Okay, so the "Franken-whatsit" thing got stale. Seems to fit this specimen, a 1959 ( or so ) EF Johnson Viking Valiant transmitter.
Not your grandad's Valiant.
This thing is the original big iron. The owner had stuff removed that didn't make it work for 11-meter AM. Wanted that "big Johnson" audio.
The VFO was in trouble, so out it came. The red digits are a PLL synthesizer. Took care of the drift issue.
Just one problem, it didn't have enough audio to suit him.
Efforts to crank it croaked the modulation transformer. Or maybe it was just time for a 55 year-old transformer to finally fail.
Had to jump through some smoldering hoops to get the new one.
The guy who worked for me at the time suggested using larger tubes than the original pair of 6146 tubes. They seemed to be the weak link in the chain.
This tube's anode is rated for 60 Watts, nominally twice the original tubes' rating.
It took a couple of tries with a new driver tube and two different driver transformers to realize these tubes run class AB2. This means you need more than just grid voltage to drive these tubes, you need current as well.
That means audio power, and even the upgraded driver tubes we tried just never would cut the mustard. No pics of those steps in the evolution. Not much point.
Not loud enough.
Decided that an external audio power amplifier would drive these tubes and would not care at all.
Had an old Rat Shack PA that uses a pair of TA7217 audio chips in push-pull.
Replaced the driver transformer on the Valiant's modulator tubes with a push-pull speaker-output transformer, meant for two 6BQ5 tubes. Hooked up in reverse, we drive the speaker winding with the PA amplifier. The center-tapped winding that was the primary now has all the drive power the tubes can use.
Installed the filtered 4-pin socket we use in RCI radios to replace the factory 6-pin socket.
A word to the wise: This amplifier has a steel panel behind the aluminum face plate. The 4-pin socket needs a thinner panel than that, or there won't be enough threads sticking out the front. Makes it impossible to tighten the mike plug.
We snipped/peeled the steel away from the corner to accommodate the new 4-pin socket. Not pretty, but effective.
Replaced all 13 electrolytic caps. Just seemed wise.
I had some misgivings about this amplifier's output transformer. After all, we're driving the 8-ohm winding on the transformer inside the Valiant.
Decided to run the push-pull output from the two audio chips directly to the Valiant. The two 5-ohm 10 Watt resistors limit the chance of overloading the PA amp. The common-mode choke on the audio wires seems to keep the RF out of the PA amp.
So far.
We'll find out where is the weak link in the chain now.
Pretty sure.
73
Not your grandad's Valiant.
This thing is the original big iron. The owner had stuff removed that didn't make it work for 11-meter AM. Wanted that "big Johnson" audio.
The VFO was in trouble, so out it came. The red digits are a PLL synthesizer. Took care of the drift issue.
Just one problem, it didn't have enough audio to suit him.
Efforts to crank it croaked the modulation transformer. Or maybe it was just time for a 55 year-old transformer to finally fail.
Had to jump through some smoldering hoops to get the new one.
The guy who worked for me at the time suggested using larger tubes than the original pair of 6146 tubes. They seemed to be the weak link in the chain.
This tube's anode is rated for 60 Watts, nominally twice the original tubes' rating.
It took a couple of tries with a new driver tube and two different driver transformers to realize these tubes run class AB2. This means you need more than just grid voltage to drive these tubes, you need current as well.
That means audio power, and even the upgraded driver tubes we tried just never would cut the mustard. No pics of those steps in the evolution. Not much point.
Not loud enough.
Decided that an external audio power amplifier would drive these tubes and would not care at all.
Had an old Rat Shack PA that uses a pair of TA7217 audio chips in push-pull.
Replaced the driver transformer on the Valiant's modulator tubes with a push-pull speaker-output transformer, meant for two 6BQ5 tubes. Hooked up in reverse, we drive the speaker winding with the PA amplifier. The center-tapped winding that was the primary now has all the drive power the tubes can use.
Installed the filtered 4-pin socket we use in RCI radios to replace the factory 6-pin socket.
A word to the wise: This amplifier has a steel panel behind the aluminum face plate. The 4-pin socket needs a thinner panel than that, or there won't be enough threads sticking out the front. Makes it impossible to tighten the mike plug.
We snipped/peeled the steel away from the corner to accommodate the new 4-pin socket. Not pretty, but effective.
Replaced all 13 electrolytic caps. Just seemed wise.
I had some misgivings about this amplifier's output transformer. After all, we're driving the 8-ohm winding on the transformer inside the Valiant.
Decided to run the push-pull output from the two audio chips directly to the Valiant. The two 5-ohm 10 Watt resistors limit the chance of overloading the PA amp. The common-mode choke on the audio wires seems to keep the RF out of the PA amp.
So far.
We'll find out where is the weak link in the chain now.
Pretty sure.
73
