Local feedback tone control ???

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Local feedback tone control ???

Postby ioginy » Tue Nov 05, 2013 10:33 pm

I ran into a bit of an issue with an amp I was building with the tone stack(s). It is a super single ended amp using a 6V6 and an EL84. I had a brain wave that I could almost separate the two tubes before they hit the OPT and use them as individual amps. One being a Princeton 5C2 and the other a Vox AC4. The theory was sound and the product ended up being quite interesting, though I still have some tweaking to do to get the bias matched on the tubes so one doesn't over power the other. The curious thing though is that the tone stacks for either tube effects the other as well. Each stage has it's own coupling cap off the input tube and the rest is basically to spec before they join again at their plates and the OPT.

After this I decided that perhaps a single tone stack might be in order and to scrap the separate/together amp thing. I stumbled across this little gem and though ti was rather interesting.


I found it here : http://amps.zugster.net/articles/tone-stacks

The thing that really interested me was Adams mention of a fellow by the name of Sean K that used a rotary switch to change out the tone caps. Having two holes from the previous setup struck me as a perfect use. One for the rotary switch and the other for the tone pot.

Here's the rub. I am using an EF86 input tube and have found absolutely no information on local feedback using pentodes. I assume that the setup would be very similar to the triode used by Garnet but thought it best to check with the folks here to make sure. I wasn't sure if there was anything I should pay specific attention to as far as the bias of the tube or the value of the pot etc.
Also, I was wondering which cap I would adjust with the rotary switch. The 1nf or the 1 uf. The Gibson GA30-RVT uses the exact same type of control for it's treble control and uses a .2 uf and a .02 us cap. If it's required to change them both together, it won't be a big deal, but I thought I would also check that out before I start chopping caps into this circuit.

Thanks a heap,
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Re: Local feedback tone control ???

Postby Geek » Wed Nov 06, 2013 2:27 am

Yep, that'll make something of a shelving bass boost/cut control. Designers of old used a lower-mu tube (like a 12AU7) and equal resistors cathode and anode.

If you add an inductor in series with the wiper to ground, you have one band of an EQ ;)

-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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