blown fuse caper...

[tubetwang]

was listening to music, minding my own business, when all of a sudden the music stopped.

Blown fuse...

Lately i had installed Uncle Ned's PA774 power transformer and put in two 1200 volts Hexfreds i had lying around...

Would anybody know how to test the power transformer?

I've put the two U74007 diodes back in, checked all connections and tested all tubes but i keep blowing fuses. Everything is brand new with no more than 50 hours on the meter.

I'm against the wall with no fuse left...

[Shannon Parks]

Would anybody know how to test the power transformer?

I just ohm'd out a PA774 (Ned's will be the same I bet):
- 87 ohms from either red secondary lead to the center tap
- 3.5 ohms across the primary
- under an ohm across both the filament taps

I would disconnect everything but the primary and try again once you get some more fuses. With some gator clips you can look at all your unloaded secondary and filament taps.

BTW, if it does look like an outright failure, be sure to tell Ned about it.

Shannon

[tubetwang]

Thanks for swift reply Shannon...

Will keep pested for the record.

Twangfully.

[Shannon Parks]

was listening to music, minding my own business, when all of a sudden the music stopped.

Blown fuse...

Almost forgot, but one 'ID-ten-T' check to do is to double-check the value of the fuse you've been putting there. 2A slo-blo is what we what.

Shannon

[Shannon Parks]

Tubetwang:
Saw your AA thread. I highly recommend Rozenblit's Beginner's Guide to Tube Audio Design. Then I'd recommend Art of Electronics. Then I'd recommend Tube CAD, since it allows you to simply test a million circuits - seeing how they react helps make the whole tube concept click - without needing a nice bench full of gear.

Leave the classics for later. I don't have Morgan's book, but I imagine it to be similar to RDH4 except more up to date (like with UL topologies and such). Just recently I picked up RDH3, which I find much more approachable than the 4th Ed. Definitely the standards and full of gold, but a combo of Rozenblit and Art of Electronics would be hard to beat for the beginner.

Shannon

[tubetwang]

Hi Shannon,

yes those are 2a slow-blo... Have managed to fing 6 at local Radio Schack.

I have Bruce Rozenblit Beginner's guide to tube audio design. A good one, but have been put off by the math,it is like chineese to me. I should just read it through i guess...but have managed to spend my time on the net reading questions and answers.

I am really surprised that nobody though about making a little booklet on how to build a simple EL84 amp.

You know...something with a pictorial...parts list etc.

"How to build an EL84 newbie' style"

1. connect resistor a to cap b

Something like your kit. 25$ no board. Just a booklet. Man, you would brake the market.

Those would sell...

[somebody]

Have you checked out your power tranny yet? Not to be the bearer of bad tidings, but it sounds like maybe you shorted out the primary side of the power tranny.

[tubetwang]

disconnected all the secondaries, plug the amp with primaries and all secondaries gave me a reading.

Tubes are all good, i've replaced the diodes, could maybe the electrolytics?

I'm putting it aside for now...

Tomorrow i'm finishing wiring rev.c with heathkit AA-151 as i've received new chokes.

Will keep you's posted!

[erichayes]

Hi All,

Simple test for a suspect power transformer: disconnect all secondary leads and apply appropriate mains power to the primary (120, 240, whatever VAC). Check the lamination temperature after ½ hour. If the temp is noticeably higher than ambient (30° C or 80° F), there's a shorted turn in one or more of the windings.

A power transformer can produce apparently viable voltage readings open-circuit even if there's a shorted turn somewhere. The giveaway is heat; the tranny becomes enormously inefficient with a shorted turn. No load, it gets warmer than normal; under any kind of load on any winding, it draws enough current to pop the fuse.

[tubetwang]

great info!

Will try it out today and report back...

Thanks Eric.