Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: vampwizzard on April 23, 2020, 01:44:56 pm
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Hey folks. Looking at a Gibson/Maestro GA2-RVT for my next hammond conversion and am curious to part of the reverb circuit. Schematic here: https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga2rvt.pdf (https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga2rvt.pdf) and ive attached the part im interested in. Why did they tie this back in the middle of a network of so many RC filters? I dont see that on other RVT models.
Is that just for the voicing of channel 2?
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just for the voicing
I'd agree, looks like Gibson's way of "mixing" the dry signal into the verb channel.
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the circuit looked familiar, looks like the VOX circuit (15 30), not sure who zoomed who :laugh:
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That's the C/R C/R C/R filter that provides frequency-selective delay for the modulator part of the circuit on the vibrato channel. (Its coming off the plate of the stage* where the trem LFO wave is injected into the signal path). Vox AC15 and AC30 have the same thing in their vibrato channel**.
*that also happens to be the input stage for the vibrato channel.
** the modulator on the AC15/30 gets mixed at the PI
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My mind is blown. Shouldve seen that.. wasnt even a consideration. I was thinking this is what inspired the $wart AST which everyone seems to think sounds more tweed like than AC15. If you chop channel one you save 2 triodes and then consolidating a half 12ax7 with a 12au7 into the 12dw7.. does that make sense to anyone else? This of course assumes using 12ax7's in place of the 6EU7's
Now I have to build it!
edit: not quite the reduction in triodes i was counting. the research continues.
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each .005uf / 1Mohm stage drops any freq. below 32Hz by -3dB. So jangly guitar signal (above 32Hz) passes each stage happily free of attenuation while low LFO rumble gets smaller and smaller with each stage.
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I don't think "delay" comes into it.
That type tremolo THUMPs. Some other amps just let the power amp and speaker thump/flap. This amp choose to try to cut subsonic thump with a many-pole high-pass R-C filter.
At some point reverb was added. Logically tapped with the main-path Volume pot. But the reverb path has plenty of bass-cut also. Maybe for simplicity, maybe for cheapness, they used the Rev pot as one of the bass-cut parts.
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My reasoning for following this amp schematic as inspiration for an SVT appears misguided but the question about the reverb/trem are still well founded and im happy for the answers here.
The reverb on this is the very familiar xmfr topology.
For those in the future looking for the 12DW7 reverb I was after, see this link: https://el34world.com/Forum/index.php?topic=21991.0 (https://el34world.com/Forum/index.php?topic=21991.0)