Cure for Mosquito tone in a Marshall Plexi

[Lewguitar]

We were talking about how when the volume is cranked and the treble turned up some of us can hear a slightly shrill and out of tune harmonics riding along on top of the guitar signal. I found this in Gerald Weber's new book: Tube Guitar Amplifier Essencials. There's a chapter devoted to correcting Vintage Tube Amp Design Flaws.

"On the 1959 Fender Bassman, the phase invertor layout is such that the plate resistors of the phase invertor are on the bottem of the circuit board. These connect via coupling capacitors to the 220Kgrid return resistors (going to the grids of the output tubes) which are mounted on top of the circuit board. This results in long grid wires going from the 220K resistors to the output tubes. Get them an inch to long and the amp will have parasitic oscillation!

It is interesting to note that when Jim Marshall copied the Fender Bassman he copied the layout! This is why on all Plexi Marshalls if you turn up the volume, treble and prescence you will hear a mosquito sound on top of the note. This is parasitic oscillation.

Here's the correction: Put the plate rsistors from the phase invertor on top of the board and put the 220K grid return resistors (for the output tubes) on the bottem of the board....this will shorten the grid wires by a few inches. The result is a stable amp."

This is a great chapter from a great book! Highly Recommended.

Lew

 

[the guy who invented fire]

Re: Cure for Mosquito tone in a Marshall Plexi

How cool!

I always wondered if there was a way to correct that...Thanks Lew!

 

[Lewguitar]

Re: Cure for Mosquito tone in a Marshall Plexi

Thanks!

Gerald autograped my copy at the Dallas Guitar Show.

After we almost got kicked out by the sound police for turning up Bruce's Aurora Amp, Gerald wrote in my book:

"To Lew! Better musicianship begins with Louder Amps! Kindest Regards, Gerald Weber 2006"

:laugh2: I really enjoyed being in the booth next to Gerald throughout that show. He seems like a great guy who gets critisized on the net by "techs" with a 10th of his knowledge and experience.

Lew

 

[Lake Placid Blues]

Re: Cure for Mosquito tone in a Marshall Plexi

I recall a phone conversation probably about 15 years ago about speakers. I was blown away by his knowlege and insights. It would be foolish to question this man's knowlege and expertize.

 

[Scott_F]

Re: Cure for Mosquito tone in a Marshall Plexi

I usually twist those wires as well. Then where they connect to the 2 watt resistor on the power tubes, keep that resistor as close to the tube socket as possible. That is where I found the oscillation to be worst when I had too much lead from the resistor going to the socket. Cut that down and it went away.

 

[kmcguitars]

Re: Cure for Mosquito tone in a Marshall Plexi

So, If I understand correctly? Just Invert your Phase Inverter.:6:

 

[Pepi]

Re: Cure for Mosquito tone in a Marshall Plexi

Is it just me or do you also hear this flaw in some modeling amps? :smack:

 

[Rich_S]

Re: Cure for Mosquito tone in a Marshall Plexi

Added to my Amazon shopping cart for the next order. I'll try to remember this when I layout my JTM45 clone-in-a-Bogen.

 

[shredaholic]

Re: Cure for Mosquito tone in a Marshall Plexi

But theoretically, couldn't you just use shielded wire? (I'm assuming the problem is that the longer the wire is, the more interference gets through)

I've only had this problem on my 1959 when using a PPIMV turned down really low, that hadn't been connected with shielded wire. Twisting the wire helps but it's nowhere near as good as shielding.

 

[BigDaddy]

Re: Cure for Mosquito tone in a Marshall Plexi

I just think you might have cured a problem I am having with a build, thanks!!!!!!!!!!!!!!!