My music man 65 is driving me nuts Hums even with out tubes fuse blows

[ericjwilliams]

I have a music man sixty five reverb, it is chassis number 2275-65, with 65 watts out of two el34's, a 12ax7 phase inverter tube and solid state pre amp. I had just bought it and it was all good at my house at low volume, then I took it to band practice....

The fuse blew and I thought maybe I lost a tube. Well that wasn't it. It hums and does not produce any musical signal, It does this even with out any tubes in it, and I have 0vdc on plates. The O.T got wicked hot and smelled a bit but didn't smell burnt after it cooled. I thought maybe its the O.T so I took It to a tech. in town to diagnose it and confirm the O.T I told him what was going on and for $40 he took out the chassis and smelled it. He said it was burnt but its not (I tried a new one), he just wanted quick cash. Any ways I bought a O.T and replaced the caps under the can because they are old. Does any one have an idea of what's going on?
heres a summery


-no musical signal, no voltage on pin's 3, and a HUM (with or with out tubes installed) until the .5 amp fuse blow's. Before I wired in a fuse the O.T was getting extremely hot.

-I put in a new O.T and wired a .5 amp fuse inline with the O.T to help prevent it burning out, I also replace the caps under the can

-if I take out the .5amp fuse so the O.T is not seeing voltage and take it out of stand by I have 480ishVDC (in low power) and 700ish (high power) to the red wire high voltage feeding the O.T. FYI these do see 700vdc on the plate.

any one have any idea's? I really can't pay to have a tech. fix it, and Im defiantly never bringing any thing to the "tube man" again I also have heard nothing but bad about that dude, and then I got burned. I should have known better.

any help would be much appreciated,
thanks,
Eric

[Ronsonic]

The OPT wasn't a bad guess. Here's a better one. THere are two flyback diodes in the plate circuit of the power tubes. I see them in the schematic, don't recall where they are in the amp. When those fail, they fail short and act as a bit of protection against some failure modes. If one shorts it will make a wicked hum with or without tubes and will blow a fuse in short order. They are marked as D7 & 8 on the map and connect between pin3 and ground on each power tube. You'll want to replace both. Either use something fancy like they did or just run 3 x 1N4007 in series with the bands pointed toward the plate on each side.

What is likely to have caused it to fail? Don't know. Check that your cab is the correct impedance and your speaker cabling is solid.

[R.G.]

The fuse blew and I thought maybe I lost a tube. Well that wasn't it.

Good on you for not immediately deciding you had a burned out transformer. The first thing to do is ALWAYS to try known good tubes.

It hums and does not produce any musical signal, It does this even with out any tubes in it, and I have 0vdc on plates.

That happens when magnetic fields drive the OT like a pickup. Happens sometimes.

The O.T got wicked hot and smelled a bit but didn't smell burnt after it cooled.

Smelling burnt is not a definitive diagnosis. Interesting, perhaps, but not all that useful.

Any ways I bought a O.T and replaced the caps under the can because they are old.

That was cheaper than a tech??

-no musical signal, no voltage on pin's 3, and a HUM (with or with out tubes installed) until the .5 amp fuse blow's. Before I wired in a fuse the O.T was getting extremely hot.

If the OT lets any current through with the tubes pulled out, something is hosed up either inside the OT or in the wiring outside it. Could be socket wiring, could be accidentally shorting wire, could be protector diodes shorted. Something is letting current through.

A simple thing to do is to pull out the tubes, pull out your fuse, then measure ohms to ground. If this is not an open circuit, you've found the problem. Find what lets current go to ground and fix it.

There may be a short to the core inside the transformer, but that's really unlikely. An external short of wiring, parts, protection diodes, RC network between plates, etc is much more likely.

Follow the current.

[Enzo]

My vote is also that one or more of those flyback diodes is shorted.

Possible arcing power tube socket.

If you have the same problem with a different OT, then it is not likely the OT that is the problem.

[andyjazz]

Does the amp have solid state rectification? If yes, check those diodes as well.

[ericjwilliams]

Im going to pick up some diodes and hopefully it does the job, thanks guys I will get back and let you guys know if that does it.

[Ronsonic]

Im going to pick up some diodes and hopefully it does the job, thanks guys I will get back and let you guys know if that does it.

The amp will run without those. You can leave them out for testing. Do they measure as shorted?

As Enzo mentions do inspect the tube sockets for signs of arcing.

[1ampman]

I had the same amp and the same problem... It was the small film cap that was across the output tube plates to grd. It was on or next to the output sockets. The cap is 3KV/500pf. Unhook one side and see if its shorted or try to bring up the voltage. It was unbelieable that this cap would take the whole B+ and not smoke. Let us know..... Cheers

[ericjwilliams]

The OPT wasn't a bad guess. Here's a better one. THere are two flyback diodes in the plate circuit of the power tubes. I see them in the schematic, don't recall where they are in the amp. When those fail, they fail short and act as a bit of protection against some failure modes. If one shorts it will make a wicked hum with or without tubes and will blow a fuse in short order. They are marked as D7 & 8 on the map and connect between pin3 and ground on each power tube. You'll want to replace both. Either use something fancy like they did or just run 3 x 1N4007 in series with the bands pointed toward the plate on each side.

What is likely to have caused it to fail? Don't know. Check that your cab is the correct impedance and your speaker cabling is solid.

Thanks for the info with the diode. Would it be better to use a single 1n5048 (it is 3 amp at 1kV). Still kinda a new guy to electronics, and saw this one next to the other. Also would 3 1N4007's in series be 3 amps and 1000v or would it be 3000v? Thanks alot for all the help guys, I may have to order the parts off the net because I work 7-6 mon-fri and the electronics store in town is only open till 5 and not on the weekends. So it maybe a week of so till I get the parts in.

[Ronsonic]

Thanks for the info with the diode. Would it be better to use a single 1n5048 (it is 3 amp at 1kV). Still kinda a new guy to electronics, and saw this one next to the other. Also would 3 1N4007's in series be 3 amps and 1000v or would it be 3000v? Thanks alot for all the help guys, I may have to order the parts off the net because I work 7-6 mon-fri and the electronics store in town is only open till 5 and not on the weekends. So it maybe a week of so till I get the parts in.

I'm agnostic on the component preference. 1N4007 is good because you can get them anywhere, even Radio Shack. And, yes, it adds up to 3K.

Now the important question, does the amp work without the diodes? Did you check them? Is one or both shorted?

1ampman suggested another possibility have you checked that. If not break out the ohmmeter and let's see what's up in there.

[ericjwilliams]

Hey every body! Sorry it took for ever to get this done but I found the problem. It was a 2kv/.001uf cap. across pin1 and pin3 and were up and running agian. Im not a "Tech" more of a hobbyist that wants to become a "tech". This was A learning experience for sure, and I feel like a moron for going straight to the O.T, but live and learn right. Thanks a lot 1ampman for pointing me in the right direction.