Peavey Classic 400 Dead PT

[Andy]

a few years ago, I bought a classic 400, and to be honest, it probably had problems right from the start, but didn't actually die until recently. when it quit working, I tested the 8 power tubes (6550's) and found that 3 of them were completely bad, and 2 others that were marginal at best, so I replaced all the tubes with brand new JJ's, to the tune of about $400USD. I powered up the amp and biased the tubes to about 60%. I then started playing it at low volume, watching for signs of trouble. I observed no problems and started bringing up the volume, eventually reaching band-practice/performance levels, and heard the best bass guitar tone of my life! then after about 5 minutes of that, it blew its main fuse, and stopped working. I replaced the main fuse, and tried again, but this time, it didn't blow the fuse, but also didn't power up. the cooling fan came on, but that was all. the cooling fan is powered in parallel with the PT primary, so it's no surprise that it still worked. in any case, I pulled the chassis back out of the box, and started looking things over, eventually testing the windings of the power and output transformer. I found that the primary winding of the power transformer had no continuity, either between the leads or from either lead to ground. all other windings had continuity between the leads, and not to ground, so at this point, I assume that they are ok.

To make a long story short, I need a new PT for my amp, and the part is no longer available from Peavey. I contacted Mercury Magnetics, and they won't even talk to me about the cost until I physically ship my PT to them, a rather expensive proposition. Does anyone on this forum re-wind transformers? I have another amp that I'm using in the meantime, but with the way the Peavey sounded with the new tubes, I'd really like to fix it and put it back into service.

[Enzo]

SO the transformer has to come out, one way or another, right? SO make detailed drawings or charts so you know EXACTLY where each wire goes when it is installed again. Disconnect them, and remove the transformer. Remove the end bell from the transformer to inspect the inner wiring. All those nice insulated wires with colors on them are soldered to winding enamel wire inside. A connection may have failed. I bet there is also a thermal fuse tucked in the winding in series with the primary. It may be open. Find it and see.

[Andy]

Fortunately, all of the wires have connectors that only fit in one place inside the amp, so reconnecting it is simple. the transformer is already out, and there are no end bells - just a copper sheet wrapped around the outside, with its ends soldered together. perhaps this weekend, I'll pull the copper sheet off of there and have a look. what might this thermal fuse look like?

[Jazz P Bass]

transformer internal thermal fuse - Google Search

[Andy]

I carefully unwrapped the outer parts of the transformer and found this device. another view. I checked it with my meter, and found that it is open. I then checked the primary winding, and found it to have continuity. so now I have a few questions:

1. is it safe to run the amp without this fuse in place?
2. if I replace this fuse, what's to stop the amp from blowing it (and/or the main fuse) again?

I don't recall the bias voltage, but I suspect it was significantly less negative than the -53v or greater that they insist is required. is it possible that I just biased the power tubes too hot, and that's what caused this? their bias setting procedure says that -52v or less will cause irreparable damage to the power tubes. is it possible that this amp truly is that sensitive to bias setting?

[J M Fahey]

I carefully unwrapped the outer parts of the transformer and found this device. another view. I checked it with my meter, and found that it is open. I then checked the primary winding, and found it to have continuity. so now I have a few questions:

1. is it safe to run the amp without this fuse in place?
2. if I replace this fuse, what's to stop the amp from blowing it (and/or the main fuse) again?

I don't recall the bias voltage, but I suspect it was significantly less negative than the -53v or greater that they insist is required. is it possible that I just biased the power tubes too hot, and that's what caused this? their bias setting procedure says that -52v or less will cause irreparable damage to the power tubes. is it possible that this amp truly is that sensitive to bias setting?

1) most amps are safe to run without ... but not yours
Clearly something gross happened to overheat the transformer so much that this thermal fuse blew.
If you do not solve it first, it will blow again and probably catch fire next time.
I also suspect you are using an improper fuse there, otherwise it should have blown ... because you definitely have some current hogging there.
2) the proper solution is to find why your amp "eats" so much current.
a) to check that the transformer itself is not bad, a shorted turn, whatever, first replace the thermal fuse (buy 2, just in case), connect its primary only to 120V but not any of the secondaries yet.
Wait 10/20 minutes. Does it hum/buzz excessively/overheat/smoke/spark/whatever?
Nothing? (beyond slight warming). Good, leave it on for 1 hour, just to be *real* sure.
b) now connect filaments only . Do they light properly?
Repeat thermal test above.
Yes, it's boring, but you already invested $400 on that amp ... play it real safe.
3) connect the high voltage and without delay, adjust bias to what Peavey asks for.
Look at the tubes: any redplating?
Check transformer temperature every 5 minutes , for at least one hour total.
It should get hotter, but *never* so much that you can't touch it.
If everything reasonably normal, play with the amp at rehearsal/stage levels, and every 10 minutes check transfirmer temperature.
As Enzo says, a smoldering transformer is usually not the problem, but the symptom.
And yes, amps are *very* sensitive to bias settings.
Good luck.

[Andy]

And yes, amps are *very* sensitive to bias settings.

the question was not whether amps are sensitive to bias settings or not. the answer to that question is obvious. I may not be the expert that Enzo is, but I'm certainly not a rookie.

the question was whether this amp is as sensitive as peavey says.

70% dissipation is commonly recommended for best tone, and I assumed I was being conservative by setting it at 60%.

[Alex R]

A push-pull amp that balances that much power across its OT will naturally and obviously be very sensitive to bias settings.

[J M Fahey]

70% dissipation is commonly recommended for best tone

Hmmmm, "commonly", famous last words.
Was it recommended by Peavey?

[Steve Conner]

The bias voltage ("-52V" or whatever) is meaningless in itself, because the relationship between the voltage and the idle current that actually flows is dependent on the tube characteristics, and these vary from tube to tube. I'm sure you could find tubes that would pass destructively high current at -52V.

The Peavey documentation should hopefully specify an idle current setting as well as a voltage. I suspect it might be a bit less than 60% dissipation: 60% of (8 x 35w) is a fair whack of power, about 160W.

The thermal fuse is there to stop the transformer from catching fire. It is non-resettable because even a single serious overheating incident can damage the transformer's insulation permanently, making it unsafe (or at least not UL compliant any more )

[Enzo]

And as the devil's advocate, sometimes fuses fail for non-electrical reasons. Drop a 1A fuse on the floor, sometimes when you pick it back up, it has opened. Sometimes thermal fuses break mechanically.

[Andy]

well I'll be ordering some replacement fuses and try again, and follow the peavey biasing recommendations explicitly. at this point, I'm guessing that 60% is just way too much idle current, and the transformer got too hot. it sounded brilliant, but maybe it's just too much for that transformer to handle.

[J M Fahey]

I'm contributing to help somebody else in another post, about a "something" 400 Peavey amp, who's complaining his runs too hot.
Per Peavey recommendations, ech tube passed *no more* than 16 mA each, which at 500V B+ means 8 watts per tube. Go figure.
As I said before, that 60% biasing madness started in Forums, it's not recommended by *any* manufacturer.
Much less Peavey who biases all its stuff cold, even SS ones.
Yes, probably biasing way too hot feels better.
Using no condoms also feels better, go figure.
Yet I wouldn't recommend it.

[J M Fahey]

Here it is: read all the post and how this guy biased his tubes even colder, to the tune of 4W per tube, go figure:
http://music-electronics-forum.com/t30627/

[olddawg]

And as the devil's advocate, sometimes fuses fail for non-electrical reasons. Drop a 1A fuse on the floor, sometimes when you pick it back up, it has opened. Sometimes thermal fuses break mechanically.

I totally agree. I have also replaced a lot of termal fuses in transformers. I usually will use a small pigtail GMA type fuse and wrap it in heat shrink tubing. I rate the fuse slightly over the mains fuse. I would not recommend doing this for a customer's repair, but have resurrected a lot of equipment for personal use that would otherwise not be economically repairable. It seems, in your case, with this beast of an amp, that you should have a variac or at the least a light bulb limiter to use when troubleshooting. Enzo would know much more than me about this, but can't you just run this amp with 2 output tubes to start once any obvious issues are corrected?