Bassman 135 Blowing Fuses

[fatboy01]

I used to be a regular on this forum back before you had to be a member, I learned to build effects and work on amps with the help of you guys. Anyways, I've lost some of my amp repair knowledge, and I hope you all can help. I've got a Bassman135 right now, which hums really loud and then pops the fuse, within a second or two of cutting it on. I've pulled the power tubes, and it still does it. I've checked over the power caps visually, and I have done the resistance test I got from R.G's page. All seems well with the power caps. It looks like a new circuit has been installed for the fan, and there's a jack and a pot on the far right of the chassis. I'm going to have to do some more digging to find out what it is. Any thoughts on the fuse issue? I'm thinking I have a bad ground somewhere....

[Amp Kat]

Have you changed the power tubes ? Are the original caps still in there ?

[bob p]

focus on the usual suspects listed in RG's "Fuse Blows" page and you'll find the problem. something in your amp is passing current to ground. even something that's seemingly as trivial as a cathode bypass cap can cause this problem. (granted, you shouldn't have a cathode bypass cap in a fixed-bias bassman, but my point is to say that any short to ground that conducts sufficient current will cause your problem).

before going any farther, build yourself a current limiter (series light bulb) so you don't melt your PT while you try to track down the problem.

[fatboy01]

Cool, I'll get on that tonight. Not sure if this helps, but it powers up fine in standby, the power tubes glow nicely. I'm still new to amps, but I'm learning. Hopefully I can find the bug tonight. Thanks for the suggestions thus far

[Enzo]

Start at the start - are any of the rectifiers shorted?

Visual examination of the caps does not tell you if any are shorted - measure them.

Measure resistance between output transformer primary winding and chassis. Better not be shorted.

[fatboy01]

Well I replaced all the caps with parts from Hoffman, because as I was measuring the old ones 3 of the leads broke off at the capacitor. Fired the amp up and same problem. Transformer test time... (i should have done this first I know )

So everything powers up fine until I flip the standby switch, and I noticed it spark. It's rated 250v@ 3A, but it's gettin 518volts. Is the 250rating for AC?

Moved on to the output because the power tranny was putting up the right voltages across. Red wire to chassis was around 25ohms, yellow/blue was 0 and yellow/brown was 0. The blue and brown were around 10M. All the secondaries were 0ohms to the chassis as well. Am I doing something wrong, or is my Output tranny shot? If it is indeed dead, would it cause the fuse to pop only when I flip the Standby switch?

[fatboy01]

Also, would a showman or twin reverb output transformer be an decent replacement? Both have a quad of 6L6s, but are 85watts

[Enzo]

Disconnect the red wire. NOW measure resistance to ground at the wire. SHould be infinite. Also measure rsistance from where the red wire was connected before. SHould charge up, not read low resistance.

DOn't worry about the standby switch, no one uses a switch rated for 600 voltsDC.


Yellow/blue and yellow/brown? The ultralinear taps on the output tranny? ANy chance the tubes have the little center peg broke off and they are in ther sockets turned a few pins from the proper orientation. I cannot imagine why both UL taps would be grounded, unless massive internal shorts exist inside the thing. Or both sockets are arcing.

10M blue and brown? WHat do they measure between blue and brown directly? And Blue to red, and brown to red?

Are the tubes in it when you do these tests? Pull them.

Your output secondaries will always read like a short. The wire is heavy and there are relatively few turns of it.

[fatboy01]

Disconnected the red wire, measured around 30ohms to ground, and had continuity to chassis. Measured from where the red wire was connected and had over 220k ohms and rising.

Power Tubes pulled for each test. Also I noticed the #2 6L6 had a crack in it after the last time I powered it up, so I've been checking that socket for arcing or other problems but I haven't found any.

Yellow/blue and yellow/brown still grounded when I disconnected them.

The blue and brown also are both grounded, sometimes my meter would read 3Mohms, but then I would retest and get grounded. I pulled the brown and it was grounded.

[fatboy01]

Ok,I pulled all the output tranny wires, plugged in 2 tubes (one of my 4 is now blown) and it fires right up, no popped fuses. I measured all the leads again and they are all grounded.

As far as replacements go, will a showman or twin tranny suffice?