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-----Original Message-----
From: ham_amplifiers@... [mailto:ham_amplifiers@...]
On Behalf Of craxd
Sent: Tuesday, December 19, 2006 2:53 PM
To: ham_amplifiers@...
Subject: [ham_amplifiers] Re: Grounded Screen Configuration

The way they're building most amps these days is to use a clam shell
type cabinet made of aluminum or like to C's placed together at 90
degree angles. The actual tube chassis is a box made with aluminum
just large enough to fit the tube sockets in the top of it. This then
is placed inside, generally at the rear of the cabinet. I would
rather have the top of this box-tube chassis made with either copper
or steel where I can solder the bypass caps directly to it or any
other ground connection. I've used steel in a lot of amps, and
personally I think it takes a bad rap over other probelms that are
not its fault. Of course copper would be the best, however there's
not enough steel here to hurt anything, and the rest of the cabinet
is aluminum anyhow. Aluminum is supposed to be better at shielding,
but I'm not convinced that is is that much better than steel. The
only difference would be the resistivity of the material, but the
area is so much that this would be really low. I've seen some big
amps built on steel chassis that were clean and shielded well without
spuriuos radiation problems. The major advantage to aluminum is that
it's a lot easier to work with and is why it's prefered in my
opinion. For years before aluminum was used, steel chassis were used
with very good results.

Best,

Will


--- In ham_amplifiers@..., "Peter Voelpel" <df3kv@...>
wrote:

Hi Will,

I wrote that the cathode is floating below ground potential.
Steel chassis I would never use in a high power amplifier at

places where

RF flows and no soldering at all.
Just copper and nuts+bolts, some times silver solder at coils.

73
Peter

_____

From: ham_amplifiers@...

[mailto:ham_amplifiers@...]

On Behalf Of craxd
Sent: Dienstag, 19. Dezember 2006 19:47
To: ham_amplifiers@...
Subject: [ham_amplifiers] Re: Grounded Screen Configuration



--- In ham_amplifiers@ <mailto:ham_amplifiers%40yahoogroups.com>
yahoogroups.com, "Peter Voelpel" <df3kv@>
wrote:

Hi Will,

All electrode voltages of a tube are referenced to the cathode.

I know this, see my last post.

If the cathode is directly grounded at the socket,

Grounded to chassis ground with the screen? The only way I can see
doing it is floating the cathode from chassis ground where the

screen

is tied to. There's no way to have a voltage difference if the
cathode and screen is tied together on the same ground. In your

case,

you would have a difference of 800 Vdc between the cathode and
chassis ground. In otherwords, your B- lead is isolated from

chassis

ground and tied to the cathode.

screen voltage is
supplied between cathode and ground (negative at the cathode).
Then normal screen voltage is supplied, you have a higher anode

voltage as

both voltage are in series and you don?t have the problem
with screen bypassing and an extremely stable condition if

driving

the tube

passive grid.

If the screen is bypassed correctly with capacitors at the tube
socket pins to chassis ground, I've never had any problems. That is
as long as the correct capacitance is used and all lead lengths are
as short as possible. Good ground connections are a must. I
personally don't like to use aluminum for the chassis around the

tube

socket, but rather use steel or copper. The reason being is I can
solder directly to the chassis. The rest of the chassis can be
aluminum though. One still has to worry about the connection

between

the two materials in how they're fastened together, etc. I've seen
too many connections come loose where a solder eyelet was attached

to

an aluminum chassis. This even when they use a wavy tooth lock
washer. A solder bonded connection is much better.

73
Peter

Best,

Will

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