I'm in the process of building a tube preamp, and I'm using a somewhat unusual power transformer (eBay find for cheap) with 175V 50V and 25V windings. At least that's what they measure out to be. It's labeled "Ferrodyne" and has X229 stamped on it, if anyone is wondering. I was planning to put the 175V and 25V windings in series, bridge rectified to end up with a B+ of around 275 VDC. I tested this and it worked fine.
The question is, I would also like to run the filaments off of the 25 V winding, (filaments from the 2x 12AU7 tubes in series). This would also be bridge rectified for DC filaments. Will using this 25 V winding for both be an issue? Current capability is not a problem, as this transformer is truly overkill for a preamp.
Thanks for the suggestions.
The question is, I would also like to run the filaments off of the 25 V winding, (filaments from the 2x 12AU7 tubes in series). This would also be bridge rectified for DC filaments. Will using this 25 V winding for both be an issue? Current capability is not a problem, as this transformer is truly overkill for a preamp.
Thanks for the suggestions.
I do not think that using a pair of windings for B+, and then using one of those same windings for DC filaments is possible.
Also, what are the current ratings of the secondary windings?
And what currents will the preamp use?
You might try using a bridge on the single 175Vrms winding. That would give a little less than 247V B+. You might use a C R C filter, or a C L C filter. Careful, the L choke will spray magnetic fields, which may cause hum if not used and positioned correctly versus the rest of the preamp circuitry.
Then use a bridge and the 25Vrms winding for the DC filament supply. That will give you about 35V DC, so you will have to use a C R C filter, with the R dropping about 10V. You need to figure that the effective load on the 25V winding is about 2 x whatever the filament draw is. i.e. 2 each 12AU7s with filaments in series is 150 mA at 24.6VDC, but the load (heating on the windings) is as if the load was 300mA AC.
If 247V B+ is not enough for the preamp tubes plus the voltage drop in the plate resistors, then consider using current sources for the plate loads, i.e. an IXYS 450V one.
Or, use the 175V and 50V windings in series (225V), that will give a little less than 318V B+.
Also, what are the current ratings of the secondary windings?
And what currents will the preamp use?
You might try using a bridge on the single 175Vrms winding. That would give a little less than 247V B+. You might use a C R C filter, or a C L C filter. Careful, the L choke will spray magnetic fields, which may cause hum if not used and positioned correctly versus the rest of the preamp circuitry.
Then use a bridge and the 25Vrms winding for the DC filament supply. That will give you about 35V DC, so you will have to use a C R C filter, with the R dropping about 10V. You need to figure that the effective load on the 25V winding is about 2 x whatever the filament draw is. i.e. 2 each 12AU7s with filaments in series is 150 mA at 24.6VDC, but the load (heating on the windings) is as if the load was 300mA AC.
If 247V B+ is not enough for the preamp tubes plus the voltage drop in the plate resistors, then consider using current sources for the plate loads, i.e. an IXYS 450V one.
Or, use the 175V and 50V windings in series (225V), that will give a little less than 318V B+.
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If you tie the heaters to the HT and the HT is more than 100volts, it will fail.
Please take a look at the valve data sheet. See page 4 of http://www.r-type.org/pdfs/ecc82.pdf
Please take a look at the valve data sheet. See page 4 of http://www.r-type.org/pdfs/ecc82.pdf
The 12AU7 / ECC82 has a maximum + cathode to - filament voltage rating of 200V.
That rating is unknown to many, and is unusually high for most 9 pin small signal tubes.
If you find a 'new production' 12AU7 / ECC82 tube data sheet which is Not rated for at least 180V, then it is Not built near to, nor at the Original Specification. In that case I would not use that manufacturer's new production tube.
I use both old production, and new production 12AU7 / ECC82 tubes that Do specify at least 180V max on their data sheets.
That is listed on the data sheet in post #3 above, page 4
Vkf = 180
Some data sheets list the maximum + filament to - cathode voltage as 100V (filament more positive than the cathode).
One of the very old 12AU7 data sheets specifies that the filament is a dual spiral assembly.
It consists of two very small diameter spirals that are then mounted vertically in the cathode.
This has 2 good effects:
The magnetic fields are vertical, and in opposite directions.
The heat coupling to the cathode is Very consistent.
JJ manufactures 12AU7 / ECC82 tubes to Vkf 180V spec, And uses spiral filaments too.
For any 12AU7 tubes that do Not have spiral filaments:
They would have to have an even number of wires up and down, or they will have a horizontal magnetic field (which could cause vibrations due to attraction between the filaments and steel plates.
Their heat coupling to the cathode may not be as consistent as the spiral filament tubes.
The high cathode to filament voltage ratings make the 12AU7 / ECC82 a good tube to implement an SRPP circuit, without having to elevate the filament voltage.
That rating is unknown to many, and is unusually high for most 9 pin small signal tubes.
If you find a 'new production' 12AU7 / ECC82 tube data sheet which is Not rated for at least 180V, then it is Not built near to, nor at the Original Specification. In that case I would not use that manufacturer's new production tube.
I use both old production, and new production 12AU7 / ECC82 tubes that Do specify at least 180V max on their data sheets.
That is listed on the data sheet in post #3 above, page 4
Vkf = 180
Some data sheets list the maximum + filament to - cathode voltage as 100V (filament more positive than the cathode).
One of the very old 12AU7 data sheets specifies that the filament is a dual spiral assembly.
It consists of two very small diameter spirals that are then mounted vertically in the cathode.
This has 2 good effects:
The magnetic fields are vertical, and in opposite directions.
The heat coupling to the cathode is Very consistent.
JJ manufactures 12AU7 / ECC82 tubes to Vkf 180V spec, And uses spiral filaments too.
For any 12AU7 tubes that do Not have spiral filaments:
They would have to have an even number of wires up and down, or they will have a horizontal magnetic field (which could cause vibrations due to attraction between the filaments and steel plates.
Their heat coupling to the cathode may not be as consistent as the spiral filament tubes.
The high cathode to filament voltage ratings make the 12AU7 / ECC82 a good tube to implement an SRPP circuit, without having to elevate the filament voltage.
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I would use the 175 + 50 in series using a three stage power supply to yield a noise free 300 or so volts. then use the 25v winding for the heaters, rectified there is enough voltage for a 7824 regulator to drive the heaters with DC.
Ignoring the breakdown voltage issue here, there is also a problem that both ends of the heater supply rail would then be swinging at the mains frequency at about 175volts peak-to-peak and possibly coupling some of that to the cathods and other parts of the circuit.
This would also be my recommendation.
DF96,
I am sorry but . . .
I saw a 12AU7 or ECC82 data sheet that listed spiral filaments less than 10 years ago.
I think it was either Amperex or Tung-Sol.
Unfortunately, I have searched for it more than once since, but have not been able to find it again.
I had wanted to get the link to show others.
I do know that after I purchased JJ 12AU7 / ECC82 tubes that I noticed the actual spiral filaments on them.
I do not even remember which I found first, the spiral filaments on the JJ, or the other manufacturer's 12AU7 data sheet that listed spiral filaments.
I am sorry but . . .
I saw a 12AU7 or ECC82 data sheet that listed spiral filaments less than 10 years ago.
I think it was either Amperex or Tung-Sol.
Unfortunately, I have searched for it more than once since, but have not been able to find it again.
I had wanted to get the link to show others.
I do know that after I purchased JJ 12AU7 / ECC82 tubes that I noticed the actual spiral filaments on them.
I do not even remember which I found first, the spiral filaments on the JJ, or the other manufacturer's 12AU7 data sheet that listed spiral filaments.
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OK. I can understand some modern 12AU7 having low noise heaters, as they are made for audio use. NOS 12AU7 were intended for multivibrators in TV sets so although audio use was possible it was not uppermost in the minds of the manufacturers.
Unfortunately, many 'modern' datasheets are merely reproductions of NOS datasheets, perhaps with a few modifications, so you can't always tell whether specific claims are being made ('my 12AU7 does this') or just a general claim ('my valve roughly does what a real 12AU7 did').
Unfortunately, many 'modern' datasheets are merely reproductions of NOS datasheets, perhaps with a few modifications, so you can't always tell whether specific claims are being made ('my 12AU7 does this') or just a general claim ('my valve roughly does what a real 12AU7 did').
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