Cool Bandmaster Reverb mod - negative feedback

Cool Bandmaster Reverb mod - negative feedback

Postby ricepr » Wed Jan 23, 2013 12:14 pm

I have always loved aspects of Vox tone, that jangly thing. They use EL tubes and different tone stack, but the jangle seems to come from not having much or any negative feedback on the power side. A friend of mine used the old ground lift switch so I can have basically 0, 50%, 100% (of Fenders normal amount) of negative feedback. Less negative feedback, more jangle and volume. I wonder if this is part of the attraction for a class A McIntosh over an A/B Twin power amp, besides just having more headroom? This is much closer to "that" type of tone than a normal Fender amp.

I have a hard time playing it at the "normal" setting, sounds constipated! I had to sell the amp I loved, a Carr VIncent, it was two 6L6's, class A, no feedback, and was just killer.....

Anyone else mod their Fenderoid in this way?
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Re: Cool Bandmaster Reverb mod - negative feedback

Postby Smolder » Wed Jan 23, 2013 12:25 pm

I've put both a pot and a multi selector switch to vary the NFL. It's pretty cool... completely removing the NFL will get you towards a tweed tone as will lifting the tone stack (replacing the 10k mid pot with a 250k pot will do this).
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Re: Cool Bandmaster Reverb mod - negative feedback

Postby mijknahs » Wed Jan 23, 2013 1:40 pm

Which McIntosh amps are class A? I believe they are all class A/B push-pull power amps (both the tube and the solid state versions).
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Re: Cool Bandmaster Reverb mod - negative feedback

Postby ricepr » Wed Jan 23, 2013 1:45 pm

I may be wrong about the McIntosh

Its the negative feedback loop or absence im wondering about. That seems to be a big chunk of vox vs fender tones to me
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Re: Cool Bandmaster Reverb mod - negative feedback

Postby TI4-1009 » Wed Jan 23, 2013 2:26 pm

I just built a Champ/Deluxe amp that has a potentiometer to allow you to control the amount of negative feedback:

"The Cut knob on the Tweed Royal is similar to a Presence knob. It
controls the amount of negative feedback from output transformer. But wait a
minute! Neither the Champ nor the Deluxe had a Cut or Presence knob,
right? Right. But what you might not know is that the Champ did have a
negative feedback loop. It was just hardwired with a single resistor so you had
no control over it. We added this negative feedback resistance, but we used
a potentiometer instead so you can dial in exact Champ tones or other varying
degrees of cut. And just like the tone knob, when you turn the cut knob full
turn counter-clockwise, it removes it from the circuit completely and restores it
to exact 5E3 Deluxe specs."

The amp is done, the cab should be here next week so I can try it out. Weber VST speaker! Chomp, chomp!

http://www.buildyourownclone.com/tweedroyal.html
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Re: Cool Bandmaster Reverb mod - negative feedback

Postby mijknahs » Wed Jan 23, 2013 2:37 pm

ricepr wrote:I may be wrong about the McIntosh

Its the negative feedback loop or absence im wondering about. That seems to be a big chunk of vox vs fender tones to me


McIntosh amps have a non-adjustable negative feedback loop.
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Re: Cool Bandmaster Reverb mod - negative feedback

Postby zambiland » Sat Jan 26, 2013 8:16 pm

mijknahs wrote:
ricepr wrote:I may be wrong about the McIntosh

Its the negative feedback loop or absence im wondering about. That seems to be a big chunk of vox vs fender tones to me


McIntosh amps have a non-adjustable negative feedback loop.


This is true, but I did have a long conversation with Ron Wickersham about 20 years ago about negative feedback loops in Mac amps (among other things) and he said they used to remove them in the Mac tube amps.

I've converted my '67 Bandmaster to a souped up single channel amp (I paid $25 for it 20 years ago and it was almost fried, so it became my vehicle for learning about Fender amps. Sadly, my first project was a kit from Torres Engineering. Bad scene that did not end well. However, that's neither here nor there). One of the mods was to create a Marshall style presence control out of the Normal channel Treble knob that essentially controls the level of negative feedback. It worked out quite well.

As far as I know, McIntosh never made a class A amp. At the very least, none of the amps that are of consideration for this forum are class A.
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Re: Cool Bandmaster Reverb mod - negative feedback

Postby Jon S. » Sun Jan 27, 2013 7:53 am

Fender already builds an amp close to what you're describing with no negative feedback. It's called the Custom Vibrolux Reverb. http://www.fender.com/series/pro-tube/c ... ux-reverb/ I've played several. The price of having no negative feedback is they're noisy - hissing and humming. http://www.thegearpage.net/board/showth ... p?t=587281 For this reason, personally, I don't like blackface amps with no negative feedback, at least at stage volumes. For bedroom practicing, I think it's great to be able to disconnect the negative feedback - it totally thickens the tone at low volumes - which is why I modded my Reverend Hellhound with an on-off minitoggle for the negative feedback. I'm also a huge fan of negative feedback-based presence controls so you can reduce the amount of negative feedback when you want to without cutting it out entirely. Below is my Pete Cage-built tweed tone stack/brownface power section one-off 1X12 combo. I had Pete build it with a presence control that is super effective for this purpose. Dig that Scumback H75HP speaker - unbelievable tone! (In the second thread I posted to, you'll note some other guy worked also with Pete Cage to ADD negative feedback to his own Custom Vibrolux Reverb.)

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