Big voltage drop with 7Z4 rectifier in P-P 6V6 amp?

[thelonious]

I recently converted a no-name PA amp into a little P-P 6V6 head. It sounds pretty good, but some of the voltages don't make sense to me. The PT is putting out 638VAC across its secondary, but on the other side of the 7Z4 rectifier, the DC voltage is only 328v, and the plates are only getting 316v. That's a pretty massive voltage drop. Is that normal? The attached 7Z4 datasheet makes it sound like it should only be dropping 40v when the amp is pulling the 7Z4's rated 100ma. The 7Z4 tests ok on my tube tester, but we all know how that can go. As of now, I don't have another 7Z4 to sub in.

The voltage drop across the OT seems pretty high, too. Of course, the OT is just about the smallest piece of junk with tiny 24ga wires (I kid you not) I have ever seen, so there's probably no mystery about why it's losing 12v to each plate. I'll probably replace it eventually.

Anyway, is that large a voltage drop normal for a 7Z4? Should I install a different rectifier? I wouldn't mind having that loktal socket for something else anyway...

All voltages measured w reference to ground.

[DummyLoad]

that's about right with that rectifier... pg. 2 of datasheet linked below. 638V /2 =319V per plate.
19V/250R or about 76mA for 6V6; 10V drop from node A to node B /1K = 10mA for all rest; or about 86mA total. my eyes say around 340V dots the curves; 328V is close enough. try a 5V4GA or GZ34 - you'll need to swap sockets tho.

http://scottbecker.net/tube/sheets/127/7/7Z4.pdf

--DL

[thelonious]

Ah - I understand now. Your math cleared it all up. Thanks!