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100W OT in 50W amp

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7/14/2005 7:29 AM
Tubefever	100W OT in 50W amp I have a 100W Tonemaster output transformer in a 50W AB763 clone. The amp sounds awesome, but bass heavy. Is the OT causing this? Angela sells Prosonic 50W OTs--should I replace it with one of these?

7/14/2005 9:35 AM
Chris@CMWamps	Hi A stock AB763 circuit is already bass-heavy but a bigger OT can enhance that a little bit. In general: a bigger OT can handle the low-end somewhat better/nicer/tighter then a smaller one. You could try lowering some coupling-caps so maybe you don't have to purchase another OT. Hope this helps, Chris Love, peace & loudness http://www.CMWamps.com

7/14/2005 9:43 AM

Generally a bigger OT will pass more bass (up to a point), so it probably would cut down the bass if you replaced your OT with a Fender-style 50-watt OT.

But the AB763 circuit itself is very bassy. So intead of replacing the OT, it'd be easier and probably cheaper to cut down the bass response by swapping some capacitors in the circuit. Try replacing some .1 coupling caps with .047 or .022, and replace some 25 uf cathode bypass caps with 10, 5, or 1 uf. I'd do it one cap at a time until I was happy with the bass response.

I assume you're using the same size filter caps as in the original circuit, but if you're using bigger ones, then replace them with stock values.

If you eliminate any woofiness and flubbiness just by replacing those caps, the amp might sound pretty cool with that big output transformer. You could end up with a big-sounding yet clear bottom end.

Shea

7/14/2005 10:27 PM
Tubefever	Who says you guys aren't the best!

7/15/2005 7:41 PM
Don Symes	Would you suggest starting from the input triode or the PI coupling caps?

7/16/2005 2:11 PM

quote:
"Would you suggest starting from the input triode or the PI coupling caps?"

Me personally? Hard to say. I don't have a reliable, systematic approach for figuring out which caps to change. I just sort of pick one that seems kind of big for the function it's serving and reduce it. Sometimes that gets me close to the desired result, so I move on to another one, or quit if I'm satisfied. Other times the overall tone will be too thin sounding, so I'll change that cap back to the original value or something in between and move on to another one. So it's pretty much just trial and error when I do it.

Someone who's more technically adept could model out exactly how each change will affect the bass response. What makes it kind of complicated is that each change not only affects the knee frequency of the lf rolloff at that stage, but also how steeply the bass is rolled off through the whole circuit.

All that said, I'd probably start with the pi, but your mileage definitely may vary.

Shea

7/17/2005 9:53 AM
Balijukka	Remember that changing coupling caps affects the overdrive characteristics of the following stages. Jukka

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