Blues Junior's smoking.....literally

[joe.dollar]

Just had a friend drop off his son's Blues Junior. Apparently started sounding scratchy and then started smoking. I haven't had much time to look into it and it's actually in pretty rough shape. I did some random testing of the voltage on the power tube pins. I'm getting very high (over 100) volts on pin #2 of power tube #1. Any suggestions off the top of anyone's head?

[Enzo]

Well pull those nasty power tubes out of there and recheck that voltage. Most likely thing is a power tube failed, then the burning is either just the tube getting red hot or the screen resistors burning - the 100 ohms connected to the pins 9.

Possible but way less likely is a shorted coupling cap to that pin 2.

I'd use part numbers but there were at least a half dozen versions of that amp and they don;t all use the same part numbers. I posted the schematics sets on www.ampix.org in the Enzo gallery

[joe.dollar]

Tried a PT swap. More or less the same situation. Voltage is present on pin 2 still.

[Enzo]

PT to me means Power Transformer, did you mean power tubes?

Is it there with NO tubes in the sockets? if so, then either the coupling cap from the phase inverter is shorted, or there is a short around the socket somewhere. Around the socket in the electrical sense, it might be located on the main board.

Look at the manual I posted or your own copy if you have it. The last drawing in the file is 57275 revision C. If you have an earlier version, please identify it. In the one I mention, pin 2 of each power tube is fed through cap C15 or C16 depending. if one is shorted, the B+ from the plate of the PI tube will get on the power tube grid.

That is why we test with the tubes removed. Then the only routes +100v has to get on pin 2 is through that cap or by way of circuits shorted together.

[Robert M. Martinelli]

Hi,
are the 100 V there as well even if removing the tubes?Swapping the tubes could not be enough because, after the fall, the internal insulation between the electrodes could have gone bad on one tube, and, since no current is drawn from the grids ( hence no voltage drop ), whatever voltage you have on one grid will be reflected on the other just as well. I would follow Enzo's advice and recheck the voltages with the power tubes out, chances are you' ll find the voltages on pins 2 going back where they belong ( about -10 V to GND on both, unless the neg bias circuit is defective too ). If this happens, then you have the proof you need new power tubes; if the 100V are still there, they' re getting there following some other route ( check for shorts ).
Hope this helps
Best regards
Bob

[Snivly]

Blues JR problem

I made the mistake of plugging my BJ into 4x12 16ohm speaker cab...
Sounded great.. than it blew something in the amp
Transformer?... Tubes still lit but nothing from the amp
Any ideas on how to trouble shoot ?

[52 Bill]

I made the mistake of plugging my BJ into 4x12 16ohm speaker cab...
Sounded great.. than it blew something in the amp
Transformer?... Tubes still lit but nothing from the amp
Any ideas on how to trouble shoot ?

Welcome to the board.
Rather than hijacking this thread why not start a new one, so that your problem can be addressed directly.

[Snivly]

sure...thanks....

[joe.dollar]

I changed the power tubes, not the transformer, and got the same voltage reading on pin 2.

I'll be back at home on the weekend and will test it with no tubes then.

Thanks for the help.

[joe.dollar]

Well, pulled out the power tubes and checked V1 again. Still getting well over 100v on pin 2. I'm going to replace the caps you mentioned Enzo. I'll let you know what happens. I was suspicious that a resistor might have burned or drifted.

[Enzo]

Well, a little nit picking first. V1 is the one by the input, the two power tubes EL84 are V4 and V5. Check the schematic. And I don't know which version you have, so the caps in question are either C15,16 OR C8,9.

If you don;t have those caps, you can still just remove the suspect ones. That will tell you if they are the source of the voltage.

[joe.dollar]

OK, I meant the power tube closest to the PT. Im guessing that's V5. Anyways, I'll let you know what happens.

As far as version, this is a green "REV_B" board. The amp is one of the first year tweed ones. It's received most of the Bill_m mods and was sounding really good before this problem came up.

[joe.dollar]

Well, here's a quick update. Finally got around to the amp yesterday. Changed c8 and c9 as well as c16. C15 had been changed a few months back. Still getting high voltage on pin 2 of the power tube closest to the power transformer.

I'm going to have to study the schematic and see what else is in the chain leading to this pin.