[MrFixr55]

Hi Dale, 

Attached is the Tung Sol Spec Sheet I described. Like RCA, they do not list different specs for 42 or 2A5 (in RCA RC12) or between 42, 2A5 or 6F6 (Tung Sol), other than heater voltage / current.  There are 2 different listings in the Tung Sol document for triode Push Pull operation.  "Typical" states 6K Ohm for triode connected Push Pull, there is a listing for "cathode bias" showing 10K plate resistance for AB2 Push Pull operation.

2A5 6F6 42 Tung Sol Spec Sheet.pdf Size: 222.18 KB  Downloads: 75

However, this is academic, as the Philco docs that Ron has stated 6K Ohm Plate resistance.  Of course, this info is all lightyears before computers, so either there are mistakes, spec differences between manufacturers or whatever.

[Arran]

If that is the amplifier section of the model 650 then it's using triode connected #42s, I think in a class A-B setup. Philco used to make a lot of sets like that, the audio interstage transformer, unlike with a typical class A amplifier, is actually a step down transformer, if it has failed your options are limited. Speaking of the audio output transformer a 10 watt may not be large enough, I seem to remember that those triode connected amps put out up to 15 watts or more.
Regards
Arran

[DaleHCook]

"Typical" states 6K Ohm for triode connected Push Pull, there is a listing for "cathode bias" showing 10K plate resistance for AB2 Push Pull operation.
...
However, this is academic, as the Philco docs that Ron has stated 6K Ohm Plate resistance. Of course, this info is all lightyears before computers, so either there are mistakes, spec differences between manufacturers or whatever.

6k plate-to-plate is correct as these are triode-connected 42s with fixed bias. If they were cathode biased they would be 10k plate-to-plate. I see no mistakes. The RC-12 data is simply more specific in naming fixed bias.

[MrFixr55]

Hi Arran,
I had posted 2 diagrams. The first one with the red boxes is the Philco 650. Fortunately, it was the output transformer that failed. Hamilton measured his interstage transformer and it was good.

Ron Ramirez has written a very good article on correcting for a failed interstage transformer primary. I don't know if it works if the primary of the interstage transformer failed in this set. A 42 is used as a driver because the grids are driven positive at peaks in AB2, therefore not just voltage but power must be generated by the driver stage. I suppose that RC coupling could be used but there may need to be a large cap to do so.

The second diagram is of a Heathkit W-6M Amp. This is a very powerful amp using Class AB, fixed bias from a separate supply as opposed to being derived from dropping resistors in the B supply return. They did not use an interstage transformer but used a 12BY7 in cathode follower mode. (I think that 12BY7s were popular as vertical oscillator / output tubes in mid 1950s TVs, therefore likely could put out enough power to drive a Class AB Amp. Heck, this beast puts out 70W RMS, 140W peak.) I am posting the W-6M Manual for anyone interested. The specs and what heath did in the manual to prove these specs are amazing, especially for 1957.

[Radioroslyn]

Heathkit got a deal on those 12BY7's. Back in the mid 1950's they use them as the audio driver tube for the modulator section in their 100 watt class AM transmitter the DX-100. P/P1625 (12v 807's) which will make about 120w class ab2. It's transformer coupled and is grid biased from the hv power supply. You'll note that there are two 1/10 of an ohm resistors in the cathode circuit of the 1625's. These are not for biasing but are meter shunts.

https://archive.org/details/heathkit_dx-100_manual

[Ron Ramirez]

> Ron Ramirez has written a very good article on correcting for a failed interstage transformer primary.

I can't take credit for that, the information came from John Grady - jgrady on ARF.

The article in question:
https://philcoradio.com/library/index.ph...nsformers/

[MrFixr55]

Sorry, Ron, I thought that at least you posted it, If you did, Thanks! If not, Thanks, anyway for all that you contribute to this hobby! John (If you ever are on the Phorum), Many Thanks!

[MrFixr55]

@RadioRoslyn, Heathkit must have gotten a good deal on a lot of tubes. I had (and still have) in my archeological dig (Garage) several Heathkit amps, including the entry level A7 which has 12V tubes (12A6s right out of ARC5 receivers, 12SN7s, etc), many branded "JAN" (Joint Army Navy) Gov't surplus. I'm sure that they saved money buying them, but I wonder if they had to blow coin on power Xformers with 12V heater windings. The more premium amps had premium tubes like Tung Sol 5881s, Genelex KT66s (I wish I had a few pairs of them) and Tung Sol 6550s (I wish I had a slew of them, the real original USA ones, not the foreign reissues) Unfortunately, I have no true "pairs" but have at least 1 each of the A7, W-3M, W-4M, W-5M and W-6M that I have to restore, or at least fix up.