JMP Master Volume 6V6 20W Tone Controls - Odd

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Littlewyan
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by Littlewyan »

Nice layout and schematic! My amp is running on 390VDC so bascially the same as your's. Have you got any clips of your 6V6 plexi?

So to get my NFB on my 20W amp the same as a 100W 2203 I did the following. I calculated the open loop gain of both amps and worked out the resistors required for the closed loop gain to be the same on the 20W amp as the 100W amp. I can't remember the figures I got from it but basically 56K on the 8Ohm tap of my 20W Amp is the same as 100K on a 4ohm tap on the 100W 2203.

As we're just trying to match NFB levels I found an easier way to calculating it. On the 100W 2203, measure the voltage on the 4Ohm OT Tap while the amp is pushing out 100W of power. Then use a voltage divider formula to work out the amount of voltage fed back in your NFB network. For instance, a 100W 2203 has 20V on the 4Ohm tap. The voltage divider in your NFB network is 100K/5K and at the end of it you get 0.952V.

My 20W amp has 12.6V on the 8Ohm tap. The voltage divider in my NFB network is 56K/5K and at the end of I get 1.03V. So my 20W amp in it's current state basically has the same amount of NFB as a 100W 2203.

I checked this was correct against my open loop and closed loop gain calculations and it seems to be correct. I also measured the NFB on a TW Express and it's basically the same as my 20W amp yet the Presence Control on the Express is more effective. It's so bizarre. I know the presence works in my amp as I can see it working on a scope.
pdf64
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by pdf64 »

Littlewyan wrote: Sun Nov 05, 2017 11:59 pm...I found an easier way to calculating it. On the 100W 2203, measure the voltage on the 4Ohm OT Tap while the amp is pushing out 100W of power. Then use a voltage divider formula to work out the amount of voltage fed back in your NFB network. For instance, a 100W 2203 has 20V on the 4Ohm tap. The voltage divider in your NFB network is 100K/5K and at the end of it you get 0.952V.

My 20W amp has 12.6V on the 8Ohm tap. The voltage divider in my NFB network is 56K/5K and at the end of I get 1.03V. So my 20W amp in it's current state basically has the same amount of NFB as a 100W 2203...
I don't think that's the right way to approach, as it apparently ignores the actual power amp gain, basing the sample ratio on the max voltage output of the amp.
If things happened to work out using that method then I think it was just coincidence.

If the goal is to have the same degree of feedback on amp B as amp A, the simple way to do it is to measure the open and closed loop gains of the amp A, then adjust the feedback sample of amp B get the same open/closed ratio.
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Littlewyan
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by Littlewyan »

Ah I'm sorry I think I see now. So are we meant to be looking at the open loop/closed loop gain RATIO rather than just the amount of NFB itself?
pdf64
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by pdf64 »

Yes, the point is 'how much effect does the NFB have'?
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norburybrook
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by norburybrook »

sluckey wrote: Sun Nov 05, 2017 5:29 pm My B+ rail is straightforward. There's a 14H choke between the plate (400v) and screen (397v) nodes. All 32µF caps. 470Ω screen resistors. All iron came from a Hammond organ AO-43 reverb amp. I don't have a 50W Plexi but I do have the November amp from ax84.com. It's a Plexi preamp into a fixed bias EL84 power amp. Both amps have a Marshall growl that I like but I prefer the EL84 amp for this sound. Here's my schematic...

http://sluckeyamps.com/6v6plexi/6v6plexi.pdf
Steve,Don't want to hijack this thread but just wanted to say your layout's, schematics and templates are absolutely first class, thanks so much for sharing.



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sluckey
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by sluckey »

Thanks for the compliment! I have a few more projects/drawings on my website. May be something else you are interested in...

http://sluckeyamps.com/index.htm
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pompeiisneaks
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by pompeiisneaks »

sluckey wrote: Tue Nov 07, 2017 2:24 pm Thanks for the compliment! I have a few more projects/drawings on my website. May be something else you are interested in...

http://sluckeyamps.com/index.htm
Same hijack, as I've told you before, your layouts and schematics are outstanding. Thanks again for the help with the AC30/4! Still love that amp.

~Phil
tUber Nerd!
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Littlewyan
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by Littlewyan »

Ha, this is a lot of hijacking!

Thanks pdf64. I worked it out using a calculator on ampbooks.com and a few calculations in Merlin's book. I wasn't too far off but basically I needed a a 75K FB resistor on the 8Ohm Tap if I wanted to match a 50w Amp with 100K on 4Ohm and a 47K resistor if I want to match a 100W amp with 100K on 4Ohm. To match the TW Express' gain ratio I need a 33K resistor on the 8Ohm tap! This will be why the TW Express has a much more usable Presence Control than my amp.
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