PlayJazzGuitar.com Forum

[randyc]

Here's an interesting project, one to which I've alluded in past posts. I'm a big fan of the Fender Champ amplifier, especially the silver-face variety with better EQ and more output power than the original. The advantage of this amplifier is small size and portability. Disadvantages are inadequate power (and headroom) coupled with the small, limited range loudspeaker of non-standard impedance (3.2 ohms).

Since the Champ is so easy to work with (there are large amounts of unused real estate inside the chassis), I've tinkered with mine often. It's been thoroughly manipulated/revised with no audible performance improvement. Some improvement resulted from replacing the speaker and output transformer with a larger 10 inch speaker of normal impedance (8 ohms), as in a Fender "Princeton". (This project has been posted before, I apologize for the repetition but in order to get to the end of the story, it's necessary to start at the beginning and chart the evolution of my amplifier.)

Replacing the speaker wasn't a trivial task, although replacing the transformer was simple. Here's a photo of my Champ with the back removed. The outline of the old 8 inch speaker cut-out is visible, replaced with a piece of plywood cut to fit the hole, epoxied into place and painted. (Note that the 10 inch speaker had to be moved off-center in the enclosure to clear a large electrolytic capacitor mounted on the chassis.)



After replacing speaker and transformer, a listening test left me wanting a more balanced response - the Champ always lacked bass. I measured the frequency response of the new configuration then experimented with partly covering the back of the enclosure until I arrived at this variation, which has a satisfying amount of bass and midrange. One of these days, I'll get around to painting that piece of cedar.



The amplifier as shown with the factory electronics is fine for practice and even for duo/trio performance (no drummer) in small venues. Once a drummer is added, the Champ is impractical. Appreciating the convenience of this small unit, I wondered about increasing the output power. I listed the implications of bringing the Champ up to "Princeton" performance levels (around 12 watts) from the measured 5 watts produced by my Champ at 5% distortion.

-The Champ is a single-ended amplifier, doubling the power implies a push-pull output circuit, requiring two output tubes and a phase-splitter tube

-Extra tubes = increased current consumption = bigger power supply transformer = $$

-Need to cut holes in chassis for additional output tube and phase-splitter tube

-Extra output power means that extra gain is required (increasing the power level from 5 to 12 watts means about one and one-half times more voltage gain)

I thought about these things, I wasn't in a hurry and wanted a simple, easy implementation (and one that was readily reversible to restore the original configuration). First, addressing the increased power consumption, I looked up the Fender "Princeton" schematic on the internet and found that the power transformer was the same used on the Champ! Obviously the power supply could accomodate the extra tube in the power amplifier and the phase splitter tube.

Then I considered addition of the phase-splitter tube. The purpose of the phase-splitter is to divide a single signal into two signals with opposite phasing. It's not necessary to use a tube for this function, it can be done with a small transformer. Since no appreciable power is being transferred, the "interstage" transformer is inexpensive and easy to obtain. I found a 1:3 center-tapped transformer for a few dollars. Only two small holes need be drilled in the chassis to mount the transformer. The transformer eliminates one tube and the filament/plate current required to power it.

Addressing the power amplifier stage, I could have replaced the single 6V6 with a single 6L6 plus a few component changes to increase the output power of the single-ended design but that approach would need about 120 milliamperes plate current, requiring a power supply redesign and more $$$. (Push-pull Class AB and B amplifiers are much more efficient, using a pair of 6V6 tubes requires about 70 milliamperes which is within the capability of the Champ power supply transformer.)

Some design changes are required, adding more voltage gain for example. Happily this isn't too difficult and accommodated by changing a few resistor values. Adding the extra transformer in place of the phase-splitter tube does create a disadvantage. Transformers have limited bandwidth and consequently more phase-shift than any other circuit in the amplifier except for the tone controls. And excessive phase shift can cause oscillation if one doesn't account for it.

The solution for this problem was fairly simple. Frequency compensation was added to the two output tubes and to the feedback loop, three more capacitors added to the circuit. Here are the schematics (excluding power supply) of the unmodified Fender Champ and the "Princetonized" version.



Since I liked my Champ in original configuration and didn't want to punch holes in its chassis, I deviated from the planned implementation of the design. I "bread-boarded" the output stage of the amplifier on a universal test chassis (home-built and useful for various tests and design verification). This chassis is small enough to fit comfortably in the bottom of the Champ cabinet, but power supply leads, speaker connections and a shielded cable to the interstage transformer must be routed from the main chassis.



(In the above photograph, the test chassis is configured with other tubes for evaluating a different circuit, however the interstage transformer and output transformer are the ones that I bought for the Champ modification.)

The modification was fun, informative and successful. The measured output power was about 12 watts at 5% distortion. Computer circuit simulations suggest that the output stage is capable of 20 watts (if cathode bias resistors are eliminated and a negative voltage is used to bias the grids of the output tubes). That happens to be my personal recommendation for the minimum amount of power required to play with a jazz drummer.

I'm not actually recommending this project for anything but reading entertainment. For most people it would be far simpler to buy an amplifier with adequate output power. I did this from curiosity as to how far I could extend the Champ concept and I was satisfied with the result.

cheers,
randyc

edited to add: the following number of extra components were required: 1 6V6 tube and socket, 1 interstage transformer, 2 capacitors and 3 resistors, the 10 inch speaker and the output transformer. Total estimated cost excluding labor: $95 at the time (2006)

[sunflower]

For interest Champ II

http://www.ampwares.com/amp.asp?id=44

never tried one tho

[randyc]