Hi Steve,
I, too, have gone through trials and tribulations with
my newly-built 6m GS35B box (see
http://www.qsl.net/k7cw).
In the past I have experienced problems with
"glitches" which occur because the tube hasn't been
brought up "conditioning the filament."
The problem had with grid current showing while in
idle is probably due to a bad protection diode - the
one that goes from the B-minus line to ground. When
you replace that one, the "leaky" grid current will
probably go away. When the diode is good, I don't show
any grid current flowing when in idle.
I don't have experience running the tube at the anode
voltage of 2600, but I agree that the grid current is
too high. I solved the same problem (along with a
low-efficiency one) by trying different values of
final tank inductance until I got it right. I read
somewhere on the internet that the length of the
DJ5RE-GS35B strip inductance needs to be reduced for
the anode tank to resonate properly. I would look at
that.
The conditions I get with my amplifier are:
Anode Voltage: 4000
Anode Current: 650mA
Grid Bias (Zener): 36V
Grid Current: 250mA
Drive Power: 80W
Power Output: 1500W
Output efficiency: 57 percent
While I'm talking about my homebrew amplifier, let me
say a couple of things that are important with respect
to safety, both to the amplifier and to the operator.
First, put an RF choke at the output connector from
the pin to ground. This will cause the power supply
breakers to trip should the blocking capacitor fail
due to shorting. I don't know how many times I have
read that warning and ignored it. Well, it finally bit
me when my blocking capacitor failed and shorted. I
was key-down on CW at the time, which made things
worse. I welded the vacuum T/R relay open, possibly
damaged the antenna feed, blew the grid on the tube,
took out the bias circuit, blew the protection diodes
and fuses in the power supply.
Another thing that is important is that the blocking
capacitor needs to be capable of handling a high
amount of RF current. The doorknob cap that I had in
my amplifier was rated at 20kV, and it look beefy
enough, but I installed it anyway, thinking 20kV was
plenty-enough safety. Trouble was, it couldn't handle
the current. It is important to use high-quality
transmitting capacitors that can handle the RF current
flowing in the circuit.
A third thing is, build a grid current protection
circuit into your amplifier. It is inexpensive and
will prevent a lot of grief should you lose anode
voltage, antenna load or some other thing that causes
operating conditions to vary wildly.
I suggest reading K8CU's write-up of his GS35B
amplifier, which appears on ND2X's web page under
"QRO." I also suggest reading the great general
amplifier information on AG6K's page.
73, Paul K7CW
--- Steve Thompson <g8gsq@qsl.net> wrote:
>
>
> -----Original Message-----
> From: Phil <PhilHSS@Breathemail.net>
> To: Amps <amps@contesting.com>
To: <amps@contesting.com>
> Date: 18 May 2001 14:13
> Subject: [AMPS] DJ5RE 2m Amplifier Tuning - Longish
>
> >The initial DC conditions are as follows:
> >
> >2.6Kv Anode Voltage (on load) and 2800V off load
> >400mA Anode Current
> >300mA Grid Current
> >80w input RF
> >300w output RF
> >150mA Anode Standing Current (giving 80mA of Grid
> Current with no drive)
> I can't claim any experience with GS35, but 80mA
> grid current at idle seems
> very high. The data suggests 30ish. I know grid
> current isn't set in stone,
> but nearly 3x different strikes me as wrong. Low
> gain and 300mA grid current
> for 400mA anode current also seems far too high,
> suggesting too low an anode
> resistance, although that wouldn't affect the dc
> conditions. I'd be inclined
> to check the grid meter and try and find another
> valve to try.
>
> Steve
>
>
> --
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