Fender Bronco problem

[monsteranus]

This amp let the smoke out not long ago. I found the 6V6 tube glass broken, removed the chassis from the case to find the electolytic cap going to it blown up. The resistor in parallel to it looks heated up. The cap value is 22uf 50v.

Im wondering did the tube fail taking out the cap or did the cap fail causing the tube to fail?

Anyone have a schematic on this unit?

[monsteranus]

I found a schematic on the unit. The drawing shows the cap value to be 25uf @ 25v. All the electrolytics in this unit were replaced in 09 based on a paper in the cabinet.

[monsteranus]

I metered the output transformer to get just under 300 ohms on the primary side. I found specs on the transformer stating 7000ohms on the primary side. Would great resistance difference cause the failure I have?

[Jazz P Bass]

300 ohms sounds about right.
The 7000 figure is the primary resistance of the tube.
Replace the resistor & cap, put a new 6V6 in & try the amp.

[monsteranus]

I went back and looked again and saw the actual values of the transformer in a chart. The values on the sticker on the replacement transformer confused me.

Tube type stuff is new to me so I do appreciate any suggestions. Its a basic circuit from what Ive been reading about and studing the schematic as well as I'm always curious as to what caused the failure.

[Bruce / Mission Amps]

Excessive current through the power tube, due to tube failure or breakage... caused excessive current to flow through your cathode biasing resistor.
If the resistor failed, and went open from too much current and heat, then a VERY large amount of the high voltage, found on the power tube's plate, would appear on the tube's cathode, where the 50v capacitor is connected, causing the cap to fail from too much voltage.
As mentioned above, replace the power tube, the resistor and the cap with like values and it should be OK.
Bruce

[Jazz P Bass]

I have seen newer amps that use a zener diode (50V?) across the resistor/ capacitor.
It is supposed to prevent exactly this scenario.

[monsteranus]

I replaced the resistor, cap and bad tube. I power it up and watch the tubes to see no unusual brightness present. It plays fine. I mount the chassis back in the cabinet the next day and power it up. Im plunking around with the guitar to notice the speed and intensity settings should have audible vibrato but there is none.

I remove the tube shields and notice V2 is whitish around the top. I remove it to see a hairline crack in the glass. I install another 12AX7 I had on hand and the vibrato is functional. I also notice the bass control has very little to no effect on the tone.

[riz]

With a stock speaker, you might perceive "bass" as merely flubbiness, with that amp.