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Fender Tone-Master / Vibro-King / Dual Pro circuits

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Fender Tone-Master / Vibro-King / Dual Pro circuits

Postby bfields on Sat Oct 13, 2012 9:25 pm

I have a Dual Professional and a Tone-Master and think they are pretty great amps. Turret board, chassis mounted sockets, classic fender construction. To my ear the Dual Pro sounds like a mellow Twin and the Tone Master sounds like a loud, hard-driven, Bassman. What I don't understand are the circuits. On all these amps the tone controls act like frequency dependent master volumes. The preamp circuits look WAY more simple than a traditional Fender.

Can any of you tech folks tell me what Zinky was thinking when he designed these circuits? Can you think of any other amps that act like this? I mean, if you turn all the tone knobs down the amp is silent at any volume.

Here are schematic links for your convenience.

http://1.bp.blogspot.com/_mgaBQf-Ljq0/T ... deramp.jpg

http://www.blueguitar.org/new/schem/fender/tonemstr.jpg
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Re: Fender Tone-Master / Vibro-King / Dual Pro circuits

Postby treasuretech on Sat Oct 13, 2012 11:40 pm

They both use extremely simple passive RC networks immediately before the phase inversion stage. This is quite a common style of network in many Fender amps, the difference being the location in the chain, and the wiring of pots to allow full bypass of the tone stack to ground (i.e. no signal). It wouldn't make any sense to me wiring it this way, although it would give a much sharper roll-off to each section of the tonestack.

This is a page that I've found pretty handy in explaining tone stacks.

http://amps.zugster.net/articles/tone-stacks
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Re: Fender Tone-Master / Vibro-King / Dual Pro circuits

Postby projectMalamute on Sun Oct 14, 2012 9:54 am

I don't know anything about the circuits in question, but I used to play with a guy who had a Tonemaster and that amp was awesome. One of the best sounding guitar amps I have ever spent any amount of time with.
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Re: Fender Tone-Master / Vibro-King / Dual Pro circuits

Postby benadrian on Sun Oct 14, 2012 12:28 pm

Pretty much any passive tone controls are just frequency dependent volume circuits. Pretty much any blackface or silverface fender will loose all volume with the tone controls at zero.

The Dual Pro looks like a 100 watt tweed bassman with an outboard reverb unit built in, before the amp circuit. There are a few differences. There's no negative feed back in the power amp, which is cool for a dirty guitar amp. There's no cathode follower before the tone controls. The tone controls are the classic fender circuit with some slight changes, they're just drawn sideways. All in all, it looks like a great amp design; awesome reverb tank and a preamp/poweramp with all the bullshit removed.

The tone master is in the same boat. The "clean" channel is very similar to the Dual Pro. The distortion channel reroutes to a different signal path which sis a bit like a JMP MKII marshall preamp. In both cases the tone controls are classic fender tone stacks with slightly different values. The weird thing about the ToneMaster is that the effects loops are transformer coupled. This is cool because it will convert from preamp level to guitar level and back again while simultaneously avoiding ground loops. The bad part is that you can't bypass it if you're not going to use the effects loops, and also the transformer on the input might give problems if you wanted to hook a preamp level device into the amp to use as an alternate preamp.

All in all, though, both those amps look awesome on paper.
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Re: Fender Tone-Master / Vibro-King / Dual Pro circuits

Postby japmn on Sun Oct 14, 2012 1:39 pm

A friend of mine had/maybe still has a Vibro-King. It is one of the best sounding amps I have ever heard or played through. I have occasionally looked for them on Ebay used but they are really really expensive. I still want one pretty bad.
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Re: Fender Tone-Master / Vibro-King / Dual Pro circuits

Postby bfields on Thu Oct 25, 2012 1:49 am

Thanks for the feedback, Ben and others. I'm guessing that the lack of a cathode follower made the circuit look simple to me. I don't recall my blackface pro reverb being silent with the tone controls at zero. Then again, I may not have tried it.
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Re: Fender Tone-Master / Vibro-King / Dual Pro circuits

Postby benadrian on Thu Oct 25, 2012 2:23 am

bfields wrote:Thanks for the feedback, Ben and others. I'm guessing that the lack of a cathode follower made the circuit look simple to me. I don't recall my blackface pro reverb being silent with the tone controls at zero. Then again, I may not have tried it.


Blackface Pro Reverbs don't have a mid control. The have a fixed resistor that is equivalent to putting the mid at half. When you turn the Bass and Treble all the way down, the "mid" is still at half. However, on Twins and later Fenders with Mid controls, this indeed does happen.
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