VVR for Express build

[aceofbones]

Hey everyone,
I’m wondering if anyone happens to have a fixed bias vvr pcb from Dana Hall they would be willing to sell? I’ve been in touch with him and he has none, and isn’t planning on producing anymore.
If not, I came across a layout for a vvr on here that Dana graciously checked over and said should work, but I’m curious if anyone ever had success using it?
As another alternative should I be looking at London Power’s power scaling? What is everyone’s preferred vvr? I’ve spent so many nights on here reading about it, that they’re all starting to blur together!
Looking to order my last few parts so that I can start my Express build!!
Thanks very much.

[norburybrook]

If you can't get one the LAR Mar type 2master volume works pretty good on the 2 express amps I built.




M

[ericlc]

When making choices about applying VVR I guess one must take in consideration:
* Where in the amp the distortion is generated (preamp/power amp)
* What volume levels are required (-20dB/Bedroom level)
* If tone consistency is important (PPIMV's can change tone )
* The required easy of use (how many controls/pots are needed, is DC pot noise an issue)
* If there is enough space in the chassis to implement VVR components (both physical and heat dissipation)

If you would ask me I would scale the plates and the screens separately and scale the bias accordingly.
And I would also implement a PPIMV so I would be able to control the amount of distortion.

With the stereo pot solution in your layout I am a bit worried that that I cannot consistently balance the plates, screens and the bias voltages.
This has an impact on tone. I have the same issue with a Whole Amp VVR on my 5E3.
I can control the B+ voltage but I do not like the tone at lower volumes or in other words, the range of control is limited.

So when I had to do it again I would use a design more in line with Kevin 'O Connors Power Scale solutions.

3rd Power (Hybrid Master) and Badcat amps (K-Master) have alternative solutions that do not require MOSFET's and BJT's
As do/did Mesa and Fender (red knob twin) with alternative transformer windings.
These can also be good solutions depending on what you are looking for.

Best,
Eric

[dorrisant]

I do not have any of Dana's PCBs, but I could make the VVR board in your layout. I can do it as a PCB with copper traces, as an eyelet board or a combination of both. I'd leave it to you to populate but could load it with parts upon request.

Lemme know.

[aceofbones]

Thanks for the input everyone.

@norburybrook. Thanks, I saw that mentioned in The Trainwreck Pages and have it bookmarked as a possibility. It’s nice to know it works well.

@ericlc Thanks, that gives me quite a bit more to look into. Seems to be Pros/cons for any option. I’ll look into some them some more.

@dorrisant Thanks very much for the offer. If I decide to go that route I will for sure send you a message. Cheers!

Anyone else with first hand experience on their own Express builds?

[katopan]

You've confirmed what I've long suspected - The fixed bias VVR shown in that layout has the mosfet up front followed by the pot on the output for the bias section. If you trace it out, the mosfet isn't actually really doing anything (it doesn't act like a buffer as configured) and the scaled bias output is no different to just having the pot and resistor. But the layout as shown will work and looks like it includes the required coupling caps on the grids feeding pots that will start leaking at low voltages (input & volume stages).

I prefer to have the pot feeding the mosfet on the bias supply rather than following it like shown. ie. Just follow the standard B+ side VVR circuit but with a P Channel mosfet and install the zener the other way around. I've installed that on tagstrip in both my Express and Bassman 6G6-B and they worked a treat.

It's common to install whole amp VVR on an Express. I followed that despite using PI/PA VVR on all my other amps. Because of the high gain nature of the Express it works pretty well with whole amp. I find the gain structure starts to fall apart below about 5W output. I never went back to tried PI/PA VVR in there to see if it was any better or worse.

[gktamps]

I haven't added VVR to either of my Express builds, but have put it into 2 Rocket types modified for more gain and tone shaping; one also has a PPIMV. I've also put it into my 6V6 K60 style amp and a couple of other designs. The VVR scales the entire amp in the Rocket types. The effect on tone in those amps is my least favorite, and I rarely use more than about a 10-20% reduction in voltage, and find a small adjustment between the PPIMV and VVR works best with that amp. Not sure how applicable these observations are since the amps aren't Express, but for what it's worth...

[aceofbones]

Thanks everyone!