Well thanks to all of you here I have abandoned the idea of using the GU-84B.
Staying with a Triode is the way to go. As it sets the amp needs a Tuned Input
and a tank circuit refinement if I stayed with the 3-400Z tubes. Not a big
deal but the tubes are hard to come by and can be quite expensive.
I now will explore using the GS-31B and the 3-500Z. The 3-500Z is too tall for
my enclosure but it could be modified I guess. The 8877 is a great tube but
$$$$!!
73,
Bill
KO4NR
--- On Wed, 4/13/11, RZaepfel@aol.com <RZaepfel@aol.com> wrote:
From: RZaepfel@aol.com <RZaepfel@aol.com>
Subject: Re: [Amps] GU-84B Conversion
To: ko4nrbs@yahoo.com
Date: Wednesday, April 13, 2011, 3:00 PM
Bill,
I rarely respond to the list, but will offer a bit of information. I have
in progress a design-build from scratch hf amp using the GU84B tetrode. I'm at
the 85% point in the job. My approach involved solving the initial problem of
the enclosure as a starting point because most commercially available amp cases
are pretty chunky and most are tall to accommodate transformers inside the
table top box. So I chose to use the case and cabinetry from a deceased HP 8660
series signal generator for the table top and the PS is located in an old UPS
box on the floor out of sight. That makes some of the problems a bit easier to
solve. Your thought to go from triode to tetrode may be too big a leap. There's
quite a bit that must be worked out. You must have a well-regulated screen
supply of 360 - 380 volts for the GU84B capable of handling negative screen
current and a well-regulated bias supply of around -70 to -100 volts. Both
are not usual requirements for gg triodes. My solution is to incorporate a set
of Tetrode Boards from Ian White, GM3SEK. Also, you will need a filament supply
of 27 v at about 4 amps. I solved this by having Peter Dahl build a 2 amp ccs
transformer for me before he ceased doing business -- weighs 46 lbs as I
recall.
It's too large for a table top installation, so the floor is a good solution.
The other big issue is the input circuit. You will need to build a swamped
input
circuit with a noninductive 50 ohm load, some inductance to compensate for tube
input capacitance and a way to deliver bias while isolating the bias supply
from
RF. Also need to install at the tube base a bleeder resistor for the screen
supply and a simple resistor-inductor combination in the screen to secure
stability. Reading the Tetrode Boards site might be a good idea, as you will
get
the steps for designing and installing all this stuff from the "horse's mouth".
There's quite a bit of work here. Staying with triode(s) would be easier. How
about a 8877?
73,
Bob, K4HJF
In a message dated 4/12/2011 8:47:41 P.M. Eastern Daylight Time,
ko4nrbs@yahoo.com writes:
I'm
considering converting my old Swan Mark 1 (two 3-400Z tubes) to use one
GU-84B. Some time ago I replaced the panel bandswitch in the Swan with a
Radio Switch Model 86 six position switch with two wafers. At that time
I also replaced the tank coil with one that was made for the National
NCL-2000
amplifier. My new bandswitch required me to rearrange the RF compartment
considerably.
The transformer was replaced with a new one from Heyborer
in Michigan. At that time I had them use the next larger diameter wire
in the HV and filament windings when they wound it. The caps, etc. were
all replaced with snap in type on a circuit board.
Ok that's the
history of the amp to date. It has a resting plate voltage of 2900vdc
and 2500vdc or so under load.
Readings with two 3-400Z tubes:
Plate
Current Readings on Operate HV Setting (SSB):
270ma using a Tuning
Pulser
Resting Plate Current is 190ma
460ma SSB Peak (Talking in
Mic)
Plate Current readings using CW Mode(1500vdc)
Key down
660ma
Resting 80ma
SSB 320ma Peak
My question is will the GU-84B
perform satisfactorily with this transformer? I know I'll need another
filament transformer and one for grid voltage.
73,
Bill Smith
KO4NR
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