Fender Hot Rod Deluxe, second PT

[tim]

Hey all,

I fixed a chap's HR Deluxe a few months back...it came to me not working at all. It took me a few minutes to realize that the PT was bad, open primary....weird. I rarely see amps with an open primary winding...no biggy. I bought another from CE Distributors and popped it in...

The amp came to life and sounded great except that the power tubes were a bit rattley so I replaced them and rebiased the amp. It sounded great and looked great on the scope. I left the amp on for a while while I did other stuff so I could touch up the bias if it had drifted.

I went back to the amp about a 1/2 hour later and it was dead. Suspecting that maybe I'd blown a weak fuse, I checked the mains fuse and it was fine. I started checking things out and found the primary open again!

The amp has been sitting on the shelf since then as the owner is out on tour and didn't need the amp back for a while. He's coming back in about a month and I suspect he'll want his amp back, preferably fixed .

I am assuming that the thermal fuse has popped inside both PT's. I plan on ripping one apart in the next few days to confirm this. Assuming it is the PT thermal fuse, there must be a circuit that is pulling too much current stressing and overheating the PT.

When I powered up the amp after replacing the PT, I had it plugged into the light bulb current limit set up and didn't notice anything unusual. I actually left it that way for about 5 minutes before I switched over to full mains so I could set bias for the new tubes.

Before I replace the transformer (again!), I need to make sure I figure out what is pulling excessive current. Does anyone know what I should expect for quiescent current in the different circuits (HT, Bias, filament, etc)?

If not, I have a friend that has an HRDLX that I could borrow for an evening and take current readings. Unfortunately, he lives about two and a half hours away.

The other thing I could do is bypass the internal thermal fuse just for testing purposes and see if I can see what seems to be pulling too much current.

Any ideas you guys can throw at me would help...Barking up the wrong tree is no fun. I know there are threads on here about open PT thermal fuses but there doesn't seem to be any follow up as to what was found to have caused it.

Thanks for any and all help and suggestions.


Edit: I found these on a website...I guess it's a place to start:

Primary supply
120 VAC @ 60 Hz (Black - White)

Secondary
#1 (Red - Red)
495 VDC unloaded
440 VDC @ 65.7 mA DC
388 VDC @ 200 mA DC

#2 (Brown - Brown)
-53.6 VDC @ 1.58 mA DC

#3 (Green - Green)
6.6 VAC @ 3.75 mA AC

[Chuck H]

To me this sounds like two bad PT's in a row!?! Why would two PT's pop their internal fuse prior to the mains fuse? You had the amp up and running with correct voltages and currents on the DC windings. Check the filament circuit and bias supply. The filter caps in some of those amps are known to fail prematurely so check the HV rail components.

I'd sub in a different PT without an internal fuse or, if possible, bypass the internal fuse on one of those broken PT's as you propose. But don't walk away from the amp this time. Check poser supply ripple to see if the filters are at least filtering. Check filament current.

There was a near identical thread started some time ago that I think turned out to be bad PT internal fuses.

EDIT: I may have found the thread but it didn't resolve as it turns out. It was a Deluxe Reverb reissue. Probably has the same internal fuse. Another thing I found is that this seems to be a pretty common problem, the PT fuse blowing before the mains fuse. The reason is that one is a thermal fuse and the other a current fuse. So I might guess that something not associated with the HV winding is heating up the PT. I'd check filament current first.

[tim]

Thanks for the reply Chuck H...I suppose it is possible but I don't think it is bad filter caps as the signal was nice and clean when played and scoped (before the second PT went bellies up). No ripple was heard or seen. I don't think it could be bad power tubes because it has a new set in there and the two PTs failed with two different sets of power tubes.

In the few minutes I was working with the amp powered up, I noticed no red plating of the power tubes and bias was fine on the tubes, at least until I set it aside...no guarantees after that because I wasn't watching things.

I have the third PT here on the shelf but I'm very hesitant to install it without knowing exactly what caused the other two to fail. That's why I was thinking of trying to get one of the bad ones going again by jumping the failed thermal fuse and using that to troubleshoot the amp.

It's hard to believe that I would have two PT's with defective thermal fuses, especially when the original PT inside the amp was made some years ago and PT number two was made relatively recently. I guess it's possible but pretty unlikely.

I'm hoping to get the amp back on the bench in the next few days...I will check current using what specs I have. I may be taking a trip to borrow my friend's Hot Rod Dlx if things look "normal".

[Chuck H]

See my EDIT: above. We were typing at the same time.

[tim]

See my EDIT: above. We were typing at the same time.

Yeah...the internal fuse opens due to excessive heat (which is usually caused by excessive current somewhere in the transformer) where the mains fuse will blow when there is too much current (only) being drawn by the amp. It would be nice if these PT's had resettable fuses but they don't...probably because of liability issues.

In my regular 9-5 job, we use them (resettable thermal fuses) all the time in high voltage, high current power supplies and Xenon light sources. It helps when it comes to troubleshooting overheating issues. Instead of killing a component for good (such as these PT's), it opens, kills the circuit until it cools and then resets itself making troubleshooting quite easy.

Oh well...I'll figure it out eventually. I'll be sure to put up here what I find as it seems to be a fairly common problem, far more common than I thought. In all the years I've been repairing amps (+25) this is the first time in as long as I can remember that I am dealing with something like this...

[g1]

It may be worth talking to CE about it in case they are aware of any potential defects.
I'll assume you checked that it is the correct value mains fuse?
If you can get the original PT running for test purposes, you can at least see if it is overheating. If yes, you should be able to see which winding is being overloaded. If it doesn't overheat, there is either a very intermittent fault or you received a dud replacement.
I would suggest you keep the new PT from CE intact in case they want it back.
One previous thread with the same issue seemed to be caused by a bad bias pot causing red-plating power tubes and PT overheating:
http://music-electronics-forum.com/t23057/

[Enzo]

And who says it will be a steady state problem? You cannot assume the filter caps are PK just because you get no hum or ripple. They still could have an intermittant short. There could be any number of things causing intermittant shorts. How about failing flybacks?

You have a dead transformer? Then stop talking about it, just cut one open any bypass its thermal fuse. Now power up the amp and don;t leave the area, keep watch. Does it start making any noises? Does the transformer start getting hot? If it does get hot, pull the secondaries off the board and see if it gets hot all by itself. Maybe you just got a couple bad transformers.

Even with a problem circuit, I don't expect a PT to melt itself in a half hour.