VCB/VVR installations - cathode biased only
barry (Administrator) 06-30-2015 at 11:00 PM.
Post: #1
If you spend any time browsing amplifier building forums on PowerScaling and/or VVR you will quickly find that there are many ways to add it to an amplifier. I will use this thread to show and/or describe typical installations.

Note: From October 2015, the Amp Maker version of VVR for cathode-biased amplifiers is known as VCB. It uses a circuit identical to VVR, with a slightly revised layout. It's functionally identical.
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barry (Administrator) 06-30-2015 at 11:09 PM.
Post: #2
Scaling the whole amplifier
By far the simplest approach is to interrupt the power supply just after the rectifier, splice in the VCB/VVR with the output of the VCB going to the rest of the amplifier circuit.

Here's an example, where a customer has added a VVR board to his WF-55 amplifier:


If you look closely and compare this to the standard WF-55 layout, you can see the following:
* the red (B+ In) and pink (B+ Out) wires run from the VVR board to the turret board
* the two turrets where these wires connect were previously connected together
* there's a black wire (Ground) from the VVR board to the ground lug by the rubber grommet near the middle of the chassis
* this customer decided to mount the MOSFET to the chassis and connect its pins with very well-insulated wires

Once installed, the VVR knob controls the high voltage (B+) supplied to the whole amplifier circuit.
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barry (Administrator) 06-30-2015 at 11:30 PM.
Post: #3
Scaling only the power section
A different approach is to control only the voltage supplised to the power amplifier and keep the preamp at full B+ voltage. Some builders prefer this, feeling that the dynamics of a fully-scaled amplifier can lead to excess compression.

I took this approach in the N5X amplifier, and you can see the full schematic here:


There are a few things to note:
* The VCB circuit is made up of VR6, R25, R24, Q1, R23, D2 and D1
* C16 and C17 smooth the rectified supply, which then splits into two: one circuit for the full B+ and the other circuit for the VCB
* The preamp is fed by the 'C' node in the power supply, and the current path for that completely bypasses the whole VCB circuit
* The power valve is fed the post-VCB output via the 'A' and 'B' nodes
* There are TWO bleed resistors - one to drain the capacitors in the power section and one to drain the capacitors in the preamp


In this example, as VCB was planned in from the very beginning, C9 and R14 are added. This ensures that the Master Volume pot and tone controls do not sound 'scratchy' at lower VCB settings. (And no such R+C additions are needed for the amplifier's Gain control nor the guitar's controls because the preamp supply is always at full B+.)

Note: When you take this approach, bear in mind that the preamp B+ voltage may actually increase a little when you lower the VCB control. This may seem odd, but there's a very logical explanation. It happens because at lower VCB settings, there's less load on the power transformer. In normal use, it's a relatively small increase, but it's worth checking in case you need to increase the voltage rating of your preamp power supply capacitor(s).
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barry (Administrator) 07-06-2015 at 10:38 PM.
Post: #4
Here's another photo sent in by a WF-55 builder who decided to use the VVR circiot to control the voltages in the whole circuit. It shows a slightly different layout to the one above, but it has been added to the circuit in exactly the same way.

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