As someone who in my younger and more foolish days homebrewed amps with up to 7500 volts on the tube, I agree with the other posters.  Assuming around 3000 volts in a typical SB-220, 1/4 inch spacing should be OK under optimum conditions.  However over time your fan will suck in dust, human hair, bug parts, etc into the amp.  Some of this will accumulate on HV parts at the worse possible locations and the spacing between them and ground will decrease.

Remember the Jacobs ladder demonstration.  Once a HV arc starts it takes much less voltage to keep the arc going.  Personally I would try for a little more spacing than 1/4 inch if possible.  As the previous poster said, inside your amp is not the place to re-inact the big bang theory.

Dick  AD4U

I have an SB-200 that I built back in 1969.  It still has the original Cetron 572B tubes.  According to my Bird 43 PEP reading wattmeter I get around 600 watts PEP on 80-15 and around 500-550 on 10.  Plate current is 500 ma and grid current is within the "white" area on the meter.  The amp is operating from 240 VAC.  It is being driven by 100 watts from a Kenwood TS-830S and is running into a dummy load.

What you are getting sounds about right, but may be a bit low.  How accurate is your MFJ wattmeter?

Dick  AD4U

I have no experience with this amp, but I have lots of experience with amps in general.  If your filter caps are overheating (hot), they are defective.  Of course this assumes the HV rectifiers are OK.  If there is a problem in the HV rectifiers you may be putting AC on the filter caps, which will (or may have already) destroyed them.

Dick  AD4U

Maybe one day I will learn to read (instead of scan) posts before I reply.  

If you have no drive to the amp and if is keyed and if there is more than normal idling plate current and if your exciter is not spurrious and if you see output on the amp's watt meter and if you actually hear a signal on a nearby receiver, my guess is that your amp is oscillating.  I know I put a lot of "ifs" in my reply, but it is hard to diagnose problems from half way across the country.

I still come back to my original question.  Did it work OK before you adjusted the 15 meter input circuit?

Good Luck

Dick  AD4U

PS:  You may also have an unwanted oscillation inside the amp.

Did it work OK before you "adjusted" the slug on the rear of the amp?  I assume you adjusted the 15 meter tuned input?  Why?

Most every amp produces "spurs" (unwanted signals).  Sometimes spurs are the result of improper tuning, excessive SWR, or many other factors.  Some spurs may be generated inside the amp even though it is properly tuned and some may be generated by the exciter.  Those generated by the exciter may (probably will) be amplified to some degree by the amp.

The FCC has regulations on how much these spurs have to be attenuated in respect to the desired signal.  Just because you can hear a spur on a receiver sitting right next to the amp does not necessarily mean you are generating a spur in excess of the FCC specs, BUT IT MAY.  The simplest way to check is with a spectrum analyzer.

Dick  AD4U

 

PS:  Some 40 years ago I got from the estate of a SK a home brew grounded grid amp that ran two 4-1000A's.  This amp used a military surplus filament transformer (7.5V @ 50A).  This filament transformer did not have a center tap.  The builder put two 10 ohm 25 watt (I think) resistors in series (back to back) across the output of the filament transformer and used the junction of the two resistors as the center tap.

I don't recommend this but the amp worked.

Dick  AD4U

I don't know anything about this amp, but a blue glow in a transmit tube is not normal and probably indicates a problem.  More than likely the tube is gassy or there is an internal problem.

Dick  AD4U

In a typical 3-500 GG amp, the center tap in the filament transformer is usually how the cathode current comes out of the 3-500.  Some call it B-.  I know that is a stupid way to say it, but without a center tap, I don't know how the GG amp would work.

Dick  AD4U

Another amp some of us "old timers" like is the Heath SB-200.  It uses "real" transmit tubes and output is around 600 watts 80-10 meters.  Usually they cost less than the Ameritron.  I built an SB-200 in 1969 and I use it a lot.  It still has the original tubes and it has never failed.  Something else to consider.

Dick  AD4U

I just did a GOOGLE search for   SWAN ASTRO 150  and I came up with hundreds of "hits".  I did not look at them, but some appear to give what you are looking for.

Dick  AD4U

IMO what you describe is exactly how to do it.

NOS 8950 tubes are still available.  Just be sure you can trust who you buy them from.  If you are determined to use the SWAN, get a pair of NOS tubes.  If you CAREFULLY tune up and maybe install a cooling fan, they should last you a very long time in SSB service.  Most sweep tubes are killed by extensive and improper tune up.  

Case in point.  I have a room full of boat anchors and I use them often.  One of them is a mint Yaesu FTDX 570 that I bought in 1979.  I installed a new set of 6KD6 tubes when I bought it.  It has a cooling fan and I can tune it up in about 3 seconds.  Granted this is not my "front line" radio, but I still use it often.  The 30 year old 6KD6 tubes still put out 275-300 watts on all bands.  It is all in how you tune the rig up.

Dick AD4U  

HV should be 2400 no load.  Keyed idling cathode current should be 90 ma.  This is what an unmodified SB 200 should be.  Not sure about what effect (if any) the mods will make.

Dick  AD4U

HV should be 2400 no load.  Keyed idling cathode current should be 90 ma.  This is what an unmodified SB 200 should be.  Not sure about what effect (if any) the mods will make.

Dick  AD4U