Re: 7623A Storage problem


Colin Herbert
 

I have just checked all of my 2N3055 stash using a magnet and they all have
steel casings.

I am afraid my electronics knowledge is too limited for me to be able to
knock-up any test-rigs to do electrical checks on them. Any hints?

TIA, Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Thomas
Garson
Sent: 04 October 2020 20:10
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7623A Storage problem

If your 2N3055s are in aluminum packages, they are likely Motorola or
Fairchild fab, and will be epitaxial (high gain). If they are in steel
alloy cans, they could be RCA, which would be hometaxial, or slower and
lower gain. There were a lot of smaller fabs making 2N3055 because that
part number had become ubiquitous. Most, perhaps all, used hometaxial
process, and steel cans. Possibly those foundries had acquired obsolete
lines which had been surplussed out by the bigger operations when they
changed over to epitaxial.

You didn't specify what collector current your tester operates at. I
suspect its in the low ma range. At 10 ma, hometaxial parts would show
hFE close to your 30 & 34. hFE maxes out at about 70 at 3a. I wouldn't
use the part with hFE of only 12, or the high gain part.

I think that when RCA was killed by GE, their power devices were still
primarily (exclusively?) hometaxial. The RCA 40436 speced identical to
the 3N3055. I used a number of them in one off power amps and linear
supplies during the early 1970s. Another RCA option would be 2N3772.

I agree that it is not a good idea to install newer epitaxial devices in
a circuit whose design hails from before about 1975. There is a good
chance the part will not behave nicely. As the unwashed might say:
"Magic smoke may appear".

Somewhere, I've got a bag of old, but mostly unused, 2N3055s, should it
become an issue. All the ones I just found in a Google search were
planar epitaxial, which work fine in newer designs that expect the
higher speed.

Anecdote: 1972. To get cheap high power for small time rock bands, I
would take 2 Peavey PA120, which was a 2 ohm capable 1ch power map
entirely built on a 5u rack panel. PA120 used house branded 2N3055
equivalent devices. Circuit probably copied from RCA Transistor Manual.
I put two of them in one cabinet, back to back with end to end fan
added, configured for bridged mono operation: Easy 500w.r.m.s into 4
ohms. If one blew up (not easy), I used RCA 2N3055s as replacement
devices. They were very reliable.

Thomas Garson
Aural Technology, Ashland, OR
By my calculation, the dynamic range of the universe is roughly 679dB,
which is approximately 225 bits, collected at a rate 1.714287514x10^23 sps.

On 10/4/20 8:26 AM, Colin Herbert via groups.io wrote:
Thanks for that, guys. I have three old 2N3055 in my "junk box". One
doesn't have a manufacturer's name but is also labelled 3055H and has (using
my MK-328 transistor tester) what I assume is a beta ("B") of 30, another
labelled ITT has a beta of 34 and one marked RS has beta of 12. Which of
these would be appropriate, if any? I also have a newer one with a much
higher beta, which I assume is not good in this application. Please forgive
my ignorance on this.
TIA, Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Tom
Lee
Sent: 04 October 2020 12:12
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7623A Storage problem

As David Partridge says, it's risky to pop in a modern replacement. It's
not just a matter of gain (I assume you meant beta). The original 2N3055
was made by a now-extinct "hometaxial" fabrication process, which
produced surprisingly rugged, inexpensive, and very slow devices. Modern
planar versions of the 3055 are much, much faster, but also less rugged.
The higher speed is not always a virtue -- it can provoke wild and not
infrequently destructive oscillations in circuits that rely upon the
slowness of original 3055s for stability. It is often possible to add
capacitances to make modern devices slower in the right ways, but it's
an ugly circuit-specific kludge that doesn't fully reproduce the
glorious sluggishness of the original.

-- Tom

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