Mesa/Boogie 400 keeps blowing mains fuse

[mauman]

Measured mains is 235V. No abnormal hum. Supply, bias and heater voltages check out.
Good hint on those varistors. I remember that I have replaced those recently and I THINK I made sure I used the same value.
It says "SanKen 130K20 82" on them. Would these be the right ones? The schematic doesn't say which ones to use.
I'll take those out of the circuit and report back...

The "130K20" varistor is rated for 130V AC RMS...

[Helmholtz]

The "130K20" varistor is rated for 130V AC RMS...

There shouldn't be more than 130Vrms across a half-primary.
What's the tolerance?

[g1]

episodic heater short on one of the tubes (is that a thing at all?

I've never heard of this.
There have been issues with variations in modern mfgr heater specs and surge currents, but I think your consecutive testing takes at least the 'cold surge' out of the equation.

[tubby]

Did some more measurements with the varistors disconnected:

Max current at power on / standby off: 6,5A (out of 25 sample measurements)
So it seems the varistors help a bit there (with varistors connected max was 5,56A)

However, the 10 consecutive measurements without varistors gave: 1,3 / 5,1 / 5,2 / 3,1 / 4,6 / 5,5 / 2,1 / / 2,1 / 6 / 2,2
That averages to 3,72A (compared to an avg of 3,81A with varistors).
It might be the methodology with 10 consecutive measurements is flawed, though. Since the varistors would heat up on power on I would have to wait between cycles to let them fully cool down (probably a minute or so).
However, I waited only for maybe 10 sec between each measurement.


Two more things: The PT gets really hot even when idling with no signal, almost to the point where you would rather take your hand off. Don't know if this is normal on those amps...
Also, at the onset of continuous sine drive clipping (~300W) the current draw on the power line is 4,5A which seems too much when there's a 3,15A fuse specified. Thoughts?

Am I right to assume the next step would involve disconnecting all the secondaries on the PT and then check power consumption / current draw of the PT and also check resistance/shorts to ground or self on all secondary taps?

[g1]

The PT gets really hot even when idling with no signal, almost to the point where you would rather take your hand off. Don't know if this is normal on those amps...

I think it's normal. I have been very surprised by the temp. and even the unloaded current draw of MB transformers. This included a brand new replacement for a dual or triple rectifier (forget which one it was).

[Helmholtz]

The tripping time of a "T" type fuse at twice the rated current can vary from 2 minutes (UL listed type) to 30 minutes depending on exact type.

https://www.eska-fuses.de/fileadmin/...raege-ESKA.pdf

https://www.eska-fuses.de/fileadmin/...ge-ESKA_01.pdf

[Helmholtz]

Did some more measurements with the varistors disconnected:

Max current at power on / standby off: 6,5A (out of 25 sample measurements)
So it seems the varistors help a bit there (with varistors connected max was 5,56A)

However, the 10 consecutive measurements without varistors gave: 1,3 / 5,1 / 5,2 / 3,1 / 4,6 / 5,5 / 2,1 / / 2,1 / 6 / 2,2
That averages to 3,72A (compared to an avg of 3,81A with varistors).

As the varistors are wired in parallel with the primary windings, it's not possible they lower the current.
Rather a bad varistor would increase current.
For reasons I mentioned earlier your current measurements are neither reliable nor reproducible.

Am I right to assume the next step would involve disconnecting all the secondaries on the PT and then check power consumption / current draw of the PT and also check resistance/shorts to ground or self on all secondary taps?

Makes sense.

[tubby]

Did a full cap job on this amp now. Also disconnected the PT and checked: couldn't find a fault there, either.

Think I've found the answer, though: I think Mesa messed up on the early 400/400+ concerning the fuse rating.
If you check the fuse rating printed on the faceplates of these amps on the web you'll find everything from 3.15A (probably early years) up to a whopping 8A (later models)!

The 3.15A fuse just doesn't make sense for an amp that draws 4.5A at 300W rms output.
Lots of headscratching for nothing...

[Helmholtz]

I guess that 8A fuse rating is for 120V mains voltage.
Still wondering why your fuse problem only happens lately.
What is the heater voltage?


Here's a very comprehensive article on amp fusing by MEF member trobbins:
https://dalmura.com.au/static/Valve%20amp%20fusing.pdf

[g1]

The 400 has 6 power tubes, the 400+ has 12. That is a substantial difference as far as fusing and current consumption goes.
The 400 schematic shows 3A fuse for 230V, 6.25A for 117V.
The 400+ shows 4A fuse for 240V, 8A for 120V.
Fuses are slow-blow

[tubby]

Maybe they just added the "+" on the early 400+ and used the same faceplates then. Because that pic below is a 400+ with a 6.25A fuse and thus doesn't fit the fuse specs on the schems.

[g1]

I think you are right about the faceplate. I guess they realized the problem and changed the labeling there, with everything else remaining the same.