Tubes Turning Red SB-220

[ouxly40]

I have recently acquired a SB-220. It works perfectly on 80m, 40m and on the higher bands I do not get over 400w and the tubes begin to glow as soon as the key is pressed. tried another pair of tubes - same thing but got a little more power from new tubes. I put the others back in until I figure this out. I tried Rich Measure's parasitic suppressors which done nothing ( I never saw anything unstable in this amp anyway - watt meter never jumped up nor did the plate current meter). After that mod 10m power dropped to 150watts. I re tuned the input circuits to get the best SWR ( nothing more than 1.3). The zener is good and measures the correct voltage. Power supply bleeders and filters have been upgraded.

My friends can bring me something broke and I sit down and find the problem, replace the failed components, and make them happy! WHY? can't I fix my own amp??? Perhaps someone has faced this problem before?

 

[W5LZ]

I wonder how much you are driving that amplifier with to get the power output that you are getting? And into what, antenna, dummy load?
- 'Doc

 

[ouxly40]

SB-220

All testing is done on a Heathkit Cantenna at 49.6 to 52.4 ohms ( depending on the band).

Thanks for so many responses! I have already printed and read (many times) that web page. From there I went to Rich Measures' web site and printed and read what he suggested. I bought his kit for replacing the zener with 1n5408s (didnt do it yet), the kit to protect the meters with 1n5408s as well and his parasitic suppressor kit. I have directly grounded the grids with thick braided copper strap.

I have been working with HV power supplies ( as much as 14KV at 15 amps) for over 20 years. My safety always comes first.

Input and output bandswitches are working and I even got a second opinion with my dip meter.

Pressing the foot pedal (no drive) I get about 5.5v on the diode and about 175ma Ip (about 120ma on 80m). On 80m and 40m I see a much more pronounced dip in Ip than the other bands. I am using 120vac but have tried 220 with no apparant changes (other than my overhead lights dont dim on 220).

My exciter is an FT-747GX which does more than 100w SSB and 80w AM.

Both tubes are glowing. Grid current appears about normal on all bands.

On the 2 bands that are working right - proper load tuning is near (not at ) the ends. On the "bad bands" the load cap is near the center. On these bands I see a very slight dip (really could use a vernier dial). More pronounced on the 40m and 80m.

 

[HiDef]

A couple of places to check.

If the amp seems OK on 80 and 40 but puts out little power on the higher bands with the tOObs blushing it's time to look at the output circuit.

The bandswitch progressively shorts more coil as it is switched higher in frequency bands. That short requires some current capability and may check OK on the bench but open up while carrying R.F. Cliplead across the coil in parallel with the bandswitch to check that.

Any coaxial cable inside the amp on the output side should also be suspect.

Good luck. Keep one hand in pockie.