RadioBanter - View Single Post - Replacing metal octal with miniature tubes

December 12th 04, 01:09 AM

In article .com,
writes:

Is it possible to replace certain octal tubes with equivalent miniature
in an older receiver and would there be an performance improvevment?

Just mainly looking to boost performance.
Any advice, thoughts, comments appreciated.

The answer is "it depends"...

There are miniature equivalents or near-equivalents of many metal tubes.
(AFAIK, all metal tubes are octals, btw). For example, the miniature 6AQ5 is a
very close equivalent to the 6V6, except the interelectrode capacitances are a
little different and the maximum ratings somewhat lower. If you can't get the
metals but can get or have the miniatures, such a substitution can be really
useful.

Performance improvement is more complicated. Development of metal receiving
tubes pretty much ended during WW2, and postwar development was almost all
aimed at miniatures. So you can find miniature receiving tubes with much better
characteristics. For example, about the best remote-cutoff pentode metal tube
you'll find is the 6SG7, which is pretty close to the miniature 6BA6 in
performance. Later miniature remote-cutoff pentodes such as the 6BZ6 and 6EH7
have more gain and less noise, and can really hop up an older receiver.

But you can't just substitute them and get a big improvement, in most cases.
The different capacitances can require realignment, the voltage/current
requirements are usually different, etc. Most of all, the higher-gain new tubes
may cause stability troubles. All of these difficulties can be overcome, of
course, but doing so requires a bit of redesign of the various stages.

There was a time when such tube substitutions were commonplace in ham gear,
because a lot of hams could better afford hopping up an old receiver rather
than buying a newer one. Nowadays, historical preservation is often the
deciding factor.

What receiver(s) are you trying to improve?

73 de Jim, N2EY