Re: My amp project continued - the story of "Cat Country" (warning: amplifier content! naked amp photos!)
- From: "Elvis Kabong" <ampscience@xxxxxxxxxxxx>
- Date: Wed, 22 Oct 2008 18:43:52 -0500
<morris.slutsky@xxxxxxxxx> wrote in message
If I may clutter up the place with some amplifier content:
First of all, the case is all drilled and partially assembled. The
four EL84 power tubes are working, as is one 12AX7 tube as a driver -
first section being a gain stage, second section a split-load phase
inverter. Schematics when I get around to drawing them, hopefully
soon. Although there will be more preamp stages and tone controls,
right now it's just got the guitar input into the driver for testing.
Photo diary - regrettably, cell phone camera pics only, nothing hirez:
Anyway, I have been suffering major RFI with this project. It's not
just this thing - my solid-state peavey is unhappy as well, and any
attempt to use the single-coil pickup on my Ibanez produces a swarm of
white noise, humbuckers only! I basically inhabit an incredibly
hostile electromagnetic environment. Not only is my apartment in an
old house, built 1830, possibly wired by Ben Franklin himself, many of
the outlets having open grounds and the ones that do have ground
simply connecting the neutral and ground, no actual ground conductor
in the place - but there's also an FM transmitter 500 yards from my
apartment. "Cat Country", it's called, plays all kinds of country
music. The combination of bad wiring and nearby radio transmitter
makes things . . . interesting.
As recommended by many knowledgeable experts, I built this amp with
star grounding. I intend to keep to this scheme as I assemble the
remaining stages, a 12AX7 preamp tube, a tone stack, and a 12AU7 FX
loop driver/return. Of course I purchased insulated input jacks in
order to do this correctly.
Anyway, the amp was playing "Cat Country" with surprising volume
(several millivolts speaker output) and rather good fidelity. I'm
amazed that it could demodulate FM in the first place, although it
could of course be some small residual AM modulation from the
transmitter that is actually being picked up. It plays "Cat Country"
even with nothing plugged in, the aluminum case all put together
including the bottom, and a steel cage set on top of the tubes and
transformers. How does "Cat Country" get into there?
Of course I suspected the power line. I even built a cheesy homemade
adaptor with 1.5 kV 20 nF bypass caps across the neutral to ground,
then I added one from hot to ground - I call this adaptor "El Suicido"
for it's obvious stupidity. That didn't really help, though. I even
tried, as a last experiment, warming up the amp and throwing it out of
standby and then yanking out the power cord real fast while listening
to hear if "Cat Country" continues for a split second after unplugging
before the tubes cool and the caps drain. Can't really be sure, but I
swear it continued to play "Cat Country".
What ended up working was replacing the insulated input jack with one
that shorted to the case, figured this out by shorting around with a
screwdriver. I really don't know why this would help, but it does.
Yeah, there are still some high-pitched squeals and the dim murmur of
Cat Country underneath. It's probably in the nanowatt range of actual
speaker output, but hearing is logarithmic after all. Even the
original "loud" interference was about 65 dB below full output, but
that's still annoying, especially if you don't plan to play it at full
volume all the time!
How can FM get through metal cases and cages? I don't know. How can
it worm it's way through the power line without being grounded out by
my El Suicido adapter or the transformer primary itself? Who knows?
I do have a wacky hypothesis though. I suspect that perhaps choosing
aluminum for a case wasn't the absolute wonderful thing I thought it
would be. True, it's very easy to work with hand tools, even for a
clumsy oaf like myself. Also, it's very conductive, the 12x8x3" case
only has about 1/100th of an ohm from corner to corner. 60 Hz hum is
not a problem - the case keeps out what might come in, the supply is
well filtered, and that is a good thing overall. However, this highly
conductive case might well be acting - I know this is crazy - as a
waveguide for VHF waves. I suspect that there are actually standing
waves of VHF throughout my aluminum chassis.
Is this the weirdest idea ever? And if so, what do I do about it?
I intend to start snapping ferrite chokes all over the place to see
what happens. Or try ludicrous things - like strapping shielded
cables from one side of the chassis to the other.
Maybe I should try to call up the engineer at "Cat Country", the guy
who handles THEIR tubes. He might know what to do. I'm not blaming
them entirely - I suspect my apartment was wired by the Mayflower's
electrician anyway - the combination is quite problematic.
Ideas? Hope you guys enjoy the photos. There's a power supply
schematic in there, I think. Gotta draw the rest as I built it,
You need to build a Faraday Cage around the room you
plan to play in, maybe. Or you need to experiement with
ferrite beads at the input as well as economize your wiring
scheme like in whatever's going on in the bottom right
of picture #17.
Presently, I'm having a weird oscillation problem even though
I thought that I thought out the complete scheme of things
and it's getting to the point of frustration which means I might
have to redesign preamp layout as well as test brand new
filter caps for defects. What a PITA!
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