Resonance Control

[fusionbear]

Hello gents,

I want to add a resonance (i.e. deep) control in the feedback section, similar to what Bogner does on a Fender twin style power amp section (4-6L^GC's) for a custom amp I'm making. Any schematics around? Recommended values.

[Firestorm]

As you can tell by the groundswell of responses, this is not so simple a question. There is a very good discussion of the subject in Kevin O'Connor's TUT 1, which is copyrighted so I can't just snag pages of it for you. (You should have it anyway -- the one book I go back to most often for confirmation of whatever wacky idea popped into my head).

Basically what you are doing with a resonance control in creating a complementary filter to the presence control (presence is typically a high-pass filter that routes the feedback loop's high frequencies around the shunt resistor so they ground out and thus aren't attentuated by the loop. A resonance control is also a high-pass filter, but in series with the feedback resistor. It lets the higher frequencies by the feedback, so they are attentuated by the loop, but limits the low frequencies so they aren't attentuated.

Now the reason there isn't one readily useable schematic is that you are going to have to calculate the two filters' break points for yourself, based on the size of your feedback and shunt resistors and the frequencies you want each control to affect. Blackface Fender values (820R and 47R/100R) aren't likely to work because you'll need huge caps and probably can't find pots in tiny values. You'll need to start working from Fender Tweed values (100K and 5K (with the parallel 25K pot).

The thing about a resonance control is that it basically undamps the speaker near the speaker's resonant frequency and lets it run loose and floppy. That's typically the opposite of what we like to hear ...

Sorry this isn't more helpful. I'll see if I can't find something I can legitimately send you.

[RHGraham]

I've often thought it might be nice to have a controll on the FB loop that would vary it in it's entirety, from "off" to "full", instead of just a switch, but have yet to give it much thought past it maybe being a cool thing.
Some amps I love with cathode bias and no FBL, some I like with both, it makes it hard to decide which way to go sometimes.

Would that require anything more than a pot inline with the fb loop?

[Firestorm]

I've often thought it might be nice to have a controll on the FB loop that would vary it in it's entirety, from "off" to "full", instead of just a switch, but have yet to give it much thought past it maybe being a cool thing.
Some amps I love with cathode bias and no FBL, some I like with both, it makes it hard to decide which way to go sometimes.

Would that require anything more than a pot inline with the fb loop?

I built one once with variable feedback -- you just replace the feedback resistor with a pot wired as a rheostat. I think I also did your feedback "off" idea too. To do that, you just open the pot and cut the resistive element with a sharp knife. On one side of the cut you have control over how big the feedback resistor is, but on the other side of the cut, you have no feedback at all.

The other interesting source I failed to mention in my original reply is Peter Baxandall. We usually think of Baxandall as a alternative type of tone stack, but the original circuit was intended to go in the feedback loop. Since the mid gain can be set flat, you can control how much gain the power amp has, but the high and low frequency filters let you adjust output treble and bass. That's probably the best way to do presence and "resonance" on the output side of an amp.

[Andy Le Blanc]

thats a neat thought.... Ive seen Baxandall tone controls that use local
feedback in a pre..... but never with global feedback...... a feedback control
that allows the user to adjust the amount is very usefull...... I've tried that too
it can be really subtle.....

[RHGraham]

thats a neat thought.... Ive seen Baxandall tone controls that use local
feedback in a pre..... but never with global feedback...... a feedback control
that allows the user to adjust the amount is very usefull...... I've tried that too
it can be really subtle.....

Hmm,you know, I find this all quite interesting... a couple of my favorite amps early on became favorites after cutting the NFL, then later putting switches on them, namely a blackface deluxe and a blackface super.
Both amps had very different but very yummy voices with the loops out.
Its a sound I like a lot sometimes. And sometimes I did like the twang with the loops in, so that's what always gave me the thought that it would be cool to have that on a controll. I suppose it would effect some amps more than others, but at least on those particular fenders, it was a very noticible difference in response, especially cranked up.

The bax info is very interesting as well, a presence and resonance together might be something that would turn my crank rather nicely.
At least to me it makes a fundamental sense to have controls like that in the power side, for amps that get a lot of thier fundamental tone from cranked power tubes. Like cathode/fixed bias switches in a way also.

I'm wanting simple anyway, I could see my favored front panel going "volume, tone, resonance, and presence"... and "power", the VVR, of course.

I think, in the end, the only real answer is to have one of everything... two of the ones that really rip...

[Alexo]

FWIW, I have an old LaFayette EL84 stereo with a bax. tone control in the feedback loop from the OT to the paraphase pi. It's really a nice sounding unit, and the EQ works great.

I did the variable NFB pot thing too in a Princeton clone, first with 6V6's and later with 5881's. I used a switched pot so it could zero out the nfb, at the other side of the pot was a series resistor of maybe 2K, so that I wouldn't affect the load on the OT by turning the pot too far.

It really worked quite differently with the two sets of tubes. With 6V6's, I would usually leave the nfb disconnected. This way, the amp gave a big, deep bluesy sound with a lot of character. With the nfb engaged, the amp would stiffen up and get much more hi-fi and flat. With 5881's, disengaging the nfb, the amp (which had a really bright speaker btw) became somewhat strident and a little harsh and aggressive, but dialing in some nfb sweetened the tone up nicely. In both cases, the nfb pot was very useful in tuning the amp to the room.

[jelle]

Check RDH4 for tonestacks in the global FB loops. There are examples there. Some use rotary switches and a bunch of caps to dump low freq NFB signal to ground instead of feeding it back in the PI. This releases lows from the global NFB and makes it louder. It can get flubby but if you use a resistor from the cap to ground to control the amount of release from NFB this should work.

Have fun and please let me know which one you decided upon!

jelle

[jelle]

Alexo,

do you have a schem of that EQ or a schem of the LaFayette EL84 amp?

Thanks,

jelle

[toneman]

Wasn't there an artical on this in the Feb 93 issue of guitar player magazine? A mod by John McIntyre's Bluesmaker for BF/SF Bassman head was in the same issue I think. 5K pot took the place of the neg feed back ressistor. Just thought I throw it out there .

[Cameron]

Wasn't there an artical on this in the Feb 93 issue of guitar player magazine? A mod by John McIntyre's Bluesmaker for BF/SF Bassman head was in the same issue I think. 5K pot took the place of the neg feed back ressistor. Just thought I throw it out there .

Do the same thing as the feedback control but use a 1M audio pot and put a .002 -.0047 cap over the 1m.

[LOUDthud]

Start with the feedback resistor that connects to the OT, call it Rf. Disconnect one end and insert a pot connected as a rheostat. The value of the pot between 5 and 10 times the resistance of Rf. Connect a cap across the pot. The value is roughly C=1/(6.28*Rf*600Hz). Adjust the value of the cap up or down as desired.

[drz400]

It is covered in the Radiotron designers handbook

Basically leave the feedback resistor in there, with a 100-47K feedback use a 1Meg pot with a 4n7 or 2n2 across it so when pot is counterclockwise the cap is shorted out, a little bit is nice but too much gets weird. The 5150III amp has 3 setting preset inside the amp with all the bass removed from the preamp, sounds weird to me, too much. Just a little is nice.