AL-811H plate current meter jumping

[N8EUI]

Hi everyone,

Was loading up my '811H and suddenly the plate current/plate voltage meter begins to bounce left and right a number of times until slowly settling on zero.  I discovered if I place my finger on the meter switch and move it, I can replicate the left and right movement.  Anyone else have this happen to them?  Before I open up the amp, do you think if I applied some contact spray that would cure it or should I forget about it and open it up anyway and install a new switch?

Thanks,
Tom, N8EUI 

[N8EUI]

Crisis averted.  I sprayed some contact spray in the switch and it's all better now.  However, another issue has developed.  When loading up, my plate current is reading high.  It should read 125ma when keyed down and no drive.  It's reading 180 ma instead.  Any ideas for a solution?

Thanks,
Tom, N8EUI

[NO2A]

What happens if you adjust the plate cap? Does the meter show any power out? Does the plate current go down? Does the grid current show any indication?

[N8EUI]

Hi NO2A

Yes, I tried that and there is no change.  I'll keep plugging away at it.  Thanks for the advice.
Tom, N8EUI

[KH2G]

Not sure what exactly when you said "yes" to the previous post but it appears you had a cruddy switch and just maybe you had 180mil all along. You may want to also double check drive but for a fact, as you rotate the Plate tune the meter should show a dip unless your coupling is too tight then you may not see a dip.
Regards
Dick KH2G

[N8EUI]

Hi Dick,

My "yes" is in response to NO2A suggesting I adjust the plate cap.  However, I'll again try rotating the plate and look for a dip.  The Ameritron AL-811H manual says that when the band is first selected with the bandswitch and the plate and load caps are adjusted per the manual and the exciter is keyed with no drive, the plate current should read 125ma.  My meter is reading 175ma.  I emailed Ameritron and they suggested I check three possible scenarios: 1) One of the tubes is in backwards. Look at the tubes and make sure all four are lighting.  2) The 0.6 ohm resistor in the circuit is open. 3) One of the grid pins has come loose from the ground point.

The four tubes are lit.  I'll have to open the amp up and check the other two suggestions.

Thanks for the advice.
Tom, N8EUI

[W8JI]

I emailed Ameritron and they suggested I check three possible scenarios: 1) One of the tubes is in backwards. Look at the tubes and make sure all four are lighting. .

Bad suggestion. That would cause low current, or cause no output, or cause backwards grid current with no drive.

2) The 0.6 ohm resistor in the circuit is open.

Bad suggestion. That would cause the plate meter to pin against the stop really hard, even at idle.

3) One of the grid pins has come loose from the ground point

That causes low output, and some really weird things.

You probably have an older amplifier before the bias system was added. Check to see if there is a string of diodes at the bottom of the board on the rear panel, b down at the bottom near the filament choke.

My bet is you have an amplifier that is older and does not have the bias diodes added. Newer tubes have higher idle current than the first tubes had, and will idle around 180 mA without drive when keyed.

[N8EUI]

Hi W8JI,
The amp was built around 2002.  The serial number is AL-811H-19356.  I'll open it up and take a look.
Thank you,
Tom, N8EUI

[NN4X]

Would "Ameritron Tech Support" be the same folks who build the amps?