Fender amp technical question

Dave Zirbel · From: Sebastopol, CA USA

Hi gang! I have a 1969 Super Reverb head that had a new transformer put in it (the one near the middle of the chassis). I know that normal load on these amps are 2 ohms. The person I got it from said he believes it was fixed to take a 4 ohm load. I took the chassis out of the cabinet and on the transformer it was written in pen:

G-2 Y-4 O-8
OHMS

exactly like that. Can anyone translate? It sounds twice as loud when I use a 4 ohm speaker as opposed to the 8 ohm. I just don't want to hurt this amp. My guess is it takes 4 ohms.

Thanks, Dave

Ken Fox · From: Nashville GA USA

Sounds like the wire colors for the transformer secondary and related load impedance.

Green 2 ohms
Yellow - 4 ohms
Orange - 8 ohms

These colors do not match up with a Fender part# 0108343, the current and correect replacement outpit transformer for a Super Reverb.
Typically a Super Reverb has 4 speakers at 8 ohms in parallel, for a total load of 2 ohms. I have seen some later models with 4 each at 16 ohms, for a load total of 4 ohms.

[This message was edited by Ken Fox on 24 January 2003 at 08:19 PM.]

Gino Iorfida · From: Pittsburgh, Pennsylvania, USA

Sounds like someone put in a Marshall 50w output transformer as a replacement. However, the color codes would be reversed unless they wired the output backwards, which is quite possible. Either case the impedance in any position may not be a PERFECT match for 6l6's.

Try to keept he impedances matched as well as you can, and if you are happy with the tone, great, if not, I'd recommend getting a good Fender replacement in there. Without checking prices, I'm estimating that it would be about $100-$120 for the correct transformer.

Dave Zirbel · From: Sebastopol, CA USA

Thanks guys. The transformer was made by Mojo Tone. There is a sticker on it that says
26L6.

So if I were to take the chassis out and look inside, the color of wire being used will determine the load, right? Two wires would be capped. Can you put a swith in there to change the ohms and use different speaker configurations or do the circuits need to be changed as well?

Dave Z

Ken Fox · From: Nashville GA USA

I saw a Bandmaster Reverb with this same setup. It had the external speaker jack removed, a rotary switch put in the remaining hole and allowed the amps output to be 2,4 and 8 ohms. Just be sure it is a switch with contacts rated for the wattage of the amp. For a 40 watt amp:

Power = (voltage)squared/resistance
voltage = square root of power times resistance

E (voltage) = square root of 40 watt x 2 ohms
E= square root of 80
E= 8.95 volts RMS

also:
Power = volts times amps
Amps = power/volts
Amps= 40/8.95
Amps= 4.5

Contacts rating is a minimum of 4.5 amps. I would use a switch with at least 10 amp rated contacts.

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