AMP 151CB Production Change

[Hi-Fi-Mogul]

I am starting the re-cap on this amp, from the Magnavox
New Horizon console.

My amp uses a 5Y3 rectifier, while the amp version in Sams
uses a 6X5GT rectifier.

There is a 1958 Sams Photofact Prod Change Bulletin 208 that
apparently addresses this amp's changes.

Does anyone have the bulletin ?

[William]

Sorry, I wish I could help!

Bill

[electra225]

I don't know whether this will help or not. The bulletin is dated 5-58 and was written by a Dirk Taekels. There should be downloads here for both the schematic and for the bulletin. Good luck!

https://www.radiomuseum.org/r/magnavox_ ... 151bb.html

[Hi-Fi-Mogul]

Greg,
Thanks, and special thanks to Dave1138 who grabbed
the bulletin & power supply schematic for me.

I'll post the sheets when I get into the New Horizon re-cap further.

[hermitcrab]

probably production change because of the problematic 6X5 tubes taking out power transformers ?

[electra225]

I think the 6X5 problem was mostly with Zenith chassis. And an old wives' tale perpetuated on ARF. Also when using a 6X5G. Ford car radios used a 6X5GT for years, some of them hanging upside down. Dad's old Newcomb record player still has its original 6X5GT still running. I read a piece years ago that said the reason some manufacturers replaced the 6X5 with a 5Y3 was to get the rectifier heater out of the circuit of the other tube heaters to prevent hum.

[Hi-Fi-Mogul]

I think the 6X5 problem was mostly with Zenith chassis. And an old wives' tale perpetuated on ARF. Also when using a 6X5G. Ford car radios used a 6X5GT for years, some of them hanging upside down.

Dad's old Newcomb record player still has its original 6X5GT still running.
I read a piece years ago that said the reason some manufacturers replaced the 6X5 with a 5Y3 was to get the rectifier heater out of the circuit of the other tube heaters to prevent hum.

Greg,
I agree with you.
I had read up on the 6X5 in the Zeniths
when I was starting on the 1956 Cobra Stroboscope.

Zenith was over stressing the circuits apparently at upper limits,
and not supplying capable enough transformers for the designs.
The tube itself was also improved by the 1950's 6X5GT type.

That's very interesting about the 5Y3 being used to reduce hum.
Thanks for that information.

[electra225]

Have you found other changes in the AMP 151 besides the change of rectifiers?

[Hi-Fi-Mogul]

Have you found other changes in the AMP 151 besides the change of rectifiers?

Greg,
I have not found any other changes in the circuit, but
read on about the differences in how power is switched to
the amp.

I am perplexed a bit on how to jumper the amp to test it
now that all the capacitors are replaced.
I think it should be b/t pins 2 & 3 of the six position plug.

You're adept at de-ciphering that, if you could assist.

The diagram for AMP 151C power supply is what I am using.

My New Horizon console uses the 5 position plug to run
power via the record changer switch
to power On/Off to the amp.

The extra 6 pin plug in the diagrams is used for the
Magnavox TP-262 consolette that has an extra On/Off switch
on the Treble control.
That is labeled AMP 151B.

Thanks !

[electra225]

Can you post the entire schematic? I need to see where C1B goes. That may be a filter cap. I don't see any in the diagram you posted. Make sure the complete diagram shows that plug. Jumping across pins 2 & 3 will provide power to the transformer, but that may not be the entire power supply.

[electra225]

I'm confused. The complete diagram only shows a five pin plug. The other two "5Y3" diagrams show a five pin plug and a six pin plug. The complete diagram does not identify the pins in the plug. I know you mentioned a rectifier tube change from 6X5 to 5Y3. If the two "5Y3" diagrams are the change, they are poorly drawn. Where is the six-pin plug in the complete diagram? Does your amplifier have two plugs, a 5-pin and a 6-pin, or only the five-pin plug?

[Hi-Fi-Mogul]

The amplifier has both 5 and 6 position plugs.
You can see them in one of the photos.

The Sams indicates there is an AC switch plug,
which must correlate to the 6 position plug.

I wrote how that 6 position plug is used in another
version of the AMP 151.

The production change diagram from Magnavox
is accurate to the AMP 151C version amplifier I have
that I explained in the post.

[electra225]

On the six-pin plug, you can jumper pins 2 and 3 and get power to the power transformer. I am concerned about the pin that says it goes to C1B. That is the output filter cap. My guess is that both of those plugs will have to be connected to make the amplifier work properly. I'm thinking that if you just jumper pins 2 and 3 in the six-pin plug, you might not have B+ to parts of the amplifier, mainly the screens of the output tubes. No screen voltage may cause the output tubes to red plate, and put undue strain on the audio output transformer. Whatever you do to test, please be sure to use a Variac, dim bulb and/or a Kill-A-Watt to monitor voltages and check current. And make sure the PT is fused. Something about that setup that makes the hair stand up on my neck......

[Hi-Fi-Mogul]

Greg,
It's unfortunate that Magnavox did not supply Sams or Sams did
not ask for a schematic showing the wiring relation of the
six position plug's wiring to the different versions of the amp.

In the chassis photo, all those small Orange wires are origin at the
e-lytic cap C1B 20 mF 300 WDCV. (Square Symbol on the can/schematic)

The Orange wires connect pins 6 on both 6AQ5 tubes.
Then one Orange wire connects from a 6AQ5 to the terminal strip
where you see the large 1500 Ohm resistor.

From that same location, there is a red wire going to
Position 4 of the six position plug.

So, the circuit is all intact.

That is all it is. There is nothing to plug into position 4, because
my AMP 151C has nothing to plug into that position.

For my amplifier, the six position plug only acts as different
tie points for the amplifier wiring.

I have found another photo of the TP262 that uses the other
version, with an On/Off switch on the Treble control, that I
wrote about initially.
The round male 6 pin plug is to the left of the vertical plinth support.

[electra225]

Pin 6 on the 6AQ5's would be the screens, so that is right. Your statement that the six-pin plug is used as a terminal strip is indeed helpful and answers a lot of questions. So, that being the case, if you jumpered pins two and three on the six-pin "terminal strip" that should let the amp run out of the cabinet. But I'd still use all the safety equipment I suggested above and monitor voltages and current, at least during initial start-up. Make sure you have correct screen voltage and nothing is drawing too much current. Let us know how you get along. Good luck!

[Hi-Fi-Mogul]

I'm going to build me a dim bulb set up, which I've
thought about for a long time.

[electra225]

A dim bulb setup is essential if you need to limit current. I like a Kill-A-Watt for monitoring wattage draw. They are not very much money, around $20 and are really handy. They won't limit current, though. My elmer, Lloyd, made me a Variac, dim bulb setup in an old oscilloscope case. He liked fiddling with test equipment. It has input and output meters, voltage and amperage, fused everything and three different dim bulb wattage settings. If it ever breaks I'll not be smart enough to fix it......