Date: Mon, 10 Dec 2012 16:50:42 -0800
From: Paul Baldock <pbaldock@frontier.com>
To: amps@contesting.com
Subject: [Amps] GS-35B Efficiency Varies with frequency?
I am building a GS-35B amplifier. The efficiency drops off a lot
between 80M and 10M. Although the gain is quite constant. Does any
body have suggestions as to why this is or whether this is typical.
The input PI network has a Q of around 2. The PI output has a Q
between 10 and 20
Here's some results with 70W Drive:
3.80MHz. Q=10, Output Power 1.1KW, Vp 3000V, Ip .54A, Ig .26A, Eff 68%
7.10MHz. Q=13, Output Power 1.1KW, Vp 3000V, Ip .60A, Ig .28A, Eff 61%
10.1MHz, Q=20, Output Power 1.0KW, Vp 2900V, Ip .68A, Ig .30A, Eff 51%
14.2MHz, Q=13, Output Power 1.1KW, Vp 2900V, Ip .70A, Ig .29A, Eff 54%
18.1MHz, Q=16, Output Power 1.0KW, Vp 2850V, Ip .78A, Ig .31A, Eff 45%
21.2MHz, Q=13, Output Power 1.0KW, Vp 2800V, Ip .82A, Ig .37A, Eff 43%
24.9MHz, Q=19, Output Power 1.0KW, Vp 2850V, Ip .79A, Ig .35A, Eff 44%
28.5MHz, Q=16, Output Power 1.1KW, Vp 2750V, Ip .85A, Ig .38A, Eff 47%
You will note as efficiency goes down the Grid current goes up (in
general) for the same drive. Is this normal?
Things I have tried:
1. Checking that the bias voltage is not affected by RF. No signal
Vp=3600V, Ip=140mA, Vb=34.5V.
2. Rebuilding 10M PI Output in a different configuration. (It is
actually an L-PI)
3. Checking that the metering is not affected by RF
Any Suggestions?
What is a safe Max Ig for the GS-35B?
Thanks
- Paul KW7Y
### what are you using for a wattmeter, have u tried the same wattmeter on
another hf amp ?
What does the tuned input consist of, separate pi nets, bandswitched, or relay
switched ?
How close do you have the tuned inputs to the cathode ? Ideally, you want them
right next to the cathode.
I forget what the input Z of that tube is. What are you using to connect
from tuned input to cathode,coax or strap ?
One method to increase eff on the upper bands is to use parallel 50 ohm coax to
make 25 ohm coax... and use the
25 ohm coax from tuned input to cathode. That has the same effect as putting
more cathode to chassis C in parallel
with the tube’s cathode to chassis C. ( you can also parallel some RG-59 to
nake 35 ohm coax.)
## You can also try adding a fixed C, like 50-200pf, from cathode to chassis.
Try increasing the tuned input Q a bit, try this on 15M, which has the lowest
eff in your case.
## If you use a T net for the tuned input, eff will suffer, and you will have
a hard time getting a flat input swr.
That entire eff issue revolves around the tuned input config. I gave up on
individual pi nets for each band...
and instead opted for a pair of broadcast variables and a tapped coil. 4 x
section types, typ 17-540 pf per section,
with all 4 x sections strapped in parallel with cu strap. No wire used
anywhere on the tuned input bandswitch,
nor the taps to the coil. I used 6 ga solid cu wire for the tuned input coil.
.187” tubing or .25” tubing will also work.
## a parallel tuned input is not much better than a T input. Unless you have
some way of tapping the coil on the cathode
side, you will find that min input swr and max grid current do not coincide.
## Your loaded tank Q is way too high on every band except 80m.. ( which has
the highest eff). ( output kw pi net )
## Loaded Q’s of 8-10 provide for WAY better eff...and slightly less harmonic
redux.
## You not only need an L-PI... you also need a vac cap for the tuned input.
Then you can easily get a lower,loaded
Q. Check the rotor contacts if an air variable used for the load cap, they
can easily heat up. Also, you don’t want the
rotor grounded in several places, only once. Use a shaft isolator on the
shaft, and float the front end of the load cap
frame. Only ground the back end of the frame. Ideally, a vac cap should be
used for the load cap as well.
## what are you using for tank coil material ? I’d suggest 8 ga solid wire
or .125” tubing for 80+40+30m.
And .25 - .375” tubing for 20+17+15m.. And .75 – 1.0” wide strap for
12+10m.
## How big are the bandswitch contacts ? Beware, the contact ratings that
mult-tech industries use are dc-60 hz ratings
not RF ratings. If you have spot heating going on anywhere, you can usually
find it with a fluke 62, point and shoot IR gun.
## do u have another tube to try out ? I think the gs35b has one side of the
cathode tied to the fil, and I forget which side.
## what are you using from load cap to TR relay ? Is it a piece of 50 ohm
coax...or is it wire/strap ? You might be introducing
a huge Z bump in there ? What is your bypass SWR....esp the upper bands ?
What are you using for a TR relay setup.
## You could also try increasing the idle current, like 180-250 ma. That
gs35b is capable of well over 60% eff on 6m.
I suspect the eff problem is in the tuned input / and or the pi output, or
wiring in both pi nets.
## IF your tuned inputs are setup for 50 ohms, you might want to try
measuring the power output of the tuned inputs
with a wattmeter, then into a 50 ohm dummy load. But if your 100 w dropped way
off to say 60 w, then your grid current
would also be way down on the upper bands, and its not, it’s higher.
## any bad switch contacts or rotor contacts will generate a lot of heat. If
input Q is too low, and a SS xcvr used, eff will suffer.
If input Q is too high, losses in the tuned input will increase. What size
coupling cap are you using between tuned input and cathode ?
2-4 x .01 uf @ 1-2 kv disc ceramics in parallel will handle the drive power.
Let us know what you fine out.
Later ... Jim VE7RF
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