Power supply problem

  • Thread starter Thread starter PhilGood
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PhilGood

PhilGood

Juice box hero
Guys, I'm bidding on a Sony C-37A on eBay. The guy doesn't have a power supply for it, but has the schematic. I've dug up schematics on eBay, but am stumped as I don't know what type of power transformer I'm looking at, as well as finding out the values for the diodes. I'm just not good at figuring out the calculations. (OK, I don't really know how.) I know I could build this if I can just get the parts right. Please help.

http://www.freeinfosociety.com/electronics/schematics/audio/sonyc37a.htm

The cryptic hand written notes I've found on this so far indicate the PT outputs at "250-0-250v @ 5ma, 6v @ .6a, 10v (6v top) .5a". Sorry, it's hard to read off of a gif.

If I need to do this with separate trafos for each, which are which? Also some of the caps are marked "2000" and "20". I'm not familiar with caps of those values. I can find 1800uF. What the heck am I missing?
 
PhilGood said:
Guys, I'm bidding on a Sony C-37A on eBay. The guy doesn't have a power supply for it, but has the schematic. I've dug up schematics on eBay, but am stumped as I don't know what type of power transformer I'm looking at, as well as finding out the values for the diodes. I'm just not good at figuring out the calculations. (OK, I don't really know how.) I know I could build this if I can just get the parts right. Please help.

http://www.freeinfosociety.com/electronics/schematics/audio/sonyc37a.htm

The cryptic hand written notes I've found on this so far indicate the PT outputs at "250-0-250v @ 5ma, 6v @ .6a, 10v (6v top) .5a". Sorry, it's hard to read off of a gif.

If I need to do this with separate trafos for each, which are which? Also some of the caps are marked "2000" and "20". I'm not familiar with caps of those values. I can find 1800uF. What the heck am I missing?
Click to expand...


It is really hard to find a transformer with these specifications. Although it is gonna be an overkill you can use something like P-T269JX from www.tubesandmore.com
You will need to know exactly the B+ and adjust the last 10k resistor in the string accordingly. You can use 4007 diodes for filament rectifier. 1800uF/25V will work, although here I'd rather use 4700uF. Instead of 20 use 47uF/350V. With different transformer filament current rating you wil need to adjust 45 Ohm resistor for filament voltage between 6-6.3V under load.
 
PhilGood said:
Guys, I'm bidding on a Sony C-37A on eBay. The guy doesn't have a power supply for it, but has the schematic. I've dug up schematics on eBay, but am stumped as I don't know what type of power transformer I'm looking at, as well as finding out the values for the diodes. I'm just not good at figuring out the calculations. (OK, I don't really know how.) I know I could build this if I can just get the parts right. Please help.

http://www.freeinfosociety.com/electronics/schematics/audio/sonyc37a.htm

The cryptic hand written notes I've found on this so far indicate the PT outputs at "250-0-250v @ 5ma, 6v @ .6a, 10v (6v top) .5a". Sorry, it's hard to read off of a gif.

If I need to do this with separate trafos for each, which are which? Also some of the caps are marked "2000" and "20". I'm not familiar with caps of those values. I can find 1800uF. What the heck am I missing?
Click to expand...

250-0-250 + 6.3V is a pretty standard tube supply transformer

http://www.eidusa.com/Electronics_Trans_Power_263_to_282.htm

You don't need to worry about the bulb so the top section could be replaced by a sperate 10V transformer. The 2000 caps are just smoothing caps, don't worry too much about value.

Just remember the power supply is probably going to cost $150 and the mic may not work in the end.
 
You guys are just friggin awesome!! Thank you so much!!
 
One last clarification. The top of the power transformer (right side) is for the heater of the 6AU6 (which is 6.3v) and that could be a 10v trafo? The middle is for the heater of the 6X4 (also 6.3 volts), so that's the 1st filament?

Is the bottom the secondary winding at 250v? Do I have the order right?
 
PhilGood said:
One last clarification. The top of the power transformer (right side) is for the heater of the 6AU6 (which is 6.3v) and that could be a 10v trafo? The middle is for the heater of the 6X4 (also 6.3 volts), so that's the 1st filament?

Is the bottom the secondary winding at 250v? Do I have the order right?
Click to expand...

Yep, you got it. The filament trafo can be even RS 12V, just make sure to adjust 45 Ohm resistor.
The circuit will need tweaking. As I told already, it is very hard to find the x-former with B+ current of 5ma, so with higher current transformer you will need to change values of 10k string of resistors. Make sure to "lift" the heaters--that's what two 50k, 80k, and two 100 are doing, and don't ground it. You will need to find a solution for audio transformer. Lundahl, Cinemag, or whatever you want, but you need to know original ratio. Needless to say, because of different transformer the mic will sound different than stock. With your trafo you will also need to tweak values of 0.1, and two 0.5 capacitors to get the bass roll off at the right frequencies.
Other than that sounds like a fun project.
 
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Eeeeww! The audio trafo's ratio is 408:1 according to those same notes. Unless there's a period I can't make out. 10k:600 is the other part of the note.
 
paddyponchero said:
Just remember the power supply is probably going to cost $150 and the mic may not work in the end.
Click to expand...

Regardless if I don't win or it doesn't work, I'm probably going to take an old MXL V63M, gut it, and build it into a C-37A-ish clone.

Thanks again for the help!!
 
PhilGood said:
Eeeeww! The audio trafo's ratio is 408:1 according to those same notes. Unless there's a period I can't make out. 10k:600 is the other part of the note.
Click to expand...

Yes there should be a period. Let's see, impedance ratio is 10k:600. Sq. root of (10000:600)=4.08, which translates into voltage ratio of 4.08:1.
A lot of transformers could fit there.
 
(Homer voice) You are so smart! You are so smart! S.M.R.T, S.M.R.T --- I mean S.M.A.R.T (*house catches fire...*)

Thanks again!
 
I ordered a couple of cinemag traffos to replace the cruddy one in one of my frankenstein preamps, definitely the best buying experience of the year - fast, friendly and efficient. Lets hope the transformers are as good :)
 
Guys,

Could you have a look at that schematic again. I have a few more questions. First, since this is powering 2 tubes that have heater voltages of 6.3 volts, does it make sense that the top 2 PT loops should just be 6.3v each? I have a separate 6.3v transformer to use in conjunction with the main one. (250-0-250 CT, 6.3v filament)

Second, since this is powering a 6AU6, which has a plate voltage of 250v (minimum 100 for class A amp) should the power trafo be a 250-0-250 or a 125-0-125?

Does the 6X4 aternate the 2 plates at 250 each to provide 250 DC? or is it going to do 500 DC? It's rated to 450v each plate. I don't want to blow the tubes. I only bought 2 of each. Just want to make sure I have it right.

Thanks again,

Phil
 
PhilGood said:
Guys,

Could you have a look at that schematic again. I have a few more questions. First, since this is powering 2 tubes that have heater voltages of 6.3 volts, does it make sense that the top 2 PT loops should just be 6.3v each? I have a separate 6.3v transformer to use in conjunction with the main one. (250-0-250 CT, 6.3v filament)
Click to expand...

No. The 6X4 heater run an AC, and mic tube is DC, which is smoothered by caps and resistor to avoid hum. Use your separate one for the tube heater, and adjust resistor for about 5.8-6.3V. You can actually play with that and see if you like little underheated filaments better.

Second, since this is powering a 6AU6, which has a plate voltage of 250v (minimum 100 for class A amp) should the power trafo be a 250-0-250 or a 125-0-125?
Click to expand...

All you need to know is a B+, i.e. actual voltage on a tube. My guess, it is somewhere around 120-150V. The 10K resistor drop the voltage down to this figure. Since the 6X4 rectifier has a warm up period you won't get a full voltage on the tube, while 6AU6 heater warms up.

Does the 6X4 aternate the 2 plates at 250 each to provide 250 DC? or is it going to do 500 DC? It's rated to 450v each plate. I don't want to blow the tubes. I only bought 2 of each. Just want to make sure I have it right.
Click to expand...

Yes, you get 2 plates at 250V, so no worry here. After rectification you get about 300V there.
 
So I'm ok with the 250-0-250. Good, 'cause I already bought it!

Underheating is probably OK. The u47 heats at 1.1v, right? I'm not saying that's what I'm trying to do!
I'll play around and let you know how it works. ...if it works!

Marik, you're a god as usual!

paddyponchero, thank you as well! Very much appreciated!
P.S. what county in Ireland are you? I have an old friend from county Clare. He was a hick, of course...
 
Maybe I read this wrong. It's from a prosound thread which, I believe, you were a part of on another page:

http://recforums.prosoundweb.com/index.php/t/574/0

Oliver Archut

quote:

Even the VF14 shares 90% of the same parts of the EF14, a tube first introduced in 1938 and the back bone of the Nazi electronic war machinery, the minor changes of cathode and 1st gird material makes all the difference. You can find about 1 out of ten EF14 that is suitable for microphone use, but that is all.

The 1.1V of the cathode in a U47 is just above the magic "Temperature Tension Voltage "of Barium Oxide of 1V with a cathode temperature of 680 degree, so electrons make it just out....
Even a U47 circuit looks quite simple, the develop engineers at Telefunken did everything they could to to make this design as best as possible. Unfortanaly a VF14 is very hard to replace....


Hope that helps out, even a bit long it is still not enough to cover everything....

Best regards,

Oliver

end quote


That's one of the only VF14 spec discussions I've found betwixt you mic gurus. If I got the specs wrong, I'm glad you pointed it out.

Phil
 
PhilGood said:
The 1.1V of the cathode in a U47 is just above the magic "Temperature Tension Voltage "of Barium Oxide of 1V with a cathode temperature of 680 degree, so electrons make it just out....
Click to expand...

This is a tube bias, which set at 1.1V.
 
Sorry. Guess I'm making a fool of myself.

Since I'm being foolish, I'll continue. See if I have this right:

If the output of the rectifier is 300v and there are 3 10k resisters in series, the output is 10mA. (amps = voltage over resistance) Do I have this right? The voltage also drops, correct? But then the two 50k in series are 100k total, and they 'lift' the voltage? What do you mean by lift?

When the 6AU6 specs talk about the plate voltage, it's refering to the annode and that's the B+? If it is, it's rated at 250v (300v max Design Center Values) The plate current specs at 7.6mA @250v up to 10.8 mA. This is dependant on the cathode bias resistor?

I dunno, it seems like this will work stock. Then again, I'm sure I've probably screwed something up.

Sorry I'm being such a pain. I guess I should have taken an electronics course.
 
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PhilGood said:
Sorry. Guess I'm making a fool of myself.

Since I'm being foolish, I'll continue. See if I have this right:

If the output of the rectifier is 300v and there are 3 10k resisters in series, the output is 10mA if the trafo puts out 60mA. (amps = voltage over resistance) Do I have this right? The voltage also drops, correct? But then the two 50k in series are 100k total, and they 'lift' the voltage? What do you mean by lift?

When the 6AU6 specs talk about the plate voltage, it's refering to the annode and that's the B+? If it is, it's rated at 250v (300v max Design Center Values) The plate current specs at 7.6mA @250v up to 10.8 mA. This is dependant on the cathode bias resistor?

I dunno, it seems like this will work stock. Then again, I'm sure I've probably screwed something up.

Sorry I'm being such a pain. I guess I should have taken an electronics course.
Click to expand...

No, it is not foolish at all. There was time when I asked similar questions...
Tomorrow I am leaving for a couple weeks, so I don't have much time right now, and I will wright only essential things, without getting too much into details.

B+ is a voltage BEFORE anode resistor and the voltage on anode will be B+ minus voltage drop across this resistor. The drop will depend on a current through the tube. Given we have 10ma PSU and lets say 1ma through the tube..... you calculate the voltage drop....
Since this is a cathode follower we don't have an anode resistor, so essentially, the anode voltage is a B+.

Now.... any tube has a cathode to filament voltage rating, which is for small tubes is usually 100V. In cathode follower with given B+ we would exceed this rating, so thats why we "lift" the filament voltage above ground, and the string of resistors working as a voltage divider, sets reference point for filament voltage.

Hopefully it is more clear now.

Oh yeah, in this arrangement cathode resistor sets the bias.
 
paddyponchero said:
I ordered a couple of cinemag traffos to replace the cruddy one in one of my frankenstein preamps, definitely the best buying experience of the year - fast, friendly and efficient. Lets hope the transformers are as good :)
Click to expand...

How did those trafos work out? I talked with Tom at Cinemag (great guy) about this project and ordered the closest one for what I'm doing. The prices are way better than Jensen and it seems they use better materials. I used Jensens in some of my Royer mods, but if the Cinemags work well I'll probably use them from here out.
 
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