Your thoughts on Power Scaling.

[leadfootdriver]

There's a couple kits on the market that I know of. 'Never tried one, but 'am curious about them. It could be a great addition to a full Express build.

Please share your thoughts and experiences.

[10thTx]

There is some useful information on Dana Hall's VVR here:

http://www.el34world.com/Forum/index.php?topic=6899.0

With respect, 10thtx

[harryk]

I have put VVR/power scaling to many amps. For some reason which I cannot tehcnically explain, VVR seems to work best in certain style amps. All these amps are cathode biased and some of them without negative feedback. Some of them were fully scaled and some only in PA. It seems that in high power fixed bias amps VVR makes power supply stiff. That is good if we seek modern metal distortion, but in most cases those distorted sounds are created in preamp in metal amps. In old school amps we try to create power tube distortion and sag/compression to get that holy grail distortion.

I think that one problem with fixed bias amps is how to bias power tubes when plate voltage decreases. In VVR both voltages are adjusted the same time with a tandem pot. We tried to simulate a situation where bias is getting hotter when plate voltage decreases, but it led to a situation where power level did not decrease as expected. These are my experiences of power scaling.

[mhartman]

I installed power scaling in a 5E3 that I built a while back. With Cathode Bias, it is a really easy mod, so I didn't bother with a kit - I just went point-to-point with the components. I have to say that, in that amp, it didn't really do much for me. It sounded "interesting", but it didn't really sound good. In a higher wattage amp, I think it would be better. That being said, if you're talking about a higher wattage fixed bias amp, you might want to install a PPIMV first to see if that works for your needs. If not, proceed with the VVR/power scaling (PPIMV is usually required for VVR anyway aif you're scaling the power section only).

[John_P_WI]

Many of the issues addressed here have been taken into account with the Power Scaling Kits from KOC / London power. KOC uses a bias tracking circuit that can be tweaked for a "fat" bias which will help retain the tone at lower volumes.

What I like about the London Power circuit is that it is installed between the caps and OT center tap, caps and screens etc. This allows instant voltage change, ie you can assign different levels of power scaling to different OD channels or instantly defeat power scaling for a clean channel.

Power scaling does have its place, it is a tool like many others that has a time and place to be used.

There are methods to achieve compression and sag with ps. Yes, it is great in cathode biased circuits, especially a JMP / JCM type circuit when the PI and Power amp or just the power amp are scaled.

One thing to watch for with power scaling just the power amp, as the amp is dialed down, the power supply voltages can rise to the preamp which can change the tone and feel of the amp. It is possible to clamp these voltages to not rise above a set level. I have not used the London Power super flexible scaling kit (only the SB kits), but in the past it had multiple nodes with their own adjustable regulator to prevent this.

John

[vibratoking]

I installed VVR in a 5E3 Deluxe with 6L6 outputs and an Express. Works great in both. I love it. No power supply stiffness detectable at all and 99% of the tone is still there. I don't scale to bedroom levels. I highly recommend VVR. It's much better than an Airbrake or similar device. I have not tried KOC's power scaling so I can't speak to that.

[bluesfendermanblues]

I agree with John_P_WI that power scaling is just one of many tools and as such it has its place.

My experience with power scaling is limited to installing Dana's VVR in a recent DIY 5E3 - where it's installed right after the rectifier tube and thereby scales the whole amp, both pre and power section. I like it from full to half, below half power I find you lose a little PSU sag and compression.

My bands rehearsal room is small (18 square meters, 193 square ft) and we are seasoned blues players, so the VVR at half speed is not enough to calm down the amp. Therefore, I have a -6dB L-Pad between amp and speaker as well. which is fine with the VVR dialled a quarter down.

One important thing to keep in mind with power scaling the whole amp is the you have to dial back back the volume pots as you dial down (or scale down) the Power or else the guitar signal will be relatively more powerful and overdrive the input and the sound will become fizzy.

Hence, at full power I like the volume pots around 9-10 (scale goes to 12) and at half power on the VVR, I dial back the pots to 5-6 in order to keep the distortion equal.

[Richie]

One important thing to keep in mind with power scaling the whole amp is the you have to dial back back the volume pots as you dial down (or scale down) the Power or else the guitar signal will be relatively more powerful and overdrive the input and the sound will become fizzy.

Never had that happen. Did you install the 1M resistor and .1uf cap on the input?
I've installed a number of these in all different kinds of amps.
Over the coarse of doing these, i have learned, lead dress or routing of wires is important. As is where the mosfet is mounted.

Some amps like fenders with reverb and trem. The trem may speed up some on the very lowest setting. The reverb still worked at lowest setting. And that is dropping the whole amp.

I know Dana has helped alot of people with these.Gave out some fixes. I've also seen about everything you could imagine talked about on many different forums about the VVR.
Like anything, some like it some may not. If it is installed correctly,i think it works fantastic.

[gary]

the telling point for me is that it is now becoming 'mainstream'. Marshall's YJM head and Laney's Ironheart, for example. Not before time, IMHO.

Have had success with KOC's and Dana's boards with 18 watt cathode-biased amps. Looking to do a homebrew tag strip effort on a Liverpool circuit soon.

[bluesfendermanblues]

....Did you install the 1M resistor and .1uf cap on the input?
I've installed a number of these in all different kinds of amps.

1m resister and .1uF cap ?? ...on the amp input or at the VVR input??

Never heard about that mod

[Richie]

On the input. Sometimes when the voltage is lowered to the lowest point. And could be depending on the tube used or circuit of whatever amp you have the VVR in. Really not a mod, i think Dana mentioned this in his info.

On some amps i just left the 1M resistor on the input jack. Adding another 1M at the tube pin to ground. The signal or input wire coming into the tube add a .1uf cap. Some people have used a .022 cap. or a .047 Doesn't have to be real high voltage rated. I usually use 200v cause i have a few of those. Very simple. Most people install little terminal on the tube mounting bolts,and attach the parts there.

What you might hear when the voltage is lowered without the resistor and cap installed. Is the amp signal get buzzy or splatty, or lose signal.
Usually this will only happen when the voltage is at a low setting.
I've seen amps do this with out a VVR, just running low voltages on the tube. I'm thinking usually around 60v ballpark where it might start to happen.

I snagged a pic to show how some wire it. Their is alot of info out there on alot of different amps they have done this in. I saw a marshall class 5 with some pics etc.

[bluesfendermanblues]

On the input. Sometimes when the voltage is lowered to the lowest point. And could be depending on the tube used or circuit of whatever amp you have the VVR in. Really not a mod, i think Dana mentioned this in his info.

On some amps i just left the 1M resistor on the input jack. Adding another 1M at the tube pin to ground. The signal or input wire coming into the tube add a .1uf cap. Some people have used a .022 cap. or a .047 Doesn't have to be real high voltage rated. I usually use 200v cause i have a few of those. Very simple. Most people install little terminal on the tube mounting bolts,and attach the parts there.

Richie, thanks for the thorough explanation.

Did a little googling to read more and its seems that the VVR can cause a little DC running to the guitar volume pot. Also found suggestions to add the 0.1uF/1M combo after master volume for the same purpose.
I wasn't aware of this. I'll definitely give a go. Thanks.

[Richie]

Your welcome.

As i posted, its not so much the VVR is causing it to happen. In a way i guess thats correct because it does lower the voltage.

But when you have an amp that runs low voltage on a preamp tube it can cause the same thing to happen with or without a VVR.
Some amps it may not happen, probably some tubes more prone to happen or not.
If using a VVR Its just alot easier to ad those,and not have to worry.

[gingertube]

Part of the arguement comes down to what in fact you are going to power scale.
That is:
- Just the Anodes and screens of the output tubes
- The Anodes and Screens of the Output Tubes plus the phase splitter
- the entire amp.

Then comes the "nitty gritty"
- If it is a cathode biased output then I have found that scaling just the anode and screen supplies works well BUT you do need drive compensation to the output tube grids to reduce the drive level at lower anode and screen voltages.

- If it is a fixed bias output then you MUST use a tracking bias regulator otherwise the tubes are not working over the same part of the Anode Current vs g1Volts curve and the sound WILL change at the lower power settings - again you need drive compensation.

If also scaling the phase spiltter supply then drive compensation is less important but still useful.
I would NOT contemplate scaling the sphase splitter supply if using a split load (Cathodyne) splitter - that would just be asking for trouble.

The thing to remember is that in an unscaled amp, the output tubes will always saturate before the phase splitter. You have to keep that when you scale the supplies down, hence the drive compensation requirement.

I have built only a single powerscaled amp - I used the KOC board to scale the anode and screen voltages and the tracking bias regulator. The phase splitter (stacked schmidt) was not scaled. Post splitter drive compensation was included. Outputs were a quad of 6V6.

Here are my test results:
Recall that I am using Hammond 370FX Power Tranny, 4 x 6V6G (ST Shape) Output Tubes and a 50W Marshall Output Transformer(Raa = 3K4).
ASIDE: A Hammond 1645 with Raa of 5K would have been a better choice for the OT, I just used what I had on the shelf.

Measured B+ unloaded (no tubes) => +378 Volts
Measured Raw Bias (no tubes) => -77 Volts

Set Power Scale Control to 10
Set Fixed Bias
Set pentode Mode

A quick calc for 70% of 12 Watts Anode dissipation gives 22mA per tube.

Set the 4 bias controls for 20mA per tube as a "start point".
Remeasure (loaded) B+, now at +349 Volts
Redo the calc above and get 24mA per tube.
Allow to warm up for 10 minutes and then set bias pots for 24mA per tube.

Checks:
B+ = 345 Volts
Bias Current/Voltages for the 4 6V6s
24.0mA @ -27.1V
24.0mA @ -26.9V
24.0mA @ -26.6V
24.0mA @ -26.7V

Set Power Scale Control for B+ = 250V
Checks:
B+ = 250V
Bias Current/Voltages
16.5mA @ -19.5V
16.9mA @ -19.3V
16.6mA @ -19.1V
16.6mA @ -19.1V

Set Power Scale Control for B+ =150V
Checks
B+ = 150V
Bias Current and Voltages
9.8mA @ -11.3V
10.4mA @ -11.2V
9.7mA @ -11.1V
10.0mA @ -11.1V

Note: that the idle current scales with the Anode and Screen Voltages because of that tracking bias regulator.

Hope there is something useful in this.
Cheers,
Ian

[husky]

I almost gave up on Power scaling in the Badger until I found the RIGHT master volume which is needed to prevent overloading the power section when turning down (Level). I didn't care for any of the Post Phase Masters with Power scaling even though they were OK on their own.

You will of course have different experiences depending on what the amps circuit is but for me in the Badger it is better than any other method of reducing volume while maintaing the feel of the amp to the fingers.

Just make sure you try all the methods of lowering the signal while turning down the Power supply voltage. You can also turn down the phase inverter B+ or the rest of the amps as well. For my amp these didn't work so great to my ears either even though they might work fine with other designs.

Basically saying "Power Scaling" can be implemented many different ways. Don't judge it until you have exhausted the options.