Ecl82 PP amp rebuild - suggestions?

[surfstu]

hi, i'm just out of noob school and have a problem with an old heathkit s99 amp. I accidently cut off air vents to the amp one evening and started to inhale a strange smell. Being slow off the mark, the amp died. There is plenty of wax drippings on the chassis now under the mains transformer that was not there previously. I have measured the voltage straight out of the transformer before the gz34 and there is no voltage. I didn't think transformers died this way? Now i'm thinking to replace the transformer and assume the output voltage is not that important as long as it is close? And i have no idea of the current demands (mA).?
I have aquired a schematic of the s88 which i'm told is identical apart from the fact that my amp has better quality OPT's. Am i right in saying that i can dispense with the preamp side of this amp and just send my line in into the volume pot? making it a power amp? I'm at a loss here as what to do, any suggestions or answers to the ?'s above would be much appreciated... i'm willing to take on some simple modifications too, upgrade caps etc...

[surfstu]

forgot this...

[nigelwright7557]

If the resin coat on the transformer windings melt they will short out and kill the transformer.

[surfstu]

thanks, so a new transformer is needed, any ideas of power ratings?

[Ty_Bower]

The schematic you posted shows a power transformer with a 440VCT high voltage secondary. Full wave rectified through the GZ34 you might get 285VDC or so. It was rather nice of them to include a GZ34 in the design - lesser amps might have tried to get away with an EZ81 or something.

Each pair of ECL82 power tubes should probably be drawing somewhere around 70mA. You'll need a little extra for whatever driver tubes you've got in the front end, including the triode section of each ECL82. The EF86 will take ~6mA. The the ECF80 could take as much as 28mA, but I can't imagine it would be running that hard given its position in the circuit. If you can find a PT rated for ~200mA, I'd expect it would be more than adequate.

You'll need 2A at 5.0 volts to power the heater in the GZ34. You'll also need a 6.3V winding on the PT. Again, if you stay with the tubes shown in the schematic you would need at least 4.5 amps (ECL82 needs 0.78A, EF86 needs 0.2A, ECF80 needs 0.45A).

[planet10]

Originally Posted by surfstu

There is plenty of wax drippings on the chassis now under the mains transformer that was not there previously

I had the same happen with a Scott... woke up to an acride smoke filled room...

dave

[surfstu]

thanks ty_bower,

will try and find a transformer close to those values, do i need the driver tubes? where could i send my line in if i wanted to bypass the preamp section of this design?

[surfstu]

planet 10 - did you get the scott back up and singing again?

[richwalters]

As you are in the UK, SOWTER AUDIO TRANSFORMERS is an immediate source but bit expensive. Currently I run an ECL82 p-p amp and it's a darned good sounder. Don't bin it.
The fact you didn't mention the condition of anode ww series resistors of the rectifier, leads me to believe the problem could be a short in the heater winding. What do they look like ?

richy

[surfstu]

I'll the check the series resistors to the rectifier anodes tonight, I've had the voltmeter on the output of the transformer before the rectifier and i get nothing. would a heater short cause this? there is also the physical observation of a lot of wax leakage that leads me to believe the transformer needs replacing. I've heard of sowter before but was hoping to find a cheap old transformer on ebay that would be close enough in specs to replace. Would love to pick your brains on a possible rebuild/ upgrade of the present circuit.... possibly losing the preamp section?

stuart