Thanks for all the input guys, i'm new here and all this is very helpful 🙂
I can change my HT current draw to 20mA and filter caps to 100uF (for C2 and C3).
I could also use a separate 'wallwart' DC power supply for the heaters as to not stress my 1st transformer, although this is a little annoying having to plug in two mains plugs but thats what I get for not doing my calculations properly and cheaping out 😆
I can change my HT current draw to 20mA and filter caps to 100uF (for C2 and C3).
I could also use a separate 'wallwart' DC power supply for the heaters as to not stress my 1st transformer, although this is a little annoying having to plug in two mains plugs but thats what I get for not doing my calculations properly and cheaping out 😆
Pairing the 6V heaters in series will cut the heater current in half and greatly improve the efficiency of the power supply. Consider: 12VAC into a bridge gives about 15VDC; you throw away 8.7V of that to get 6.3. And you need about 20VA input to get 6 watts of output. With 12.6V heaters, you only drop a few volts. Just use a resistor for that - DC heaters don't need regulation (though a low-dropout regulator could work)
As far as capacitor size, RMS current changes very little with capacitor size. And that's the only value the transformer cares about. Yes, peak current is higher, but conduction period is shorter. Might matter in other respects, such as more noise with improper grounding (connect one output terminal of each supply to ground, nowhere else, and you'll be fine.
As far as capacitor size, RMS current changes very little with capacitor size. And that's the only value the transformer cares about. Yes, peak current is higher, but conduction period is shorter. Might matter in other respects, such as more noise with improper grounding (connect one output terminal of each supply to ground, nowhere else, and you'll be fine.
Hi As I see it you are powering a <11 Watt circuit from a 12VAC, 3A, 36 watt power supply, it would be a shame to have to plug in an extra supply. Chris stated in post 22 the efficiency benefits of a 12 volt heater supply. In post 10 suggested a 12.6V heater circuit and you stated in post 12 it was not easy.
If it is difficult to rewire for 12 volts one could use a cheap ebay buck converter something like this.
There may be a high frequency noise issue to filter out.
https://www.ebay.com.au/itm/355421436683?itmmeta=01HPTMTA35EE4VDE8WP94Q8Q7C&hash=item52c0c4bb0b:g:XTgAAOSwezZlt4Mj&itmprp=enc:AQAIAAAAwKex3wb0xKp2rUlTqZPrtrEU6JZ3PnV1X8BBxFfKt+UIAgsjXbal6niUmPpRRVbg0eelPvqTJabnt9Uj6t2UEszn1GRMpO9ITKYvDXsGQVAEBOQ9d3a9H4zG3JSxk6vWZ1dhKyvySbsbrk6aktSg6UUWWAPzAxcs4eMl9ZjsbLp0PvMcNXN3lyrzz1TUnAXJAlSe6EXoPUQjIjH8/0f237JcuSxYnuZr5OQT4A56U44CxTfRIlkSSd9BQMZg+hYdrA==|tkp:Bk9SR9ih6dS2Yw
The efficiency can be up to 90% input voltage 3 to 40VDC the input voltage must be a couple volts above the output voltage. The output voltage could be 6.3VDC.
I used some of these buck converters to power some 12 volt LED lamps from a 24 volt supply, they have been running fine all night for years.
For B+ there are also cheap ebay 12 to 250VDC modules that have high efficiency, shielding and filtering would be required. Maybe an efficient heater supply will enable you existing backwards transformer to work fine.
If it is difficult to rewire for 12 volts one could use a cheap ebay buck converter something like this.
There may be a high frequency noise issue to filter out.
https://www.ebay.com.au/itm/355421436683?itmmeta=01HPTMTA35EE4VDE8WP94Q8Q7C&hash=item52c0c4bb0b:g:XTgAAOSwezZlt4Mj&itmprp=enc:AQAIAAAAwKex3wb0xKp2rUlTqZPrtrEU6JZ3PnV1X8BBxFfKt+UIAgsjXbal6niUmPpRRVbg0eelPvqTJabnt9Uj6t2UEszn1GRMpO9ITKYvDXsGQVAEBOQ9d3a9H4zG3JSxk6vWZ1dhKyvySbsbrk6aktSg6UUWWAPzAxcs4eMl9ZjsbLp0PvMcNXN3lyrzz1TUnAXJAlSe6EXoPUQjIjH8/0f237JcuSxYnuZr5OQT4A56U44CxTfRIlkSSd9BQMZg+hYdrA==|tkp:Bk9SR9ih6dS2Yw
The efficiency can be up to 90% input voltage 3 to 40VDC the input voltage must be a couple volts above the output voltage. The output voltage could be 6.3VDC.
I used some of these buck converters to power some 12 volt LED lamps from a 24 volt supply, they have been running fine all night for years.
For B+ there are also cheap ebay 12 to 250VDC modules that have high efficiency, shielding and filtering would be required. Maybe an efficient heater supply will enable you existing backwards transformer to work fine.
If you are going with a switching supply, Tayloredge has some great ones (there are cheaper options on that auction site)
https://www.tayloredge.com/storefront/SmartNixie/PSU/comparison.html
https://www.shop-tes.com/power-supply/
https://www.tayloredge.com/storefront/SmartNixie/PSU/comparison.html
https://www.shop-tes.com/power-supply/
Thanks for the links quadtech.
For those that like to play with spice files. Plug in your own transformer values. Note my own brand of diode 🙂
For those that like to play with spice files. Plug in your own transformer values. Note my own brand of diode 🙂
Hi Guys - hey this is really interesting
Years ago I used to manufacture a DAC with a tube based output stage.
I used a R-Core transformer in reverse & fed 16.5v from an alarm plug pak into one of the 9+9v CT secondary windings.
I then used all the R-Core outputs including the 115+115 primary to power the DAC.
I think I used to get about 200v out of the 115+115v primary which I used for HT.
It worked very well & was quite reliable.
Years ago I used to manufacture a DAC with a tube based output stage.
I used a R-Core transformer in reverse & fed 16.5v from an alarm plug pak into one of the 9+9v CT secondary windings.
I then used all the R-Core outputs including the 115+115 primary to power the DAC.
I think I used to get about 200v out of the 115+115v primary which I used for HT.
It worked very well & was quite reliable.
I'll get round to taking measurements in the next couple of days and see how much current headroom I get from my first 3A transformer and go from there.
In the mean time here is the circuit I plan on powering:
I will be adding a switch at R11 to control the gain, possible a potentiometer to gain match left and right channels as it is a feedback-less design.
Note that I will be using 6N2P tubes not 6N1P (I couldn't find splice models for it)
In the mean time here is the circuit I plan on powering:
I will be adding a switch at R11 to control the gain, possible a potentiometer to gain match left and right channels as it is a feedback-less design.
Note that I will be using 6N2P tubes not 6N1P (I couldn't find splice models for it)
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Thanks euro21!
The new model has a lot more gain than the other! (33.3db vs 24.7db).
Still good distortion levels at 0.5% for 18v p-p. all second harmonic of course!
BTW the MOSFET is an IRF840. rated VDS is 500v. (Datasheet: https://www.vishay.com/en/product/91070/)
The new model has a lot more gain than the other! (33.3db vs 24.7db).
Still good distortion levels at 0.5% for 18v p-p. all second harmonic of course!
BTW the MOSFET is an IRF840. rated VDS is 500v. (Datasheet: https://www.vishay.com/en/product/91070/)
For a project I needed a PT with 200V AC at the secondary: I used a VPT230-110 (this is a 230V to 230V PT), with the secondary connected to the net. 230V to 230V is under load, unloaded it is rather 260V at the secondary, so the transformer is rather a step-up. Connecting the 260V to the mains gives about 200V AC at the primary. The transformer loses a bit of the VA, but for that it runs easier (less magnetization current).
Once I want to try the same with this transformer, https://www.mouser.ch/datasheet/2/410/VPS230_350-781627.pdf As it has less good regulation than the toroidals I expect that there will be around 180V AC at the primary when connecting the secondary to the mains.
From a safety point of view I see no problems, the isolation between primary and secondary works both ways.
Once I want to try the same with this transformer, https://www.mouser.ch/datasheet/2/410/VPS230_350-781627.pdf As it has less good regulation than the toroidals I expect that there will be around 180V AC at the primary when connecting the secondary to the mains.
From a safety point of view I see no problems, the isolation between primary and secondary works both ways.
Buffering a cathode follower (mu-follower) with a source follower seems a little unnecessary, no?
If you have such output tube, which requires significant amount of grid current (in A2 mode), CF or SF is necessary.
In every other case -relatively- low output impedance VAS stage is enough.
In every other case -relatively- low output impedance VAS stage is enough.
This project is just the preamp. I have built a MOSFET single ended class A output stage that needs the current to overcome the high input capacitance (high frequency roll-off).
I also plan to build different output stages to experiment with 🙂 probably heavily bias class AB next for more power.
The preamp will have multiple outputs (simple relay based) to easily switch and compere between them, or just for preference 😎
I also plan to build different output stages to experiment with 🙂 probably heavily bias class AB next for more power.
The preamp will have multiple outputs (simple relay based) to easily switch and compere between them, or just for preference 😎
Maybe it's just me, but I'd rather use voltage multipliers than running transformers back-to-back. Just seems hokey to me.
I agree! I like to use what I have handy, so if there is some VA to spare from a low voltage filament supply and a small trafo available for some mA for a driver stage, lets go. But never buy new to do this. And indeed, do not be afraid of voltage multipliers.
I don't think is would be possible to meet the 20ma current required with a multiplier unfortunately.
I could be wrong but going from 12Vac to 200Vdc at 20ma seems like a lot. and like 20x stages?
I could be wrong but going from 12Vac to 200Vdc at 20ma seems like a lot. and like 20x stages?
Why not? Here is a schematic by Broskie for a cap multiplier.
https://tubecad.com/2024/01/blog0593.htm
https://tubecad.com/2024/01/blog0593.htm
I agree that the solution by Broskie is probably a bit too extreme, but it should work, specially in a class A preamp. Do not use wimpy caps. I use doublers, sometimes quadruplers, it really depends on the needed voltage and what I have at hand. Here in Switzerland there is barely surplus stuff available, and shipping, even from neighboring countries, is expensive and entails risk ok VAT and administration costs (UK is probably similar, but you have a bigger internal market). I do not need a lot of stuff, but when I do and have nothing at hand, I check what is available at e.g. Mouser or Digikey and order from them.
And then indeed isolation transformers, as I described in post 30.
And then indeed isolation transformers, as I described in post 30.