McMurdo Silver Power Supply/Amp, Unidentified

[Russ Webb]

I have a version of the MPV - MPVI power supply that has 5 tube sockets on it rather than the 6 normally seen on the MPV or the 4 used on the MPVI. Has anyone seen one of these? I have not identified what tube goes in the 5th socket yet.

[Carlos Insa]

Pics please!

[Gerald Steffens]

That’s a new one on me, as a Silver modification.  Does it appear to be a factory installation?

I have heard & seen some discussion from some ham types (on the old Boatanchors reflector somewhere in the late 1990s) of putting a voltage regulator on the B+ feed to the tuner chassis on Silver radios (that would be on the 345 volt range if I recall correctly).  The obvious thing is stability.

Some guy was bragging up the conversion of a Silver power supply for use with his home-brew receiver made from an article in the Radio Amateur’s Handbook.  I believe the discussion centered around circuitry, or chassis, originally from the illusive 14-15 ham radio for which nobody I know has ever seen a schematic but we know at least a limited few were produced.

The 14-15 had TRI-BAND IF selectivity (3kc flat-top, 5kc flat-top and 10kc flat-top). Nothing variable, rather switched caps. This produced 60 db rejection for stations as little as 5 kc., 9kc. or 16 kc. away from the desired signal when using the 3 passbands. If Oldtimers Disease hasn’t affected me, some ham(s) came up with this modification to aid in stability in the narrow selectivity position, obviously for cw operation in times when transmission & distribution voltages were very unstable.  After WWII the distribution voltages would get as low as 90 to 92 volts in some areas – my mom told the story that at noon her burners on her electric stove wouldn’t even get red hot.  The base 14-15 radio was, reportedly,  quite stable but the varying input voltage kind of messed things up.  I will check my Silver brochures to see if there is any mention of a VR in the 14-15.  I found none but there is an extensive discussion of the extreme IF selectivity.

I will see if I have that discussion from the old Boatanchors group before it became Qth.net (somewhere in the late 90s I would guess).

Shoot me a message with a phone number and we can talk.

I am a Scott and McMurdo Silver collector.  I have all but five known Silver chassis (4 tuners 5A, 5D, MP-VPA and the MP-IIIX and one amplifier/power supply – MP-IVA three tube version).  I also have extensive Silver literature.  I should clarify, no known 14-15 or Orpheums exist today to the best of knowledge,

Talk with you later,

Gerry

[Russ Webb]

Gerry, thanks for the reply.

I should have taken some pictures and I will ASAP. That radio belongs to a friend that has quite a collection of MPVs and MPVIs. None of the radios is complete - yet. Actually, what he has is a large collection of parts and cabinets which we are trying to reassemble. I guess that having only one questionable part is OK. I was honestly more interested in the 25hz MPVI PS/amp at the time and did not examine the underside of the 5-tube chassis. I will add that the 5th tube is mounted in the center between the "normal" 4 tubes of the MPVI unit. Again, there were no tubes installed at the time.

I have also heard another theory about the propose of the mystery tube, but rather than quoting it here, I will investigate since it should be easy enough to verify.

[palegreenthumb]

So I'm pretty curious about this, Russ, keep watching for updates.

Have you solved the mystery of the 5-tube amp yet?

-Rodney

[Russ Webb]

Rodney, I have decided to spend less time at most of the radio sites. Maybe I can get more done.

This is a long story, as you might expect over 80 years.

The fellow that had been tasked with the restoration several years ago, never completed these. He was under the impression that this amp was unmodified. That is certainly not the case.

Of the 6 MS radios I have been working on, from that group, a similar theme occurred in most of them - at some point the tech/owner/whoever had a problem with audio level. Tube functions had been changed, bases had been rewired and in this case, a 6SN7 (or similar twin triode) was added. It appears that a failure of the interstage may have inspired this OR AND MORE LIKELY this amp was used by its self and additional amplification was needed. In any case the tube was added.

This modification also incorporated an additional audio input using two banana jacks wired to the cable. The additional conductors required the speaker plug to be changed to an octal, making the addition of an MSVI speaker NOT plug-and-play.

The extra audio stage results in very loud operation at about "3" on the volume control and at least some audio at "0". This extra gain does nothing for noise and hum. The only possible benefit might be a bit better freq response.

The MSVI receiver had been "recapped" at some point and the owner had been hoping that a good amp/speaker was all that was needed. So I only addressed the 18" SG and the amp. If it was mine I would have restored the interstage transformer, but since it is to be sold I only made the amp work which included replacing the original speaker plug (compatible with the SG 18 MSVI) and reinstalling an on/off switch. In an attempt to reduce the drive to the 6L6s, I substituted a 12SN7 which was an improvement. I also tried a 12SL7 with not much change from the 12SN7.

It is possible that I may need to work on the receiver if not sold "as is". At the moment a MSMV is all that remains to be restored.

The V and VI that I put into service for the same owner work very well and look great flanking the Z1000.

Russ

[Russ Webb]

More on the MSMV and MSMVI restorations:

https://www.russoldradios.com/blog

[Jiri Placek]

Russ, thanks for the 18" speaker info in your MSMV / MSMVI blog.  That is is exactly the information I have been looking for.  Regards, Jiri

[palegreenthumb]

Wow, that's a lot of McMurdo Silvers all in one place!

Thanks for the information about the 5-tube amp, mystery solved.

One of those Super Giants was originally paired with the MPVI receiver and amp that I own.

Of the 4-tube MPVI amps you have, how many have 2 chokes and a 5-pin speaker connector, and how many have 1 choke and a 7-pin speaker connector?  Mine is the latter variety, and would pair to a speaker through a 7-wire cable. Although maybe by the time you got it, the speaker was converted to a 5-pin socket.

[Russ Webb]

None of the ones that I had/have has that configuration.