Blown tube or blown amp?

[jtxdriggers]

I have a '93 2-channel Dual Rec (sn#1313) that I completely retubed with 12ax7's, el34's, and 5u4gb's less than a year ago. A few days ago, my friend asked if he could use it for band practice, so I let him. I made sure to keep the cabinet cable in the 4 ohm output so he would know which one to use. However, he's obviously not the brightest tool in the shed. He connected to the cabinet to the effects loop (I don't remember if it was the send or return jack), and called me saying my amp was not turning on. I took the amp back home and replaced the fuse with a 4A 250V slow blow fuse from RadioShack (probably not the best idea on my part) and heard a really loud buzz immediately upon turning on the amp, even though the volume was at 0. I made sure everything was connected correctly, and tried again. Same result, except this time I noticed a flash of light in one of the el34's, so I assumed I would just have to replace that tube.

Desperate to play through my amp, I looked up on the internet how to drop the amp to 50 watts by removing 2 of the tubes. I removed the blown tube and another (both being the 2 inside tubes) and tried to play. The loud noise was gone, but I got no sound from my guitar. I know my guitar is outputting noise because my noise gate opens up whenever I play. I played with the volume and settings a little, and then I tried moving the 2 outside tubes to the inside positions, and I got the same result. I also tried replacing the fuse again (same brand) and using only 1 rectifier tube (tried both positions).

Is there something very wrong here?

 

[domct203]

This is not sounding good. It sounds like this guy fried your output tranny (or more).

I would say contact a tech before you cause more damage trying to troubleshoot.

Dom

 

[clutch71]

You learned the same lesson I did. Unless you know that someone has knowledge of how to hook up tube amps properly, never loan out your gear. A ROV I had was hooked up to the slave out and "ran for a couple of minutes" before they realized something was wrong.

When they brought it back and I hooked it up right, powered up and it popped the fuse. I replaced the fuse and it blew the power tubes. I replaced the fuse and power tubes and fortunately it worked fine. Any idea how long he was running it like this?

Have to agree with Dom...If you replaced the fuse and then it blew one of the two tubes. Then replaced with a fresh fuse and then replaced the power tubes with a known good pair (outer or inner) and no sound...you've got problems. Most likely a grid resistor was cooked and or the output tranny.

Grid resistors are fairly cheap + plus the cost of labor. Maybe $60 all told. The out put tranny are not cheap $259-$300. Best of luck!

 

[jtxdriggers]

Well I thought my friend was a knowledgeable person when it comes to these things. He's been working in a music store/repair shop for months now, but obviously he knows nothing about ohms or output :x

I have no idea how long he ran the amp. I just got a phone call right when his practice started that said my amp was not turning on.

Thanks for the help guys. I guess I'm going to have to make him buy me a replacement (rev. F or older). Or else...

 

[3124+]

Yeah, you've got a serious fault in the output section. Could be a blown output transformer, output tube, resistor, damaged tube socket, or all of the above. With any tube amp, always be sure the head is properly connected to the speaker before powering up. Flyback voltage is an instant killer.

 

[jtxdriggers]

After examining the amp thoroughly, I found one blown power tube, one blown preamp tube, and a blown resistor just behind the treble control knobs (outlined in green here: http://home.jtxdriggers.com/downloads/will/int1.jpg).

I'll see if I can get those fixed/replaced over the next week, and hopefully the amp will work. If not, then something else is wrong

 

[3124+]

After examining the amp thoroughly, I found one blown preamp tube, and a blown resistor just behind the treble control knobs (outlined in green here: http://home.jtxdriggers.com/downloads/will/int1.jpg).

Interesting. Which preamp tube position?

 

[jtxdriggers]

Unfortunately, I don't remember. I removed all the tubes when I was taking the chassis out of the shell, and once I set them out, I noticed that the top of one of the preamp tubes was charred black from the inside. It's possible that the tube still works, but I have no way of testing it at the moment. And honestly, I'd be kind of scared to try.

 

[nomad100hd]

fortunately, though it's an expensive fix if the OT bad, it's an easy fix. If mesa will sell you the OT reasonably you might be able to change it yourself.

 

[jtxdriggers]

Well if my revision F has superior transformers to newer dual recs, then wouldn't my amp sound more like a newer one if I got a new transformer? That's really what I'm worried about; I'm not a fan of the revision G or later, especially next to my rev F

 

[nomad100hd]

Could even be no two transformers sounds the same. You could go the expensive route and order mercury magnetics transformers. Either way buying another revision F wont give you exactly the same sound. Besides it's probably no the OT that gives the biggest tonal difference between the revisions.

 

[jtxdriggers]

Well I'm back at home for Winter break now (finally). I'm going to take my pride and joy down to my friend who works at a music repair shop and see if he can replace the resistor. Then I'll pop some new tubes and a new fuse in and see what happens. If I still get no sound, then I guess I'll start looking up output transformers

 

[jtxdriggers]

Sorry to bring this thread back up, but I just got my amp back from the shop, and unfortunately, after replacing the blown resistor, it seems the output transformer is fried. The numbers on mine are 562100/EIA606-033. Could someone help me out in finding a replacement that would very minimally affect the sound my amp once had.

 

[alumataz22]

From what I hear the old rectifiers had the same transfomers as the mark III. Maybe you could find an old mark III for a decent price and get the transformer from it?

 

[jtxdriggers]

I actually looked for a Mark III tranny too, but couldn't find one. But MonstaTone bought and repaired the amp, and there was much more wrong with it than the transformer. Thanks for your input though

I'm happy with my new blackface DR now

 

[Monsta-Tone]

Hey Justin,

It seems that after all the heartache, it worked out well for both of us!

Here's a re-cap of what was wrong:

Blown preamp tube was most likely the Phase Inverter, but was marked to go in position 2 when I got it.
O. T. was good, but measured strangely. I could not get a good reading between both Primary taps.

The entire filament circuit was damaged.
Basically, the 2 100 ohm resistors that provide a reference to ground were cracked in half.
This and the blown tube were most likely the culprits in the humming.

Also, the DC rectifier for the filament supply and a few of the resistors for the channel switching power supply, which is fed from the filament taps of the Power Transformer, were fried.

The entire Bias Supply was toasted.




I replaced the entire Bias Supply (I also added a bias pot where the Slave pot used to be since I never use that function).
I replaced the filament resistors.
I replaced the Footswitch Power Supply, with the exception of the 2 4,700 mf caps, which I will need to remove the circuit board to do.
I will replace them in the next week or so.
I replaced all of the tubes.
I replaced all of the Power Tube Grid Resistors because they all measured out of spec, ranging from 800 ohms to 1.5k ohms. This is pretty common with carbon comp resistors, but I didn't want to chance it, so I put some heavy duty wire wound ones in.
I replaced all of the power supply caps. The 2 220 mf caps looked ready to pop. The ends of them were really sketchy looking, which basically says, 'Replace me!'


In the end, it would have most likely cost around $300 for the repairs if someone had brought it to me or another tech with the same problems.


It's interesting though that the O. T. was not the 1st thing to go. Almost would have been easier to fix if this had been the problem.


I thank you again Justin for the great amp!