Discussion in 'Amateur Radio Amplifiers' started by K6GB, Jul 28, 2018.
And therapeutic, too.
I doubt very much if it would have been cheaper, even if this Amp was sold as fully working.
Given that it was faulty, I'm presuming the OP bought it cheap !
Real cheap, cheap enough I may just pull out the tuned input and put in a swamping resistor, or convert for 6m? This is for fun.
Tom W8JI has something to say about doing that (link):
Why is a tuned input necessary?
An RF power amplifier's tuned input circuit is more than just impedance matching. While reasonable impedance matching can be important for proper exciter operation, the tuned input circuit of a grounded-grid or cathode driven radio frequency power amplifier provides multiple important functions for the amplifier:
The tuned input system prevents harmonics generated in the PA tube from passing back to the exciter
This prevents false high SWR or exciter power readings, since cathode harmonics show as reflected power
The tuned input circuit provides a stable low impedance to the cathode of the PA tube
This helps amplifier linearity and reduces IMD, and can improve PA efficiency
The tuned input circuit matches the cathode impedance to the exciter system impedance
This lets the exciter work into a proper SWR
A tuner between the amplifier and radio, a matching circuit some distance from the cathode, or a high pass or broadband network will not do all of the above.
I don't think you would have very good luck trying to run 572B tubes on 6 meters. The tubes internal capacitance is too high to reliably get them to work properly.
The wires from the internal parts (especially the grids) to the pins are very long... what you don't want at higher frequency. It's the same deal with 811s and 813s.
My amp I used for years had no input matching (it had all been removed for spare parts, as had the PA Coils) . . .
So I put a close-wound PA coil in for 160m, and later tapped it for 80 & 40.
I found that a series 5000pF capacitor straight to the filaments gave me a 1:1 swr on 160m, and less than 3:1 on 80 & 40m. (no swamping resistor at all)
I mainly drove it from a valve PA rig . . . but even when I got a rig with a transistor PA it still worked fine (and didn't produce any distortion, despite W8JI's comments)
^I agree and never got 572Bs to work well at 6m; however I didn't try all that hard.
Lou W1QJ tells us he can get SB-200s converted to 6m and they work. I'm not sure exactly what he does, but evidently it's possible.
The only SB-200 I got working on 6m used only the chassis and HV plate supply and no 572Bs -- I installed a 4X150A socket and chimney, pressurized the chassis, added a small blower and small bias and screen supplies (these only need to be good for 20mA or so, so they can be really small) and built input and output tanks for 50 MHz and it came out fine. About 350W output on 6m from one tube with ~5W drive. My old Icom IC-502 3W portable SSB rig could drive it to 200W output power.