First build!

[EtherealWidow]

I'm now able to give back at least a little to TAG. Thanks to everyone for your immense knowledge that I've been able to glean enough understanding to an amp I feel is at least a little worthy of playing. For now is just a very long modification log. I bounced around a LOT on this amp trying to get it where I wanted it to. Still an "interesting" sounding amp. Definitely not like what's commercially sold (at least not in my experience). Not too bad though. I'll try to update this thread in a timely manner to include a schematic, gutshot, and sound clips.

Started with High Octane circuit with the following mods:

-10k/500pF 1st stage input load (instead of typical 1M)
-500pF 1st stage snubber
-2 x .68uF || 1st stage Ck
-.0033uF peaker cap
-47k CF Rk
-.0047uF filter cap bypass
-680pF treble tone cap
-47k slope resistor

Amp sounds like I turned my tone knob all the way down

-Changed input load to stock 1M
-Removed snubber

Better. Very "sweet" sounding amp. A little tubby.

-Removed one of the .68uF Ck's on the first stage

Better. Very Santana. More gain, please.

-Changed 1st stage Ra to 220k

Much better. Very rich. Good amount of gain. Would like more bite.

-Added 1.5k Rk to 2nd stage for total Rk of 666R

Somewhat Marshall. Rich, still sweet. Was trying to get more odd harmonics by warm biasing the stage. Might have worked, but probably also added a lot of even harmonics due to 1st stage being cold biased (clipping only one side of wave) Want more bite still.

-Changed peaker cap to stock .001uF

This thing has a lot of bass surprisingly. Sounds better with Mesa tubes rather than Gulbransen ones. (Too bloated) A little ratty on the high end. Might just be the low volumes I'm playing at. Stiff amp. (Go figure, it's SE)

-Added 1M local feedback resistor on 2nd stage

Might be useful on a switch for a clean tone with the master all the way up and gains to taste. Takes away a ton of preamp gain. Reverted back to "normal".

-Changed 1st stage Ck to 60nF

Much better frequency response. Bass has kind of fuzzy texture by nature. Not sure what to do about it.

-Changed 2nd stage Ra to 220k and Ck to .1uF

Horribly unstable at higher gain levels, particularly with gain 2 (gain 1 set to moderate and master up high.) Sounds like a bad fuzz pedal and oscillates depending on signal level and playing dynamics. Still very bassy. Big volume change when backing off guitar volume knob a little bit, but then evens out. Mid pot doesn't seem to do much.

-Changed 2nd stage Ra to 68k, Rk to 6.8k, and Ck to .01uF.

Much better. More clarity and stability. Not much distortion, but a lot of chime. Much quieter hum.

-Removed grid leak on 3rd stage voltage divider leaving only 220k grid stopper.

Better, but still not enough bite. Enjoying the medium gain and clarity. Still a lot of bass, but that's okay. Mid pot sounds better over the entire range now.

-Added 100k || Ra to 2nd stage for total Ra of 40k

Neat, but too clean. Nice jangle.

-Changed 2nd stage Ra to 100k

Better. Need more distortion.

-Changed bass cap in tone stack to 2nF

Much better. Don't have to dime treble and mids to get a decent sound. Can also turn up bass without it flubbing out. It's the lower mids that have the weird fuzz pedal quality, I think.

-Put 220k resistor between gain pot 1 and ground.

Got more gain. Sounds like a decent amp. This amp has surprisingly good clean tone. I like it.

-Put another 220k resistor on gain pot 2.

Got a little more gain. I think I'm done modding for now. Thing sounds decent enough.

-Changed 220k grid stopper to 1M

Mistakenly thought it was a grid leak. Wanted more gain. Oops. Changed back to 220k

-Disconnected Ck and added NFB w/ 15k resistor from 8R tap to 3rd stage.

Awesome. NFB does what it's supposed to. A little more percussive, but the mids aren't as complex. Still like it, though. Not a bad amp overall. Would like more distortion, but I'm not too upset. Takes a compressor pedal extremely well and gets great, greasy distortion. Love it a lot. Almost like a small Fender Supersonic 22.

-Removed NFB, added 4.7uF Ck on 3rd stage, changed EL84 Ck to 25uF

Obviously more gain. Raw and rich, but oscillates at higher gain. Oscillation pitch gets lower with more gain.

-Turns out I have a UL tap on the OT. Bypassed screen resistor and attached UL

Oscillation stopped.

[matt h]

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[EtherealWidow]

Thanks man! Definitely a huge learning process. Taught me that you definitely can't judge an amp's sound just from looking at the schematic. I really do dig the UL. I actually tried it after stumbling upon your old thread again. I do like the global NFB though. There are a lot of good options for obtaining a great sound for sure. I'll see if I can further document this thing later this week. Just a wee bit slammed at the moment.

[matt h]

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[EtherealWidow]

I'm actually planning on doing another PP build based off this one with a 330V CT PT and experimenting with that one quite a bit. Definitely considered bootstrapping the CF and also messing with the value of Rk. This one's a pain to work on, though. Rk values were chosen for the first two stages in large part for the frequency response. I like a lot of chord note separation and articulation. I don't need all that much bass.

[EtherealWidow]

Alright. So here's the guts and schem. The resolution doesn't need to be any higher on the guts. It only gets uglier.

https://www.dropbox.com/s/qqiw0nwq1y5n4 ... s.jpg?dl=0

The schematic is tidy, though.

https://www.dropbox.com/s/tqumgl7v212f8 ... m.jpg?dl=0