(Note: This post was edited on 2010-01-02, reason: toned down the editorial commentary.)
It must have been the changed antenna impedance; we had some sun the few days before the vids below were made and things have started to dry out a little. That, and perhaps the close proximity of the transmitter (and ‘audio chain’ et al) to the RF ‘field’ of the antenna.
What exacerbates the problem is the ‘architecture’ of the Gates BC-1G RF PA (Power Amplifier) stage. The Gates BC-1G RF PA stage uses 833A tubes which are triodes and in an RF application the effect of inter-electrode capacitance (producing grid-to-plate feedback within the tube itself) must be effectively ‘cancelled’ using circuity external to the tube using a technique called “neutralization”.
Neutralization in the BC-1G is done by coupling energy back from the 833A PA plate circuit into the opposite end of the driver stage tank circuit. This creates (under ideal circumstances) out-of-phase energy which then along with the driver tube’s output drives the RF PA’s 833A tube grids. The image below shows this schematically:
See also Eimac tubes – SECTION 5.3 SINGLE-ENDED NEUTRALIZATION Circuit.
So the chain of events is as follows: The complex (R + jX) load presented by the antenna changes, the phase of the RF voltage on the plate changed and the ‘neutralization’ feedback that keeps the PA stage stable (from breaking into uncontrolled oscillations) changed magnitude and/or phase angle and allowed the PA stage to figuratively “go off into the weeds” quite dramatically.
Apparently, if the complex load of the antenna changes, a relatively simple Triode PA like the Gates BC-1G can literally ‘take off’. Again, the Gates BC-1G uses a neutralization scheme coupling an out-of-phase voltage from the PA plate circuit back to the grid to keep the PA stage from breaking into uncontrolled oscillation.
Points to consider:
- The Gates BC-1G sales brochures lists the frequency range as “540 to 2000 Kc as ordered”. Hmmm. “as ordered”. So, it can be supplied to work over that range, but I’ll bet it takes Gates Engineering to make those changes 100% effective (with no side effects e.g. PA oscillating etc).
The sales brochure also doesn’t state over what ‘range’ of VSWR the Gates will load into; normally in AM Broadcast work the broadcast engineer/consulting engineering firm takes great pains to bring the impedance seen by a broadcast transmitter quite close to 50 Ohms. I’ve personally used a vintage GenRad bridge to perform these msmts while adjusting the ATU at the base of the antenna to achieve 50 Ohms.
- It is operating at nearly 4 MHz now. What was changed to accomplish this … don’t know. Were considerations made to accommodate the neutralization circuit operating at 4 MHz? Don’t know that either … but reading some accounts on the ‘net some people have revised the original Gates neutralization circuit to use something different.
- Circuitry (bypassing, decoupling, chokes, coild) that worked well at 1.6 MHz might not be optimum for performing bypassing, decoupling and neutralization at 4 MHz.
Perusing through pictures of the Gates BC-1G one can see what most of us would consider ‘long’ plate and grid wires connecting the 833A power triodes to their respective circuits
See also: HF Amplifier Stability
- Probably 99.99% of the antennas that Gates BC-1G’s ‘worked’ into had little change in characteristics (compared to the changes an amateur-band 75 Meter band ‘bazooka’ antenna 20 feet above – and parallel to- ground might see) day to day and week to week; With significant moisture changes in soil affecting the near-ground antenna Impedance (Z) the BC-1G is the requires extra supervision and special care and handling to assure the PA stage remains stable during operation.
Now, observation of kilowatt-class transmitter with what looks like PA instability problems on the ham bands.
1) First, a problem is observed by the operator of the modified Gates BC-1G broadcast transmitter:
2) The operator tries some test transmissions on the 75 Meter band -
- A little tuning of the transmitter (dipping the plate and adjusting the neutralization maybe?):
3) And now, from all indications of the metering it probably appears tuned (until conditions change again e.g. the next time it rains, or the antenna is moved, the tuner is adjusted, etc.) so a few test transmissions are tried:
Note: The spectrum analyzer was set for peak-hold on Trace A, capturing those occasional transient, spurious RF output events. Those transient ‘events’ sound like buck-shot splatter up and down the 80 Meter band.
4) Back to normal service on the ham band:
1. 833A Tube
2. Eimac tubes – SECTION 5.3 SINGLE-ENDED NEUTRALIZATION Circuit