MOTORBOATING!!!

[ViperDoc]

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.

Thank you, xtian! Now I am very curious as to the differences between motorboating, howling, and what I heard in the garage. The sound I heard before switching the wires sounded like Herbie on crack crossed with the strangulation of a canary, mic’d loud! Such an objective description, I know!

[xtian]

Motorboating is low frequency oscillation, like 2-10 Hz. See 2:40 in this video: https://youtu.be/A0m65Wkprms

[ViperDoc]

Motorboating is low frequency oscillation, like 2-10 Hz. See 2:40 in this video: https://youtu.be/A0m65Wkprms

OK, that's definitely not my problem. I've got PFB howling, looks like. That's why it went away when I switched the wires.

If I can warm up my amp and confirm proper voltages as per the diagram, is there any benefit to the limiter at this point? I can see it's utility, and I think I'll make one. I put my chassis on my variac and raised the voltage on it and the lamp light didn't turn on until I got closer to 120V. That may be unnecessary, but hey, what do I know? I'm not sure how to use it properly.

Thanks.

[xtian]

If I can warm up my amp and confirm proper voltages as per the diagram, is there any benefit to the limiter at this point?

No.

[Stevem]

Motorboating sounds like a slow non metallic jackhammer.

[ViperDoc]

Thanks all for your help! I’ve fixed all my issues and the JTM 45 is now fully functional. See the results here:

https://ampgarage.com/forum/viewtopic.p ... 05#p407205

[MakerDP]

Here is a video of one of my builds motorboating... just the sound, no other distractions... lol.

It would be cool if there was a sticky thread that had samples of BAD sounds coming from amps so we could all speak the same language when issues come up. Like samples motorboating, reversing the OT leads, blocking distortion, etc.

[ViperDoc]

Here is a video of one of my builds motorboating... just the sound, no other distractions... lol.

It would be cool if there was a sticky thread that had samples of BAD sounds coming from amps so we could all speak the same language when issues come up. Like samples motorboating, reversing the OT leads, blocking distortion, etc.

That is a very good idea. I'm still a noob, so I definitely made a wrong call describing my problem. I'm 2 for 2 on my last two amp builds with positive feedback, so Im getting accustomed to that one! Hah!!! I wonder who could vet all the terms and examples, but I agree, that would be very useful.

[pompeiisneaks]

Here is a video of one of my builds motorboating... just the sound, no other distractions... lol.

It would be cool if there was a sticky thread that had samples of BAD sounds coming from amps so we could all speak the same language when issues come up. Like samples motorboating, reversing the OT leads, blocking distortion, etc.

Start the thread, I'll sticky it

~Phil

[Bergheim]

I've done a couple of builds, all of them designed from scratch, basically. On the first amp I used Paul Ruby's start up guide, found here (http://www.paulrubyamps.com/info.html#FirstPowerUp). I like to build one part/system of the amp at a time. For example I always do the heater wiring first, then I check all the solders, polarity and voltages before continuing with the next part, which for me is the grounding. Always confirming that the one thing you just did/installed is correct is so much easier than wiring everything before checking anything. Still, just before every first real power-on with tubes in and all that my palms get sweaty and I end up tracing every part of the amp thorougly one more time. So far it has been worth it, never gotten the magic smoke to this day.
To list it up I usually do this:
1. Install all pots, jacks, switches etc.
2. Wire heaters and main grounding layout.
3. Confirm correct wiring, confirm no shorts.
4. Wire fuse, rectifier, filter caps, dropping resistors and drain resistor across reservoir cap.
5. Measure resistance across caps, confirm no shorts.
6. Fire it up, confirm correct b+ and heater voltage.
7. Wire one gain stage at a time, starting with the power stage. Fire it up and confirm correct resistances and voltages after wiring each stage.
8. Measure the whole amp, following the signal path down stream.
9. Obsessionally measure everything one more time.
10. Insert tubes, power it up and check all voltages.
11. Confirm that it makes guitar sounds.

[ViperDoc]

I've done a couple of builds, all of them designed from scratch, basically. On the first amp I used Paul Ruby's start up guide, found here (http://www.paulrubyamps.com/info.html#FirstPowerUp). I like to build one part/system of the amp at a time. For example I always do the heater wiring first, then I check all the solders, polarity and voltages before continuing with the next part, which for me is the grounding. Always confirming that the one thing you just did/installed is correct is so much easier than wiring everything before checking anything. Still, just before every first real power-on with tubes in and all that my palms get sweaty and I end up tracing every part of the amp thorougly one more time. So far it has been worth it, never gotten the magic smoke to this day.
To list it up I usually do this:
1. Install all pots, jacks, switches etc.
2. Wire heaters and main grounding layout.
3. Confirm correct wiring, confirm no shorts.
4. Wire fuse, rectifier, filter caps, dropping resistors and drain resistor across reservoir cap.
5. Measure resistance across caps, confirm no shorts.
6. Fire it up, confirm correct b+ and heater voltage.
7. Wire one gain stage at a time, starting with the power stage. Fire it up and confirm correct resistances and voltages after wiring each stage.
8. Measure the whole amp, following the signal path down stream.
9. Obsessionally measure everything one more time.
10. Insert tubes, power it up and check all voltages.
11. Confirm that it makes guitar sounds.

Thanks for the link, Bergheim!

[Fritzer]

Just a tip, Start with Power Supply and then work back from the speaker to the input. It will save your sanity. trust me on this.