Yes, he'd need to generate a negative bias, usually with another winding on the power transformer, although there are other ways.

Let's take that previous 250V example again, but at cathode bias. They spec an 830 Ohm cathode resistor with 61 mA going through it. That's 3 Watts going to air in just the cathode resistor to get 4.85 Watts of audio and B+ has to be increased to 300 V to make up for the cathode bias loss.

Now, I don't know exactly what bias should be with his already low 200V supply but it's going to take one heck of a chunk out of it, probably around 30V, so he'd be looking at a 170V B+. And assuming one finds an OPT 'right' for that B+ it would probably do no more than around 2 Watts.

That seems to me like an awful lot of expense for not a lot to show for it and, if spending that much money, I'd be tempted to go 275V B+ fixed bias Class AB2 for 18 Watts which, with a little tweaking, looks like it could be done with a 'standard' 3400 Ohm PP OPT.

How much current should the power transformer be able to supply? I found one that puts out 70 mA at 325-0-325VDC, 2A at 6.3V, and 2A at 5V. Is that enough?

Well, the B+ is maybe enough for one channel, one mono amp, depending on how much preamp you need. Using a tube rectifier B+ would come out near the voltage rating, 325, so you'd go back to the cathode bias to burn off the excess. I hadn't brought it up till now but with only one low mu 26 it's going to take a rather large input to drive the amp to full power. It wouldn't be a 'line input'.

The 5V could power a rectifier but I don't know what you'd use to power the tube filaments because they aren't 6.3V heaters.

It might be worth asking what the real goal is. I mean, are you trying to use 45s because you have some and figure that's 'inexpensive', or are do you want a 'tube amp', of any kind, or is there a 'true to period' reason for the 45s, or... ? Is it supposed to be 'hi-fi'?

Please don't misunderstand, I'm not saying there's anything inherently 'wrong' with making an amp with 45s but even with the tubes in hand it's not 'inexpensive' to design around DHTs. Good interstage transformers aren't cheap and even power transformers are more expensive because the 6.3V heater variety, like the one you mention, are much more common. And later generation tubes have more gain, so it takes fewer of them, and the design is easier.

Couldn't I lower the 6.3 volts down to 1.5 and 2.5 volts with resistors?


I'm trying to build a basic amp that is powered by a pre-amp I've got and will drive an average 10 inch speaker. This isn't hi-fi or any kind of high-end audio.

Any comment to the PM I sent you?

Seems you don't want to re-invent the wheel. I thought that the schematic info would be of interest? Any details please PM...

Chas

Sent you a PM.

Yeah but that means you have to drop 3.8v @ 1.5A for each 45 which equals 2.5ohm at 5.7W, so you'd need approx 10W resistors to keep from from eventually burning out...

Tom

In theory yes but you'd be throwing away heaps of power and generating heaps of heat, 5.7 Watts per 45 and 5 Watts for the 26 (over 16 Watts), not to mention the combined 4.04 Amps is over twice the transformer’s 2 Amp rating.

Not needing 'hi-fi' makes things simpler and less expensive.

How much audio power do you actually need? What kind of "10 inch speaker," an old style accordion cone or modern rolled edge? Why 10"? Is this a mono amp?

Do you care what tubes are used? I mean, if free to use whatever works well you can easily tube, and more, a whole amp for the cost of a single interstage transformer.

I need a maximum of 5 watts audio power.
New rolled edge speaker.
Because, 10" is the size I need.
Yes, this is a mono amp.
Yes, I care what tubes are used. I would like to use 45 and 26 tubes.

Then you'll need at least 250V B+, not counting loss to self bias if you go that way, to get 'almost' 5 Watts and 275V for 5 Watt or over.

Edcor has 2.5V filament transformers here http://www.edcorusa.com/c/36/lvpseries
high voltage transformers here http://www.edcorusa.com/c/37/xpwrseries
and PP output transformers here http://www.edcorusa.com/c/59/pushpull

You could make a 250V B+ supply with one of these wired backwards http://www.mouser.com/ProductDetail/Triad-Magnetics/N-68X/?qs=%252bfhPKvh1fqMgz%2fL8G7F80A%3d%3d For over 250V you'd need two in series wired forward (or use an Edcor).

http://www.tubesandmore.com/ has an interstage transformer, P-T156, as well as some Hammond models.

The 10" speaker will have no high end and probably rolls off after 5 kHz or so but if you just have to have a 10 inch speaker then a 10" guitar speaker would be twice as loud for the same power. They also aren't cheap. Otherwise, this 8" has much better frequency range http://www.parts-express.com/pe/showdetl.cfm?Partnumber=292-430

It doesn't take many turns of wire to get 1.5 and 2.5 volts off a transaformer. Many are wound 4 turns per volt. For 1.5 volts might need 6 turns, 2.5 volts - 10 turns. Some transformers have enough room to add turns of wire.

Thank you for the links.