Hammond 1760W

[Jazz P Bass]

So the consensus is to run the 8 ohm load on the 4 ohm tap?

[g1]

Yep, or 16R load on the 8 ohm tap. Either way you are reflecting 4K impedance back to the power tubes.

[Jazz P Bass]

Thanks for the input G1.

I had to believe what I saw in the testing.
But the doubt would not recede.

Still, it doesn't make any sense, unless Hammond wired the transformer incorrectly.

[g1]

It doesn't make sense if you assume a 2K primary is correct for this amp, but I don't think that is the case.
I would guess the original Univox OT was 4K primary.
(see Nick's post #28)

[Jazz P Bass]

The amp sounds way better ( & louder) at 4K.

Thanks again.

[nickb]

The amp sounds way better ( & louder) at 4K.

Thanks again.

Excellent, heh, heh, heh!

Tell me JPB, what brand of 6L6's are you using that can withstand that kind of punishment?

[Jazz P Bass]

I installed Sovtek 6L6WXT's in it.

[deci belle]

What an inspiring exchange!

[nickb]

Some time ago there was a topic about measuring the OT inductance and it seemed like there was no any standard how to measure it. Most manufacturers wouldn't even provide a number.
You say the "hammond inductance" is probably correct but compared to what? Not to the formulas for sure. They measure it at 240V but I've seen other manufacturers measuring it at other voltages. I assume those numbers are derived from measuring the transformer idle current as a way to check fast that it has no shorts or any other problems.
If you check some datasheets you'll notice that their 100W OTs inductance will vary several times for approximately the same output power, transformer size and primary impedance. In the same time the formulas produce almost the same result or at least in a very narrow range that has nothing to do with those datasheet numbers. On the other side the low signal seems to produce values that are close to the formulas.
So the question still remains how to properly measure the OT inductance and compared to what?

Jazz- if you'll indulge us just one more time...

Gregg, you are sooo right.

I decided to actually ask Hammond for the answers. I was expected them to define the test conditions for each measurement and got some pretty nonsensical replies, despite my best efforts to word the questions well, or so I thought. The replies included such gems as "the primary inductance is proportional to the primary impedance". Well, we all know that's not true for starters.

The only thing that really did cast any light was "The bandwidth is expressed as minimum not maximum. " This would mean that our estimate of inductance is also going to be a minimum.

They didn't know why the ratios are not maintained between models so I guess the test conditions are not well standardized.

Does it matter? Probably not much

[Gregg]

Behind all those mumbo jumbo numbers if we're talking guitar OTs there's a well established practice.
The average 50 watter has 1600-1800 turns of primary.
The average 100 watter has 1000-1200 turns of primary.
What is different is wire size and winding configuration. Knowing this, the iron data and package size everyone can calculate whatever they want.

[GLPro]

If I had a suspicious transformer I would measure the primary inductance first. How to measure properly an OT primary inductance by itself is a long and debatable subject but to make things simple I use my DMM which has such capability. In this case (depending on the DMM test signal) expect somewhere between 3 and 5 Henry from end to end. Once I make sure it's in the ballpark I connect both ends of the primary in the AC line socket and take measurements on the secondary. It's as simple as that. If you can't measure the inductance you can calculate what voltage to expect and then compare it with the actual results. All measurements are taken without load.

A simple test that I have always used to determine inductance (L) or capacitance (C) without a device designed for such, is determined by measuring the resistance across a pot placed in series with an unknown reactance element (coil) that is in circuit with a known AC voltage source frequency; just adjust the pot until you have an equal voltage drop across both the inductor (or capacitor) and the pot, measure the resistance across the pot once the drops are equal and this resistance valve is equal to the impedance of the inductor's reactance (or capacitor); now, solve for inductance such that L = measured resistance of the pot divided by 2 x 3.14 x frequency used; this should be a very close determination using the relationships for series circuits and L = 2 pi f L

[pdf64]

The measurement error due to such a tiny test signal (compared to the working signal level) may be significant, see post #21 http://music-electronics-forum.com/t40692/#post401716