1952 Newcomb RC-12

[Hi-Fi-Mogul]

Another of the many Hi-Fi gals almost completed.

This is an uncommonly found 1952 Newcomb RC-12 with a
V-M 950 record changer.

Power transformer model using 6X4, 6V6, and 6SQ7 tubes.
The 6SQ7 triode is the only section of that tube used.
AC line fuse.

Speaker: 6 x 9 inches.

The cabinet covering needed gluing down in some
spots.

To treat the flea-bites, I tried a little Howards Restor-a-Finish
mahogany on the bottom to test the results.
It looked good after curing a few days outside, so I went
over the entire cabinet, and it is much better in appearance.

Outside of the e-lytic filter caps, there was only one
bumble-bee 0.047 mF installed across the cabinet On/Off switch.
Other caps are ceramic.

The V-M 950 changer dates from Sept 8, 1952.
Per Gary at V-M, it has the earlier RMA stamp code vs. the later
V-M factory code.

After cleaning/lubing the motor, I used a square O-ring replacing the
old rubber on the idler wheel.
For the 33.3 rpm speed turret, I used a cut-down rubber
grommet to replace the old rubber.

6X4, 6SQ7 tubes had production dates of 1952.
The 6V6 was from 1954.
The 6SQ7 had a bad diode section, and I
thought it best to find a tube with all sections good.

Did an audio bench test today, and the amp is quiet,
and the changer is almost there.

I need to tweak the record support arm, as
it is not dropping easily, though its been
cleaned and lubed.
And the AC cord needs a new plug.

When it's all back in the cabinet, this is one
heavy player, but a nice one.

[William]

Nice job, Mr. Mogul, I really like the way Newcomb did their in cabinet schematic. Very easy to read and lots of information there. How does it sound?

Bill

[Hi-Fi-Mogul]

Nice job, Mr. Mogul, I really like the way Newcomb did their in cabinet schematic. Very easy to read and lots of information there. How does it sound? Bill

Bill,
I used another 6 x 9 speaker for the bench test,
but it was not in an enclosure.
The output though, was nice and clear, no hum.
Vol & Tone controls work well.

After I get the changer 100%, I'll see how the
Newcomb speaker sounds.

[electra225]

Do you have the EIA code off the Newcomb speaker? I have a Newcomb player that needs help. Parts aren't easy to find, at least the ones I need. Nice job, Mr. Mogul. You find the good stuff.....

[Hi-Fi-Mogul]

1. Greg, the Newcomb speaker has no EIA markings
I can see.

2. I have found the 6X4 rectifier plate voltages to be high.

Schematic values in the photo I posted
are 270 VAC for Pins 1 and 6, and I read 328 VAC.

The amplifier chassis On/Off switch will energize the amplifier
with the record changer power plug dis-connected.

That is how I took the readings, with the changer
dis-connected.

Would the rectifier voltage drop if the record changer power
plug was connected and the changer motor energized ?

Thanks !

[electra225]

When you take readings of voltages, particularly on the plate of a rectifier, consider the line voltage. Most specs are for 117 volts AC line voltage. IF your line voltage is higher than that, the rectifier plate voltages will be higher. Also, there is normally a tolerance value given for voltage readings. 20% is typical, so consider that. Voltages are also specified to be taken with certain instruments, typically a VTVM with 20,000 ohms per volts (sometimes 1000 ohms) sensitivity. IF you use a digital meter or a regular VOM that may affect readings. Lastly, (and please don't think I'm trying to insult your intelligence) make sure you are reading AC volts when you are measuring plate voltage. I have been guilty of leaving my meter on DC when I measure a rectifier plate. The rectifier plate has the highest voltage anywhere in the chassis. The rectifier cathode has the highest DC voltage. Since the phono motor is powered by the same plug as the power transformer, if the motor was running, voltage might be lower on the primary, hence the secondary, of the power transformer.

20% of 270 is 54. 54 plus 270 is 324, so your 328 volts would be pretty close. With a 10% tolerance, you would be 29 volts high. I wouldn't worry about that.... What is the voltage on the plates of the output tubes? I won't swear that I'm seeing right, but I THINK it shows 305 VDC on the plates of the output tubes. Check the voltage on the plates of the output tubes, then check cathode voltage. If those are close, I really wouldn't worry about it.

The schematic you posted doesn't specify if the readings are with the phono motor on or not. I'm guessing "not" but maybe taking them both ways would satisfy your curiosity and provide more information.

[TC Chris]

For my 1936 GE console, the voltages were quite high because of modern supply voltages. As I recall, I was getting some red-plating. So I made up a bucking transformer for it. You can read a clear description of the bucking transformer circuit and advantages here:

https://sound-au.com/articles/buck-xfmr.htm#s30

You'll find others online. It avoids using a power-dissipating resistor (heat) and it can be used with other radios too, as long as they are in the transformer's VA range. I built mine with a plug-in receptacle for that reason.

Chris Campbell

[electra225]

Red-plating is not typically the result of high plate voltage, but, rather high plate CURRENT. MOST power transformers are rated safe to 125 VAC.

[Hi-Fi-Mogul]

I appreciate the inputs.

I have also done AC voltage measurements with meter on DC,
and I remembered to check that when I took readings.

All the other plate voltage readings on the 6V6 and triode plate
of the 6SQ7 are very close to schematic values.

My mains voltage is 123 VAC.

Tomorrow I'll hook up the record changer, to see if
that affects the rectifier readings.

The 20% tolerance value Greg noted aligns with 6X4 tube data
sheets that indicate 325 VAC max application value on the plates.

The cathode voltage read off the 6X4 is 347 VDC; schematic value 338 VDC.

I took off the record support arm of the changer, and the small
amount of old grease/dirt I found had really kept the arm from
dropping easily down.
Now it's smooth as silk and working well.
I look forward to putting it all back into the cabinet !

[electra225]

At 123 VAC line voltage, your readings are very respectable. 347 volts at the rectifier cathode vs. the 338 spec is really close as well. Putting a large-wattage resistor in one leg of the power cord, ahead of the PT might be a good stunt in this case. I'm not a fan of bucking transformers, since they are another thing to have to mess with. The line voltage in my shop is about two volts lower than in the house for some odd reason. I have a whole-shop RF filter on the circuit breaker panel, so that may be part of it. I checked my shop voltage after you posted, and found it right at 118 volts. Resistance to high line voltage is one advantage of "hot chassis" type of construction, with no PT. I wonder what would be the result if you ran a 12X4 rectifier in place of the 6X4. Would that lower B+? I have a couple of AA5's that run 45Z5 rectifiers in place of the 35Z5. I don't see a difference in performance.

[Hi-Fi-Mogul]

I checked the 6X4 plate V's today with the motor on,
and VAC was 327 V, so no real change there.

I wanted to note this amp has a fuse.

Placing an in-line resistor would be arduous, due
to the layout of the chassis components.

Would you all say this is an acceptable condition
to leave as-is ?

That is, measured plate voltage today of 327VAC on the 6X4
is close to the 325 VAC of a "capacitor input filter" application
per the data sheet design limits.

[TC Chris]

Red-plating is not typically the result of high plate voltage, but, rather high plate CURRENT. MOST power transformers are rated safe to 125 VAC.

But generally higher voltages lead to higher currents, all things remaining equal. E=IR. Most red-plates I've seen are from leaky coupling caps causing skewed bias, but on the GE, getting the voltage under control solved the problem

Chris Campbell

[electra225]

I'd leave it alone if it was mine. Record player chassis don't leave a lot of room to add a bucking resistor in many cases. You have done really all you can, given high line voltage.

[Hi-Fi-Mogul]

Ms. Newcomb is back in her cabinet.

The 70+ year old speaker sounds fine.

There is not as much bass as I would enjoy,
but the Pfanstiehl P-228 stereo cart and this
amp are a good match.

I used a 22 mF cathode bypass cap for the
original 25 mF value, and that may/may not
have made some difference.

Though both R/L channels go into mono, I
can really hear them distinctly melded.

Gosh, she's heavy now all back togther !

[electra225]

As a suggestion for the wimpy bass, you might try bridging the cathode bypass cap on the audio output tube with one of a larger value, say, 50uf. Many times, increasing the value of that cap can enhance bass response. It won't hurt anything to try it. Also check the value of the 2200 ohm resistor in the feedback loop to the 6SQ7 first audio tube. Then check the 100 ohm resistor from the cathode of the 6SQ7 to chassis. These resistors are in the feedback loop, so they need to be pretty close to right on value.