Hello all,
I recently bought a transformer that had formerly been in a Sony STR-DH190 solid state receiver. The specs say that this receiver was capable of about 100 watts per channel. It's quite large and heavy. I've read online that these transformers can be repurposed as PT for lower-voltage tube amps, given that their secondaries are frequently rated in amps as opposed to milliamps. Some have even argued for using voltage doublers, which I've done myself in a different tube amp; it worked fine.
The PT has a pair of primary wires and two sets of secondaries. One is a set of five thin wires, divided into 2 and 3 (center tapped). The second is a set of five larger wires, with one center tap for the remaining four wires.
I measured the primary resistance at 1.6 ohms. The secondaries all measured between 1 ohm and .3 ohms, as shown in my pencil drawing.
I put a 24 volt (no-load was 28 volt) AC transformer on the primaries and measured the secondaries. Then I applied a factor of 4.35 to get to what the approximate no-load voltage would be at 122 VAC.
The smaller secondary pair is 8VAC. The center-tapped smaller secondary is 21.83 VCT. The larger secondary is 65VCT and 89VCT.
So here's my question. I was playing around with a pair of 32L7GT tubes. They are combined rectifer/beam power tetrode. They need 32.5 volts on the heaters at .3 amp, and 90 volts on the plates and screen, drawing a combined 29ma per tube.
I was thinking I could derive my heater voltage from the 65 VCT secondary running the heaters in parallel, and then get my (approximately) 90 volts from the 89VCT secondary. However, I don't know if you can or should take power from both legs of a single center-tapped secondary; I've never done that before. I was thinking I could use the smaller secondary's 8 VAC output to run a solid-state pre to feed the tube input.
I realize this is all an exercise in WTF, but I'm just playing around for fun. Can I get away with it?
I recently bought a transformer that had formerly been in a Sony STR-DH190 solid state receiver. The specs say that this receiver was capable of about 100 watts per channel. It's quite large and heavy. I've read online that these transformers can be repurposed as PT for lower-voltage tube amps, given that their secondaries are frequently rated in amps as opposed to milliamps. Some have even argued for using voltage doublers, which I've done myself in a different tube amp; it worked fine.
The PT has a pair of primary wires and two sets of secondaries. One is a set of five thin wires, divided into 2 and 3 (center tapped). The second is a set of five larger wires, with one center tap for the remaining four wires.
I measured the primary resistance at 1.6 ohms. The secondaries all measured between 1 ohm and .3 ohms, as shown in my pencil drawing.
I put a 24 volt (no-load was 28 volt) AC transformer on the primaries and measured the secondaries. Then I applied a factor of 4.35 to get to what the approximate no-load voltage would be at 122 VAC.
The smaller secondary pair is 8VAC. The center-tapped smaller secondary is 21.83 VCT. The larger secondary is 65VCT and 89VCT.
So here's my question. I was playing around with a pair of 32L7GT tubes. They are combined rectifer/beam power tetrode. They need 32.5 volts on the heaters at .3 amp, and 90 volts on the plates and screen, drawing a combined 29ma per tube.
I was thinking I could derive my heater voltage from the 65 VCT secondary running the heaters in parallel, and then get my (approximately) 90 volts from the 89VCT secondary. However, I don't know if you can or should take power from both legs of a single center-tapped secondary; I've never done that before. I was thinking I could use the smaller secondary's 8 VAC output to run a solid-state pre to feed the tube input.
I realize this is all an exercise in WTF, but I'm just playing around for fun. Can I get away with it?
I think you could aim higher. Push-pull 8CW5s or 10CW5s at 200V from a doubler could do 15 watts per channel or so. Problem (as always) is the output transformer. A pair of 120/120 to 12V toroids would be about right ($16 each from Antek). Those output tubes are expensive, though - $3 each at findatube.com.
You could also go single-ended (21LU8 / 21LR8s for a spud) but output transformers will cost more.
You could also go single-ended (21LU8 / 21LR8s for a spud) but output transformers will cost more.
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You will better off by straight building regular "tube" amps fed from standard "tube" PTs instead of jumping through loops and hops to use something you found for cheap.
Or build an SS amp.
You will spend $1000 to save $10 or something like that.
A guy once wrote me: "I have a TIP32, which amplifier should I build?"
True story.
Or build an SS amp.
You will spend $1000 to save $10 or something like that.
A guy once wrote me: "I have a TIP32, which amplifier should I build?"
True story.
200 volts from a doubler? Nah, go for 400 using Tubelab’s “Too Many Diodes“ power supply. Big transformer like that can take it. A couple of 21LG6’s, and make some real watts. Make the whole front end out of 8BQ7’s (Done this, it works nicely).
The original idea with those 32V tubes may have h-k voltage problems. Ground will be moving around with respect to heater, as it will come of the negative half of a rectifier, probably a doubler or quadrupler. You can’t ground the center tap, which is what you really want to do for the heater supply. I’d use 8V or 21V tubes. Even if you didn’t go for crazy/stupid watts.
The original idea with those 32V tubes may have h-k voltage problems. Ground will be moving around with respect to heater, as it will come of the negative half of a rectifier, probably a doubler or quadrupler. You can’t ground the center tap, which is what you really want to do for the heater supply. I’d use 8V or 21V tubes. Even if you didn’t go for crazy/stupid watts.
And if you used those LU8’s in P-P at full rated 260 mA peak cathode current - on a 200V supply…. Let’s see 140V swing x .26 A, divide by two, still gives 18 watts. 15 after losses. Requires 2150 ohm Ra-a. 16.4 to 1 ratio with 8 ohms. Antek 7V toroid work just fine at that low flux density (secondaries in series). 40 some bucks plus what’s on hand and youre in business for 15 watts per channel - nice practical and easy to build amp, assuming you have any experience at all with tube amps. The only “trick” is dialing in the screen voltage to get exactly what you need. Those *BQ7’s are intended to be used as cascodes at 150V per element, so they will work well on a 200V B+, and make very nice LTP splitters. Regular audio stuff will have a lot higher distortion if starved like that. The usual 12AU7 wants more. The triodes that come with the LU8 for free aren’t the best for LTP splitters. They don’t match well and aren’t as happy on low B+ trying to deliver 20 or so volts. So use them in the front end where a 60 mu triode is welcome and voltage swings are low.
Spending $1000 to save 10? Don’t thinks so. Thinking outside the box will get you going for cheap. No over priced audio tubes coming from conflict regions around the world, no “tube specific” transformers that carry a premium price tag just because they KNOW audiophiles will pay it. And not building the same 3 to 5 basic amps everyone else has (and all offered as kits on E-bay).
Will your result SOUND (or measure) as good as a classic design costing $400 for the kit? Maybe, maybe not. What I do know it it will sound better than $50 (your additional expense) has a right to.