output transformer fault finding.

[Dmoney]

SO.

I've blown an OT before and may have come across an amp with a similar problem...

voltages on the pins of all the valves look good, but the output is REALLY quiet.
as in gain & master vol needs to be maxed out just to really start hearing it, and it should be 100watts. At those dial positions id expect to be stripping paint off my walls.

so.
last time i had this it was the intermittent OT in my Avenger, but I just want to know the procedure for pin pointing a fault OT.

[Dmoney]

any local amp guru's help me out?
i'd like to work on this today if possible

[martinw]

Tricky. You need to run a signal through the preamp up to the grids on the output valves, and scope it. If the signal is on the o/p valve grids, and all the voltages are ok on their pins, then the problem must lie with the OT, or in that area at least. Check all the associated wiring especially the output jacks.
You've got 4 o/p valves I assume, so to get no sound (or very little out) would require all 4 to have faults.

If you haven't got a signal generator and scope, plug in a CD player and measure the AC voltage at the grids. You should be able to get it up tp 20-30 Vac easy enough, and if that's there, and on all 4 grids, there's a limited amount that can cause zero output. (Assuming Va, Vs, Vbias all ok.)

Continuity and insulation tests on the OT are of limited value unless you've got a Megger.

Be careful obviously!  :)

[Dmoney]

these are the DC voltages on the octal bases

Pin3 463
Pin4 458
Pin5 -47.5
Pin6 462

If i just switch my MM from volts dc to volts AC, I don't get anything on pin5.
my MM is set to a range of 200 V~

I can hear something though.

I did get two pop while no touching the amp.
1 dropped the volume and stopped the resonance control working but presence was ok.
the next pop reversed the above.

[jpfamps]

This is the procedure I would use assuming you don't have a 'scope.

Firstly I use a dummy load rather than a speaker (I don't particularly like listening to a 1kHz sine wave at 100W).

You will need to do this with the amp on, so be SAFE.

I have assumed that the output valves are functioning correctly, and that there is no problem with the output jacks/ connectors to the speakers, and that the speakers are working (!).

Stick a signal into the amp and measure the AC voltage on the control grid of the output valves (typically pin 5) using a DMM. You can get a kit from Maplin for about £5 that will generate an approximately 1 kHz sine wave. If you really want to push the boat out you can also get a cheap aluminium box from Maplin to house it in.

You should be able to get 10s of volts AC onto the control grid. I adjust the AC voltage to have roughly the same peak value as the grid bias voltage (ie divide the grid bias voltage by 1.4, so in your example I would shoot for around 34VAC on the grids, which should drive the amp to full undistorted volume).

If by manipulating the amps volume controls you can't get any AC voltage onto the valves control grids, then the problem is elsewhere in the circuit.

I now test the voltage across the dummy load. This should allow you to roughly calculate the output power of the amp (power = Vsquared/R). If this is suspiciously low, then it is likely that the output transformer is defective.

Now the dangerous bit! As a final check, measure the AC voltage on the plates of the output valves (typically pin 3). This should be significantly larger than the grid voltage. Again if this is suspiciously low (it can often be much less than the control grid AC voltage!), then the OT is defunct.

Using a 'scope employs exactly the same procedure except that the waveform is visualized on the 'scope rather than using a DMM.

Hope that this is helpful.

[HTH AMPS]

have you tried swapping in a 'known good' preamp valve in each position one by one? - could be a simple fault.

also try measuring the resistance of the OT's primary between each side of the OT to the CT (obviously with the amp off).  you should get resistances on each side of the OT of around 50 to 100 ohms.  for example, I've got an AC30 OT here and get...

* 67.5ohms from one side of the OT primary to the centre tap
* 69ohms from the other side of the OT primary to the centre tap
* 136.5ohms between both ends of the primary (i.e. across the whole winding)

also check for shorts to the frame of the OT from each winding; using the continuity tester function on your DMM.

[Dmoney]

If i measure each side of the primary to the CT (with no speaker attached and no valves in place)
I get 9.6ohms on each side to the CT.
so its very low, but symmetrical.

This is it maybe?
a short?

[jpfamps]

If its a 100W OT, that could be the correct reading, although it does seem a bit low.

I've seen plenty of duff OTs where the primary is shorted but the DC resistance seems OK.

I've assumed that this is because the insulation only breaks down at the high voltages seen when the amp is operating and is OK when measuring DC resistance with the low voltages employed by a DMM.

[martinw]

I've assumed that this is because the insulation only breaks down at the high voltages seen when the amp is operating and is OK when measuring DC resistance with the low voltages employed by a DMM.

Hence what I said earlier about this test being of little value, and using a Megger (or similar).

[jpfamps]

I've assumed that this is because the insulation only breaks down at the high voltages seen when the amp is operating and is OK when measuring DC resistance with the low voltages employed by a DMM.

Hence what I said earlier about this test being of little value, and using a Megger (or similar).

And hence why I only discussed this in reply to a question be the original poster..........

[martinw]

A simple "as has been said" would have done. A little acknowledgement oils the wheels.

[Dmoney]

its all good gents.
helps me understand a few things.

ive been quiet on here cos ive been working on this amp. I got it ALL done, and the schematic I had, had a little phrasing I misunderstood.

My housemate flicked the thing on without a speaker or any test kit attached.
the original OT i used made some crazy noises and the tubes red plated and the HT fuse blew.

the note I had said if the bias voltage is fed from a tap on the mains winding, that i should use a 220K resistor before the diode in the bias circuit, when I should actually have just used a jumper. it effectively made the bias voltage nonexistent. If the test gear was attached id have seen the idle current race sky high and i could have been ready to turn it off. I would also have measured the bias voltage without turning the standby switch on first of all. The bias pot was set to max resistance, but that didn't matter.

anyway. I got that all solved.

I decided to let a colleague scope it, since he's into this stuff too and is planning on building a rig for prototyping amps to some degree. So while he's having fun with that I'm waiting for the info on the OT as to what resistance I should read across taps etc.

I'm hoping the extra testing will prove it should be OK up until to output, and then I guess by process of elimination and a few other things we can blame the OT.

just my luck!
anyway, i NAD will be here shortly, and other than this (rather dumb, yes i do feel like a massive idiot) set back, I've quietly impressed myself with this project so far.

thanks Martin, HTH, and Frank for all the help as usual. wish i was as knowledgeable about these things as you guys, I actually would enjoy working on amps and such a lot more than what I do now. kinda feel its a bit late to get into it.

[jpfamps]

If an amp has an HT fuse, usually that blows to save any collateral damage from bias failure.

In amps without an HT fuse (think Fenders here), in my experience, bias failure causes the power transformer to expire rather than the OT.

I wouldn't worry about "getting into" valve amps too late. They really aren't that complicated. Indeed what you need to know to play the guitar to any degree of competance, in my opinion, far exceeds the sum of knowledge you need to know to fix/ build valve amps.

[Dmoney]

HT fuse was 1amp.
I don't know if anything was going awry inside the valves to damage the OT.
Still haven't had word on the OT spec.

At least I only blow my own stuff up, and nobody elses.

That Resonance Control I asked for advice on for the Laney GH100TI worked like a charm by the way!

[jpfamps]

I personally like to have the HT fuse after the first filter cap from the rectifer, and use a fast blow fuse. 1A would be a HUGE fuse to use here in a 100W amp, as the HT is probably at least 450 VDC, so you would need be pulling at least 450W through the power valves for the fuse to blow!

I like to have the HT fuse after the first filter cap (and before the OT centre tap obviously) because, in my experience, bias failure and valves shorting out (especially) are relatively common occurances, and will cause a fuse located here to blow, ie do its job and prevent any collateral damage to either of the transformers.

Some amps have their HT fuse before the first filter cap, or  before the rectifier, in which case you will need a much larger and slow blow fuse to accomadate the fact that the first filter cap is charged in pulses from the rectifier. These pulses are usually about 5-10 times the static current draw of the amp.

The arguement for having the fuse before the first filter cap and/ or rectifier is that if the cap or rectifer shorts out then the fuse will blow and save the mains transformer. Again from my experience, if the rectifier or main filter caps  go short circuit, then the amp's mains fuse will blow and save the mains transformer, however bias failure (usually) won't blow an adequately rated fuse in this position.

I have to admit that this is one issue where I would disagree with Merlin Blencowe (http://www.freewebs.com/valvewizard/fuses.html). I recommend you read his views on this and come to your own conclusions.

For the ultimate protection you probably would want a fast blow HT fuse after the first filter cap and a larger slow blow fuse/s before the rectifier.