blue jr to scoop

[Andy Le Blanc]

I have a blues jr to scrap

The transformers are good, only 70 ma. to play with, but that'll be fine.

It'll be a simple head, el86 pre, 12au7 inverter. AC15 pre, AC 100 inverter.
getting away from the long tail cathode coupled tone.

The inverter is a variation of self balancing paraphase, you see them in few
other things like early fenders... deluxe 5D3. The fender example is missing
the coupling cap the feeds the inverting side, you can do this with 6v6 and 6l6
but not el84 or el34. A grid current sets up the plays havoc with the inverter.
One of the perils of assumption, just because its a fender doesn't mean it any good.

The ef86 has plenty of gain, even through a 12au7 the break up in the
power side is very nice. The draw back is the high impedance of the pre
limits the practical choices for tone control. But I've chosen a set from
early wards and supro, and bright control that seem to play well together.
the first swings between normal and brilliant and the second controls the top.
I've had success with this using ef86, 6sj7, 6au6. The component values
aren't fixed yet, feed back is implemented in the power side with a cathode
resistor in addition to the fixed bias suggested

I'm little bummed to be stuck with JJ tubes, I'd much rather use NOS 6bq5
but its only a beat little fender

but I do have one of those GT classic clone cc-12 things, should be interesting.

it be a little like cramming a JBL E-120 into a princeton, but with a VOX lisp.

[Andy Le Blanc]

chassis...

1/8" alum., I had a local shop fab it 20 years back

when the economy was good, before the jobs went all china..

50 bucks, bent and welded, funny what lays around

the thick aluminum actually makes DIY fab more harder, seems like good idea
but unless you can afford a CNC shop, its a pain

bits swim in it, there's di-electric issues, it holds heat like cook ware...

not too bad to cut logos out of

figured I'd use it up, You get faster cleaner results on flat stock 16ga mild steel

[M Fowler]

Andy,

On my Allen Hot Fudge with Nuts build I used a ebay chassis and that aluminum was damn hard to drill through but got it done.

Your use to drilling your's out flat and then folding, this should be fun to hear the report on

[Andy Le Blanc]

I've grown to prefer work in the flat

This is the soft stuff, gets the bits all gummy and all that.

In general... a pain in the arse... every step through fab takes more time

even with drill press, the drift in the material makes it difficult.

and theres a durability issue with dents, scratches and gouges too

I started out with aluminum, the whole marshal thing, but mild steel
is so much easier to fab DIY, less expensive and easier to source
easier to finish and paint with easy to get spray, just easier

And once you have a fabrication method that works, go for it...

The chassis is actually my old bolts and screws pan, cleaned that out
and was sick of looking at it any way...

[Andy Le Blanc]

sparkly...

why buy it when you can fab it....

[M Fowler]

Ah the engine turn effect, that looks good!

Hey the holes are drilled and cut! Does this mean the cussing is over?

Mark

[Andy Le Blanc]

Oh ya... fun couple days... had to get to town to get a 15/16 socket to
rotate the drill press table there. Then figure a poor bob to get the work
secured to it so I could get a little accuracy. Trying to get 5 axis plumb
on wobley floor is a hoot...just to get a few straight holes

Should go pretty fast, but I got 8 gigs in 10 days starting tomorrow.

I was thinking the jr. carcass would make a good wreck

And I've been thinking of may be using a deluxe opt..
or maybe one of the allen amps opt, hes got a nice jr. upgrade....

[Andy Le Blanc]

jeez... finally get a few moment to get after it

I figured I'd try a 6.6k opt, there's datasheets that suggest a predominance
of 2nd order harmonic distortion with a distributed load or ultra-linear at 6.6k

I was surprised by this, because most builds use cathode coupled long tail
inverters which minimize 2nd order products, I'm going with a paraphase
variation, which will color the tone, limit the bandwidth, and introduce phase
distortion, and there's only going to be local degenerative feed back
should let even order tendencies of the tubes go unchecked, get a lot of
harmonic distortion at the onset of clip.... real musical tone color

[Andy Le Blanc]

heaters .... OH boy....

I figured I'd give those big ac mp thing a whirl for bypass values around
a ef86 pentode pre, the values are about right for cathode and screen...

love scrap builds, mostly just bench leavings, pt, tubes and filter caps from the jr...