get rid of it, but it's not worth the effort IMO.

I got rid of the pop by putting a 470 ohm/2 watt resistor from the switch to ground. It was a tip from Merlin

[wizard333]

I've experimented with both volume boosts, mid boosts, and both combination.

Switching in a very large resistance or a large value pot there set high is going to give you a HUGE midrange boost if your normal setting and tone stack values have a low or moderate midrange.

Whether or not that is good depends on your non-boosted tone. If you scoop a lot of mids for your normal tone, and all of sudden you switch in what essentially amounts to a huge midrange pot turned up all the way, the change in tone may come accross as quite shocking. You want your guitar to be more forward in the mix for solos, but there is a point at which the change in tone just comes across to the audience as bizarre. If you set your tone to sit nicely in your band's mix with a moderate amount of mids, and you hit that button, all of a sudden its going to sound like an unrelated instrument tossed into the mix and turned up loud.

The more mids you use normally, the better that is going to work for you, because there is a point beyond which using a bigger mid resistor doesn't really change things that much. If you have a 25k mid pot and you play with it at 5; then you switch in a 100k resistor, the "holyshit" factor is going to be high. If you have a 100k mid pot and you set it at 5, then you switch in another 25k resistor, thats going to be more reasonable.

The point here is: Don't over-do it. From my experience with this, which goes back about 20 years or so, there is definitely a line you don't want to cross in terms of how much more "extra" mids you can throw in and not sound like a non-sequiter to your mix. Its less than you probably think. Unlike most people, I generally play with a lot of mids, and even going from a 50k value to a 100k value there can easily come accross as "too much". A 25k bump in the mid control is a reasonable bump and won't make you all of a sudden sound 'weird'. You can experiment and see if you can push that, and whether that is enough to give you the mix cutting increase that you want.

In my experience, to really stand out, you generally need both the moderate mid boost coupled with some overall volume boost. Its easiest to accomplish that with relaying in an different mid pot or switching in a larger mid resistor, coupled with switching in a cathode bypass cap on the last stage before your PI.

[Hardtailed]

Which is why my plan is to have a pot wired as a varistor to provide the boost, not a fixed resistor. I've planned the front panel to include that.
I somehow wished I could get away with one pot shared by both dirty channels, which is something I cannot do if I go with the TS (tone stack) lifting method because the two TS are very different (a typical BMT, and a James).

However, the switchable bypass cap on the last gain stage is something uberclever that I hadn't thought of.
I usually don't put a bypass cap on that last gain stage before the CF, and that gain stage is shared by both dirty channels (the 3rd channel basically just adds a gain stage after the input stage, and switch to a different tone stack) so it would work on both, and through experimentation with various cap values, I could get the right amount of "cut" boost. It wouldn't boost mids per se, but it simple and effective, just the way I like it.

Perhaps in combination with a rather small resistor lifting the ground on both TS (or a trim style pot on the board so I can fine tune, then set and forget). Just one pot on the front panel that would be in serie with the extra cap (since I usually like less than 6dB of level boost... and because I have to have a control on the front!)

[hasserl]

I got rid of the pop by putting a 470 ohm/2 watt resistor from the switch to ground. It was a tip from Merlin

Can you elaborate a little? How did you wire this up, this may be very helpful to me.

Thanks!

[Gaz]

Can you elaborate a little? How did you wire this up, this may be very helpful to me.

Sure! Here's a link to my schematic. Of course it doesn't have to be grounded at the speaker jack, but it was just convenient for me.

http://img189.imageshack.us/img189/5066/ampu.jpg

[hasserl]

Sure! Here's a link to my schematic. Of course it doesn't have to be grounded at the speaker jack, but it was just convenient for me.

http://img189.imageshack.us/img189/5066/ampu.jpg

Cool, thanks!

[DIY_Guy]

Sure! Here's a link to my schematic. Of course it doesn't have to be grounded at the speaker jack, but it was just convenient for me.

http://img189.imageshack.us/img189/5066/ampu.jpg

Sorry to go off topic, but what do those 220pF caps on powertube pin 5 to ground do?