mA handling of this toroid HT

[Guitarist]

Just wondering how many 12au7 or 12ax7 plates one can hook up to this toroid (after it get's laddered up by the filament transformer)? I think a 12au7 uses 20mA (and the 12ax7 45mA) per tube (more gain, more mA right?).

Digi-Key - TE70001-ND (Manufacturer - 70001)

Hoffman Vintage guitar tube amplifiers, tube amp parts, tube amps, tube amplifier

from Digikey: Series Output Voltage @ Current 18 VAC @ 89mA
Parallel Output Voltage @ Current 9 VAC @ 178mA

I suspect this is the one EH and Hoffman (wow, huge site) uses.

[R.G.]

Just wondering how many 12au7 or 12ax7 plates one can hook up to this toroid (after it get's laddered up by the filament transformer)?

I think a 12au7 uses 20mA (and the 12ax7 45mA) per tube (more gain, more mA right?).

No, that does not follow. The 12AX7, for instance, is the higher gain tube, and typically runs under 1ma per plate, maybe 2ma average per tube. The tube data sheets show much smaller plate currents as gain goes up.

from Digikey: Series Output Voltage @ Current 18 VAC @ 89mA
Parallel Output Voltage @ Current 9 VAC @ 178mA

Digikey thoughtfully gives us a link to Amveco's web site with the specs on that transformer, indicating that the transformer produces 11.6V x 2 with no load. That would indicate that it works fine with a nominal 12Vac input to its secondaries. The datasheet also tells us that:
- the no-load primary current is less than 1ma
- the efficiency is about 77%
- the no-load secondary voltage is 11.6V
- the designed output power is 1.6W

The primary current for max power out is the 1ma of magnetizing current plus the secondary current referred to the primary. The secondary power out is (9/230)*0.178A = 6.96ma, so we know the primary will not overheat with about 8ma through it. We can presume that the primary will not overheat when it provides this much current, either.

It also tells us the secondary-referred resistances. At 178ma, the output is 9Vac, at 0ma, it's 11.6. The resistance then must be (11.6V-9V)/0.178A or 14.6 ohms referred to the secondary. We can refer this to the primary side by multiplying by the square of the turns/voltage ratio: on the primary side, it's (230/11.6)*(230/11.6)*14.6 or 5739 ohms. The transformer will act like it has a 5739 ohm resistor in series with a "perfect" 230:11.6 transformer.

A quick trip to the circuit simulator shows the following behavior:
1M load resistor: 309V, 50mV ripple with a 47uF filter cap: Pout = 96mW
240K load resistor:289V, 150mV ripple: Pout = 348mW
2x240K load resistor:271V, 250mV ripple: Pout = 612mW
3x240K load resistor:257V, 380mV ripple: Pout = 825mW
4x240K load resistor:245V, 4.08mA, 400mV ripple: Pout = 1001mW

From this we can guess that the maximum load it will reasonably do is the 1.6W it's speced at, or something like a 5ma load on the high voltage. At that point, the DC output has sagged to 232Vdc. Get much over that you're going to overheat the trannie. I would not like to go much over 4ma with that device. Still, good enough for two 12AX7's.

I suspect this is the one EH and Hoffman (wow, huge site) uses.

Yeah, and the guy with the biggest site wins!

[Guitarist]

OK, only two 12ax7s then, fair enough, and then durable as seen by these guys over the years. Thank you for all that hard work R.G. you have an amazing mind, from my perspective, a guitarist. I've used a few of these little transformers in the past, usually for a Cornish/VHT-style buffer.
I'll have to look elsewhere for a dedicated preamp transformer (toroid). Something that can muscle up to 7 preamp tubes out the door, but not designed for output tubes as well. Where are people getting these?
Antec's (Antek - Transformers - Grid View[]=0) seem made for amps and are a bit tall for a 1U rack space.
But, Amveco (Acme Electric/Actown) has others too. Let's try this one then: Digi-Key - TE70031-ND (Manufacturer - 70031)

[R.G.]

OK, only two 12ax7s then, fair enough, and then durable as seen by these guys over the years. Thank you for all that hard work R.G. you have an amazing mind, from my perspective, a guitarist.

Thanks for the kind words - I use techie-ism to make up for my lack of talent as a guitarist.

That would be be funnier if it weren't mostly true.

Something that can muscle up to 7 preamp tubes out the door, but not designed for output tubes as well. ...
But, Amveco (Acme Electric/Actown) has others too. Let's try this one then: Digi-Key - TE70031-ND (Manufacturer - 70031)

With the analysis under our belts, we can make quick guesses. As transformers get bigger, they get less encumbered by their internal resistances, like the 70001 was. The 70031 you suggest is rated on Amveco's data sheet as 9Vac at 0.778A versus the 0.178A of the 70001. That suggests that if we went through the math again, we'd get a primary current capability that's 0.778/0.178 bigger, or roughly 4.4 times bigger. It's a fair bet that the 70031 will power four times as many 12AX7s, or about 8 tubes; you may be where you want to be already with the 70031.

There are some other tricks. While it's not good to parallel transformers directly unless they have been designed specifically for that, you can parallel two small, high impedance transformers like the 70001 and probably up through the 70031 after their diode bridges. These have appreciable internal resistances, which buffer out any slight voltage differences, so you get good sharing of the load. You could then fit in more of them up to your height limitation by putting a diode bridge on each one and feeding the diode bridges to the same filter cap. It won't be perfect, and one will get hotter than the other(s), but it will get you more output DC in a low-height package. It's costlier, but it does preserve the physical form factor.