Hello all,
Yes, I am sure this one has been asked before.... I am building one of the classic Princeton 5F2 circuits. Google it and will find the circuit. I have built about 6 tube amps over the past decade so this is not my first party. However, this is the first time I have had an issue with motorboating!
The circuit board is actually a PCB (from tubedepot for a 5F1 ) and not point to point. first for me...
upon first power up, I get a horrible flabby motor boat low freq sound. As I increase the volume pot the frequency increases. at full volume the feq is probably close to 18Khz!
AND yes I have tried reversing the OT primaries, and yes I have tried to removal of the feedback resistor. None fixed the issue.
So the only things I am doing differently on this build from the original fender is the following:
1. PS filter caps are 47uf, 47uf and 10uf. and the first filter resistors are 5K and 10k instead of the stock 10k and 22k.
2. The main power xfmr is a much lower voltage at about 265VAC
3. I am using a two diode full rectifier to get the DC, and the B+ is about 168vdc under load.
4. I added a Princeton tone circuit (pot and two caps)
all the rest of the circuit should be the same.
Any suggestions or help is greatly appreciated!
Yes, I am sure this one has been asked before.... I am building one of the classic Princeton 5F2 circuits. Google it and will find the circuit. I have built about 6 tube amps over the past decade so this is not my first party. However, this is the first time I have had an issue with motorboating!
The circuit board is actually a PCB (from tubedepot for a 5F1 ) and not point to point. first for me...
upon first power up, I get a horrible flabby motor boat low freq sound. As I increase the volume pot the frequency increases. at full volume the feq is probably close to 18Khz!
AND yes I have tried reversing the OT primaries, and yes I have tried to removal of the feedback resistor. None fixed the issue.
So the only things I am doing differently on this build from the original fender is the following:
1. PS filter caps are 47uf, 47uf and 10uf. and the first filter resistors are 5K and 10k instead of the stock 10k and 22k.
2. The main power xfmr is a much lower voltage at about 265VAC
3. I am using a two diode full rectifier to get the DC, and the B+ is about 168vdc under load.
4. I added a Princeton tone circuit (pot and two caps)
all the rest of the circuit should be the same.
Any suggestions or help is greatly appreciated!
Last edited:
Is it this one?
You can use clip leads to start grounding grids until the noise goes away.
Based on your description, I would be looking very, very carefully about how you have that pot wired and how well connected the ground connection is on said pot.
You can use clip leads to start grounding grids until the noise goes away.
Based on your description, I would be looking very, very carefully about how you have that pot wired and how well connected the ground connection is on said pot.
schematic
yes, this is the circuit basically. except like i mentioned i added a tone circuit from the 5f2 circuit. and i used this pcb
5E1 / 5F1 Tweed Printed Circuit Board
| TubeDepot.com
yes, this is the circuit basically. except like i mentioned i added a tone circuit from the 5f2 circuit. and i used this pcb
5E1 / 5F1 Tweed Printed Circuit Board
| TubeDepot.com
I also might add that this motorboating occurs when I have a guitar cable plugged in. unplug the input and I get a sharp squeal from the speaker! and I have tried grounding the peamp grids and all I get is more interesting sounds...
If you're not going to ground out the grids of the first two triodes, just remove them and power up the amp with the 6V6 and 5Y3 and let us know what you get.
I have already tried reversing the OT primary side. No effect. and I also already tried to remove the feedback 22k, no effect.
I have also removed the 12ax7 and powered it up and the motorboating goes away. so not sure if this means the issue is in the preamp section or not.
I have also removed the 12ax7 and powered it up and the motorboating goes away. so not sure if this means the issue is in the preamp section or not.
not sure what you mean by 43 ohms??? which resistor? I don't see a 43ohm anywhere in the circuit?
okay I just double checked my 10K resistor (marked R12) who ever replied and said it was a 43ohm. you read it backwards! you read it left to right. brown, black, orange (and the yellow is wattage)
Yes, I was backward, I realized it in minutes, removed my post. Sorry you saw it.
The circuit is well-proven. Too-small resistor there would be unstable; but I saw it wrong, and you have proven it right.
This gets down to *maybe* a bad filter-cap, or the ever-popular wiring error which is either very well hidden or too big to see.
The circuit is well-proven. Too-small resistor there would be unstable; but I saw it wrong, and you have proven it right.
This gets down to *maybe* a bad filter-cap, or the ever-popular wiring error which is either very well hidden or too big to see.