Is there a way to determine if an output transformer is "good"

[dbishopbliss]

I have a Marshall 9005 stereo power amplifier meant for guitar.  It was made in 1990.  It stopped working around 1994 and I never bother to figure out why. 

After 18 years in storage, I pulled it out but I'm not really interested in trying to fix the amp.  It uses circuit boards, the wiring and solder jobs aren't that great, lots of flux all over, etc.  Not to mention, I don't have any EL34 tubes for it and I don't really need a 100 watt guitar amp.

I want to pull the transformers out and sell them to fund another project.  There was never any smoke or anything when it stopped working.  The tubes would light up, but it just didn't make sound.  I'm wondering how to determine if the transformers are any good or not.  Can I just measure the primary/secondary impedences and check for shorts with a DMM?  Thanks for your help.

[JC]

You can measure for opens and shorts with a resistance meter, but to really check an output transformer, in my experience, you will need to be able to measure impedance at audio frequencies.  Such meters exist, but they are not all that common.

In addition, most output transformers have high Voltage running through them, so at some point, they may need to be checked at operating Voltage to make absolutely sure of them.

Again, this is just my experience working on guitar amps and commercial amps which use output transformers to drive 70.7V and/or 25V speaker lines.  Someone else might have some nifty, easy way that I have not run across.

Having said that, I have to wonder if it wouldn't be more lucrative to just see if you can get the amp running, then sell the running amp?  It would take some research on your part to determine what such an amp would fetch on the market, but I would have to think it would be more than just the output transformers would fetch as "pull-outs".  And, your problem may not be all that difficult to solve.  For instance, many, many Marshalls used two fuses, one for the "Mains" and another for "H.T.", or the high Voltage supply.

And, even if you don't want to work on the amp, I would expect you could still get money for it "as-is", if you can locate a buyer who is into working on older guitar amps.

[ssssly]

I would second JC.

A working 9005 will fetch about $600.

An as is about half that.

The pull out trannies, even if you can prove they are good, maybey $75-100 if you are lucky.

[dbishopbliss]

Alright... you've convinced me.  I put the amp on the bench and it ends up that the tube plate voltage fuses are blown. 

So, I hope the simple answer is the guy who owned the amp before me had it biased way too hot... which I have read that many people did with this amp.  I don't have any 12AX7 tubes right now (imagine that... with all the tubes I have no 12AX7's).  I will re-tube, re-fuse and turn up the bias pot (up right, higher value should make for lower voltage) before turning on again. 

I'll keep you posted.

[Grainger49]

I would determine the middle of the pot travel and set it there.  Amps are designed so that it should run at the center point so when the tubes age you have travel in both directions to adjust for aging.

[JC]

Yes, a blown HT fuse would do it.  HT stands for "High Tension", a common European reference to high Voltage.

So, basically, if that fuse doesn't hold, you could have anything from a bad filter cap to a bad negative bias supply (separate winding and rectifier/filter) to just a shorted tube somewhere or a bad rectifier diode.  You can make sure that the negative bias supply is functioning without having the output tubes in place.  It is vital that it measures as "there" and at roughly the correct bias for the output tube type.

The preferred Marshall "factory" method for setting bias is to run the amp into proper load(s) with a sine wave input.  IIRC, tone controls to "zero" so they don't effect the sine wave.  A scope on the output to check the sine wave.  Then, increase the input signal/volume control until just before the amp clips.  If you see distortion at the crossover point, the bias is too negative.  Adjust until the crossover distortion just goes away with the amp at full power output.  Well matched output tubes, of course, are a great help.

As Grainger says, when in doubt, just shoot for the middle of the bias pot travel.  That may not be right, but either end of the travel is usually wrong, too.  And/or measure the bias with your Voltmeter and set according to what your tube manual says is nominal for the tube type.

[JC]

Update:  I was curious, so I did a little digging on the internet, and found the schematic and other information on this amp.

It looks as if I was in error when I assumed that the negative bias supply would be fed by a separate winding on the power transformer; it appears to get its AC from one side of the high Voltage winding.

In addition, I see that Marshall included resistors in the cathode circuits of the output tubes.  These will simplify setting the bias, as you can use your Voltmeter across each resistor to determine Voltage, then use Ohm's Law to calculate current through the tube with no signal.  Very handy, and not done all that often.

There also appears to be a balance pot which looks to adjust signal balance between the output tubes.

Quite a bit of information available on line, as it turns out.