Ok thanks but if I'm doing this right:
The PT states 3A available for the 6.3V winding. I would be running 2x6V6 (0.45A each) = 0.9A plus 4x12AX7 (0.3A each) = 1.2A. Combined these are 2.1A but we multiply by 1.4 (convert RMS to peak I'm guessing?) = 2.94A. Do I need to include the pilot light?
The PT states 3A available for the 6.3V winding. I would be running 2x6V6 (0.45A each) = 0.9A plus 4x12AX7 (0.3A each) = 1.2A. Combined these are 2.1A but we multiply by 1.4 (convert RMS to peak I'm guessing?) = 2.94A. Do I need to include the pilot light?
If you use a small switching power supply to power the heaters, your problem is solved, and you can use your existing transformers.
Since it is your own DIY build, simply wire the power supply up to the same IEC inlet as everything else. From the outside of the amp, no-one can tell that there are two separate power supplies inside.
I suggest using a 12V DC power supply, wiring the 12AX7s for 12V heater operation, and putting the two 6V6 heaters in series, so you can run the pair of them off the same 12V DC.
Not only will this let you use the transformers you have, but as an additional bonus, you will have no worries about AC hum from the heaters or heater wiring.
-Gnobuddy
Not for sure, though I read somewhere on some forum that there were some Peavey valve guitar amps that used switching power supplies (for everything, not just heater power.)
There has got to be some commercial outfit that has thought about this, but electric guitar is now mostly a grandfather's market, and grampa often wants things the same way they were in 1960. If Fender used a switching power supply in their 2019 Deluxe Reverb, many customers would immediately refuse to buy it.
But you're building for yourself, so you don't have that problem. If you find a solution you like, there's nothing to stop you from using it.
I've used DC heater power (from dirt cheap thrift-store power supplies) a few times now. The ones that have a 2-wire AC power cord usually have a floating output (neither of the DC output wires is grounded.) This means you are free to ground either end (+ or - ), whatever suits you best. No need to use two resistors to create an artificial centre tap like you do with AC heater power - you can just ground one end, since there is no AC voltage.
If the budget is a little bigger, or the local thrift-store doesn't have a power supply that suits your needs, you can instead buy a little board-mount AC-DC converter from Digikey or other electronic suppliers. Stick it on a scrap of proto-board and mount it inside the amp, and there you go. Like this one (it will be cheaper in the USA): IRM-45-12 MEAN WELL USA Inc. | Power Supplies - Board Mount | DigiKey
By the way, you can always use a series power resistor to knock down the voltage a wee bit if you end up with a power supply voltage that's a little higher than what you need. I used an 8.4V DC power supply in one of my projects, with a 5W resistor in series to drop the voltage to 6.3V at the heaters.
-Gnobuddy
Mozz is right - if your power transformer is rated to supply up to 3 amps of 6.3V heater power, you are good to go.
As an engineering solution, using AC heater power is a poor second to DC heater power. But it does have the advantage of convenience - if you already have it, you might as well use it.
-Gnobuddy
Just as a side comment, from a manufacturer´s point of view, it does not pay to use an SMPS power supply just for the filaments.
Once you are comitted to a large EI transformer, a few pounds of iron and a couple pounds of copper, adding a filament winding is an easy and relatively inexpensive deal, just a few turns of thickish wire, won´t materially alter the bill of materials or the work time.
Now if you are repurposing a ready built transformer, be my guest.
Once you are comitted to a large EI transformer, a few pounds of iron and a couple pounds of copper, adding a filament winding is an easy and relatively inexpensive deal, just a few turns of thickish wire, won´t materially alter the bill of materials or the work time.
Now if you are repurposing a ready built transformer, be my guest.
Understood, and what you say makes total sense. The Peavey amp I heard about used a custom SMPS for the entire power supply.
Exactly. For us hobby / DIY types, we have the problem that 350 VDC - 400 VDC switching power supplies are not easy to find, and there are very few choices available. Meantime, low voltage, high current power supplies are everywhere now, and cheap ones can be found in every thrift store if you don't already have a few unused ones lying around your house.
Meantime, we hobbyists can find inexpensive 240/120V AC isolation transformers that can be used to generate about 340V DC. But those transformers don't have 6.3V AC windings for heater power. We can still buy dedicated valve power supply transformers instead of 120/240 isolation transformers, but the price jumps from $20 to $100+.
So - again, for hobby builders - it makes a lot of sense to marry an affordable 120/240 or similar isolation transformer for the HT with a cheap switching power supply for the heater power.
-Gnobuddy
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