mosfet cathode follower - tone stack-fxloop buzz

[swingarm]

Greetings all!


I just finished a fender super 5e4a clone (6v6). Sounds great! used an electrohome pa-300 amp as the conversion platform. I decided to put an effects loop in too like the dmars ods using  a cathode follower to drive the tone stack and fxloop, I used a mosfet namely the irfp350. I can hear a buzz at higher volumes and  did trace it to the mosfet. This is my first time using one and used the one from merlin's preamp book. I know most use the 820 but the 350 was on hand. I'm getting 129 volts out. I have 410 on the plates. all the other voltages are close to typical for this design.


 I was wondering if anyone has run into this problem before? Is there any "best" mosfet in these wonderful current times ? Thanks for any input.

[Bieworm]

same old question/remark:

post the schematic for us to get a clear view on what the circuit you built looks like

cheers!

[tubenit]

Fender_super_5e4a_schem.pdf (el34world.com)

Yes, please show how you inserted the mosfet cathode follower into that schematic. And also include a few good resolution photos in your post.

Did you use a mosfet so you'd have a triode for the recovery in the FX loop similar to the "Minimalist"?

Are you using a 12AY7 in the V2 position?  If not, what are you using?

With respect, Tubenit

[swingarm]

Fender_super_5e4a_schem.pdf (el34world.com)

Yes, please show how you inserted the mosfet cathode follower into that schematic. And also include a few good resolution photos in your post.

Did you use a mosfet so you'd have a triode for the recovery in the FX loop similar to the "Minimalist"?

Are you using a 12AY7 in the V2 position?  If not, what are you using?

With respect, Tubenit

Hi guys,


yes to both those questions  tubenit.


I made the fx loop circuit  board after the fact.


I will post more pics a little later, thanks!


edit : Did a few things different too. I have cascaded inputs and individual power tube bias pots.
other than that its very close to the original schem. Also the Paul c mod from Rob's site


edit: actually i went off this schematic, but yours is  better. I used Merlin's mosfet follower from his book.

[swingarm]

Here a few pics. I drew a quick and dirty layout from the schematics. As I said
the fxloop decision came after it was done. Also there is a boost buck switch using the 5volt winding, a triode switch and variable negative feedback too.

[swingarm]

Here  are a couple more pics. This was a "what I had on hand" build and the donor amp was 125 dollars on Kijiji, not bad.

[tubenit]

Do you have an actual schematic that is a true representation of what you amp is?


With respect, Tubenit

[swingarm]

Thanks for your help fellas, I will sort this. Just thought I'd throw it out there in regards to the mosfet. Like I say its the super with cascaded inputs and seperate bias controls TUT style, other than tha the effects loop follows the demars. I understand its difficult on your end. Cheers!

[PRR]

The 'before' schematic takes off from the Treble pot wiper and then goes MANY places. I'm not going to squint the photo to see how much wire is hanging there. But that node is HIGH impedance and really should not be driving many things or much wiring.

I bet if you carefully poke a finger in there "live" (don't get killed) you can make it buzz worse and at many places.

[swingarm]

Hey thanks PRR, so that applies to the demars also. no?

[tubenit]

I've not had a problem with mosfet cathode follower & a psuedo FX loop recovery stage on the D'Mars amps I've built.


With respect, Tubenit