[DaleHCook]

But opt on the H-13 are. That's been about a 50/50.

That is good to know. I hope the OP can find a UOT for a reasonable price - major brand NOS 10 watters are selling for $50 and up on the auction site. At those prices he might be better off buying a Hammond P-T125D from Antique Radio Supply at $55.35.

A word to the wise - some sellers on the auction site are listing 70.7 and 25 volt line transformers as UOTs. It pays to have lots of old electronic parts catalogs to look up part numbers and see what a part really is.

[Radioroslyn]

Here's a good source for old transformer catalogs. https://www.bunkerofdoom.com/xfm/
Main site  https://www.bunkerofdoom.com/ You may find it interesting, lots of electronic info in there!

[DaleHCook]

Bunker of DOOM has long been a favorite source for electronics catalogs. My favorite parts catalog is Radio's Master / The Radio Electronic Master, published from 1935 to 1972. It was a hardcover which could be customized for each electronics parts jobber by including only the lines which they carried, and often had that jobber's name printed on the cover. Many editions are at World Radio History.

That reminded me that I haven't recently updated my online list (with hyperlinks) of online parts catalogs. I will work on it and post a link later today.

[DaleHCook]

I have updated my partial list of my parts catalogs in print, in photocopies, and in electronic form, with links to online copies.

[MrFixr55]

Hi Dale, 

I corrected my earlier post to make it clear that since Ron had specs for the transformer, to go with them instead of the specs that I gleaned from the RCA and Tung Sol documents.
Yes, I got it out of RC12, and a Tung Sol spec sheet (and they have different specs.  RCA wanted everyone to think that they invented every tube, but I believe that the 42 was invented by Sylvania (who manufactured tubes for Philco after they stopped using RCA.  I don't know who invented the 2A5 or which came first (Philco was an early advocate of using 6V tubes in AC sets, possibly because they used them in car radios.  RCA was still using 2.5V tubes in 1933 in their home AC sets.  

When looking up the 42, in RC12, the reader is directed to the 2A5.  Both RCA and Tung Sol claim that, other than base (in the case of the 6F6) or heater specs (in the case of the 2A5), all the other specs are the same.  It seems to me that in the 650, the tubes are" cathode biased".  In the method popular in the 1930s, the cathodes are at chassis ground potential, but an unregulated C- analog (for lack of a better term)is developed by having resistance between Chassis ground and the center tap on the power transformer HV Secondary.  The center tap of the interstage transformer secondary goes to the desired tap on this resistor.  This is the same as putting the resistors between cathode and B-. IMHO, fixed bias involves a separate rectifier on a separate winding, like many Fisher HiFis, Ampeg B-15 Guitar amps, the Heathkit WA 5 and WA6 Williamson amps of the 1050s, etc.

The specs I gave were for triodes in Class AB (it does not mention AB1 or AB2), cathode bias (self bias) as opposed to a fixed C- bias) but the listed B+ voltage is 350V.  Definitely use any published Philco specs.  I posted the RCA and Tung Sol specs before Ron R,or another posted the specs for the speaker and output transformer.

Terry, you are right about the sockets.  I have seen some rather gnarly PCBs on AA5 radios and definitely on TVs around the Hor Opt tube.  Hamilton could find some 6 pin sockets on ePay., some of them ceramic.  hopefully that is just dust. If taking a brush and dry cleaning the socket does not remove carbon, worst case, drill out the burned area, so long as it does not dislodge the pin. Replacing the socket is the better idea.

[Radioroslyn]

Replacing the socket with a phenolic molded or ceramic one is a pain! The hole in the chassis is a bit under sized to drop it in from the top so I ended up grinding out the hole to enlarge it. It's not much fun. I thought about using my Greenly punch but the chassis is pretty thick and steel. Looking back I would grab a wafer socket from a Philco parts set.

[Hamilton]

After reading the exchange between Ron, Radioroslyn, Dale and MrFix, I realize I’ve got a huge learning curve ahead of me. Thank goodness for the Phorum to help guide newbies like myself.

I got the Hammond 125D last night. Just as a point of interest I ordered it from Digikey, a Hammond distributor, in Montreal on June 9th, shipped out of Grand Forks North Dakota the morning of the 10th, thru Memphis and delivered by Fedex last night. Impressive service!

With that being said I need advice as to how to properly wire it up. If you could use the my Pin numbers from previous posts as a guide it would really help. I will be able to attach it to the existing speaker housing no problem but I really don’t want to mess the connections up. Thanks again.

[Radioroslyn]

Looks like you'll want to use terminals 1 and 3.

[DaleHCook]

... I got it out of RC12, and a Tung Sol spec sheet (and they have different specs.

Do you remember which Tung-Sol sheet you used? I have a couple of their tube books and it would be enlightening to see what the 42 numbers look like which you compared with RC-12.

RCA wanted everyone to think that they invented every tube

RCA wanted everyone to think that they invented everything including the electron.

It seems to me that in the 650, the tubes are" cathode biased".

I don't see any cathode bias resistors..

[Hamilton]

Thanks Radioroslyn but my schematic abilities are severely limited at this time and this is the first time I’ve replaced one. I have read the pdf file but that doesn’t help me. I would need a more detailed picture of what needs to be done.

I would need almost a “hold your hand” approach …connect the red here, the brown here, the blue here etc.

I hope one of the learned members doesn’t mind taking the time to lead me thru this. Once I’ve done it, I will learn the process and be able to trace it thru the schematic but in the interim I will require more specific advice.

[Radioroslyn]

Ok the red wire connects to your #2. It's the center tap of the primary. On the Philco transformer it also connects to it to field coil (yellow wire). The brown and blue wires connect to #'s 3 and 4. They go to the plates or the 42 tubes. The two connections at the bottom are for the voice coil. They go to terminals 1 & 3 on the top of the new transformer. Your terminal 1 is a tie point to connect the field coil.

[MrFixr55]

Hi Dale, 

Though the cathodes are grounded, there is something more negative than ground.  Resistor # 66 in the drawing goes from chassis ground to the center tap of the B Supply secondary, making the B supply center tap the most negative point in the radio. Note also that the driver and 1st audio stage get their grid bias off the same tapped resistor. Note how the 2 main filter cap negative terminals do not go to ground but to the B supply secondary center tap.  Although it looks like fixed bias, variation in current draw will vary that voltage.  This is why I am calling it cathode bias.  Newer radios usually have chassis ground as the most negative point and resistors from that point to the cathodes of those tubes that require bias. It may be less complicated and cause less hum that way.

See the annotated pic below:
   
In contrast is this diagram of a high power Heathkit W6M Amp below.  Note that the cathodes of the 6550 output tubes are also at chassis ground potential, but so is the B Supply Minus terminal.  The negative grid bias comes from another winding on the power transformer (I should say the extremely substantial power transformer).  This is an example of fixed bias:

Heathkit W6M Williamson Amp.pdf Size: 30.77 KB  Downloads: 86

BTW, I have one of these. They are rated at 60 watts But I don't think RMS, but even more conservatively. I bet that this thing does at least 100. I haven't fired it up in almost 50 years. One of my projects is to recap it. I may sell it. I wish I had 2.

IMHO, the difference between fixed and cathode bias is not whether or not the cathodes of the output tubes are at chassis ground, it is whether a separate power supply of good regulation supplies the grid bias.

[Hamilton]

Radioroslyn. Perfect ? ?

[Hamilton]

Radioroslyn. Those ? ? marks on my post were supposed to be thumbs up emoji’s. Not sure why they showed up like that but thanks ever so much!

[DaleHCook]

Newer radios usually have chassis ground as the most negative point and resistors from that point to the cathodes of those tubes that require bias.

Yes, and that is called self-bias or cathode bias. What the 650 has is fixed bias, derived from a point in the power supply which is negative with respect to the grounded cathodes.

Some variation in the fixed bias voltage always occurs in the ways in which it is normally implemented in consumer equipment such as the 650. That is why commercial equipment using fixed bias (such as the broadcast equipment and instrumentation) that I have worked on for more than 50 years generally uses a separate bias supply.

Self-bias (aka cathode bias) has its own drawbacks where varying plate current causes varying bias.

I do not expect rock-steady bias in any consumer equipment - providing it costs too much.