Saturday, October 27, 2012

Irrational Amp Tinkering

This Champ is ridiculous. No sane person would ever spend so much time and money on such an inexpensive small amp, would they? It is irrational.

I got the amp in Sacramento in 2005 from the estate of a woman who bought it new in 1971 along with her new electronic organ. The amp sat in her living room, occasionally singing church music, for 35 years. The picture above was taken the day I brought it home.

It was the coolest little amp I had ever heard for harp, but I just could not leave it alone. First I tried several new input tubes, and that is when I learned that the 12AT7 tube is a terrible tone generator and the 5751 is great. The Champ is a good test bed for things like that because its circuit is so simple.

Next I replaced the speaker with a Jensen reissue P8R. Hated it. Then I altered the baffle so it would accept a 10-inch speaker and tried a Jensen P10R. Hated it. I tried several other various speakers and finally settled on the Weber 10A-1250 with H dustcap. Now we were getting somewhere. The Champ settled into that configuration for several years. 

Then one day I lost my mind and decided to trick the amp out more. (Read: spend more money on it.) 

First I installed a pot in the negative feedback circuit so I could adjust it to find the optimal mix for crunchy tone. I discovered the optimal mix was ZERO, so I removed the NFB circuit and the pot (which was mounted on the back part of the chassis) and used the hole where the pot had been to mount a line out jack.

More tube swaps followed, finally ending with the 5751 in the input section, a vintage black plate GE 6V6 in the power section, and an NOS “Coke Bottle” Sylvania 5V4G rectifier. Again, the Champ settled into this configuration for some time. It was sounding GOOD.

But not good enough. So I asked Bruce Collins at Mission Amps to voice the tone stack for harp using NOS Soviet K40Y paper in oil tone caps. Big improvement, but I wasn’t finished quite yet. A few months later Bruce installed a Hammond 15-watt output transformer and a choke. There! The amp was finished. A sleeper amp, with big punch and crunch in a small package.

Which brings us to today. Before guitar player Steve Mignano headed off to tour with Cassie Taylor and the Soul Cavalry he gave me a Warehouse Veteran 10-inch speaker and said, “It crunches like a motherf---er.” Hmmmmm, this I had to hear.

I looked around for a 10-inch cabinet and finding none I decided to swap it into my Champ, removing the nice Weber alnico speaker. The Weber speaker made the Champ darker and warmer, and Steve was right: The Veteran speaker crunches like a mutha with a brighter, raspier tone. It’s fun. Ratty!

I really don’t know how much I have invested in the Champ now, but it is a ridiculous amount – certainly more than the value of the amp. I have learned more about harp amp tone from the Champ than from any other amp. It is now the amp I have owned longest. Purists might sneer at me for altering a classic but it has been a blast. And the amp is a little beast.

Choke and 15-watt transformer

Silver bullet-shaped Soviet military PIO caps

Rectifier and power tube, and Veteran speaker

The Champ at work

No comments: