MOTORBOATING!!!

[ViperDoc]

OK, so I've got some motorboating on my new JTM45 amp after installing all the tubes, hooking up the speaker and taking the amp out of standby. This happened on my Bassman, and all I had to do was swap the wires that come out of the output tube section into the gridstop resistors (?) (red and blue twisted wires). Is this all I have to do, then?



Thanks.

[ViperDoc]

UPDATE:

I swapped the red and blue output tube wires, turned the amp on and out of standby and found the following:

1) No more motor boating.
2) No DC voltages on any test points, although I did hear faint popping in the speaker as I probed the points towards the preamp side.
3) After about a minute, there rose a slight electrical smell that I've never smelled before, so I turned everything off!

What do I do now?

[pdf64]

How does your light bulb limiter respond to the amp?

[Stevem]

You swapped the wrong wires!

You can do it two ways the easiest being to swap the Blue and Brown feed wires on the output tube sockets, but in doing what you did you should not have lost your V+ voltage like you did.

Unhook your output wire from the V+ fuse and confirm that you still have your + 450 something volts of D.C. Coming into the fuse holder.

Your OT red wire gets fed off of that HT fuse and then the Blue and Brown wire go to pin 3 of each output tube socket respectively.

[martin manning]

You swapped the wrong wires!

I don't think so. Note in the photo above the power tube grid wires are blue and red.

[Stevem]

Sorry, I did not look, I just assumed that since he was talking about a blue and red wire that a brown wire would have to be the other plate wire.

[xtian]

Stop! This is a new build, so follow new-build startup procedures, starting by pulling out ALL the tubes and measuring voltages first (preferably with a bulb limiter to prevent burning stuff up...which sounds like may have happened already).

[thetragichero]

lightbulb limiter has saved my bacon a number of times
was messing with too many octave fuzzes so instead of a bridge rectifier i put in a diode ring....

as exciting as it is to get to play a new build, an extra ten minutes can save ya a lot of F words

[ViperDoc]

Stop! This is a new build, so follow new-build startup procedures, starting by pulling out ALL the tubes and measuring voltages first (preferably with a bulb limiter to prevent burning stuff up...which sounds like may have happened already).

Good call. I am a rookie, and come to find out, this could be explained by a multimeter ground disconnect. I quickly detected voltages on the chassis this morning but will confirm values later today. I have a Variac, so I assume I can just use that. All the tubes and fuses are in and they look good.

[pompeiisneaks]

a variac is a good way to keep low voltages and check nothing is going wrong, but there's a cutoff point where if you go 'too high' you can still damage things, so be very careful/slow on AC volt ramp up... start very low, check voltages everywhere, go up a few volts etc. The light bulb limiter is way safer at ensuring you don't burn something up at full voltage.

~Phil

[ViperDoc]

a variac is a good way to keep low voltages and check nothing is going wrong, but there's a cutoff point where if you go 'too high' you can still damage things, so be very careful/slow on AC volt ramp up... start very low, check voltages everywhere, go up a few volts etc. The light bulb limiter is way safer at ensuring you don't burn something up at full voltage.

~Phil

There’s enough info online, maybe I’ll build one. Thanks for the advice! How will the VDC readings differ when using the limiter? Fractional?

[pompeiisneaks]

basically think of what expected max should be

say 450VDC

if you're inputting 12VAC instead of 120VAC you should see a linear representation of about 45VAC on the secondary etc.

~Phil

[xtian]

There’s enough info online, maybe I’ll build one. Thanks for the advice! How will the VDC readings differ when using the limiter? Fractional?

Voltages will be significantly lower with bulb limiter (20-50% maybe). But we use the limiter only during first start up, to make sure there are no shorts or problems that draw too much current--fault conditions. After determining that the circuit is functioning properly, set the bulb limiter aside, put tubes in, and continue testing.

[ViperDoc]

There’s enough info online, maybe I’ll build one. Thanks for the advice! How will the VDC readings differ when using the limiter? Fractional?

Voltages will be significantly lower with bulb limiter (20-50% maybe). But we use the limiter only during first start up, to make sure there are no shorts or problems that draw too much current--fault conditions. After determining that the circuit is functioning properly, set the bulb limiter aside, put tubes in, and continue testing.

So you would do your sans-tubes chassis power-up with the limiter and then pull the limiter when putting in tubes? All looked well when putting in the rectifier and preamp tubes . I only got motorboating when I put the kt66 pair in, connected the cab and then went out of standby. I’m not sure how to check that the circuits are functioning properly. How do I do that? What I mean is I know how to check voltages against a chart, but they’re never on point until all the tubes are inserted, right?

[xtian]

I like a 40w bulb. at 120vAC, 40watts is 333mA--enough to power up your caps quickly, but not enough to burn your PT. With no tubes installed, and with the bulb limiter in line, at power up you should see the bulb flash brightly (as the reservoir caps fill up) and then go dim (because very little current is being drawn with no tubes installed). That's it! If you have a short somewhere, caps installed with reversed polarity, or other faults to ground, the bulb will stay brightly lit.

In an amp with NFB from the OT, having the OT's primaries reversed can cause howling from positive feedback, but not usually motorboating. Motorboating comes from too little filtering in the reservoir, too little decoupling between nodes (that's the function of the power dropping resistors), or parasitic coupling between wires/components in close proximity.