[ARC5] Modifications...

[Neil]

> Those folks were no dummies, and in most cases, were thinking of things 
> and situations that would never, ever even enter the modern hams' heads.
> 
> In my somewhat limited experience, most "ham" mods do far more harm 
> then good, unless VERY carefully thought out. One of the FIRST criteria 
> before contemplating a modification is to clearly and completely understand 
> what the original designers' ideas were, and WHY he/they did what they did.

The designers of the ARC-5 receiver were under pressure to reduce
tube current consumption, which as a byproduct would keep heat to a
minimum inside the receiver tube compartment and ensure longer life
for all tubes. They were not under pressure to provide a lot of audio,
since the audio stage would be driving headphones.

They chose the 12A6 (12v version of the 6A6), probably because it had
a heater current of 0.15 Amp making it eminently suitable for use in the
series-parallel heater string, was capable of far greater audio output than
needed, and by throttling its current consumption back with high cathode
bias resistor it could deliver the necessary audio while giving longer life
than if run at normal bias resistor/current values.

The original output transformer is not suitable for speaker operation and
most receivers have had a normal single-ended transformer fitted, of the
type used in domestic radios. If you get adequate audio into a speaker
with the original 12A6 cathode bias resistor and one of these transformers
then you don't need to change a thing.

However, if needing more audio eg. in a mobile or other noisy application,
you could consider changing the resistor to 390 ohms to give greater
undistorted audio capacity. As pointed out elsewhere, this will increase
the current drawn and the heat dissipation inside the tube compartment,
but these effects can be offset by reducing the receiver's B+ voltage.

I'm not pushing any particular set of circumstances. I'm simply offering
advice about the 12A6 bias conditions to those who may find it useful.

73 de Neil ZL1ANM