Need help, problem with Octal Cornet

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Need help, problem with Octal Cornet
« on: 23 Nov 2003, 04:56 am »
Everything was working fine, I replaced C13 and C14. Thats when the problems started. I have 5.8V now on all tube heaters. But.......

The Cornet has CCS. On V3 I have 315V on both plates, 137 on both cathodes and 140V on the grids. On V2 I have 136V on both plates and 0V on both the cathode and grid. On V1 I have 150V on both plates and again, nothing on the grid and cathode.

I tried changing both V1 and V2 but no change.

Any ideas?



Need help, problem with Octal Cornet
« Reply #1 on: 23 Nov 2003, 06:09 am »
Same tubes as before and now your heater voltage dropped a few tenths because of changing caps?  That is quite odd.  It's possible they have a much higher internal series resistance resulting in a larger ripple.  

More likely the tubes have changed too.  Like a 5814A instead of a 12AU7.  You can always tweak the value of R21, but that shouldn't have to happen.

Cathodes of V1 and V2 should be at about 0.9V, not zero.  Also, cathode of output should be about 4V higher than its grid.

If you have 315V on V3 plate, that is a proportional drop from nominal same as the heaters.  Has ac input voltage changed?  I am wondering if this is how some of the transformers have gone bad.  Tiangus noticed a loss of sonic quality before his  died.  I have his transformer here for measurements.  Resistances and inductances seem to be ok, but firing it up on a variac showed normal operation until about 40Vac, then something triggered and output voltage dropped.  At about 60Vac the fuse popped.  All I can guess is that there is some insulation breakdown in the primary - shorted turns.  The drop in inductance pulls too much current.  Hopefully that is not what is happening to you.  So far we've had three Hammond trannies blow amongst us.  Odd, because everything is whithin spec.  My only guess here is that there might be a primary side inductive back EMF during turn off that breaks down the insulation.  I am presently investigating this theory with some instrumentation.  If true, then a simple ac-rated 0.01uF cap across the primary should fix it.  I'll be posting results here in a few days.

jh :?


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Hammond Tranny
« Reply #2 on: 23 Nov 2003, 11:41 pm »
     It's good to know I haven't lost all my critical faculties. My resistance readings were comparable to the new one, but it kept failing anyway. Your test facilities are a bit better than mine!

Larry Welsh