Was trying out my 400RB to power amp my SGT-DI today and noted that the RB’s power amp volume was affected by both the master volume and boost circuit knobs. Does anyone know what knob positions of these two offer unaltered full power amp volume?
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GK 400RB Effects Return
- Thread starter Lowbrow
- Start date
- Feb 12, 2006
- 60,951
- 5
- 220,594
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
Not sure which version you have. Here's a vintage one.
Image from: Invalid Link Removed
The Boost control is after the preamp, tone network, and Effects Loop (see block diagram below). The Boost control feeds the boost circuit. The boost circuit is connected to the Master.
Think of Volume as input gain. This is adjusted to get a nice hot signal through the tone network. If you turn it up too much, you may get some harsh distortion.
It's important to grasp that the Boost control and boost circuit are two different things. The Boost control adjusts level and the Boost circuit responds to level. Volume, tone controls, and the Boost control effects the level going to the boost circuit. For example if you turn up the Volume and turn down the Boost control, the signal hitting the boost circuit can stay at the same level.
I couldn't find a block diagram for the vintage amp. This is from the Mk IV, so there may be some variation...but it's close enough for the point of discussion.
I would start with the Boost control all the off (fully CCW). Try turning up the Volume until you clip the front end of the preamp, and then back it off so the signal stays clean. Next adjust the Boost control for the desired amount of compression or drive. Finally adjust the Master for the desired output level or loudness.
If you want the most dynamic and clean sound possible, turn Boost all the way down. However if you push the amp to the limit, you will still get compression and drive from the output section. By varying the Boost and Master controls you can create different compression envelopes and distortion characteristics.
Getting a good signal to noise ratio with a vintage amp may be challenging. Experiment and do the best you can.
Image from: Invalid Link Removed
The Boost control is after the preamp, tone network, and Effects Loop (see block diagram below). The Boost control feeds the boost circuit. The boost circuit is connected to the Master.
Think of Volume as input gain. This is adjusted to get a nice hot signal through the tone network. If you turn it up too much, you may get some harsh distortion.
It's important to grasp that the Boost control and boost circuit are two different things. The Boost control adjusts level and the Boost circuit responds to level. Volume, tone controls, and the Boost control effects the level going to the boost circuit. For example if you turn up the Volume and turn down the Boost control, the signal hitting the boost circuit can stay at the same level.
I couldn't find a block diagram for the vintage amp. This is from the Mk IV, so there may be some variation...but it's close enough for the point of discussion.
I would start with the Boost control all the off (fully CCW). Try turning up the Volume until you clip the front end of the preamp, and then back it off so the signal stays clean. Next adjust the Boost control for the desired amount of compression or drive. Finally adjust the Master for the desired output level or loudness.
If you want the most dynamic and clean sound possible, turn Boost all the way down. However if you push the amp to the limit, you will still get compression and drive from the output section. By varying the Boost and Master controls you can create different compression envelopes and distortion characteristics.
Getting a good signal to noise ratio with a vintage amp may be challenging. Experiment and do the best you can.
- Feb 12, 2006
- 60,951
- 5
- 220,594
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
Yes, mine is the version pictured here (had it since maybe ‘93.) Excellent description of how this amp operates and the boost function in particular. Question: does a similar circuit exist in a modern amp for comparison?Not sure which version you have. Here's a vintage one.
True, and many thanks for your advice.
The boost circuit is pretty much a single FET that is set up to emulate a pushed tube. GK has described this as Gate Induced Valve Effect (G.I.V.E.) This circuit was a feature of the RB series, which has been discontinued.
The Legacy series appears to have something that is similar.
From the Legacy webpage:
I have not seen a schematic and AFAIK the Legacy manual does not include a block diagram. So that's about all I know.
Thanks. I owned a Legacy 800 and will say the amp’s Drive circuit is really quite unlike the RB boost, and from reading the marketing description it seems to be more ‘in the spirit of’ the RB boost. It’s a full-on drive channel with multiple controls for drive effect level, drive signal level, drive tone (labeled ‘edge.’)
It's been long rumored the GK will eventually bring out a new RB series. No idea on the timeline.
