Couldn't find a great answer in the forums...
So, I'm having a hard time sourcing a power transformer for a preamp build. I'm adapting a Japanese design so the primary input voltage is the genesis of this quest... The specs for the original transformer are 100V primary with 260V 30mA and 15V 1.5A secondaries (see first image).
I found a transformer on a surplus site that potentially seems like a good fit... with some tweaking (see second image).
The primary voltage is pretty straightforward with options for 108V, 117V, and 125V.
The secondary voltages appear to be 30V 1.2A, 5V2A, 6V 1.5A and 6.3V 3.5A.
My question is regarding secondaries...
Can I use only one leg of the secondary to achieve 260V?
Can I combine the secondaries - even though they're of different values - to get close to my 15V secondary? 5V, 6V and 6.3V... to achieve 17.3V.
I guess alternatively, I could use the 30V 1.2A secondary and lower the voltage.
Thanks in advance!
So, I'm having a hard time sourcing a power transformer for a preamp build. I'm adapting a Japanese design so the primary input voltage is the genesis of this quest... The specs for the original transformer are 100V primary with 260V 30mA and 15V 1.5A secondaries (see first image).
I found a transformer on a surplus site that potentially seems like a good fit... with some tweaking (see second image).
The primary voltage is pretty straightforward with options for 108V, 117V, and 125V.
The secondary voltages appear to be 30V 1.2A, 5V2A, 6V 1.5A and 6.3V 3.5A.
My question is regarding secondaries...
Can I use only one leg of the secondary to achieve 260V?
Can I combine the secondaries - even though they're of different values - to get close to my 15V secondary? 5V, 6V and 6.3V... to achieve 17.3V.
I guess alternatively, I could use the 30V 1.2A secondary and lower the voltage.
Thanks in advance!
Is this a valve (tube) amp?
Normally the 260-0-260V secondary would be used with two rectifier diodes to operate as a full wave rectified voltage. Valve diodes were often dual so that this was normal, you can use two silicon diodes instead. Of the right reverse voltage rating, of course. I'd use 1000V devices myselft. You could use a bridge rectifier on one side but I would suggest using the twin rectifier solution.
If the 15V is for a valve heater it's an unusual voltage. Most valves were 6.3 (rectifiers 5V) or some crude mains-fed devices with 100mA or 300mA series connection. If it is a valve heater it's important to run heaters at the right voltage as over-running them shortens the life, under-running reduces efficiency.
If it is for a solid state circuit then a regulator can be used to reduce the voltage, or indeed for heaters for that matter.
Normally the 260-0-260V secondary would be used with two rectifier diodes to operate as a full wave rectified voltage. Valve diodes were often dual so that this was normal, you can use two silicon diodes instead. Of the right reverse voltage rating, of course. I'd use 1000V devices myselft. You could use a bridge rectifier on one side but I would suggest using the twin rectifier solution.
If the 15V is for a valve heater it's an unusual voltage. Most valves were 6.3 (rectifiers 5V) or some crude mains-fed devices with 100mA or 300mA series connection. If it is a valve heater it's important to run heaters at the right voltage as over-running them shortens the life, under-running reduces efficiency.
If it is for a solid state circuit then a regulator can be used to reduce the voltage, or indeed for heaters for that matter.
It appears you could use the CT and one end of the 260VAC secondary with a full wave bridge rectifier
(4 diodes) to get 260V x 1.414 = 367VDC at 30mA, if that's what you want to do.
(4 diodes) to get 260V x 1.414 = 367VDC at 30mA, if that's what you want to do.
thanks for the reply! yes it’s a tube preamp. i’ve attached the full schematic below.
the 260V secondary is rectified with diodes and the 15V is for the heaters after some processing.
the 260V secondary is rectified with diodes and the 15V is for the heaters after some processing.
To avoid burning a lot of power with the 30VAC winding, use the 5V2A, 6V 1.5A and 6.3V 3.5A in series for 17.5VAC.
Then increase the 2R and 4.7R resistors to get the filament 12.9VDC.
I would use 35VDC capacitors for the filament supply.
Then increase the 2R and 4.7R resistors to get the filament 12.9VDC.
I would use 35VDC capacitors for the filament supply.
One way to test your setup almost safely.
Find an old AC wall wart, one that outputs 5, 10 or 15 Volts AC.
Use that low voltage AC as your line voltage. Measure and multiply by the ratio of low voltage to real line voltage.
Find an old AC wall wart, one that outputs 5, 10 or 15 Volts AC.
Use that low voltage AC as your line voltage. Measure and multiply by the ratio of low voltage to real line voltage.
Thanks for your replies! All super helpful.
Also, is there an easy way to test the polarity of secondaries if they are unmarked?
I'm particularly interested in the results of combining different value secondaries of this specific transformer to get something close to the 15V as specified in the schematic.
Thanks again for all of your help!
How does series wiring affect the total current - particularly when they are of different values? Do you just go with the lowest value of the combined secondaries?
Also, is there an easy way to test the polarity of secondaries if they are unmarked?
Are you suggesting I construct the circuit as drawn, but use an appropriate AC wall wart in place of the low-voltage secondaries from the PT? If so - and I might be missing something - what would this achieve?
I'm particularly interested in the results of combining different value secondaries of this specific transformer to get something close to the 15V as specified in the schematic.
Thanks again for all of your help!
Series connection means the lowest current rating of the three windings is the limit.
Connect two pairs of windings in series and measure the total voltage. It will be either 0VAC or the sum.
Flip one winding if needed to get the sum.
Add the third winding and repeat so you get 17.5VAC total.
Since that is a bit high, increase the two series resistors to compensate. There's no problem doing this.
Thank you! Yes I believe I read something about parallel secondaries being an issue if the polarities were mucked up.
Having a hard time with this calculation as I’m not sure of the current and how the voltage is affected by the bridge rectifier. Any suggestions as to how I might approach this and calculate the resistor values?
Thanks in advance!
Each PCL83 draws 300mA, so stereo needs 600mA.
If you have 17.5VAC, rectified that will be around 23.6VDC.
You need to drop 23.6V - 12.6 = 11V.
Then for stereo the total resistance needed to drop 11V / 600mA = 18.3R @ 6.6W.
Use one resistor in each line to the capacitor, 8.2R @ 5W and 10R @ 5W.
Suggest that you use 35V rated capacitors for the filament supply.
If you have 17.5VAC, rectified that will be around 23.6VDC.
You need to drop 23.6V - 12.6 = 11V.
Then for stereo the total resistance needed to drop 11V / 600mA = 18.3R @ 6.6W.
Use one resistor in each line to the capacitor, 8.2R @ 5W and 10R @ 5W.
Suggest that you use 35V rated capacitors for the filament supply.
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For a full wave (4 diode) rectifier, the rectified DC voltage is
( Vsecondary x √2 ) - ( 2 x Vdiode_drop )
= 17.5 x 1.414 - 2 x 0.6
= 23.6VDC
( Vsecondary x √2 ) - ( 2 x Vdiode_drop )
= 17.5 x 1.414 - 2 x 0.6
= 23.6VDC
This is why I’m here! Thanks for your insight. I’ll replace the 2R 3W with an 8.2R 5W and the 4.7R with a 10R… and good call raising the voltage rating to 35V for the caps. I’ll post the results when I finish the build!
If you get much different output voltage from 12.6VDC, more than say 0.3VDC off either way,
with both of the tubes plugged in, change one of the 5W resistors accordingly.
If you get a too low voltage, increase the 8.2R resistor.
If you get a too high voltage, decrease the 10R resistor.
with both of the tubes plugged in, change one of the 5W resistors accordingly.
If you get a too low voltage, increase the 8.2R resistor.
If you get a too high voltage, decrease the 10R resistor.
Toroidy manufactures a 200V 100mA and 15V 2A secondary transformer.
https://www.tme.eu/nl/details/tsta50_002/toroidale-transformatoren/toroidy/tsta-50-002/
https://www.tme.eu/nl/details/tsta50_002/toroidale-transformatoren/toroidy/tsta-50-002/
What is the advantage of starting with almost double the heater voltage and burning off the excess rather than starting with closer to 12.6V?
Using 5+5V would produce 13V
The schematic says the secondary voltage should be 15VAC, so that's what he wanted.
A little more, dropped back down, is better than not enough.
A little more, dropped back down, is better than not enough.
Another option might be some of the recent SMPS offerings. This is being explored over at GroupDIY: https://groupdiy.com/threads/ht-transformers-for-vacuum-tube-preamps.75793/page-3#post-1067004
They might not be as robust as old school windings, but might be worth it 🙂
They might not be as robust as old school windings, but might be worth it 🙂