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PCB for LFO & Bias (newbie questions)
#1
Old AX84 user here, just finding my way to this forum...

For my next project, I'd like to build a deluxe reverb amp -- mostly just AB763 except using bias-wiggle instead of the usual opto-trem. I'll use turret board construction for most of it, but for kicks I'd like to try a small PCB for the LFO and bias supply. For the LFO I'm thinking of using KG Keen's mosfet tremolo http://www.geofex.com/Article_Folders/ss...remolo.htm and the bias supply would be more or less what you'd expect.

Now I've never made a proper PCB before and I don't want to also dive into the whole home etching thing just for this one project, so I was thinking of using one of the services that does low volume prototypes catering to the DIY crowd.

My basic question is what design rules do I need to follow (I guess it's mostly just spacing?) for "tube amp" voltage levels? The LFO will take voltage from a B+ node and regulate it down to 200V for the oscillator. Is there any good intro/primer I should be reading for this stuff?

Also any suggestions regarding the PCB service? I'm not so worried about finding absolute cheapest or quickest, I more interesting in improving my odds of success since I'm new to this whole thing. I've been looking at the purple boards, but in truth that's mostly based on their web site looking warm and friendly. Should I be looking elsewhere?

Thanks!

Regards, David
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#2
(03-08-2020, 06:04 PM)David Staepelaere Wrote: Old AX84 user here, just finding my way to this forum...

For my next project, I'd like to build a deluxe reverb amp -- mostly just AB763 except using bias-wiggle instead of the usual opto-trem. I'll use turret board construction for most of it, but for kicks I'd like to try a small PCB for the LFO and bias supply. For the LFO I'm thinking of using KG Keen's mosfet tremolo http://www.geofex.com/Article_Folders/ss...remolo.htm and the bias supply would be more or less what you'd expect.

Now I've never made a proper PCB before and I don't want to also dive into the whole home etching thing just for this one project, so I was thinking of using one of the services that does low volume prototypes catering to the DIY crowd.

My basic question is what design rules do I need to follow (I guess it's mostly just spacing?) for "tube amp" voltage levels? The LFO will take voltage from a B+ node and regulate it down to 200V for the oscillator. Is there any good intro/primer I should be reading for this stuff?

Also any suggestions regarding the PCB service? I'm not so worried about finding absolute cheapest or quickest, I more interesting in improving my odds of success since I'm new to this whole thing. I've been looking at the purple boards, but in truth that's mostly based on their web site looking warm and friendly. Should I be looking elsewhere?

Thanks!

Regards, David

Having not used a PCB service before I can't comment on it really. But tremolo I have definitely done before. First thing I'd suggest since the circuit you linked to is based on the classic Fender circuit is that the circuit can be improved by adding in  a couple of extra .01uf caps parallel to the ones in the feedback network, switchable if desired, to give a slower speed range. If made switchable you will have a high/low speed range. A common complaint with the stock circuit is that it doesn't go slow enough.

Second suggestion would be to make that third MOSFET a source follower if your are going to go for bias-mod tremolo. A poke around some of the brownface era schematics will give you some ideas for how to paste it all together. If you are going fixed bias then your depth control could be a small (10k) pot right after the source follower, with a coupling cap connecting it to the bias supply. Use a fairly high value cap as you need to pass signal down to around 1hz, and place a fairly high resistance between the bias supply and LFO, otherwise your LFO will be in effect grounded at AC by the bias supply filter cap.

A neat idea I used a few builds ago on a cathode biased output stage was to "ground" a post phase inverter master volume through the tremolo depth pot. Best of all worlds solution, you get a master volume that includes PI distortion and that awesome bias mod tremolo.
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#3
To be honest, most of the cheapest (like JLPCB, PCBWAY, Itead etc) are all really fine.

For distance, google something about creepage distance.
There are even a bunch of online calculators;
http://www.creepage.com/
https://pcbdesign.smps.us/creepage.html

Unless you're going to work with very high voltages, I mostly just keep something like 2-5mm
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