Mesa SOB Blowing Fuses

[rf7]

This amp calls for a 3A slo-blo fuse, but it's drawing 4.5 amps at full clip. The B+ voltage is 496V, but the schematic shows 450V, and I'm getting 128Watts RMS out of it (should put out 100W). I tried a 6A slo-blo, but that actually blew when it was being played through.

What could cause the amp to be drawing excessive current like that?

[Enzo]

Pull the power tubes and see what it draws.

Leave it in standby - ie the B+ turned off - and see what it draws.

[Amp Kat]

The extra voltage is caused from higher household voltage. If you used a variac to tweek it down the power and voltages would drop. Amps don't generally put out exactly what they are rated at. There is a window of % +/- 10 % always. Did you check those tubes yet ?

[rf7]

Pull the power tubes and see what it draws.

Leave it in standby - ie the B+ turned off - and see what it draws.

It's drawing 350mA in standby and 380mA fully on, with no 6L6's.

It draws 550mA in standby and 950 mA fully on with the 6L6's.

[rf7]

The extra voltage is caused from higher household voltage. If you used a variac to tweek it down the power and voltages would drop. Amps don't generally put out exactly what they are rated at. There is a window of % +/- 10 % always. Did you check those tubes yet ?

The power tubes are brand new. I've tried other sets, they pull at least the same amount of current at idle.

When I drop the B+ to 450V, by dropping the input AC with a Variac, it pulls 2.5A at full clip. So I guess I need to drop the B+ internally.

[Enzo]

At this point I would disconnect the OT from the power tube plates and the B+. I would clip some other OT in its place. DOn't worry about NFB, just clip the OT secondary to a speaker.I am not even concerned over impedance. Wrong impedance won;t triple the B+ current. I have an old Fender Bassman OT I use as a universal test OT.

Transformers ar eusually the last thing on the list, but you seem to have excess curents.

[rf7]

I put a Fender OT in there and it's only pulling 2A at full clip at 120Vac, so I guess it's OT replacement time?

[Enzo]

Before you go out and replace it, go over to RG's Geofex web site and check out his very simple transformer tester. It is very simple to make and will detect shorted turns in a transformer.

Big time shorts in transformers will blow fuses or something, but a couple adjacent winding turns shorted to gether will just sap the thing, kinda like driving down the interstate with your parking brake still on - you can do it, but the car works a lot harder.

But in my estimation if all you do it swap between two transformers and one draws twice the mains current of the other, well it is likely at fault.

[rf7]

Got back into this. I was running it on the bench and it was only pulling about 2 amps at full clip. Then I ran it at 20% power with a sine wave for a while, with an ammeter on the ac line. Well, lo and behold, at a certain point, it started to draw heavy current and blew the fuse. The ammeter peaked at 17 amps! I had a 3 amp slo-blo fuse in there.

I tested both the OT and the PT using the neon bulb tester and they checked out fine. Something seems to heat up and then draw current like crazy. It's tricky. Any suggestions? Hard wire the fuse and see what burns up?

[edge59]

If you remove the power tubes, check pin 5 for - voltage, is it about -47-57? I would check the bias caps, they may be dried out or shorted out.

[52 Bill]

I'd suggest a light bulb limiter on this one. See if you can run it long enough for it to go into failure mode and start taking voltage readings to see where the big current draw is coming from.

[Enzo]

I suspect your OT is arcing inside. RGs transformer tester checks for shorted turns, but can't find arcs that only occur at high voltages. WHen a high level of signal is present, the instantaneous voltages within the transformer can be WAY higher that B+.

The fact some other transformer doesn't act this way is telling.

[rf7]

The guy finally agreed to spring for an output transformer. He kept waiting for one to show up on ebay, which wasn't very likely. The amp works flawless now!!! Thanks for the help guys!

[xjbear]

At this point I would disconnect the OT from the power tube plates and the B+. I would clip some other OT in its place. DOn't worry about NFB, just clip the OT secondary to a speaker.I am not even concerned over impedance. Wrong impedance won;t triple the B+ current. I have an old Fender Bassman OT I use as a universal test OT.

Transformers ar eusually the last thing on the list, but you seem to have excess curents.

Thanks for this Enzo. Ive been screwing with a '69 Univox 1001 guitar amp for on and off for a year, finally got in it again and tried this and wow, it worked. So next step...find out what was up with the output from the OT. Guess what, it had shorting jacks on two of the output jacks, So it needed to have TWO speakers hooked up to the head for it to work. Guess I see something new everyday, I had no idea some old amps had things like this in them. I bet this design has saved the OT a few times in its life, I fixed a few pawn shop heads that have blown OT's cuz these dipshits dont know you have to have speaks hooked up. I guess grounded is better than open HUH lol Anyway thanks man