I recently purchased a power transformer and a pair of single ended output transformers relatively inexpensively. The PT is rated at 250 volts at 200ma and 6.3V at 3.5 amps. The output transformers are 15 watt, 3.5K to 8 ohm, 40% screen tap.
My initial thought was 12ax7 and EL34 tubes. But I'm open to suggestions. I won't be building this immediately, but it will probably be my winter project. Please give me your thoughts. Thanks!
My initial thought was 12ax7 and EL34 tubes. But I'm open to suggestions. I won't be building this immediately, but it will probably be my winter project. Please give me your thoughts. Thanks!
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Does the power transformer have 2 x 250v windings rated at 200ma or just a single winding?
If it's just a single winding you would be better off buying a beefier Antek power transformer and reserving the one you have for a preamp.
Do you already own EL34s? If so, just use those. They sound nice enough that there isn't a reason to buy something else. If not, we would need to know how much current the transformers are rated for.
If it's just a single winding you would be better off buying a beefier Antek power transformer and reserving the one you have for a preamp.
Do you already own EL34s? If so, just use those. They sound nice enough that there isn't a reason to buy something else. If not, we would need to know how much current the transformers are rated for.
3,5k is a bit on the low side for many tubes. Perhaps EL36/6CM5 or its russian equivalent 6P31S would be a good option. I've had some good results from 25E5 (the same tube but with 25V 300mA heaters) in Ultra-linear SE with cathode feedback using 3,5k transformers.
A 250VAC PT should be about right, assuming cathode bias and some voltage drop across the OPTs and a filter choke.
A 250VAC PT should be about right, assuming cathode bias and some voltage drop across the OPTs and a filter choke.
PT is 250V @ 190mA
Is that center-tapped? The primary will have three leads, usually something like red-yellow-red, but could be blue-yellow-blue, or similar.
Or is that a single winding? The primary will have two leads, usually red.
The 6.3V winding will usually have green wires. If center-tapped, they'll usually be green-yellow-green.
Is that center-tapped? The primary will have three leads, usually something like red-yellow-red, but could be blue-yellow-blue, or similar.
Or is that a single winding? The primary will have two leads, usually red.
The 6.3V winding will usually have green wires. If center-tapped, they'll usually be green-yellow-green.
Jesus. Those must be some real monsters.
You have lots of options if you want to try paralleling several tubes together. You could easily do 3 el34s, a couple kt88s, 6550s, etc. Or you could do a couple transmitting tubes. You could just barely get away with 2 810s in parallel.
@Tjj226 They are these:
https://edcorusa.com/products/gxse1...ube-output-transformer?variant=41523062014139
Edcor GXSE15-3.5K 8 ohms
The PT is this:
https://edcorusa.com/products/xpwr050-250v-200ma-6-3v-3-5a?_pos=1&_fid=e26c96503&_ss=c
Edcor XPWR050 - 250V@200ma, 6.3V@3.5A.
https://edcorusa.com/products/gxse1...ube-output-transformer?variant=41523062014139
Edcor GXSE15-3.5K 8 ohms
The PT is this:
https://edcorusa.com/products/xpwr050-250v-200ma-6-3v-3-5a?_pos=1&_fid=e26c96503&_ss=c
Edcor XPWR050 - 250V@200ma, 6.3V@3.5A.
You'll want to use a diode bridge, which should yield about +325V DC under load, after the reservoir capacitor.
If you add a choke (recommended) then you'll need to figure the loss of voltage across the choke's DC resistance.
Then comes the filter capacitor.
You should be able to use 400V rated caps for this.
Let's say you use 220uF - 10H 200mA - 220uF. That should attenuate the ripple pretty well.
An Edcor https://edcorusa.com/products/cxc125-10h-200ma-10h-200ma-choke (10H 200mA) has 75 ohm DCR. So,,,
200mA * 75R = 15V dropped across the choke, so your B+ is now 305V DC.
If you cathode bias your output tubes, you'll need to subtract the positive cathode voltage from the plate-cathode voltage of the output tubes. This looks like the voltage may be too low for a pair of 300Bs, but it looks perfect for a pair of 2A3, 6B4G, or 6AV5GA-triode wired (or similar). The bias will be roughly -50V (depending on the value of cathode resistors you choose), which means the plate-cathode voltage will be about +255V.
Edit to add:
I figure a stereo pair of fancy driver stages could end up consuming 40mA of plate current.
A pair of 2A3s running at the full 15W plate dissipation with 255V plate-cathode will consume about 118mA.
That's 158mA of plate current drawn from a 200mA rated power transformer.
That sounds reasonable to me.
The tricky part will be making a driver with enough voltage swing to cleanly drive the 2A3, etc. to full power. It can be done, though. It will take some work to make it sound good, but that's half the fun, right?
A 2A3, 6B4G or 6AV5GA-triode will make about 2W out (maybe a little more) loaded by the OPT's 3.5k ohm primary.
If you want/need more power, look at running the 6AV5 in pentode and with fixed bias, or using one of the other TV sweep pentodes (of which there are many).
But I don't think that power transformer is enough for a pair of 300Bs plus driver stages. IMO, of course. YMMV.
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6AV5GA is probably a good choice for a cheap, simple amp that still can sound very good. Not too different from the tubes I suggested earlier, but probably easier to find in the US. Lots of info here: https://www.diyaudio.com/community/threads/the-edcor-meets-the-6av5.74650/
Those are rated to 190ma MAXIMUM. Normally transformers are rated based on the maximum allowable bias current. Edcor does things slightly differently, so what you would need to do is find a tube that has a bias current of about half the maximum allowable current through the transformer.
That being said you can still use any of the "mainstream" tubes that can work with a 3.5K load.
So the OPT should be used with the output tube(s) biased to no more than 95mA? That's not much of a limitation.
Perhaps a single EL34 or 6L6 per channel, wired 'ultralinear' using the screen grid tap?
With only 285V or so plate-to-cathode, the 6L6 could be biased up to about 70mA. That would be easy to drive, and should sound pretty good too.
For 6AV5GA, I think you'd want to either use it triode (for only about 3W max out) or use it pentode with the screen grid fed from a regulated 150V DC (or thereabouts). I'm not sure how a 6AV5GA in ultralinear config would fare with >250V on its screen grid. Its datasheet states the maximum screen grid voltage is only 175V.
Check out this thread for output tube ideas -- https://www.diyaudio.com/community/threads/those-magnificent-television-tubes.211254/
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Those are decent OPTs but the power transformer is the weak link. I would invest in an Antek power transformer so you can get the most out of the OPTs. You could easily run KT88 or 120s on them. Go with SS rectification and you’ll save money and complications. The Antek have 2 6.3v @4amp windings so the heaters are no issue. Start with something simple like the Mikael Abdullah KT88 schematic and later you can reuse the transformers for something more ambitious.
Agree. Edcors OPTs are generally accepted as good value for the money. Their power transformers not so much...
6P41S make a nice SE amp with those output transformers.
I have built several amps running them at 250Vak, 60mA, which is well within the limits of the power transformer.
Build thread is here: 6P41S amp build
Use Grid Stops and Screen stops as these tubes may oscillate.
I have built several amps running them at 250Vak, 60mA, which is well within the limits of the power transformer.
Build thread is here: 6P41S amp build
Use Grid Stops and Screen stops as these tubes may oscillate.
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