Typical motorboating problem

[EETStudent]

I just plugged in my finished 5F2 project for the first time, and everything sounds wonderful minus one thing. When the volume gets to about to about 8, it starts to develop motorboating. My understanding is that its usually a filter cap issue, but I wouldnt think that would be my problem with these new spragues. Any other sources I should look at? Ground loops?

[Jazz P Bass]

Putt Putt

What circuit are we working with? The original Fender Princeton 5F2?
Are you using a tube rectifier?
Can you identify what the frequency of the motorboating is.
Use a scope & look for a real high oscillation on the output.
There may be more than one frequency.
Did you try anything to get rid of it?
Ground out each successive grid.
Remove the PI tube.
Sometimes just probing voltage levels will effect the oscillation.
That in itself can provide a wealth of information.

[EETStudent]

Yep, the original 5f2 schematic exactly, even a helpful layout guide provided by Angela Instruments. I do not have an oscilloscope to view the frequencies. I did not notice that grounding the first grid did nothing, but grounding the grid after the volume control removed it. So it must be between those two spots?

[Jazz P Bass]

Tone Stack

Are you sure that you built the tone stack correctly.
Grounding the second grid is the same as turning the volume off.
Sure sounds like it is in the stack.

[EETStudent]

I disconnected the tone control completely, and motor boating still occurs when the volume hits 8. The tone control is functioning as it should, and the symptons are the same with or without the stack. I'm trying to think of things I can do to isolate certain sections and yield some results

[Jazz P Bass]

OPT

Try reversing the output transformer primary leads.

[EETStudent]

Even though its single ended?

[Alex/Tubewonder]

It does have NFB so correct phasing of OT matters.

[Chuck H]

You could try reversing the OPT leads... It can't hurt, but that's probably not it.

The ground scheme layout for the 5F2 shows the volume control grounded to the back of the pot and all others welded to the chassis. This is lame, but I guess it worked, I've never owned an original. You might try re-doing your ground scheme based on a two star type system. All amplifier grounds on one star near the controls and another for the power supply at the main chassis ground. That should fix it.

Chuck

[EETStudent]

Ok I solved the motorboating, heres how: In the schematic, you will notice that the cathode at the first stage is not bypassed. Simply out of experimentation, I stuck a 22uF cap in there and viola, no more motorboating and I happen to prefer the slightly thicker tone. Maybe the cap helped to smooth things out a little?

[Chuck H]

Hmmm... That may have shifted the phase angle just enough to solve the problem, but it doesn't mean the ground scheme couldn't be improved to eliminate the wrong parts of the circuit cross talking. You may get similar results by just grounding the first stage cathode in a different location.

Did you follow the schem exactly, including the two prong cord and death cap? Or did you use a proper grounded power cord?

Does the amp seem to have a lot of hummmmmm? Just guessing. Fixing the ground scheme can help with this too.

Chuck

[Jazz P Bass]

22 uf bypass cap

As an aside, you do not have to use 22uf as a cathode bypass cap. It just passes "all" frequencies (kinda sorta).
I have tweaked a lot of Fender amps that have a nasty boomy, farty tone.
10uf, 4.7uf. Whatever sounds the "best".
Some Marshalls use a 1uf. The Marshall honk.
I have also seen Fender use a 330uf on there Master Volume amps. Don't know what that's all about.

[Chuck H]

As an aside, you do not have to use 22uf as a cathode bypass cap. It just passes "all" frequencies (kinda sorta)

A smaller cap may not create the phase shift needed to avoid oscillation though. But it's worth experimenting.

Chuck

[EETStudent]

The amp has a whole is pretty quiet, considering the nature of the design. No excessive noise whatsoever, no unexpected hum either. I did a lot of research on grounding schemes, and I followed the original wiring layout almost exactly.

I used a 3 prong cord, and no death cap. The thing is that the amp sounds great, with or without the bypass cap. I think I prefer the tone with the cap, so the problem is "fixed". Thanks for everyones input

[Jazz P Bass]

5F2

Job well done , heah?
I do not see a first stage cathode bypass cap on the original schematic.
Maybe you hit on something.
Maybe Leo's crew figured it out later.
Enjoy the tone:
John G