[Amps] HIGH INPUT SWR PROBLEM
g8gsq72 at gmail.com
Thu Dec 12 04:21:12 EST 2019
I think the GI7 is indirectly heated with a cathode, like 8877. If
there's no coupling cap between the cathode and filaments, it
shouldn't need a big heavy duty filament choke like you need with
3-500Z and the like.
The input impedance of the tubes varies with drive level and
operating voltage etc. and might not be near 50ohms. The input
matching needs to be set up to suit that:
One example will serve to illustrate the problem of valve
impedence swing. Some years ago I constructed a 6m linear using
the Russian GI7-BT triodes and needed to establish the cathode
impedance over the working drive range. This data wasn’t included
in the data sheet so I had to determine it experimentally. A test
amplifier was built and driven by a very good high power driving
source made by Hewlett Packard and a dual directional coupler was
used to sample the amplitude and phase of the input signal. From
this I was able to measure the impedance as the drive was varied.
At 1W drive the cathode exhibited an impedance of over 600Ω and
the grid current was very low. At 5W the impedance was 150Ω and
the grid current was 50% of the safe maximum according to the data
sheet. At 10W the grid current had reached the maximum value, with
an impedance of under 30Ω.
>> I have a commercial amp from Europe running a pair of GI7B
>> tubes. The amp works well and loafs along at 500W output with no
>> problems on any band, but it has a very high input SWR on all
>> bands. Design of this amp is such that drive is fed into the
>> filament choke via a BNC connector and cable from the input
>> matching circuit so it was easy to check the input pi networks
>> for each band. On all but 15M the input SWR indicated when
>> tested this way was quite acceptable: 1.3-1.7 as tested with an
>> MFJ analyzer. Similar results were obtained by attaching the
>> input matching circuit BNC to a dummy load and feeding a small
>> amount of power into the front of the input circuit. So, the
>> problem does not appear to be in the input circuitry.
>> The first thing I discovered when I opened up the sub-chassis
>> was that the coupling input capacitor appeared to be too low in
>> value: only 1 nf compared to many designs for a pair of these
>> tubes which I have seen using 10 nf. Second, there were no
>> filament by-pass caps installed and no cap across the filaments.
>> Therefore, as an experiment I installed by-passing using common
>> values and changed the input cap to 10 nf. These changes
>> produced no lowering of the input SWR but did increase the plate
>> current drawn for a particular level of drive, probably due to
>> much higher drive.
>> This amp design uses a large fan beneath the tubes which does
>> not leave very much room for a rod-type filament choke inside
>> the enclosure. The choke is wound with 19 bifiliar turns of what
>> looks to be #16 wire on a toroid, 1.75" in diameter by 3/8 inch
>> thick. No information is available on the core.
>> My questions is this: if this filament choke is undersized,
>> could this be causing the high input SWR? If not, does anyone
>> have other ideas about the problem?
>> Bill, VE3NH
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