Need Shamanistic Guidence From The Chart Guys

[rp]

Not to copy... only to marvel. I have been doing a lot of p-t-p on some mic preamp prototyping and would love to incorporate a few of the fine points...

https://goo.gl/photos/cxkvSPPVUWkZS9sk7

Copy all you want, I do Yippie values, "everything for everybody" plus I'm getting too old and weary for the I, me, me, mine thing. Don't want to go out like a Pharaoh: take all I gathered up and bury me with all my shit so I can wind up in a glass museum case 3000/ys later with snotty grade school kids starring dumb-assed at me.

Might want to copy this: 2XEL34 / 250Rk / ~400Vp / 6.6K big iron OT sounds just killer to me, I stumbled upon it, but found some confirmation over on Hoffman by PRR there. I mismatched the secs to 3.3K and no comparison, 6.6K much better. The Edcor is a little weird (too revealing) but growing on me, bell like piano tones are pretty nice.

Martin, one more thanks, amp really opened up with that little tweak, much more 3D and in the pocket. Nice finally.

[martin manning]

[Might want to copy this: 2XEL34 / 250Rk / ~400Vp / 6.6K big iron OT sounds just killer to me, I stumbled upon it, but found some confirmation over on Hoffman by PRR there.

Sounds like a cathode biased Express power amp to me.

Martin, one more thanks, amp really opened up with that little tweak, much more 3D and in the pocket. Nice finally.

I'd never thought much about the early Fender inputs, so that was fun. It was you who found the right Rk by listening, but it's nice that the theory backs it up too. The 5C8 didn't separate the inputs very well. 5D8 and 5E8 fixed that, as you pointed out, but the AC load is still pretty heavy at about 69k ohms.

[martin manning]

I'll likely wire the inputs to single and parallel.

If you do this, see what a 2k7 Rk in place of the 1k5 sounds like. Maybe you can find a way to switch it with the input jack...

[rp]

The 5C8 didn't separate the inputs very well. 5D8 and 5E8 fixed that

Oh, now I understand the mixers on the 5D8. Looks odd on the schematic, I saw it was a mixer pair but didn't really get what was up. Had I gotten this sooner (as well as foreseen the switch to 6SL7s) I would have used a bigger terminal strip and split the plates and avoided all this. But my goal was to clone not improve.

If you do this, see what a 2k7 Rk in place of the 1k5 sounds like. Maybe you can find a way to switch it with the input jack...

Now you're making trouble. Mods on pause for a while, I'm pretty happy I got this amp modern quiet and sounding nice. However, now that the trouble is done, it looks easy-peasy. I won't do k-ugly, but I have a free lug right where it should be - destiny?

I could do 2.7k or 3.3K (or hunt down 2.84K metal films) in parallel, one to the lug, lug to the ground switch on one of the input jacks (gotta think through which, could do either make or break too) and I'm cooking with gas. The jacks have home runs to the bus, and it's all close by, easy to test with a alligator jumper. I have shielded dual 22awg if I want to finalized it and make it nice.

Btwn: 2.7K & 1.35K or 3.3K & 1.65K which is best (or splitting hairs?)

[martin manning]

I totally get the clone idea. You have to hear it as it was first.

2k7 paralleled with a 3k3 gives 1.49, eh? Could you arrange the jumped-input jack to disconnect the 3k3? Or the non-jumped one to connect it?
I would try it with a jumper to see if it's worth the effort. That would be interesting in itself.

[rp]

I would try it with a jumper to see if it's worth the effort. That would be interesting in itself.

I first need to redo the jacks to get rid of the low 68k/68k set up, the // inputs with a Y-jumper sounded dark, so test that first. I'm pretty sure I can use the shield switch on either of the two Cliff jacks to work open or closed, in or out, for any two parallel resistor combinations. I could think it carefully through but inputs make my brain hurt.

Pretty cool if fussy occult pre-amp bias switching trick. Anyone looking in this thing some day, along with the LED, tubes swaps, raised heater, resister/cap tweaks, will think I'm either a super genius or a persnickety nut case. Both good, I should try it just for that.

My resistance is the plastic Cliffs don't like to be over worked - k-ugly.

[rp]

Couldn't leave you hanging Martin, and man do I ever have a soldering jones.

So?

Fantastic! Brilliant! First I tested the single/parallel jacks with only the 1500R to keep any problems limited. The parallel jack sounded big and FAT and punchy. Very nice. Fun.

Then I did the 2.7K/3.3K switch, checked that the resistance change was working, plugged in tube, powered up - and slamming! Noticeably better, not so much a bass or treble or a tone thing, best I can say is with your calculations the amp's just in the pocket whether single @1500R or parallel @2700R. With that tubed biased this way the amp just has that extra mojo, bit fuller, more vim, draws you in.

It's late must stop, test more tomorrow and replace the jumper. Again, thanks, great guidance. Who knew a tiny bias tweak on a preamp tube could matter so much. Maybe it's the clarity of the Edcor, maybe all the patient tweaking I did on this amp helped improve my ear. Wish I was better at tube math.

Was gonna send some very warranted jaguar pelts to go along with the quetzel headdress but the dead pelts freaked me out, even virtual, so here's a jaguar cub - and Screaming Jay Hawkins with Henry to inspire further amp voodoo for everyone.

FYI single: 1450R/80Vp/1.0V - parallel: 2590R 109Vp 1.5V - 248 node.

[martin manning]

Well great! What did you come up with for the input grid resistors and the jack wiring arrangement?

That's a cute kitty. Spotted cats are cool.

[rp]

What did you come up with for the input grid resistors and the jack wiring arrangement?

Updated (again ) the schematic on pg 1. Did the best I could with the inputs, think I drew it right, right enough anyway.

This wasn't easy. At least no extra noise, no bugaboos, no frying bacon, no melted parts.

I hate bells and whistles in amps, this is bad-ass as it's a "silent process" invisible and automatic, and old school no relay. I feel advanced.

[martin manning]

I don't see the switching for Rk and the single-parallel inputs. I made a sketch and will post it later.

[rp]

Check the first photo again. It works, so it must be right

[martin manning]

I think I see something like the circuit shown below. I don't think you want to keep both 1M to ground. One will do, and the input impedance will be 1M either way. You can just delete the upper one, I believe. I like the simple elegant solution too. Dare I say it's Apple-like?

Here you can get all the theory you need to become an expert at plotting preamp tube load lines: http://www.valvewizard.co.uk/gainstage.html

[rp]

Kudos, Martin, I added into my schematic as a detail, plus it’s now in TAG, maybe someday it’ll help someone though this odd bias issue should be rather rare. But the internet is forever, one day someone will run into this.

Final thoughts, to me the paralleled input sounds much better than the Fender version that has the second gain stage as a paralleled 6SC7. I also had V3 paralleled plus V2 (6SC7) set up w/ single/parallel and that was way too much for this old circuit. V3 paralleled seemed to exacerbate the negatives, while the present set up w/ 6J5 plus the 6SL7 seems to ameliorate the negatives and the paralleled input actually fattens up and smooths out the brash treble a bit.

Seems to me these really old octal tweeds (and Valco, Supros, etc) were really all in one PA amps and didn’t have the musician input that helped develop the later “tone machine” amps that they would become. Early tweeds sound really good clean - open, immediate, and wide bandwidth, but their distortion is what it is - a found sound. Good and cool in its own right, but if you are looking for compressed smooth ‘60s rock distortion better stick to late ‘50s circuits.

I’m running it through a vintage C15N and it’s really nice now - loves the fat parallel input and loves EL34s, punchy as heck. I’ve been using a Leo era G&L, the ceramic pickups can be a bit brash full up, turning them down a bit smooths them out (as Leo intended) and brings the Edcor more into line. I’ll have to try and find a proper Strat and Gibby to try through it. Wish I was in NYC w/ some access to “real” guitars.

I actually read that valvewiz a long time ago, I sort of skipped the graphs - guess that’s why I didn’t learn anything - can't skip over the pictures in engineering. I will try again.

Maybe someone should do an Uncle Doug type load line YouTube lesson??? Got a crotchety old dog?

[martin manning]

Sorry, had to correct the value of the 2k7 (was 2k5).