Discussion in 'Amateur Radio Amplifiers' started by AC0LA, Aug 20, 2012.
Unfortunately I do not have access to that handbook. I do have a copy of Bill Orr's Service Newsletter on application of the 3-400z. It shows what appears to be a homemade filament choke made out of copper tubing that doubles as a tuned input????????? I will have to try figuring that out.
Thanks for the input. I am pretty careful with high voltages and have had some experience in the military with tube gear from before bleeder resistors were standard. The drill was to turn the power off, remove the cover, and short the filter caps with a screwdriver. When we worked in "hot" gear it was with one hand in the pocket. Of course that was all on receiving and recording gear with most voltages below 1KV.
It would probably be prudent to add bleeders to this power supply since there are non at this point as pointed out by WB2WIK.
Wow, that's a lousy design and practice if there really aren't any bleeders. I'd bleed that thing with about 250K/50W made up of a string of five 50K, 10W resistors or similar, depending on the value of the filter capacitor(s). In any case, I'd want 5T (tau) to be 30 seconds or less, so by the time I could take the cover off the power supply it would be 98% discharged.
This is why I have concern.
He seems not to realize these things.
Take some time and look into some older ARRL handbooks at the designs using the same tube family and see how the circuits are designed and layed out.
For a 3-400 or a 3-500 the total design will be so close in all them that it's a reference to go by in looking at what you have and seeing it's short comings..
Indeed. I can not make out what he writes with my NEC monitor. Dark blue might work but black is best.
My Tempo 2000 2x3-400 amp does not use a tuned input. Manual states its close to 50 ohm on average (paraphrased). Worst case input has been 1.7:1 on 10M. I don't see any fold back of exciter output here but any meter I use would be hard pressed to keep up if there was. Input can vary from as low as 25 ohms to a couple hundred over a single SSB transmission because the cathodes are directly fed by the exciter (an FT450AT in this case).
The problem is when using a solid state exciter. The input impedance varies with the drive angle, and a solid state exciter needs to see a constant 50 ohm load, otherwise the iMD will suffer. It isn't a matter of SWR, it is the flywheel effect provided the tuned input matching network. Older amps worked well with transceivers with Pi Net outputs, since the PA tank in the rig provided the flywheel.
Just FYI. It could be just the nature of the beast but I have never had a problem using my Swan MKII with exciters that have pi networks. For the past 10 years plus I have used it almost daily with a FT1000MP on all bands 80 thru 10 including WARC bands with no problems at all. I have had it since the early 70's so it's seen a lot of transmitters & transceivers
Thanks for reminding me about this amp. It is still on the bench waiting for me to get the power supply connected to the RF deck. The initial test will be without any changes since i do not even know if all the components are functional and any changes would only confuse the issue. If it operates as is I can then start making things better which will include a tuned input since it is recommended by many who should know.
To me this amp is more about a learning experience than about getting more power output. Most of the people I hear can hear me with the exception of 160 meters when the band is marginal. I come here to get input and then go do my homework which at the present time is working on the schematic, gathering components, and getting things ready for initial testing.