Fender Twin 135 Watt

[bsco]

Need some info if possible. I have this amp and it is blowing the mail 4A fuse...I removed the tubes and it still does it. So I labeled the secondary wires and dis-connected them and installed a new fuse...blew it again...I get resistance readings on the secondaries which I think would appear normal.....however on the primary winding I get something like a half ohm....so I think the power transformer is shorted in the primary winding...before this happened, the output tubes red-plated and one of them sparked.anybody know what the resistance reading of the primary winding should be??...It appears to be the original xmfr with the cloth type wiring....not the shinny plastic stuff.....
Cheers,
Bernie

[stereomonostereo]

If ALL of your secondaries are disconnected, and you're still popping the fuse, that's pointing to a bad power transformer. Do you have a current limiter?

[66tele]

Take a look at R.G. Keen's article on testing power transformers.

Power Transformer

This will help you determine if the PT is blown.

[Enzo]

I see no caps across the primary or any MOV or similar to short, so I have to agree, you got bad news.

[Jazz P Bass]

If the transformer secondaries are floating & the transformer primary blows a 4 amp fuse, the transformer is bad.
1/2 ohm for a primary resistance sounds suspiciously low.

http://acruhl.freeshell.org/mga/sche...ate70s135w.jpg

[bsco]

Thanks guys. I do have a current limiter.....I suspected that the transformer is bad....I just wanted to check here...I have all the secondary windings floating....so it is just the primary.....before this happened, the amp did work but the volume was very low even at full and I noticed the tubes would red-plate...turn the volume down and the red plating would go away....I shoved another set of tubes in and tried to check the bias.........I had also installed two 1.2 ohm, 2W resistors to check the bias current.they were up around 80mA for each side...plate voltage was at 500V Screen grid voltage 498V and negative bias voltage at the grids was -58V on one side and -53 volts on the other side...then one of the tubes started to red-plate and then sparked internally....I shut it down immediately, removed the tubes and powered it up...no power.....fuse blown...so I put in another one and fuse blown....that was when I dis-connected all the secondaries and re-tried it...fuse blown....so I suspected the transformer primary was shorted....now.....where do I find one of those at a sensible price?? any suggestions?????
Cheers.....
P.S. Still don't really know why it red plated..the tubes that I used were definately good.....I am now suspecting tube sockets??

[stereomonostereo]

Just to make sure, I'd hook the power transformer primaries directly to the limiter and see what happens.

Hammond makes decent replacements. Mercury Magnetics are nice

[bsco]

OK. I'll do that later tonight and get back to you.....
Cheers

[g1]

If cost is an issue I would suggest you verify OT is ok before replacing PT.
Redplated tubes, fried PT, maybe someone gave it tinfoil treatment or something.

[bsco]

The amp was working..it had extremely low slightly distorted volume.I installed my tubes in there...it was the same....actually this has the output transformer replaced back quite a few years ago....one of the tubes started to red plate and the tube sparked inside then I shut it off right away but the fuse went...so I had replaced the fuse and this is where I am at right now....
Cheers

[pontiacpete]

I have one of these PT for the 135 if you are interested. I picked up this shell of an amp and will someday convert it to a regular AB763 circuit with a lighter PT.

[bsco]

OK. I have a local guy who thinks he might have one...if he does not, I will be in touch later today as he is going to check and get back to me before the end of the day...Thanks for the offer...as soon as I hear back from him I will let you know.....
Cheers

[bsco]

Hey pontiacpete.....how much did you want for your transformer?? the local guy here doesn't have one for that amp...
Cheers

[pontiacpete]

I sent you a personal message.

[pontiacpete]

Don't think that went thru. Anyway $80 plus shipping.