{Collins} Re: Collins 30L-1 Help

Your webpage on this topic says to tune for maximal drive. I can push my Collins 32S-3 exciter beyond the recommended operating output of 230ma plate current and Collins recommends only 200ma when driving the 30L-1 amp.

The only way to tell what is going on SSB is with a real peak envelope reading meter with good storage time, a spectrum analyzer, or a scope (in that order). Here is how it works.

If we only TUNE for maximum output or rated grid current (or any other parameter) at a given drive level, that is the virtual limit of instantaneous peak power we can allow without going into non-linearity.

The reason is speech (and sometimes even a CW envelope) has very short duration peaks. It is those peaks that the amplifier has to handle without nonlinearity.

If we underload or undertune for those peaks two things happen:

1.) The impedance seen by the PA tube will be too high. This means the tube will pull anode voltage down below the limits of the linear area of plate voltage. The PA will become non-linear and not follow the peaks.

2.) With even slightly more drive than the initial non-linear drive, the tube rebounds on the positive anode swing to more than double the anode supply voltage. The abnormal voltage stresses components in the PA, like tuning capacitors and band switches. You can see how high voltage can get here:

Tuning to xx mA is just fine, so long as we NEVER allow the peak current to go past the same xx mA. We cannot depend on the tuning to limit the power, or we have RF envelope clipping. We have to decrease audio input (or early stage gain).

If you have a REAL peak reading meter with a fairly long storage time (like an Ameritron AWM-30) you can load up to some power level and then watch peaks and never allow the meter to exceed 90% of those peaks. Collins had an excellent idea when comparing drive power to output power, but 30 years ago a true peak reading meter was not practical to build. This means we can't tell if peak linearity is good, we only see the average on our old boatanchors.

With my AWM-30's I can actually see transients from bad relay timing! If I load to 1500 watts PEP, I can see if any peaks no matter how short reach or exceed 1500 watts. It is impossible to do that with average or meters that do not register and hold the real peaks.

73 Tom

the 32S-1 at 230-250ma plate current I can then push the 30L-1 to almost 850w on my highly calibrated Powermaster meter. Is tuning to that peak output really healthy for the amp (and the 32S-3) ?? Also, what do you recommend when driving the amp with a higher output rig like most modern solid-state... how do you decide how much drive is too much before turning
to an antennuator before the amp?

Thanks for the great post.

73 Jerry

-----Original Message-----
From: Tom W8JI [mailto:w8ji@xxxxxxxx]
Sent: Sunday, December 10, 2006 5:48 AM
To: jsternmd@xxxxxxxxxxxxx; d12190@xxxxxxxxxxxxxxxx; COLLINS@xxxxxxxxxxxxx
Subject: Re: {Collins} Re: Collins 30L-1 Help

I'd just like to add a few points to the good instructions
already given.

If you have an antenna or antennas that require a tuner, the
proper proceedure is to install a high power tuner between
the antenna and the amplifier.
Antenna tuners for reducing antenna SWR belong between the
amplifier and the antenna, not the amplifier and radio.

1.) Automatic antenna tuners are, in general, very bad news
between a radio and amplifier. The reason is the input
impedance of most ampiliers is dynamic, it changes with
drive level and amplifier tuning. It also may, with tubes
less than class A conduction angle, contain some harmonics
generated in the amplifier input. An automatic tuner will
often not find or lock on the correct matching conditions
because of these problems.

2.) You should always load or tune the amplifier slightly
beyond the peak envelope power you intend to operate at.
This is to ensure peak linearity. If you load at lower power
and stop without slightly overcoupling, it is rough on the
amplifier (it can cause arcing or excessive grid current)
and you will splatter. See:

3.) It is perfectly OK to rock capacitors slightly at full
power for a final minimum SWR adjustment when tuning a
tuner, but do NOT rock any switch contacts!!  Also a T
network tuner, if you have that type, produces maximum
efficiency and handles the highest power when using the
maximum capacitance and minimum inductance that allows
matching the load.

4.) Finally, my finger nail on a blackboard reaction.
Antennae are on bugs and insects, antennas are on our
radios. Antennas by definition are for reception or
transmission of electromagnetic waves. Antennae by
definition are "feelers". Collins and everyone else uses or
used the plural antennas.

73 Tom

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