Gibson BR-9 Tube Amp Issues

[pcmurphypc]

I picked up an old Gibson BR-9 amp from a friend. It was missing the preamp tube and rectifier. I bought a new set of tubes. When I powered it up, all tubes lit up, but after about 10-15 seconds, the rectifier - 5y3 - caught fire. The flame was blue and contained inside the tube glass.

I'm relatively proficient with solid state electronics, but this is my first jump into a tube project. Is it possible that I just got a bad tube? If not, can someone point me in the right direction as far as troubleshooting the problem with a multimeter.

Many thanks in advance

[VictoriaGuy]

I'd put a series light bulb device in the line before powering up an amp like that.
Light Bulb Current Limiter

My first guess (likely wrong?) is that you have shorting filter caps which are pulling too much current through the 5Y3.....

If the amp's been sitting around unused for a long time, the filter caps probably need replacement. (coupling caps too). Check the wiring as well- some of the older insulation can get quite 'crumbly'....

Oh, and check to make sure you have the right size fuse in it.....

[HollowState]

OK, I'll second everything VictoriaGuy said and add that the electrolytic capacitor(s) are probably the culprit. If left to sit unused, the electrolyte in them dries out. When high voltage is applied they leak current, get hot and even short out. They can sometimes be brought back to useful life by appling voltage very slowly over the course of several hours with a varaic® This will reform the electrolyte. (A solid state temporary replacement rectifier helps here) But at their age, I'd replace them anyway as a matter of course. If you try reforming, make sure they don't get hot. They shouldn't even get warm during normal operation.

It seems that the BR-9 uses an electro-dynamic field coil speaker. If so, this really dates it's age. A diagram is here.
http://www.freeinfosociety.com/media/pdf/3193.pdf

Generally, tubes are very forgiving. You may or may not have damaged the 5Y3 rectifier. But todays tubes may not be so forgiving as the older production.

[DJDestiny]

I would first say that your BR-9 is old , but who cares , once it's fixed it would be useful .
Anyway , Change the fliter caps first , very important , but if you like , swap the tube for a SS Rectifier , not sure if it has a profound effect on the sound , but hey .

ADD-ON : Yesteryear's TUBES were truly forgiving , they won't give up even catching fire , a little bit , that is .

[RTF671]

Changing the recto for diodes will most certainly change the tone. 5Y3 has the some of the highest anode resistance and drops the most current among the common vacuum tube rectos I'm familiar with.

You should change the capacitors. If you like you might even be able to increase the smaller ones to improve filtering. The 5Y3GT datasheet I have says that 10uF should be the biggest that 'should be' used.

Here is the schematic for the BR-9.

[pcmurphypc]

Thanks for all the replies. I'm gonna start looking at it this a.m. I already purchased replacement caps but when I pulled out the chassis and looked underneath, the caps had been recently replaced and looked fairly new - definitely not the originals. I thought I could at least try to fire it up w/o replacing and that's when the rectifier flamed up. That led me to believe that there may be something more at work here than just cap replacement. I'm going to take care of the cap replacement and check again with the light bulb limiter. I'll report back with any new problems or, hopefully, simply report that it works perfectly....wishful thinking.....

[pcmurphypc]

My amp wiring differs from the schematic in that pin 2 of the 5y3 tube is feeding the field coil spkr and has the 20pf electrolytic cap vs. pin 8 (as shown on schem). After reading about the 5y3, it seems it may not be a big difference - something about B+ power.

Also, I noticed that the top of the tube has white residue on the inside at the top - I read this indicates that maybe some gas leaked out and the tube failed. True, False or maybe?

Thanks much

[HollowState]

Since the 5Y3 has a directly heated filament/cathode, power can be taken from either pin 2 or 8. Makes no difference. Sorry, if the top has turned white, it has lost it's vacuum and is bad. It should be shiny. Time for a new tube.

Just make sure there are no shorts or very low resisrance readings from the B+ point to ground (B-). You'll need an ohmmeter for this. If there is, you need to determine why before damaging another tube. But it could have been a defective tube. Use the light bulb trick to protect it. The light should light only very dimly if the circuit is ok.

Added: Possible causes of short circuit could be the output transformer primary, the speaker field coil, the filter capacitors, a tube socket or even another tube. Particularly the 6V6s. These are the common areas to check.

[pcmurphypc]

I replaced most of the capacitors - the 20uf on pin 2/8 of the 5y3 was shorting. The only one that wasn't replaced was the .02uf on the preamp tube. I fired it up with only the 5y3 (new one) in it's socket with the light bulb limiter and everything looked good - the 5y3 started glowing and the light bulb was steady and dim. I then put in 2 new 6v6 power tubes and a NOS 6sn7gtb preamp.......and got no signal. I could hear a quiet hum from the speaker if I put my ear close, but nothing from the guitar input. The hum increases with the volume knob, but it is not as loud as I would expect from a tube amp.

I have seen posts where the problem was an pen coil on the speaker - could this be the problem? If so, how would I test that? Any other suggestions of where to look for the problem?

[Wavebourn]

If you got no hum, it is fine. But if you got no signal, it needs repair.

If you hear hum the speaker works.

First of all, I would measure voltages using a DMM. This schematic shows them:

http://www.gibson.com/Files/schematics/BR-9%20Amp.pdf

Also, when nothing is plugged into one of inputs, both of them are shorted to ground: they have switches on them.