[AMPS] Re: 3-500Z/ G

[Tom Rauch (W8JI)]

km1h@juno.com wrote:

> >What is the breakdown V of the AL-82 bandswitch?
> 
> Ameritron rates it at 6KV 20A but  it is just a standard
> Centralab/Electroswitch JV series (new E4 series) as used by Alpha, Amp
> Supply, MLA-2500, etc. The old Centralab and new Electroswitch catalog
> rates it a 3KV AC and 17A max.

Two sections are paralleled, and the contact arrangement is changed.

It is not a standard JV series.

>  > Is the AL-82 bandswitch  similar to the AL-80 bandswitch?
> 
> No, the AL-80 uses the 2500 series switch as was in the SB-220, SB-1000,
> Clipperton -L.

Again the contact arrangement is changed. The ten meter tap is moved
away from the grounded mounting screws, and the rotor is cut
differently.

> I agree, but it is unlikely to be VHF either, it is more likely to be
> quite close to the operating frequency and a design flaw associated with
> the Pi-L.

What does a pi-L "design flaw" have to do with a overheated choke?

Think about that a while. 

The ONLY reason the chokes burn up is when they are operated too close
to series resonance. Series resonances are "parked" at 12, 16, and 26.5
MHz in the AL-82.

There was a brief periood of time when someone on the assembly line
decided to NOT test plate chokes for series resonant frequency. That is
a bad idea, because even a material change in the ceramic can move the
series resonant point into a ham band.  

> Why is the 3-500Z so squirrely when the 3CX800, 8877, 8874 and the like
> are not. VHF gain is not the answer since those latter tubes will run
> full bore in the 250-450 MHz range. Why are some 3-500 designs bullet
> proof? Can anyone see a common thread?

The common thread is obvious Carl, if you slow down and think about it.
The AMOUNT of high voltage has a lot to do with the number of problems,
look at the Vectronics PA.

The reason 3-500Z's are less stable than tubes with high VHF gain is the
self neutralizing frequency of a 3-500Z is lower. The internal grid
leads are long, and the grid becomes poorly grounded at VHF. The AL-80B
and AL-80A oscillate at about 180 MHz, if the suppressors are removed.
The AL-82 at about 165 MHz. 

The 3CX1200A and D7 oscillate at about 100 MHz with a good socket and
layout.

The 811A oscillates at about 40-70 MHz.

Most VHF capable tubes will not oscillate at all if the grids are
grounded with very short leads and the layout is clean.

The longer and thinner the grid leads, the more headache the tube
becomes to stabilize. The problem is aggrivated by high anode voltages.
Components downstream of the plate tuning cap have nothing to do with
VHF stability. 

The tuning capacitor is a virtual dead short for VHF energy (unless the
tube oscillates at low VHF). That makes bandswitch arcs due to
parasitics virtually impossible, unless the PA is oscillating near or
below the operating frequency. 

None of this is that mysterious.

73 Tom


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