(SOLVED) SB220 wont load up

[357]

I just rebuilt my SB221 (220) and it shows good idle current but only make 60w with 20w in and the load is at zero. Grid current is off the chart.
I tried cleaning and inspecting the band switch and relay to no avail.

What would do that? dead tubes?

 

[543_Dallas]

When the tubes start going soft the input swr will go up and it will take more grid current to make the same power as they used to.

Was the amp working before you rebuilt it? I would test the tubes in a known good amp if that's possible. If the tubes aren't bad it won't take long to destroy them if the grid current is high.

 

[Captain Kilowatt]

Check the meter shunts for the grid and plate current. I had a Heath TX that I thought had problems but it was the plate meter circuit that was bad. The shunt went WAAAY high and I was not loading it nearly as much as I thought I was. It showed very little output with proper grid and "proper" plate currents.

 

[357]

Would the shunts stop it from making power though?
I wont make more than 80w

Input SWR on TX was 1.4

 

[Captain Kilowatt]

Would the shunts stop it from making power though?
I wont make more than 80w

Input SWR on TX was 1.4

No but in my case the meters made it look like I was driving it properly according to the currents however the meters were reading wrong and it was not being loaded nearly enough. Plate current meter said about 200ma when in reality it was drawing about 50ma. Not much more than bias idle current. Plate tune and load controls were a bit off from normal too because of thje reduced loading.

 

[nomadradio]

If the idle current is good, the tubes probably are not flattened.

Flat tubes will behave the way 543 described. This is a frequent result of running the amplifier from a 120-Volt outlet without throttling down to about half power. Low line voltage reduces the temperature of the tube's filament. Keying it with that temperature too low can screw up the chemistry that allows full current from the cathode.

I would turn the Load control to the halfway point, key it and see if the Plate Tune control will show a peak that way. If it does, go back to the Load and set it for max modulated power, then back to the Plate Tune.

Lather, rinse repeat until both knobs are at a peak position.

Could be your Load control is just set totally wrong. It should never have to be turned that far unless there is another problem.

73

 

[357]

OK I put the load at half and was able to get 400w on cw with 1 tube, swapped and same thing.
I obviously need to set the load somewhere other than 1.
I was ready to but new tubes.
Thank you for the advice

I'm getting 982w with reduced drive due the the input swr at 1.7
Grid is happy at 225, plate 625. I hope this helps someone else cause I was stumped.
Last 2 pics are SWR and the power on RTTY
It peaks sometimes to 1100 on SSB

Merry Christmas to you all, thanks again.

 

[Captain Kilowatt]

OK so in this case the meters were reading correct but you just were not loading it nearly enough. Same results from the same problem but for different reasons. I take it you are not too familiar with tube amps? No offence meant BTW it's just that most operators familiar with tube amps would check out the performance while running thru the range of those controls. Sometimes they will find that the shaft has slipped and the amp performs normal but with vastly different positions of those controls.

 

[nomadradio]

But look at the bright side.

This is the sort of lesson you won't forget any time soon.

Congratulations for sticking with it!

73