That is also a common method or shunt the choke with a 3K or so carbon
resistor.
A homebrew amp could also mean it is full of WW2 style HV mica caps which
are often leaky by now and make poor bypasses, coupling, and tune/loading
padders. For 160M included I use a .01 3KV disc ceramic for up to 2KV, or
a pair of .0047 at 6-10KV in parallel for big amps, as the plate choke
bypass and a .02 1KV disc for all the LV and input coupling.
Carl
KM1H
----- Original Message -----
From: "Vic K2VCO" <k2vco.vic@gmail.com>
To: <amps@contesting.com>
Sent: Wednesday, March 27, 2013 12:11 PM
Subject: Re: [Amps] instability issue
I had a similar LF oscillation issue with my 813 grid-driven amplifier. I
fixed it by replacing a grid RF choke with a resistor. If you can arrange
the biasing so that you do not have an RF choke in the grid circuit, that
will probably help. Or use a different choke, perhaps with a ferrite core.
On 3/27/2013 8:18 AM, Carl wrote:
Did it ever work well on 12M? If not or not tested Id also suspect the
plate choke and hope the big Ameritron fits.
As far as the LF oscillation I hope the bias is not fed thru a RFC as
these tend to turn the tube into a TNT ocillator. I also suspect the
antenna choke is open as that would likely be self resonant down there.
Carl
KM1H
----- Original Message ----- From: "N1BUG" <paul@n1bug.com>
To: <n8de@thepoint.net>
Cc: "'' amps ''" <amps@contesting.com>
Sent: Wednesday, March 27, 2013 8:26 AM
Subject: Re: [Amps] instability issue
Wow! Further to this... my frequency counter seems to believe the
oscillation is occurring at ~400 to ~450 kHz! The frequency does shift
substantially as I adjust the load cap, and to a much lesser degree as I
adjust the plate tune cap. The band switch seems to have very little
influence, though I cannot say it is zero.
73,
Paul
On 03/27/2013 07:18 AM, n8de@thepoint.net wrote:
Sounds like a resonance in the RF Choke.
Have you checked for that?
Do you have a frequency counter to detect the oscillating frequency?
73
Don
N8DE
Quoting N1BUG <paul@n1bug.com>:
I'm trying to understand a problem with one of my amps. It is
homebrew,
160-10 meters, using a 4CX1500B with screen grounded, grid RF
grounded,
cathode drive. It uses a conventional tapped coil pi network with
vacuum variable capacitors.
The amp was going into oscillation when using it on 12 meters
(frequency of oscillation unknown, but not in-band). When it did this
it was also destroying the vacuum relay on the output. The relay would
fail with contacts stuck in the NO position (contact connected to amp
output).
While investigating I discovered the amp will always go into
oscillation as soon as operating bias is applied IF there is no load
on
the output AND the LOAD capacitor is set to anything MORE than about
700 pF. Oscillation will stop if the load cap is reduced to something
under 700 pf. This happens regardless of band switch position or plate
tune setting.
On most bands the amp does not break into oscillation during normal
operation. That was only happening on 12 meters for reasons not
understood by me. I managed to get it to stop doing that by adjusting
the coil tap so that more output capacitance is needed. (That coil tap
needed to be optimized anyway, as it was never in the ideal spot)
Clearly this amp is not as stable as it should be. I am trying to
understand what is going on here. I am having trouble grasping why the
load cap seems to be the critical parameter, with coil tap and tune
cap
having essentially no influence. Any suggestions?
Thanks,
Paul
--
Paul Kelley, N1BUG
RFI Committee chair,
Piscataquis Amateur Radio Club
http://www.k1pq.org
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Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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