Simple Tools: Detecting Oscillation Above The Audible Range

There are a lot of folks around who think that a scope and a signal generator are the answer to every problem large or small, and you should have one before you go to brush your teeth or eat breakfast.....where was I? Oh, I remember.

On a website which shall remain nameless, a person had a problem that was related to tube rattle and a poster suggested it was "hyper sonic oscillation" and that the guy had to have a scope and a signal generator to suss it out.

Problem is, most amp owners, unless they're going to get into the trade, don't have a scope or a signal generator and wouldn't know what to do if they did have one.

Here's what I answered.

You don't need a scope and a signal generator to detect any high frequency oscillation above the audible range. All you need is a little common sense and some simple tools.
Here is what you do.
Hook up your bias checker and start at a low level with no signal input. While you're doing this go and get a transistor AM radio with an antenna (remember them?). Tune it between a couple stations and turn it up about half way. Lay it on top of the amp with the antenna extended.
Then advance the volume in your amp until you're reached full tilt.
If there is any HF oscillation going on two things will happen.
First, your bias current will go crazy and all the frequencies radiated by the oscillation will make your transistor radio hiss like a cobra.
I figured that out all on my own while building a tube matcher and have since used that trick on several amplifiers. I'm sure the old radio men knew all about it too. 

The moral of the story is this. All of the problems we have with guitar amps today have existed ever since there have been amplifiers with tubes. Most of the old radio men like Everitt Lincoln have passed on to the great ham shack in the sky, but they knew that there were plenty of ways to skin any cat you might come across. This is one I figured out on my own but I am sure they were using it too.

Revoicing a middle seventies Fender Bass Amp

Back in the early seventies Fender produced a number of eminently forgettable amps based on Super Reverb architecture. The Bassman 10 is one of them, and the Bantam Bass is another-which is the subject of our disquisition.

The original CFA 7003 Bantam came from Fender with, of all things, a trapezoidal shaped Yamaha speker with a styrofoam cone.  Think I'm kidding? That was the day that the crack truck collided with the LSD wagon and both overturned outside the Fender factory no doubt.

To say the Bantam in its 35w configuration was not a commercial success may qualify as the understatement of the twentieth century. The speakers lasted a week or so.

However we're not here to second guess what some marketing mavens cooked up out of spare Super Reverb cabinets and chasses. Yes, Virginia that is the plural of chassis.

The one I have came to me with a Jensen P15L speaker installed and a Fender Twin Reverb output transformer, which made it into a passable amp for guitar with more volume. Kinda like the Fender Pro they never built.

The P15L was not a great speaker although being well able to soak up the extra 20w or so of power being generated. Gerald Dishon, my accomplice, produced a JBL for me from only he knows where, and that put the thoughts of crossover networks to rest.

I hauled it out yesterday for some noise and I said "This just has to sound better." I had revised the normal channel of a friend's Bassman 10 to look like standard AB763 a/k/a Super Reverb.

On my own Bantam which is the subject here I went a little farther. After routine maintenance I gutted the normal and bass channels and built them to look like an AB763 Super Reverb on both sides. The easiest way to do this is to just remove all the old stuff, nobody'll miss it and it only takes a few minutes. Then following the AB763 layout you can duplicate all the tonal stuff as you wish.

One thing I've been puzzling over is whether, since I have plenty of room for expansion as the real estate boys say, if I should incorporate a second gain stage or a vibrato circuit stage before the phase inverter gets involved. I'll mull it over tonight.

Film at 11 as they say.