Discussion in 'Amateur Radio Amplifiers' started by KN4CTD, May 15, 2019.
Keep in mind the SB-200 has tuned input issues. Mods are shown on the internet.
My AL-80B has this for a suppressor. It seems fine.
image stolen from Tom's website
For the OP:
Here's the page on W8JI's site where he goes over certain points of stability. Interesting reading that, for better or worse, made me want to directly ground the grids on my 572b amp!
Does anyone know a “better” way of removing/installing the bandswitch in this sb-200??? I was able to take the screws holding the rear input wafer out and slide that whole shaft backwards and remove the 20-80 meter tank coil and bandswitch as an assembly....but it was a tight fit trying to wiggle that shaft upward enough to allow the complete assembly to pull out the top of the chassis. I read on here somewhere that someone posted it was possible to slide just the shaft out completely but that doesn’t appear possible to me. I may just cut that shaft in half and add a coupling to reassemble.
I would directly ground the grids on this amp if I could find the correct information online on what changed need to be made with the amp. The bias would need to be applied to the cathode and someone said changes in the metering circuit would also need to be made??
I searched, and found no Application Note. W8JI may like them but, Ohmite's datasheet says nothing about inductance or reactance. So, I measured a couple, and at the same time, a couple NOS A-B resistors. I used my AADE L/C Meter II, and a HP8711 VNA, and a HP LCR meter. They all showed the A-B resistors at about 17nH, and the Ohmite #OY471KE resistors indicated 1.6 microhenries so, I used the A-B parts. To a cheap ham, the MOST important characteristic is the cost. Suitable A-B parts cost less.
Note that the resistors each have two capacitors in series...
The 811 and 572B share a not so good characteristic... long internal grid leads. That makes 'em not so good for VHF-up use, and not so stable at HF. Hard-grounding the grids is worthwhile.
Jim W7RY offers a PCB for that.
An interesting point on Tom's page is that he uses the inductance of the OY resistor as part of the network in his spiffy suppressor.
Note he also did that with carbon comp resistors.
The "VHF tubes" like 3CX/4CX stuff are so stable they generally don't need anything. My 6m 8877 amp is unconditionally stable with zero parasitic suppressor -- never used one.
It’s official. I’m gonna have to give up on this thing. I’m ordering several different values of NOS carbon Allen Bradley resistors to rewind the parasitic suppressors. I’m gonna replace the plate blocking cap just for the heck of it.....and I’m giving up and passing this thing on lol! I accidentally hot switched (oops) the bandswitch from 20-15 and burnt one side of the 15 meter contact but amazingly it still put full power out with just the contact on the other side of the wafer. A friend had a few of the Harbach replacement wafers so I pulled the switch completely out and replaced the whole wafer. The other one had quite a bit of corrosion anyhow. Tested into a 10 meter dipole.....no dice! This thing is still oscillating (I think). Even when tuned for max out, which isn’t much on 10 BTW, SWR goes full tilt when keyed either by a hand switch, or via the transceivers keying circuit on ssb with the mic gain cut completely off. I’m sure someone who has way more technical knowledge and experience could figure this out, but I’m done after getting some different resistors for the suppressors. All of my previous experiences working on/tinkering with tube amps have been with, obviously, more stable tubes that were truly grounded grid. I’ve never seen or had an amplifier act like this before. I may replace the input and output SO-239 connectors with brand new ones also just for good measure. If someone knows what inductance the plate choke in these amps are, I guess I could unhook it and check it out of circuit also. I did see that the coax that feeds the tuned input wafer from the relay is only grounded on the relay side. The schematic even shows it that way???? Because there is no solder lug or hole for one back at the tuned input wafer. Probably wouldn’t be causing these issues, but I figured I’d point that out.
I did advise, several pages back, just to avoid 10 m because of the bad "voodoo" on that band, but ...
The problem is that even on 15 meters.......only when keyed with no rf drive....it does the same thing. I can get it tuned up using RTTY and as soon as I go to SSB (what I generally use) and key with no rf drive, SWR goes full tilt.....also shows pulling grid current.....which shouldn’t be. I’m gonna post some pics of everything as a last ditch effort. Some of y’all can pick stuff out like that.