Question about AMP185 Rectifier Circuit

[mackinthebox]

I dont quite understand the rectifying circuit in my amplifier
Im confused as to what the yellow wires are for...
is this info listed on the transformer anywhere? I dont see it on the transformer, maybe the sticker was removed?

Does anyone know what the yellow wires on the transformer are?



Thanks for answering my newb questions

[stratus46]

The cathode/filament of the 5U4 is the high voltage output of the tube at 310V. That filament winding is useless for other tubes because of the 310V so the 5U4 has its own private winding. Since you obviously are new at this, be careful as those voltages can be lethal.

G²

[mackinthebox]

thanks for the warning
Ive worked on tesla coils and other high voltage equipment so i take high voltage seriously
theyre dangerous and can cause severe injury and death!!

so what are the yellow wire on the transformer for then? do they supply voltage as well and what voltage do they supply?
the schematic isnt clear...
do the yellow wires supply the 5v for the cathode/heater as well as piping the DC voltage to the choke and 310V supply?

yes im new to tubes and thanks again for answering my newb questions

[20to20]

do the yellow wires supply the 5v for the cathode/heater as well as piping the DC voltage to the choke and 310V supply?

Correctamundo.

It looks odd but the 5v AC line has the B+ DC on it as well.

[mackinthebox]

wow, it is very odd looking indeed, i just didnt know it could work that way

now i just need to understand how to read the graph for the rectifier tube so I understand the math on how it gets the 310 volts

[stratus46]

Originally Posted by mackinthebox

wow, it is very odd looking indeed, i just didnt know it could work that way

now i just need to understand how to read the graph for the rectifier tube so I understand the math on how it gets the 310 volts

Well it's been done this way since the '20s and '30s and likely before that. I'm surprised the output is only 310. I'd expect it to be 1.4 (square root of 2) x 290 (at least with no load). I assume the 'droop' is due to transformer loading and voltage drop through the 5U4. And yes, the yellow winding is a 5V 3A supply, the requirement for a 5U4 filament.

G²

[mackinthebox]

where would i find information on how much voltage would be dropped through the rectifying tube? would that be something found on the graph for the tube?
I want to eventually build a duplicate of this amp and may not be able to find exactly the same power transformer so would like to know how to get the same voltage from a different power transformer.

you can even point me to some online resources where I can find my answer and I will gladly read them

[korey]

I know its weird, but the yellow wires are the 5 volt filament voltage for the 5U4 rectifier tube. The high voltage is actually produced at the red wires, but taken from one yellow wire.

If you take voltage measurements with the 5U4 removed, you get the correct voltages as being 5 volts from the yellow, and 290 on each of the red wires with respect to the center tap. Both red wires measured together measure 580 volts.

I know, pretty strange huh! All 5Y3, 5U4, 5R4, 5V4 etc, are utilized like this.

A circuit which uses the 6CA4, 6X4 rectifier tubes, uses the same filament voltage of all the other tubes, but the high voltage tap is taken off of a seperate cathode pin as not to interfere with the 6.3 volt operation. With these tubes (and others) the cathode is internally independant of the heater.

A 5AR4 tube can be used in place of the 5U4, but its cathode is connected internally with the heater and gets its high voltage the same way as the 5U4 tube.

When looking for a replacement, you would see specs something like this:

PRIMARY: 117-125V, SECONDARY 1: 580VCT (290-0-290)@ .093ADC, SECONDARY 2: 5VAC @ 3A, SECONDARY 3: 6.3 VAC @ 2.1A, and SECONDARY 4: 6.3 VAC @ 2A.

[20to20]

A 5AR4 tube can be used in place of the 5U4, but its cathode is connected internally with the heater and gets its high voltage the same way as the 5U4 tube.

Not a good idea in these Magnavox amps. They drive the B+ on many of them at or above the design max voltage for the output tubes. The 5AR4 has a much lower voltage drop and so the B+ will be @ 25v or more higher. Considering the line voltages we have nowadays is above 117v, too, we are already starting out high.

[mackinthebox]

i cant find one with exactly 580 so would this one work?
EDCOR Electronics Corporation. XPWR064-120

Secondary (output) Voltage 570V(285-0-285)@250mA, 6.3V@5A & 5V@5A

im assuming I would need to use a different choke as well
is the 250ma too much?
the original was 150ma