OT primaries 22k? Am I reading this wrong?

[Gainzilla]

Hey all,

A guy I know was asking if he can upgrade the transformer from his Jet City 50h with a Hammond 1650P. He sent me the attached pic showing his transformer specs. My question is, 22k primaries? Is that right? And if so, this is from an amp guys often use with EL34's or 6L6s. I would've expected something in the 3-6k range, but 22k seems excessive!

I know the jet city 50 and 100 amps are essentially SLO Hot Rods. Aren't those untralinear? If so maybe this one is too, but I'm not aware of any differences in transformer primary impedance. Any ideas?

Also, what would the secondary implications of this be? Since the secondaries are just ratios wound to give you the impedance relative to the expected primary impedance, doesn't that make the secondary taps much higher than their expected impedences?

Cheers guys!

Ps, this guy is looking to run this with EL34s.

IMG_4047.JPG

[Phil_S]

That seems so wrong that I think I'd want to test the turns ratio. In an existing amp, this shouldn't be too hard to do. Pull the tubes. Disconnect the CT. Jumper the 6.3V supply onto the secondary. (Actually, I'd check all 3 secondaries.) See what voltage comes out the primary. You can meter the primary at the tube sockets. From there, it's just math, you know, the square of the turns ratio is the impedance ratio.

Whether you want to say 22K primary gives something besides 4-8-16Ω secondary is somewhat arbitrary. If you put an 8Ω impedance speaker on the 8Ω tap, then its 22K:8. I suppose this being so far out of what we normally see, there must be some consequences. What they are, I'm not sure.

[johnnyreece]

Whoah...that's the type of OT one would use for a little P-P 12au7 amp or something...Definitely double check.

Edit: Spent a couple minutes on the Google machine and found this: http://www.guitarampboard.com/forum/vie ... 53&t=12660

Long story short, it appears to be a typo (2.2k, not 22k).

[Gainzilla]

Thanks for the response, guys! Wow, ok, the typo makes the most sense. It might be worth doing the testing phil mentioned just for piece of mind though.

So, to clarify, when looking at transformer specs such as the diagram above (2.2k - 0 - 2.2k), is that to be read as 2.2k impedance measured across the two outer primaries? So if EL34s like around 3.5k, this above OT is maybe a a bit low?

[Ten Over]

I would interpret it as 4.4K anode-to-anode. If you take it literally that it is 2.2K from center tap to anode, that would make it 8.8K anode-to-anode, which I am sure it isn't. 4.4K anode-to-anode is right in there if you have around 500V for B+ and you want 50W.

[RockinRocket]

Hmm the 20 watt JCA 20 with el84s read 1.8 if I'm not mistaken. So any clues what kind of typo might make cense for that?

[Phil_S]

That referenced thread says on the 8Ω tap he found turns 23:1. 23^2 * 8 = 4224. That makes a primary of 4.2K. Multi tapped OT's sometimes have a variance in the primary:secondary; that is, they don't always tap at the exact spot that would keep the primary constant. I'd say that label is more than strange. I'd call it 4.4K and move on. You might use the 1650P but, really, it isn't the appropriate replacement. Look at the 1650N.