Is this .mp3 file the sound of motorboating?

[mike_mccue]

Greetings,
I've played electric guitar and tube amps since apprx 1981. When I was younger I always seemed to have a nasty old amp that was always fritzing out so now I have a small collection of amp repair books and have frequently read about motorboating, bad power supply caps, and the need to replace the caps every decade or so. I replaced caps on some of my older amps as a matter of course but I can't say that I've ever really heard the "motorboat" sound.
A few days ago a silverface Fender Champ I own started making a weird noise. I was set up to record with it so I made this .mp3 sample file of the noise it's making:

http://harmoniccycle.com/hc/sounds/m...boating_2b.mp3

I'm wondering... is this the classic "motorboat" sound?

thanks for any insights you can offer,
mike

[Alex R]

I suppose it's not the kind of put-put that usually gets the name but I must say that it sounds a lot more like a motorboat than motorboating does. It really does sound exactly like a motorboat doesn't it.

But it is probably oscillation, so treat as motorboating and see if it goes away.

[mike_mccue]

Thanks for taking a listen,

So this isn't what amp techs call "motorboating"? Darn, I was hoping I was finally getting to hear what I've read so much about.

Just to be clear, the sound I recorded is present with all 3 control knobs set to "1" and nothing plugged in to the input.

It seems heat related because it only becomes apparent after the amp has been powered up for a while.

The recording is highly amplified so it's just a quiet drone... but unlike system noise (hiss) or hum that I am familiar with.

thanks again,
mike

[Alex R]

I listened to it again. Kind of relaxing...!

Might be worth trying a new tube or two?

Motorboating is not a precise term - but usually the oscillation is so low-frequency that it's almost like a tremelo. Put-put-put. Yours is oscillating just a bit quicker I guess.

[mike_mccue]

Thanks again Alex,
I've got several spare tubes but the power cap(s) is the Mallory can with a 40-20-20mFd 450vDc combo so I'll have to order that.

best regards and Happy New year,
mike

[Alex R]

Happy new year to you too Mike.

Now if I were checking that amp for bad filter caps I would turn on the amp, take a 450v cap, maybe 33uF, clip the negative wire to the chassis, and poke the positive contacts on the can in the amp with the positive wire. There are sparks aplenty, and you have to discharge the free cap afterwards - but if the noise stops when you do it, you've found the problem. But be careful man! A less spark-prone way is to clip it all up before you turn the amp on, but since the caps stay charged till you discharge them it's high-voltage work either way and I prefer the real-time method.

The can could easily be bad.

I've given up trying to source and replace can caps. Modern ones are often not all that anyhow - low production runs, compromises. But much smaller modern types are easily available in radial or axial wired formats, rated 450v. I leave the can in place, shave off all the tags inside, and bolt in a tagstrip with whatever of the freely-available radial electros it needs soldered in a row on it. Those caps are good. They only go up to 450v but if you need more than than, two 350s in series will do it, just double the value. But you can I think get those Mallory types still... much simpler to fit if you can get one, I've just got used to making up tagstrips.

[mike_mccue]

Hi Alex,
I think I'll try your diagnostic method... and I DO appreciate the need for safety with this procedure. Thanks for taking the time to suggest it.

There was a period here in the U.S.A. when I wouldn't know where to source a "canned" cap but Antique Electronics in Arizona now stocks these:

CAPACITOR, ELECTROLYTIC, 40/20/20 µF @ 500 VDC
C-EC40-20-20
Made in the USA, to the same specs as the Original Mallory, on Mallory's original machinery.



Which I thought I might try.

best regards,
mike

[Alex R]

Yeah simpler to fit that & neater too.