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Re: [Amps] AL-80A tube glow

To: "Mark Beckwith" <>, <>
Subject: Re: [Amps] AL-80A tube glow
From: "Tom W8JI" <>
Date: Sat, 7 Jul 2007 20:15:22 -0400
List-post: <>
> Hmm.  Okay.  I have used amps (all GG) with these kinds of 
> tubes and
> sometimes they glow more than other times.  Why is this? 
> It seems like it
> has happened when one of these conditions exist:
> 1. out of resonance
> 2. overdriven
> 3. bad match

Hi Mark,

The only significant thing that makes the anode glow red is 
power dissipation in the anode. That is purely a function of 
the DIFFERENCE between plate input power and RF output power 
at the tube anode. A reasonable estimate is found by taking 
the anode dc power input and comparing it to the RF output 
power. This would be under a long term average of power 
conditions, not peak power or peak envelope power. This is 
because the thermal inertia of the anode averages things 
over a few dozen seconds or longer.

If an amp is averaging 1000 watts dc input and averaging 600 
watts RF output, the anode dissipation would be 400 watts so 
far as long term heating and color goes.

The problem is most meters won't integrate the power over 
long (20-30 second) periods, so we often have to use carrier 
conditions and know the real dissipation is less.

This is actually the basis for IVS, intermittent voice 
service, ratings. The low duty cycle allows tubes with 
larger grids (non-plated, like the 3-500Z) or anodes to be 
abused without harm. This is why 811A's with 60-65 watts 
dissipation can output 200 watts with reasonable life if 
operated voice or CW, and why tubes like the 3CX1200 can run 
10 or more kW pulse output power. The average power is low, 
the elements are large and have thermal inertia, and so the 
heating is significantly less than the same dissipation with 
longer duty cycles.

FET's can work this way also. I've used small FET's like 
MRF150's in pulse applications where two 150W devices ran 
over 1200 watts output.

The big Ham myth is that SWR affects heating or life of the 
tube. It does not affect a transistor or a tube, unless the 
tank circuit is fixed tuned or adjusted out of range. All 
our solid state radios, unless they have built in 
auto-tuners, require close to a 50 ohm load or linearity and 
efficiency can suffer. Tubes are the same, except the 
adjustable tank eliminates the need for the tuner.

The only components in the amp that are affected by load SWR 
are the loading cap and things from that point downstream to 
the antenna. Inductor current can also change, but it is 
generally a more minor change. The tube and the tuning cap 
never see the SWR **if the tank is properly tuned to the new 

Overdrive mainly affects the control grid. The amount of 
loading greatly affects the amount of overdrive, because the 
tank tuning establishes the load line of the tube. If I 
mistune an amplifier with excessively light loading ( too 
much capacitance for the power) the loadline is much too 
high an impedance. This increases grid current and tank 
voltage (tank voltage can be several times dc voltage) and 
destroys linearity. It's very tough on components, although 
anode heating is much less.

If we load the PA too heavy linearity generally stays good, 
but the load impedance on the tube is too low. This 
increases dissipation in the anode, but reduces stress on 
other components.

Watching the grid current while talking on voice, unless you 
have a peak reading grid meter, is almost useless. We really 
need to apply a quick carrier with full drive and look at 
the grid meter. A 3-500Z is typically approaching rated grid 
dissipation at 150mA during a steady carrier,  although that 
varies with the particular operating conditions. The 3-500Z 
is not sensitive at all to exceeding the grid dissipation 
for short periods because the grid is a material that can 
withstand a very high temperature, almost incandescence.

On the other hand a tube like the 3CX800 or 3CX1500 can be 
ruined with only seconds of excessive grid current, and the 
damage accumulates over time even if it isn't ruined 
instantly. That's why any tube like the 3CX800 or 8877, 
using a metal oxide cathode, requires a very fast electronic 
grid overload trip circuit. I'd personally never own or 
build an amp with a MOx cathode without a positive acting 
electronic overload, not having one is just plain stupid. 
I've got three Ten Tec amps here now with blown 3CX800's 
from excessive grid current during operation. One amp owner 
had an antenna fail, and before he saw the LED flashing he 
cooked the grids. That's an 800 dollar problem that would be 
easy to repeat over and over caused by lack of a $1 op amp.

73 Tom

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