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Super Hot-rodded 37-620 Schematic - Printable Version

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Super Hot-rodded 37-620 Schematic - AI2V - 06-12-2012

A few months ago I showed photos of this super hot-rodded 37-620 but didn’t have a schematic to go with it; this is the schematic and description. It began as a junker model 37-620 and slowly evolved into what can be seen on the schematic today. Since I'm not a pro at drawing and laying out schematics, you will have to accept the somewhat unusual convention.

There are no major changes in the 6K7 RF amplifier and 6A8 Oscillator/Mixer stages with the exception of a BNC oscillator output connector on the rear skirt; that signal goes to an external freq counter module which gives direct reading of the dial freq (these counter modules are available on Ebay for around $50). There is a high Q tank circuit in series with the antenna input designed to kill the local 1250 KHz station; it is not on the schematic; think of it as being an optional circuit.

Beyond the 6K7 and 6A8 circuits there is very little that remains original. The first IF 6K7 is now a much higher gain 6SG7; the 6Q7 was replaced with a high output low distortion 6J7 detector and is located in the center of the RF deck in the place formerly of an electrolytic cap; the AVC detector is now solid state; the detected audio from the 6J7 is fed to a 6K6 audio pre-amplifier; the pre-amp feeds the 6L6 output stage; and the 6L6 drives a 60 watt broadband 8 ohm PM speaker. Great attention was paid toward audio fidelity and bandwidth at all levels of output, the maximum rated output being 11 watts. Distortion is extremely low; the sound quality is extraordinary. Except for the 5Y3 and 6L6, all tubes are metal; that required tube socket changes.

The power supply uses an 8 Hy choke mounted on the speaker. Tube voltages throughout the radio have been adjusted for improved linearity; all voltages are within the proper tube parameters.

The next time I feel like taking out the RF deck, the 6K7 (and socket) will be replaced with a 6SG7. A variable bandwidth IF is under design using high Q ferrites; this circuit was installed earlier but the Q was too high and I hadn’t thought about making the Q or bandwidth variable. (The Q of a Philco IF is usually around 8-10; the ferrite Q was 280). In all of the re-design, the LED replacement for the shadow meter was left out but will be re-installed during the next round of changes.

Gas tube regulators were integrated in a previous version; they didn’t really serve any necessary function so they were removed.

There is still room for two more tubes; three if I get rid of the 5Y3. The only future limitations are from drawing too much from the power transformer; at that point the new tubes would need to be subminiatures. Maybe I'd better quit on it now- nah!

Pete AI2V

RE: Super Hot-rodded 37-620 Schematic - TA Forbes - 06-12-2012

I LOVE IT!!! Pete, that was some swell engineering! I saved a copy of your schematic and may attempt a clone this winter when my garage cabinet shop is closed due to it being just too d--n cold to do lacquer work.

How about finding a trashed 16 or 116 or 37-670 and doing another one????

RE: Super Hot-rodded 37-620 Schematic - AI2V - 06-12-2012

I've got to laugh, T A. This weekend I came home with a ratty 37-670 and thought about "updating" it. Right now it's in pieces and I'm doing the block rebuilding. Someone had been redesigning this beast already but not with any engineering, assembly or soldering skills. I know that if I say I'm going to "update" this 37-670, there will be an uproar. My hot-rodding is fun and educational and is limited to junkers; the 37-670 is still in the non-junker class but right on the edge.

Pete

RE: Super Hot-rodded 37-620 Schematic - Radioroslyn - 06-13-2012

Gosh it seems like you just got rid of one of those!
Terry

RE: Super Hot-rodded 37-620 Schematic - AI2V - 06-13-2012

Terry- about a year ago I gave away a 37-675 console; I may have been offering new homes to a couple of large tabletops since then. This 37-670 is a tombstone with five rippling and bubbling coats of brushed varnish; I gave up counting the "re-engineered" alterations by a previous owner. Luckily its a very easy chassis to work on.

Pete

RE: Super Hot-rodded 37-620 Schematic - Ron Ramirez - 06-13-2012

Terry - I thought you ended up with Pete's 37-675 console?

Pete - That's great work and thank you for sharing it with everyone here! I have to tell you folks, Pete shared a preliminary schematic with me some time back, but I stay so busy that I forgot to share it here. I'm really glad you did so Icon_thumbup

RE: Super Hot-rodded 37-620 Schematic - Radioroslyn - 06-13-2012

Yes he did I was just poking a little fun. Did do a little work on it but still has the two major problems. Squeals like a pig when you first turn on, it's somewhere in the low level audio after the volume control. And no RF amp on the SW bands only on the BC band. Somewhere in the band switch.
Pete, if you need to find a new home for any of you tabletop sets please shoot me a PM.
Have fun with your new hot rod project!!
Terry

RE: Super Hot-rodded 37-620 Schematic - AI2V - 06-14-2012

Terry- I had forgotten where that console went, and you are right - it is another 37-670. About the only improvement on this tombstone will be speaker reconing and maybe plate/screen voltage changes to compensate for my 128 VAC line.

Gotta check Ron's site to see if there are any ckt. changes on this new 37-670; I'm seeing several factory discrepancies from the original circuit.

RE: Super Hot-rodded 37-620 Schematic - TA Forbes - 06-14-2012

You might want to make sure that it is a 37-670 and not a 37-2670. If it has a longwave band it's a 2670.

RE: Super Hot-rodded 37-620 Schematic - AI2V - 06-15-2012

It's a 37-670 for sure; the label says so and there's no low band. When I first saw it I too thought it was a 37-2670 but checking under the hood and looking at the dial proved differently. It's getting a slow but precision chassis restoration: spaghetti on all component leads, wrapped joints, Alpha solder, and acetone-cleaned joints.

RE: Super Hot-rodded 37-620 Schematic - TA Forbes - 06-15-2012

Interesting. What are the differences you noted?

Philco is famous for this....