guidance

[Dafudd]

last night amp worked ok .
my ameritron al811 has plate volts1800
grid current moves when keyed
but has no plate current valve,s light up ok
any sugestions as where to look for the fault.
this a 1996 model

 

[Shockwave]

last night amp worked ok .
my ameritron al811 has plate volts1800
grid current moves when keyed
but has no plate current valve,s light up ok
any sugestions as where to look for the fault.
this a 1996 model

If you're not showing plate current or output power when driven and only show grid current in proportion to drive power, this is a strong indication that plate voltage is not actually arriving at the tubes. Especially if the grid current is abnormally high, because all drive power is then dissipated in the grid, in the absence of plate voltage.

I don't see a lot in between the plate voltage meter and the actual plate of all four tubes that would be in common other than L4. It's at least worth checking to see if that plate choke has an open. Make sure the filter caps are discharged before you do anything.

 

[linearone]

If you're not showing plate current or output power when driven and only show grid current in proportion to drive power, this is a strong indication that plate voltage is not actually arriving at the tubes. Especially if the grid current is abnormally high, because all drive power is then dissipated in the grid, in the absence of plate voltage.

I don't see a lot in between the plate voltage meter and the actual plate of all four tubes that would be in common other than L4. It's at least worth checking to see if that plate choke has an open. Make sure the filter caps are discharged before you do anything.

An open plate choke or lead somewhere is where I'd look, something on that little parasitic board above the tubes, or look on the supply board for a spot where a diode used to be. Usually they evaporate and you get 0 reading on plate.

 

[loosecannon]

ive had to replace the plate choke in two al811s over the years.
both times the wire had just broken.
LC

 

[linearone]

ive had to replace the plate choke in two al811s over the years.
both times the wire had just broken.
LC

W8ji speaks alot about how and why he designed that choke the way he did. He basically said a series resonance was inevitable on that amp so he chose to "park" a resosnance on a band where it wouldn't be used, 27mhz. It makes me wonder if that's why all the broken ones Ive seen have been used on CB.

 

[Dafudd]

Many Thanks to all of your suggestions.
Found a resistor had shorted on a mod made to amp as sugested by
M0UKD in 2013 ,the 630 mil amp fuse had not blown which totaly threw me
thanks again AMP is now working big smile hello dx .

 

[Shockwave]

W8ji speaks alot about how and why he designed that choke the way he did. He basically said a series resonance was inevitable on that amp so he chose to "park" a resosnance on a band where it wouldn't be used, 27mhz. It makes me wonder if that's why all the broken ones Ive seen have been used on CB.

Not for nothing however, at this frequency how hard do you think it would have been to shift the resonance point one megacycle above 10 meters, rather than one megacycle below 10 meters? I just find this to be a little bit more than coincidental.

 

[linearone]

Not for nothing however, at this frequency how hard do you think it would have been to shift the resonance point one megacycle above 10 meters, rather than one megacycle below 10 meters? I just find this to be a little bit more than coincidental.

My thoughts exactly.

 

[linearone]

For anyone interested:

RF Plate Choke

Designing and testing RF Plate choke for power amplifier

www.w8ji.com