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Anyone understand exactly how the VRM impacts the circuit enabling such variations in breakup. I’m loving the variability, but am trying to understand the engineering behind it. How does the change in circuit voltage impact tube performance and distortion?

-Brian

Note: MIM Telecaster upgraded with Fender Pure Vintage 64 pickups is great match with Tweed mode. Only thing holding me back now is talent and experience!

OK…..so I’m trying to figure this out….is the VRM modifying plate voltage which increases or decreases plate dissipation and impacts at what level clipping and distortion starts?

The VRM is a variation of the VVR, which I think was popularized by Dana and Ritchie Hall over on 18watt.com. Hall Amplification used to sell the little VVR boards for about $30, and I still have a few in the parts drawer.

Basically what this does is takes your B+ (rectified high voltage) to a circuit that essentially "dumps" some of the voltage through a mosfet to the chassis or other heat sink. I have no idea how a mosfet works, but it is my understanding this extra voltage is released as heat through the chassis.

This "scales" the voltage of the entire amp down but basically keeping all the "proportions" the same. VVR preserves to tone fairly well until (in my opinion) you get into lower settings.

One of the options with Hall's VVR instructions was to scale only the power amp, keeping the pre-amp "stock". While I never implemented this in any of my builds, it is thought to preserve the tone of the amp a lot better especially at lower settings.

The VVR (and VRM) are for cathode-biased amps. Hall did sell a board for fixed bias but I've never seen/used it.

Thanks PharmRock. Sounds similar to the Variac made famous by the one and only Edward Van Halen.

I just Googled VRM and found this:
https://www.ampmaker.com/shop/vcb-vvr-v ... ntrol-kit/

"VCB is a method for controlling the power supply voltages within a valve amplifier. By controlling this voltage, you can control the amplifier’s power output. VCB provides a continuously controllable voltage, and you can rotate a knob to get as much or as little power output as you like.

The reason why this is a popular upgrade for valve amplifiers is that you can get the amplifier’s natural power valve distortion at much lower room volumes. Just dial the voltage down and the amplifier’s maximum volume drops. VVR was designed and refined by Dana and Richie Hall of Hall Amplification. The Amp Maker version is called VCB, and this version, VCB1, is for cathode biased amps. It’s been successfully fitted to many amplifiers, from Champs, Deluxes, 18Wers, AC30s, etc."

that's cool that they still make them. You can also buy the stand-alone VRM from Trinity. What I like about Trinity's version is that it is switched, so it essentially becomes the on/off switch as well as the voltage scaling knob. https://www.trinityamps.com/product/voltage-regulation-module-vrm/


The VRM is a bit different than a Variac, as a Variac scales the AC voltage coming INTO the amp. I think EVH dropped the voltage down to 90VAC or so to get his brown sound.

The "problem" with scaling the line AC is that there is a lower limit to how far you can go before the amp stops working. For example, the heaters need to operate at a certain AC voltage (~6.3VAC). Scaling the B+ with a VVR/VRM/VCB works only on the DC voltage WITHIN the amp after the rectifier.

These transistors act as electronic switches, turning on and off rapidly to regulate the voltage supplied to the CPU.
Mini Crossword

What a worthy way it is! I definitely try it.

This is a pretty good option when scaling up a power amplifier, keeping the preamp "stock" intact. But the way to do it is quite complicated.

What do you think about reducing the voltage to about 90VAC to get the brown tone? Is that stable and are there any dangers?

Thanks for your advice about it, I will try it
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I have that feeling too

that's cool that they still make them. You can also buy the stand-alone VRM from Trinity. What I like about Trinity's version is that it is switched, so it essentially becomes the on/off switch as well as the voltage scaling knob. https://www.trinityamps.com/product/voltage-regulation-module-vrm/ connections


The VRM is a bit different than a Variac, as a Variac scales the AC voltage coming INTO the amp. I think EVH dropped the voltage down to 90VAC or so to get his brown sound.

The "problem" with scaling the line AC is that there is a lower limit to how far you can go before the amp stops working. For example, the heaters need to operate at a certain AC voltage (~6.3VAC). Scaling the B+ with a VVR/VRM/VCB works only on the DC voltage WITHIN the amp after the rectifier.