Stoopid Question re PT datasheet

[shaun]

Heyo. So, I'm looking at a Hammond datasheet. It seems to cite 115mA as it's current draw capability, but in tiny letters in the lower box, it says 230.4mA Max.     

Datasheet attached.

This implies, to me at least, that I could run an amp with, say, 150-160mA draw and have no problems in terms of the PT melting down, because 230mA is apparently what it could handle intermittently, as long as the max draw never remained close to 230mA. Is that correct?

The reason I ask is because I've always considered the 115mA figure as the max draw I should allow, which means I've been spending way more than I need to on PTs and that the execs at Hammond are holidaying on my dime. Again.

Feeling rather stoopid over this. Please advise.

[tubeswell]

Heyo. So, I'm looking at a Hammond datasheet. It seems to cite 115mA as it's current draw capability, but in tiny letters in the lower box, it says 230.4mA Max.     

Datasheet attached.

This implies, to me at least, that I could run an amp with, say, 150-160mA draw and have no problems in terms of the PT melting down, because 230mA is apparently what it could handle intermittently, as long as the max draw never remained close to 230mA. Is that correct?

The reason I ask is because I've always considered the 115mA figure as the max draw I should allow, which means I've been spending way more than I need to on PTs and that the execs at Hammond are holidaying on my dime. Again.

Feeling rather stoopid over this. Please advise.

115mA is the maximum draw on the HT winding.


Its a 2-phase 115mA winding. When one side is pulsing at +300VAC, the average current available to be drawn is 115mA, and although the other side of the winding is pulsing at -300VAC in the opposite direction, this other side is cut-off from the load on the power rail by the rectifier diode, so no current is being 'drawn off' through the power rail during the negative pulse. Then when the signal is in the other phase, its vice versa. So after the rectifier, you have a whole series of +ve pulses drawing 115mA maximum ave current. If you run a higher current draw than this, the PT will run hotter than intended.

[acheld]

As I understand it, the "230" mA is for the primary winding.

No need to feel stoopid, these charts are not that easy to sort out.

[shaun]

Okay. Thanks guys. That means I understood it correctly the first time. I seem to be at that stage of development where I am reviewing much of my early learning, which started about 5 yrs ago. Generally speaking, I'm feeling a lot smarter now than I was then about most things tube, but sometimes I second guess myself.

And most important, I would be very upset if I thought I'd spent more $$ than necessary ;).

[PRR]

...it says 230.4mA Max.

No-load input current AT 120V 60Hz. Ideally this would be zero but we can't have ideal things. If the transformer had an internal short the no-load input current would be (much!) higher than 230.4mA and that would be a reject. (point four??)

It is a big 2*6V6 or small 2*6L6 part. About 15-20 Watts output, and favoring a 6-8KCT load.

[shaun]

Thanks for that info, PRR. The question arose for me because I'm building a 6L6SE, using a 6U8A in the preamp, a 12AX7 cold-clipper OD channel, and a 12AT7 as the driver. Below, you'll see how the math adds up, if I understand my datasheets correctly (and it's possible I don't):

6U8A:    23mA   (total for both sides)
12AT7 in parallel:   20mA   (total for both sides)
12AX7:      12mA   (total for both sides)
6L6:         69mA
TOTAL:      124mA    total draw.

I bought an Edcor XPWR107 - 600V@150mA, just to have decent iron in the amp. But when it arrived, the PT is kinda big, so I may be way over spec. The next size down - either Edcor or Hammond - seem to be around 115mA at 600V, so I went up in size instead of down, but the PT is kinda heavy, so I'm wondering whether I could have gotten away with the 115mA@600V.

[tubeswell]

building a 6L6SE, using a 6U8A in the preamp, a 12AX7 cold-clipper OD channel, and a 12AT7 as the driver. Below, you'll see how the math adds up, if I understand my datasheets correctly (and it's possible I don't):

6U8A:    23mA   (total for both sides)
12AT7 in parallel:   20mA   (total for both sides)
12AX7:      12mA   (total for both sides)
6L6:         69mA.

Without having a schematic to see how your gain stages would be set up, my guess is preamp tube allowances are too high. For instance, a typical 12AX7 triode in a normal common cathode stage in a normal Fender amp draws about 1mA. A typical LTP 12AT7 triode is also about 1mA (tops). 8mA as a reverb tube driver.

[shaun]

Without having a schematic to see how your gain stages would be set up, my guess is preamp tube allowances are too high. For instance, a typical 12AX7 triode in a normal common cathode stage in a normal Fender amp draws about 1mA. A typical LTP 12AT7 triode is also about 1mA (tops). 8mA as a reverb tube driver.

Hmmm. Yes, I see. It seems I have not been been interpreting the datasheets correctly. When I look at the attached sheets, I see 10mA on the 12AT7. Then I assumed it was 10mA per side. So, 20mA. But you're telling me to expect much less. So I'm attaching the schematic. For my schematics, I need to get with the modern era, but I find it quicker to draw by hand. More fun than sitting at the computer, too :). Just fyi, I used a Hoffman relay PS and switch off the 6.3V supply - just don't know how to draw that in yet.

[PRR]

> interpreting the datasheets correctly.

That's how they get you.

They want real high transconductance number. That means high current. "Show-off numbers".

Especially high for 12AT7 and similar, which will do radio frequencies, with coil-loads and high current to swing parasitic capacitance.

Now look at your circuit. Supply voltage, 100k resistor, tube, some minor resistance. The current and voltage will not be "zero" anywhere, and you can start by penciling "about half" everywhere.

I assumed 300V supply, so 150V across tube and 150V across 100K resistor. Which is 1.5mA. (And this is both halves of 12AT7, which you budgeted 20mA.)

"About half" is not written in stone. Tubes do not conduct well so we have to hit them with voltage. The popular 12AX7 stage with 1.5K and 100K tends to bias at plate at 70% of supply (30% of supply across 100k resistor). This makes 90V across 100k so a hair under 1mA.

Supply voltage does not matter a lot. If I had assumed half of 400V on 100k we'd have 2mA, not 1.5mA or 6mA or 10mA.

My car is rated a long half-ton. Me and my sweetie are much less, even with groceries. Machines don't always run at full load.

In your plan, I see 8mA for all the little stages. Not 55mA.

[shaun]

Wowza. That is some fantastic info. Thanks, PRR. I appreciate the support. I have probably spent hundreds...nay, thousands...more than I ever needed to.

Those trannies do sometimes get hot though ;).     (Is it trannie, or tranny?)

I now have knowledge that I didn't, and I'll save a buck or two. Plus, my amps won't be so heavy. And my chassis won't buckle from the tranny weight. And maybe one day soon, I'll know what I'm doing!

[tubeswell]

(Is it trannie, or tranny?)

Might as well ask, what is one tranny not able to do that two or more trannies are able to do? Answer - I prefer not to know.