5F1 Motorboating

[TIMBO]

Hey guys,
I've had some 5F1 PCBs made and having a problem with motor boating sound when the volume is maxed with no guitar plugged in.
Thought it might be the power valve plate wire needing to be reversed, no joy
Pulled the 12ax7 and the volumed maxed and no motorboating
So I narrowed it down to the 12ax7.
Replaced the 12ax7 with 12au7 and volumed could be maxed with no problem.
Replaced the 12ax7 and put a 100k grid stopper between the pot wiper and the grid of the second triode, worked great no motorboating.
Thinking the second triode was getting hammed so removed both 100k grid stopper and 25uf bypass cap on the first 12ax7 triode, this also worked great.
The PCBs followed the layout and schematic of Rob Robinetts 5F1 which has a 25uf bypass cap on the first triode
Fender schematics don't have this cap.
I know many have built Robs 5F1 with no problems

Any thoughts...

[Latole]

Motorboating (amp sounds like a boat engine) is usually caused by a weak or damaged power supply filter cap.

 You can temporarily alligator-clip an extra filter cap in parallel with the caps, starting with the first filter cap. If the temporary cap fixes the problem then replace the original cap. Also verify the power supply voltage dropping resistor values.


https://robrobinette.com/Tube_Guitar_Amp_Troubleshooting.htm#Weird_Noises

[tubeswell]

Motorboating (amp sounds like a boat engine) is usually caused by a weak or damaged power supply filter cap.

Usually the smoothing cap

[sluckey]

motor boating sound when the volume is maxed with no guitar plugged in.

Are  your switching jacks wired correctly?

The PCBs followed the layout and schematic of Rob Robinetts 5F1 which has a 25uf bypass cap on the first triode
Fender schematics don't have this cap.
I know many have built Robs 5F1 with no problems

Hoffman's 5E1/5F1 also shows a 22µF but he qualifies it with a note that says "5E1". The original 5E1 does have a 25µF bypass cap. I bet that if you just lower the value of that cap to something like 1-5µF that the motorboating would go away.

Personally, if I were building a 5F1 I would not use the bypass cap.

[TIMBO]

Thanks for the reply's
I'll check it out