I need to come up with a moderate-level HV supply for a small 1-watt 1-tube audio test/signal tracing amp. I do not have any spare tube PT's for this project, but I do have a couple of 120V to 12V center tap PT's. I'm planning on using a 12V 2A center-tap filament transformer winding to power the full secondary of smaller 120V to 12V/40mA transformer for the HV supply. I'll also be using one side of the filament transformer's secondary to power a 6AV8A tube filament. I'd appreciate thoughts on the feasibility of this idea as opposed to using a voltage multiplier scheme. Thanks for any and all input!!
Here's a rough schematic of what I mean.
Here's a rough schematic of what I mean.
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It'll work but you will lose a few volts, you won't get 120VAC on the output side, maybe 110VAC.
Don't know how critical that is.
Jan
Don't know how critical that is.
Jan
And for an inexpensive alternative, a proper transformer can be used instead of putzing around with two transformers.
https://www.tubesandmore.com/products/transformer-hammond-plate-filament-or-bias-63-v-auxiliary
https://www.tubesandmore.com/products/transformer-hammond-plate-filament-or-bias-63-v-auxiliary
The cheapest alternative is one that you already have, not one you have to go buy something for.
Thank you for your replies thus far! I should have specified that this needs to be built entirely from junk box bits. Buying parts, used or otherwise, is not an option.
As for some voltage loss, that was assumed already in my thought process, and shouldn't be an issue. This only needs to develop a watt or so across a small 8 or 16 ohm speaker coil.
As for some voltage loss, that was assumed already in my thought process, and shouldn't be an issue. This only needs to develop a watt or so across a small 8 or 16 ohm speaker coil.
I built up the power supply circuit in a make-shift chassis with an 18uf filter cap, and with a 2.2M bleeder resistor as the only load, I'm getting a bit over 170V. Good enough for this project! I can add another 47uf if needed, but I doubt I'll need to since I only need about 1 watt of audio.
When I first started with tubes I built a few power supplies like this. They worked fine. I had twenty or so various 12.6CT Radio Shack transformers that I was given with a literal trunkload of parts and gear, nothing high voltage but I made do.
Even did a few voltage doublers off of them for 300 volt supplies.
Even did a few voltage doublers off of them for 300 volt supplies.
A single ended output, with an 18uF cap, a drop resistor, and a 47uF cap (CRC), might have a fair amount of ripple (hum).
And a 2.2 Meg Ohm resistor is not a good bleeder resistor.
With 18 uF + 47 uF, it will take 143 seconds to drain from 170V down to 63 Volts. Ouch!
1 Watt of output power with junk parts . . . A 6AV8A? (I can't find it on Franks Tube Data pages),
and what output transformer?
And a 2.2 Meg Ohm resistor is not a good bleeder resistor.
With 18 uF + 47 uF, it will take 143 seconds to drain from 170V down to 63 Volts. Ouch!
1 Watt of output power with junk parts . . . A 6AV8A? (I can't find it on Franks Tube Data pages),
and what output transformer?
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I'm guessing 6AU8-A through an old pair of bifocals.
https://bms.isjtr.ro/sheets/093/6/6AU8A.pdf
Though that would more likely be a third-Watt. And trying to get from g-pickup to speaker in one bottle is begging for howling. Reverb transformer Fender 12AT7 style.
Don't miss the chance to reduce noise, when you build it ...
Connect the two 12V centre-taps together and to the DC output's 0V/Ground.
Now the two 12V AC outputs will be balanced low voltage, and if those windings are connected together with 3-core cable (one core grounded), the risk of coupling is reduced.
The grounded CT will also drain away the common mode noise current.
I have used this method for experimenting with very low noise supplies; though the added leakage inductance may need careful snubbing.
If one of them is a toroidal trafo, locate it at the rectifier end.
This way, the usual problem of mechanical hum in toroids (caused by DC offsets on the mains supply) is also eliminated.
Connect the two 12V centre-taps together and to the DC output's 0V/Ground.
Now the two 12V AC outputs will be balanced low voltage, and if those windings are connected together with 3-core cable (one core grounded), the risk of coupling is reduced.
The grounded CT will also drain away the common mode noise current.
I have used this method for experimenting with very low noise supplies; though the added leakage inductance may need careful snubbing.
If one of them is a toroidal trafo, locate it at the rectifier end.
This way, the usual problem of mechanical hum in toroids (caused by DC offsets on the mains supply) is also eliminated.
I hadn't thought of Rod's mention for low noise power supplies, but EI transformers will shield some of the AC line crud better than toroidal.
We used back to back in ham radio for low power stuff, but in the era of my youth you could get quite beefy transformers out of B&W TV's -- and it got even better when the first gen color TV's were getting scrapped. You could get a couple hundred watts Class C operation!
We used back to back in ham radio for low power stuff, but in the era of my youth you could get quite beefy transformers out of B&W TV's -- and it got even better when the first gen color TV's were getting scrapped. You could get a couple hundred watts Class C operation!
I used such a power transformer as modulating transformer several years ago. Primary (220V) used as secondary to RF tubes, center tapped secondary as primary (push pull 6KD6) and the heater secondary to feedback and a 4" speaker to test modulation.
Thanks for the reply. 6AU8A is a typo, that should have been 6AW8A, not 6AU8A. The 6AW8A is a triode/pentode combo, with the pentode plate dissipation rated at 3.75 watts. It's perfect for what I specified as my project goals in the original post, IE 1 watt of general purpose audio amplification for signal tracing, microphone testing, etc. Basic testing of a guitar's output could certainly fall into that category, but its not intended to be a guitar amp.
6AW8 and 6AU8 are pin to pin compatible, and both are designed for same function, sync separator and video out. So not too great error.
The HV winding that will power the tube can only handle about 45mA, and I don't want to load it down with too small a bleed resistor. I will drop that resistor value if I add extra capacitance.
Agree 100%! I've already tied the taps together, and will be tying them to the DC ground. Because the chassis is rather small, I'm just gonna use star grounding for now. I'll do gorund bus later if I think its needed for noise reduction. I'm new enough that I have no idea (yet) how to snub leakage inductance, but will cross that bridge if needed. Goal #1 is get it working with basic good grounding practice, then get picky with any additional noise once I know the basic circuit works as intended.