fresh paint

[M Fowler]

Really kool Andy.

A white cliff jack

Is that a cap can stuffed into a Mallory jacket

Lots of room in that chassis.

Fine job.

[M Fowler]

dynaman,

I love those old cage type amps and you seem to fine plenty.

[Andy Le Blanc]

yes a white cliff jack, new this year maybe I was surprised too

no that's a Mallory PF, don't see those too often any more.

So.... shake down revealed an issue with the power side farting out.

I've be thru most of it and it doesn't appear to be the PS, the pre, caps, etc...

One of those where I'm running out of possibilities, tuff with a known circuit.

still could be the PT, OPT, sockets.... every now and again you get yourself good.

the hardest part is not being too pissed to work at it, most likely something
ridiculous.

cabinet day must wait

[Andy Le Blanc]

It was the bypass cap for the power tubes, not a bad cap but any cap.

Its EL84, around a 8.5k load plate to plate, thank you dartanion, And I'm running
the plate volts fairly low, but every thing is 5/5 with published RCA AB1
push pull with around 300v (335 in this case). Anybody got a good explanation?

I've read arguments for and against cathode bypass of power tubes but
mostly from a fidelity/distortion view point. I've not had the pleasure of
such a dramatic example of distortion caused by bypass.

Anybody else have an experience(s) with this?

I'd like to bypass, there is a sonic benefit.

[Andy Le Blanc]

interesting issue, appears to be related to a high transconductance.
I would suspect loading too, EL84 and EL34 share a high trancontuctance,
But .... most practical experience with EL34 used a load 50% smaller.
6v6 an 6l6 have transconductances 2 to 3 times less than EL84.

Can't wait to find out more. First time I've run into this in such a critical way.

[Andy Le Blanc]

The real issue is that I like paraphase and seesaw inverters and made
assumptions about their application. The high transconductance encouraged
just enough grid current to frig up the inverter bias and raise bloody hell.

I've been using a seesaw very similar to vox, ampeg, and assumed that
it would act like any other paraphase, same dress works fine for other tube types.
I finally found a N. Crowhurst reference on it. Still get to use a seesaw, just not that one.

[Wayne]

Glad you're nailing down the problem, Andy.

I like your "no-shit" approach to construction. It looks neat & tidy, while at the same time looking very handmade.

W

[Andy Le Blanc]

Appreciated the feed back....

I've been flat broke for so long I've had to unlearn fabrication.

[Andy Le Blanc]

Finished voicing... the EF86 has ample gain, making lower gain tubes for the
following stages accessible and very useful as tone sources.

This thing is based on a Vox AC 15 with the EF86 pre and EL84 power side.
The client like very much an earlier amp I made that had a paraphase inverter.
So, instead of the very common cathode coupled inverter like that in a AC30
I used the one from an AC100 keeping the Vox design theme.

Now this is where I tried to screw myself into a corner in the build.
The high transconductance and my design assumptions created a condition
that sounded like a good many things, with EL84 and a 8 to 10k load
a fixed and self balancing paraphase inverters, and seesaw inverters with
the inverting signal derived after the coupling caps might see some
power tube G1 grid current on the inverting stage grid if you don't use the
extra coupling cap. Its easy to make assumptions based on build successes.
A known circuit tried to bite back with a different tube type in the power side.

The tones are interesting, and cover two common VOX design features.
The bass control swing back and forth between a "normal" and "brilliant"
channel tone, and the treble covers the "brite" control territory fairly well.
Both are high impedance and play well in the EF86, keep the layout simple,
and more importantly don't carve out the gain structure.

The cathode bypass values are critical, small, 10 uf on the EF86 and ...
0.47 uf respecting the high tranconductance of the EL84.
This is the opposite approach of most current design trends, there just is
no real need to maximize gain with a EF86, the low end is tight enough
and the usable bandwidth is appropriate to the instrument, no farting out
and no trespassing on the bass player. The only feedback is degenerative
through the inverter cathode and from the inverter it self because the
inverted signal is differential in nature.

I'm kinda bummed that its going out the door, don't get that too often.
The clean tone color is unreal.

[dartanion]

Cool Andy! Glad you figured out the issue. How do you like the transformers?

[Andy Le Blanc]

The pt and opt are good, no issues, the opt is very good tone wise.
Thank you again. might need another set pretty quick.
Its nice to not have to make concessions with your chosen operating point.

[Andy Le Blanc]

I have other.... vices too...

[M Fowler]

Funny as I was looking at the picture I was thinking I got you beat I use more clamps than that on my cabs.

Then I read your post

[Andy Le Blanc]

glued and pegged... cheaper and stronger than mechanical fasteners.

Its very simple and looks great executed in hardwood.

pneumatic nailer's and hot glue are great for MDF and hardboard,
but are weighty compared to simple wood, and they don't hold with weather.

[Andy Le Blanc]

Roll the edges and corners with a router.

Clean and blend with the sander.

Gone over by hand for aesthetics and to blend any missed tooling marks

Almost ready for upholstery.