My Stupid Mistake on the Cornet, Help!

pretzel_logic · « **on:** 3 Oct 2003, 07:54 pm »

I just put this info in a reply but I accidently while tube rolling put a 5814A/12AU7 into one of the 12AX7 sockets. I was letting the pre and amps warm up and then had other things to do. A few hours later I heard a bit of a crackle through my speakers and found I had a blown fuse in the Cornet. Of course now I blow a fuse immediately and am trying to figure out what I burned up.

As I'm still new to this, and of course Jim is at VSAC, I was wondering if anyone else here could help. I am not sure exactly how to test the trannie and I don't have any burn smell but I did notice one of the diodes was not glossy anymore, wondering if I burned one up. Anyway if anyone has any suggestions I would very much appreciate it. I did go ahead and ordered new diodes but was also wondering if say a Radio Shcak diode would work. They have some 3 amp diodes but none with matching numbers of course.

TIA,

Brian

GarMan · « **Reply #1 on:** 3 Oct 2003, 09:03 pm »

My understanding is the the 12A_7 tubes are all very similar, and therefore, are interchangable (so long as you don't stray too far from the intended gain factor). The 12AU7 has a much lower gain factor than the 12AX7, so putting the "U" in place of the "X" should not overload your circuit, but just underpower it.

Check out thetubestore.com (http://www.thetubestore.com/gainfactor.html) for gain factor chart.

Of course, I don't have multiple years of tube experience yet, so I could be wrong.

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pretzel_logic · « **Reply #2 on:** 3 Oct 2003, 10:03 pm »

The gain of the AU7 compared to the AX7 is much lower I found out, AX=100 and AU=19. I don't know what may have cuased my fuse blowing problems but it does look like I may have fried a diode and I think it corrosponds to the channel I had the wrong tube in. I guess I'll have to wait for Jim to get back.

Brian

GarMan · « **Reply #3 on:** 6 Oct 2003, 06:41 pm »

Brian,

Looking as the schematics of the Cornet, there does not seem to be any right channel/ left channel tubes. Each of the tubes are dual triodes (two separate halves), and from the schematics, both channels go through all three tubes. Also, the rectifier diodes that burned up are not channel dependent either.

You mentioned that you inserted an AU7 into the AX7 socket. Mind if I ask what you put into the AU7 socket?

Gar.

pretzel_logic · « **Reply #4 on:** 6 Oct 2003, 07:17 pm »

Garman, I had an AU7 in the AU7 socket. I have been checking things over and tpk the diodes out and checked them. They seem to be bad but am not sure. All 4 measure 3.5k anode to cathode and 37k cathode to anode. Now they may be good but I read in another post that diodes should read meg-ohms in one direction. I'm also not sure how to test my trannie, where should I connect the neg meter lead? I have the trannie disconnected right now.

Brian

GarMan · « **Reply #5 on:** 6 Oct 2003, 08:17 pm »

Brian,

Those measurements sound really off. Typically, diodes measure fractions of an ohm in one direction and mega-ohms in the other. I'm sure Jim will be able to straighten this out when he returns.

Gar.

Braden · « **Reply #6 on:** 6 Oct 2003, 08:27 pm »

Brian, I just measured my diodes. I'm not an expert with a multimeter, but one direction measured 0.17 and the other slowly worked its way up to around 1.7 or so. Hope that helps. Now if only I knew how to measure my capacitors. My meter has selections for 20uf, 2uf, 200nf, 20nf, and 2nf but nothing ever shows up on the display.

Braden

pretzel_logic · « **Reply #7 on:** 6 Oct 2003, 11:56 pm »

I'm not sure how to measure capacitance either. I don't think any of mine are bad, checked some important ones for shorts and found none so I think my diodes are bad, just have to figure out why.

Brian

pretzel_logic · « **Reply #8 on:** 8 Oct 2003, 05:22 am »

Looks like it was the transformer all along, Hammond 270BX. I pulled the greens and yellows and still the fuse blew immediately. Hopefully I'm assuming correctly that the yellows and greens are the secondaries and that removing those leads told me the trannie is shorted.

Live and learn, move on, and enjoy the music. The Cornet sounded great even with limited hours on it, can't wait to get it running again.

Wondering of course why it went, I had no signal going through the unit for some time, several hours. When I have my amps on I don't like turning off preamps as it produces a loud pop. Any suggestions in that area?

Brian

hagtech · « **Reply #9 on:** 9 Oct 2003, 05:43 pm »

Gentlemen, thank you for helping debug this problem while I was away. The community support is great.

We did finally chase this problem down with some offline emails. Seems to be a bad tranny, but still not sure why it failed.

I bet the diodes in heater circuit are ok. Measuring in circuit can be problematic. And the reading you get varies with DVM. By measuring ohms, they inject a small known current, measure voltage, and display resulting resistance (divider circuit). The current varies with range and turns on diode to different levels. I find the most useful feature to be the "diode checker" which displays the voltage across a junction. Anyway, 3.5k is not an outrageous reading for a good diode at low current.

Also, putting the 5814 into an X7 socket is ok. That will not cause damage. Just lower gain and messed up EQ.

Our final test was to disconnect all secondaries. Fuse still blew. The tranny is the only thing in circuit. My guess is a short in the primary or short between secondaries. Each winding should measure some resistance, but no connection to any other winding.

jh

pretzel_logic · « **Reply #10 on:** 9 Oct 2003, 06:48 pm »

Checked for shorts yesterday and have a direct short in the primary. Why this happened I don't know, defective maybe? I'm hoping it wasn't something I may have done wrong building up the board as I surely don't want to blow another trannie. I'm going to go over the board again and check what I can.

Brian