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    Building, troubleshooting and testing of these amplifiers should only be
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PCL Vintage Amp VA1956 - transformer issue

Overgaard

Member
2009-06-01 2:54 pm
Hi you all,

A friend at work asked me a while back if I could help him fix his broken
guitar amp. He said that the fuse had blown and that he'd put a new fuse in.
But after a while some smoke came from the amp. He checked the fuse
and somehow he had put a 10 amp fuse in there instead of a 3.15 amp fuse.
:rolleyes:

Anyway, since he knew I like building and restoring tube stuff, he asked if I could have a look, and I said I will help him.

A quick look and I found the two resistors (100 ohms) on the heater chain towards ground was cracked open, probably from overheating.

It did not take long to find out the mains transformer was shot.
Primary winding shorted somewhere. To check the transformer, stand alone, I hooked it to my variac and put about 50 volts on the primary and meassured the secondaries. Output voltages was way too high already. Cranked up the voltage some more (maybe 100 volts) and it started cooking. :D (this is a 230 volts transformer)
Some calculations on what would happen if the primary winding shorts, changning the turns ratio... Lots of more voltage on the secondary and cooking transformer. Not too good!
I sure hope nothing got too much over voltage on the amp.

What could have caused the two 100 Ohms resistor on the heater chain to break? This is an old amp that has been unused for a long time according to my now sad friend... Did the two resistors short out due to bad quallity and old age and thus overloading the transformer? And since he put the wrong fuse in there the primary winding could not take it and decided to...well... release it's magic smoke.

I will go through the amp before even considering hooking a new transformer to it. But to the main question here...
What will be a good replacement transformer. I have NO idea what
the broken one is when it comes to current output.
From the schematics I know B+ is 460 volts. And before the rectifying
it would be in about 325 volts, or in this case it's 650 C.T.
Yes I could just put a BIG transformer in there, but he already payed a lot
for his amp and only played it for a week before breaking it. So no
bigger transformer than needed.

It's a 50 W amp using two 6L6GC's in PP for output. I will try put the
schematics and some picture of it here.
Any suggestions for a good transformer? I have been looking at Hammonds 373CZ (650V C.T. @ 173ma) or 373EX (650V C.T. @ 345ma).

Best regards
// Jorgen
 

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yamatele

Member
2011-01-30 6:54 pm
Hi !

I also own this amp in the 1x12 version. I have had the tubes arc - possibly caused by poor soldering in the tube base; the same fuse problem etc. Mine have Engel trannies EV 2542, EV 2549. My tech corrected these problems - mine puts out 45 watts. I am in Seattle. These amps on rare occassion show up on Ebay here in the States - they basically are a copy of the Mesa Boogie Mark I. If you know anything about Boogies, they are very hard on components, tubes etc. My only suggestion aside from replacing the blown transformer with the Engel would be to contact PCL in Germany PCL-musical-instruments Vintage-Amp Rath-Amp PCL-Audio FORUM Board Customshop Lautsprecherboxen Röhrentechnik Gitarrenverstärker Musikelektronik Beschallungstechnik Röhrenverstärker tube amplification tubeamps guitaramps guitar amplification Audiotec and see what they say. Jurgen Rath is one of the main designers - involved with Berringer as well, I believe. They are hit-and-miss in responding to email. Best of luck & let me know what you find ! Gil