5F6A popping

[jpbondo]

I am out of ideas for my bassman build! I need some help on the smart way to troubleshoot this amp

here's the story,

I have just built a MOJO bassman kit, and did my usual startup, adding rectifier(GZ34), power(2x5881), pre-amp(12Ay7 and 12AX7), and finally the 12AX7 cathode follower, one by one and checking voltage and for smoke, noise, etc.........all was fine till the cathode follower was installed.......with no speakers, the amp, after a few seconds started making some noise and then the rectifier and power tubes flashed internally.....wow!....


voltages pretty much match the schematic's notation, although a bit high, say 480 v at the high end......with all but the last tube installed....that would probably lower it to where they would look right on....

painstakingly checking the work I had done revealed the second pre-amp tube (12AX7) did have pins 2 and 3 reversed........I corrected this and tried again, same problem...

by now, maybe I have damaged the tubes.transformer never got hot........caps look good but I don't really know how to check them

I am no electronics expert, but I have built a couple of 5E3 deluxes and an AX84.....so I'm not a total novice.....but this one has me baffled


HELP!

[Satamax]

You've put the power tubes in without a load on the output trany??? That sounds real bad!!!!!!

[jpbondo]

Thanks for your help!

That sounds like just what I have done!

Is it possible that I have damaged the rectifier and or power tubes in doing this?

I have hooked up the whole thing, speakers and all (4xP10Q's in parallel) and had to swich off after an audible pop and another flash in the 5881 tubes........

In my limited experience, I have never seen a tube spark internally,

I have spare tubes to try but have been reluctant to put them in

Dumb question, what is the purpose of the "ground" switch on this old Bassman circuit?

[jpbondo]

found the problem(s)...thanks to satamax re don't turn it on without a load! dah............anyway....after getting it all checked out and new 5881 tubes, it still squeled but no sparks.progress..........I just swirtched the brown and blue wires of OT on the 5881 power tubes...........perfect..prior to that it just started sqealing louder and louder........feedback I guess.....I don't understand it all, but it works now and sounds great!..........this circuit has feedback from the OT output and I guess I had it backwards......not sure how I would know without hooking it up and turning it on

so now the feedback is "negative feedback"..correct?.......

[Satamax]

Yep, corect.

And yes, never use a tube amp without a load, or you get blown output transformers! You got lucky here, and escaped away from this as it seems.