Tuesday, December 23, 2008

Fender Champion 600 Output Transformer

Another difference between the new Fender Champion 600 and the older Fender Champ 5E1 and 5F1 circuits is the higher supply voltage (also called the B+ voltage) and an accompanying higher impedance primary on the output transformer in the Champion 600. I've never measured an original myself but replacements like the TF103-48 have a 5K ohms primary impedance. My measurement for the Champion 600 transformer at 1kHz is about 11K ohms.

I was a bit surprised by this so I called Mercury Magnetics to ask them about the impedance of the output transformer in their C600 Champion Upgrade Kit. They said their measurement was around 9K for the original and that they raised it a bit to 10K for the upgrade.

I don't have the details for their measurement, but here's the details of mine:

The tone generator (center) is set to input 1 volt @ 10Khz into the secondary of the output transformer (shown on the left side meter). The right hand meter shows the voltage output on the primary with 1 volt on the secondary. So here's how the primary load is calculated.

A transformer impedance ratio is equal to the square of it's voltage ratio.
Here the voltage primary to voltage secondary is 59.4:1
59.4 squared is 3,528
So for a 1 ohm load, the load reflected at the primary would be 3,528 ohms.
But the Champion 600 doesn't have a 1 ohm speaker for the load, it's 4 ohms.
That will raise the the impedance reflected at the primary 4 times.
So 3,528 times 4 is 14,112 or about 14k ohms.

But didn't I say I measured about 11K?

Yes - but at 1kHz. The above measurement was for 10Khz.
Transformers (especially cheap ones) are imperfect devices and their response varies with frequency.

Below is the same procedure, but at 1k.

With 1 volt on the secondary at 1 kHz, the reading on the primary is 52.94 volts (rounded to 53).

Here the voltage primary to voltage secondary is 53:1
59.4 squared is 2,809
So for a 1 ohm load, the load reflected at the primary would be 2,809 ohms.
So 2,809 times 4 is 11,236 or about 11k ohms.

Here's the reading at 100Hz:

With 1 volt on the secondary at 100 Hz, the reading on the primary is 44 volts.

Here the voltage primary to voltage secondary is 53:1
44 squared is 1,936
So for a 1 ohm load, the load reflected at the primary would be 1,936 ohms.
So 1,936 times 4 is 7,744 or about 8k ohms.

From this you can see that if you load the 4 ohm secondary of a TF103-48 with a 8 ohm speaker you raise the primary load to 10k - same as the Mercury Magnetics. Of course there's a whole lot more to a transformer than just the loading, but if you need to replace the OPT in a Champion 600 and are short on dough an 8 ohm speaker on the 4 ohm tap should get you back in business.

UPDATE: For comparison, I've posted measurements for an AB764 Vibro Champ output transformer here.


Fernando (Barcelona) said...

once again extremely useful, a million thanks!

Anonymous said...

... and what happens changing the OPT from stoch to TF103-48 but use the stock speaker with a load of 4 ohms... that means 5k for impedance. Does this work with the champion 600 or will the 6V6 need a different bias-resistor?

Wayne D. said...

Any specs for someone looking to replace the OT on a Champion 600 with better transformer and retain a 4 ohm load?

akavalve said...

Well, better is subjective to a certain extent. On the relatively inexpensive side, you could try the Magnetic Components 40-18030. It gives a 5k and an 8K primary with 4 and 8 ohm secondary. That would let you switch primary taps so you could hear the difference with a higher plate load. I haven't tried it myself but I've used their transformers in other repairs and been quite happy.

Anonymous said...

TF103-48 and 40-18030, which are much better this replacement the stock trans?

Tiberiu Ciocirlie said...

I have installed the ClassicTone 40-18031 OT and the amp makes a high pitch noise all the time. I used the 8k lead of the transformer.. what's the matter? thanks

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