• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Question about AMP185 Rectifier Circuit

mackinthebox

Member
2010-10-19 11:03 am
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?

[IMGDEAD]http://afterlifedallas.com/modules/coppermine/albums/userpics/10002/amp-185_power_circuit.jpg[/IMGDEAD]

Thanks for answering my newb questions :p
 

mackinthebox

Member
2010-10-19 11:03 am
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 :)
 
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 :p

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.

 

mackinthebox

Member
2010-10-19 11:03 am
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 :)
 
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korey

Member
2008-03-04 2:00 pm
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.
 
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20to20

Member
2010-06-23 9:25 pm
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.
 
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ok thats what i figured :)
and the reason i said a different choke was that the transformer i chose will be putting out less voltage so I would probably need a choke with a little less resistance to keep the 300V source at 300V now?
I guess lower voltages wouldnt exactly hurt anything...
 
A 5V4 rectifier will give you a little more output voltage. About half the voltage loss of a 5U4 (25 volts vs. 50 volts).
But there again, I wouldn't be too concerned that your transformer delivers 5 volts less than the original. It's in the ballpark. If you find that you want more voltage from your power supply, you can always try a 5V4 rectifier tube and see how it works for you.
 

20to20

Member
2010-06-23 9:25 pm

korey

Member
2008-03-04 2:00 pm
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.

Nevertheless, it can still be used for testing purposes, and it won't hurt a thing as long as its not being used for hours on end. My point being was the use of the individual cathodes, compared to the heater tapped cathodes!
 

korey

Member
2008-03-04 2:00 pm
yeah, all in all if i recreate the amp all the new parts are going to run me around 400 bucks i believe not including the chassis...

YIKES, 400 sounds a little steep :eek: .

I modified my AMP 175-67 with a little over 50 bucks. Sounds like you are not rebuilding the amplifier, but building a completely different and more powerfull amplifier with that kind of money!

You don't need a new power transformer, but its the output transformers that will cost you. Four coupling caps and the electrolytic may be all you need for it to really sing!
 
YIKES, 400 sounds a little steep :eek: .

I modified my AMP 175-67 with a little over 50 bucks. Sounds like you are not rebuilding the amplifier, but building a completely different and more powerfull amplifier with that kind of money!

You don't need a new power transformer, but its the output transformers that will cost you. Four coupling caps and the electrolytic may be all you need for it to really sing!

yeah pretty much, its all new parts from thetubestore and edcor
the tubes and sockets are only about 150 but the iron is running 200+ at the moment
with the beefier power supply and beefier output transformers it should push out a bit more power than the original