[Moon-net] Amplifier technical questions

[Leif Asbrink]

Hi All,

I have not followed this thread, but I must react on
Kims statement! 

If the tank is degraded to the extent that the bandwidth 
is notably affected when copper is replaced by brass, 
the amplifier falls into the category I would label 
as "not working".

Surely one can use brass on 144 MHz - but only if one 
knows what one is doing. Copper is better and requires 
less skill in the design of the tank. Iron is very 
dangerous if it is at points carrying large currents.
Even small screwa may be really harmful. A thin layer
of zinc on an iron screw makes it totally harmless.

Assume the tube is loaded by the antenna and the ohmic 
losses of the tank only. Then the losses will be the 
square root of 3.49 times higher with brass compared 
to copper. The surface resistivity goes with the square 
root of the bulk resistivity due to the skin effect.

Only if nearly all the power is dissipated in the 
tank circuit it will be possible to approach a bandwidth 
that is 1.86 times larger with brass compared to copper.....

If we assume a tank efficciency of 80% with copper, 
the efficiency would drop to something like 63% with 
brass but the bandwidth would not be much affected. 
With copper the loss resistor referenced to the 50 ohm 
output would be 250 ohms while it would be 135 ohms for
brass. The total load for the amplifier would be 50 
ohms in parallel with one or the other loss or 41.66 
and 36.49 ohms respectively. The associated increase 
in bandwidth is 14% only.

LOSS OF POWER AND EFFICCIENCY COMES MUCH FASTER THAN 
INCREASED BANDWIDTH!!

To make good amplifiers, look for highest possible 
L/C ratio. Do not add needless capacitances. Tune 
with a small capacitance and do not couple the low 
impedance output through a capacitor to the high 
impedance point at the anode. That would cause a 
lot of capacitive current that has to be balanced 
with a smaller inductance to ground. (Exactly as 
excess tuning capacitance) Lower inductance means 
more current (at the same voltage). Losses go as 
current squared!!!!!

Excess capacitance makes loaded Q higher with 
stability problems as a consequence in addition to 
loss of efficiency.


Allow the current to flow onto as much area as 
possible directly as it comes out of the tube. 
Losses are proportional to the square of the 
current density (integrated over area). More 
area means that you need a longer path for the 
current to flow (to get the desired inductance
that is needed to balance the tube capacitance), 
but further out the area can be even larger. 

The ideal tank is a cavity where the current 
flows radially out from the tube at the anode 
and RF-grounded control or screen grid. The 
distance between the two current-carrying surfaces 
should increase softly to maximize the enclosed 
volume inside the cavity at a minimum current path 
for the desired resonance. On 144 MHz a flat cavity 
with 10 cm separation has a diameter of about 110 cm
and gives a tank efficciency of about 99% when 
aluminium is used. Surely overkill on 144 MHz...
A a reasonably sized brass design is surely possible 
if a tank efficiency of something like 90% is 
acceptable.

73

Leif / SM5BSZ

 
> Re: Regarding the use of Brass as a anodeline
> 
> I know the use of Brass is not the vy best choise ! Thats correct Chris - 
> but this is for 2M, so you will only get a bit lower tank Q, and hereby
> a bit broader tank, which absolutely is to prefer.
> 
> 
> 
> OZ5IQ, kim