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Old 16th January 2018, 05:52 PM   #261
mjd_tech is offline mjd_tech  United States
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Join Date: Apr 2002
Location: Boston, MA
Quote:
Originally Posted by Gnobuddy View Post
This is the route I'd probably take:

1) 12V, 2A switching power supply:
2) 45V - 390V DC-DC boost converter for the high voltage:
3) 3.3V, 5W switching power supply for the -3.3V rail:
I like that idea.
If things work out, I will likely go with switching supplies and a suitable enclosure.
The Hammond iron is way too big and heavy for the finished product.
Right now I just want to get a power supply going for prototyping purposes and I already have the needed components on hand.

Quote:
Originally Posted by Gnobuddy View Post
I like voltage doublers, they are a great solution for relatively lightly loaded supply rails.
Ok that's what I'll do, use a voltage doubler on the 6.3v filament winding.
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Old 16th January 2018, 09:24 PM   #262
Gnobuddy is offline Gnobuddy  Canada
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Join Date: Mar 2016
Quote:
Originally Posted by mjd_tech View Post
However, the eyelet board of 5F6 and 5F6A is identical.
The tail resistor was a convenient place to attach the NFB, having rejected the idea of going to the bottom of the tone stack.
It is a very minor change to the layout, almost unnoticeable unless you're really looking for it.
So that would support Merlin Blencowe's theory.
Nice detective work! I think you've nailed it.

Quote:
Originally Posted by mjd_tech View Post
But now, one wonders what was wrong with the 5F6?
Was it oscillating?
<snip>
5F6: 1k5 grid stoppers, 5F6A: no grid stoppers
<snip>
...Also possible weird interactions based on tone control settings. This may have caused oscillations.
Again, I think you've nailed it. I think Blencowe actually mentioned some of these issues in his book (tone controls in the feedback network, added grid stoppers, presumably in an attempt to combat instability.)

Quote:
Originally Posted by mjd_tech View Post
So I just play with the feedback resistor until I get about 10db NFB, and call it a day.
Some things are awkward to calculate, and quick to do on the test-bench. Other things are the opposite. To me, it makes sense to use whatever method works best.

I once worked on a motional-feedback woofer project, with the feedback voltage coming from a piezo disc mounted to the speaker voice coil. I really had no idea how much signal to expect from the piezo, or how to calculate it, if that was even possible.

So I did the same thing as you, first an open-loop frequency response sweep, then carefully dial in negative feedback until the gain drops by the appropriate amount.

Quote:
Originally Posted by mjd_tech View Post
I will say that when you wire it "correctly" and stare at your schematic, it's a lot easier for your brain to figure out what is going on.
I think you just put your finger on a key point: people who read schematics won't like the weird Fender feedback. But people who read physical layouts (and usually aren't comfortable with schematics) won't care.

I was pretty surprised some years ago when I found out that a large proportion of the DIY guitar amp building community don't actually read schematics, and work directly from layouts instead. Same thing for DIY stomp-boxes.

-Gnobuddy
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Old 17th January 2018, 02:32 AM   #263
mjd_tech is offline mjd_tech  United States
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Join Date: Apr 2002
Location: Boston, MA
Quote:
Originally Posted by Gnobuddy View Post
I think you just put your finger on a key point: people who read schematics won't like the weird Fender feedback. But people who read physical layouts (and usually aren't comfortable with schematics) won't care.
That's a great observation.

And it may have something to do with the 5F6.

I took an additional look at the changes in NFB between 5F6 and 5F6A, but from the layout point of view, rather than the schematic.

Here is the 5F6 layout with the 5F6A mods in red:
Tube Emulation &amp; EQ-5f6-png

When you look at it on the layout, it kind of looks like you are doing the "right" thing, as in "we need to put the NFB to the unused input of the LPT"

It is entirely possible Leo was indeed intending to do it the "right" way.

To "complete" the mod, he needed to lift the 10k tail resistor and run it to ground, but there was no extra eyelet to do so.
There was no "visual cue" that anything further needed to be done.

It's easy to overlook the 10k tail resistor in this situation.
I can totally see how that could happen. You are focused on getting the NFB to the right place, the tail resistor is not of immediate concern.
You mod a little, test a little, mod, test, and stop when it works.

Heck, I've done stuff like that a bunch of times, although my errors usually end in a puff of smoke rather than the most iconic guitar amp of all time.

Here is the 5F6A circuit with the "completed" mod shown in red.
This is the "correct" way to do NFB to a LTP
Tube Emulation &amp; EQ-5f6a-png

The 5F6A method worked good, looked good on the eyelet board and Leo may have thought it was the "right" thing, because it kind of looked like the right thing, at least on the eyelet board.

The new schematic was drafted after the mod was performed, and Leo may have said
"Is that what we came up with? Well that's odd, but it fixed the problem. I'm done fooling around with phase splitters, this is what we are using from now on."
Attached Images
File Type: png 5f6.png (123.3 KB, 22 views)
File Type: png 5f6a.png (118.3 KB, 22 views)
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