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The 70W model from 1978 with master volume and push/pull boost had a huge power transformer and big filter caps.It reminds us more of a classic 80w Twin Reverb than a Bandmaster Reverb, and it shared its circuit design with the Pro Reverb and Super Reverb at that time.Yes of course some have the mid knob and bright switch while others don’t.All these ab763 amps with reverb has the gain stage in the reverb recovery circuit which the non-reverb amps don’t have.The Bassman has a slightly bigger output transformer resulting in a firmer tone and more attack.A smaller output transformer will introduce sag and compression in the power amp section.
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The vibrato channel in the Bandmaster is even more clean than the Bassman because of the vibrato circuitry loading the signal chain and reducing the gain level in the premp section.
Hence, the AA864 Bassman normal channel has more preamp “juice” and reaches the sweet spot at an earlier volume knob setting.
This mod does not alter the tone significantly when amp is played clean or when only the power amp section distorts.
You’ll have to increase the volume setting to achieve a similar volume as before.