london power scaling

[schmoog]

I realize there are a lot of strong feelings regarding london power scaling, and that I may open a huge can of worms here. That being said, anybody here have experience addind the london power scaling kit to a fixed bias amp? Was it a big pain in the neck? How did it sound? I have a 60 watt fender super-sonic I've been thinking about adding it to.

So, your thoughts?

[jaysg]

I have no experience with the kits. The fine print problem is that...and I think it's the biggest problem...is that heat dissipation almost requires an aluminum chassis. Since you've got a steel chassis, you're going to have way more to do mechanically than if it was aluminum. Essentially, you'll want to create a window in the steel and mount a large heat sink in that window. The MosFet contacts the heat sink. hth

[John_P_WI]

Schmoog,

I have purchased numerous SB kits from London Power. The newer SB (Super budget) DC power scaling are very easy to install and will work in virtually any amp. The DC kits are easy to install between the filter caps and transformer center tap and screen caps and screen grids. It is easy to add multiple control pots for channel switching, or defeat for instant headroom etc.

I have said before and will again Power Scaling is another tool to lower volumes to make amps usable. Power Scaling generally sounds good until the speakers just don't push air. You also have to realize that your ears response changes with volume level and may contribute to the "difference" in tone. There are many methods available to make it performance switchable etc.

IF you have specific questions, visit PowerScaling.com and post your questions. Kevin frequents over there and answers many questions. You could also try emailing him.

John

[schmoog]

Yeah... I applied for an account at powerscaling.com but it still hasn't been activated.

I've been using attenuators for years (Weber and THD), and am really getting fed up. The weber is a good attenuator, but it doesn't really provide a smooth increase/decrease in volume as you turn the dial, which really kind of annoys the hell out of me. The other thing is I initially had a 25 watt weber, which I then had to upgrade to a 50 watt weber, and now I need more than that. The MASS III seems to have solved the issue of a smooth sweep, but thats ANOTHER 265 bucks to drop.... THD doesn't really give any sweep at all since it just has notches, and not having switchable impedance is really a bummer.

I figure power scaling seems to provide a pretty good range of volumes and tones, without affecting the tone (other than the affect that lower volume, lower speaker cone distortion, and frequency/volume dependant damping that speakers have on tone).

But if I have to cut a window in the back of my amp and mount a big ass heatsink, that's kind of a bigger undertaking than I thought it would be....

[John_P_WI]

You should be OK with the steel chassis as long as you have a good thermal connection using a silicon pad or mica spacer with thermal grease. In fact the included instructions with the kits show "bolting" the mosfets to the chassis with the thermal pad between the chassis and mosfet.

You can always use a small fan too. I just ran an experiment and found out that a 12v dc fan will operate at a much lower voltage. Also IIRC, the maximum heat dissipation from the mosfets is around a "scaling setting of 1/2".

[Firestorm]

You can mount the mosfet on bolt-on heatsink, then mount that to a cool part of the chassis (away from the tubes). Should be adequate unless you're scaling serious wattage.

[schmoog]

What about mounting the whole kit in an aluminum chassis, bolting the chassis to the inside of the combo, then just running the wires up into the amp chassis?

I was looking at the amp, and the other issue is that everything is pcb mounted (thanks fender!) So there's not a lot of room. Maybe doing it that way kills 2 birds with one stone?

Does anybody have a scan of the instructions for the kit? I can't find them to download anywhere...

Also, I don't know if I mentioned it before, but the amp in question is a fender super-sonic 60 watt 112 combo...

[John_P_WI]

Space should not be too big of concern. The SB-1 power scaling kit for fixed bias is around 0.8 x 3.5 inches. The long length can be broken (pcb is scored) into 3 separate sections if needed. The mosfets can be mounted directly to the chassis and the board would then be vertical.

I would not put it outside of the chassis (if this is what you were asking above) as the longer wire lengths could possibly invite noise or other issues. You will need to account for Va, Vs and bias lines.

Email Kevin with specific questions, I use his kits on ground up builds and plan for their placement from the start.

[schmoog]

Space should not be too big of concern. The SB-1 power scaling kit for fixed bias is around 0.8 x 3.5 inches. The long length can be broken (pcb is scored) into 3 separate sections if needed. The mosfets can be mounted directly to the chassis and the board would then be vertical.

I would not put it outside of the chassis (if this is what you were asking above) as the longer wire lengths could possibly invite noise or other issues. You will need to account for Va, Vs and bias lines.

Email Kevin with specific questions, I use his kits on ground up builds and plan for their placement from the start.

Sounds good. I will email Kevin. You don't happen to have a digital copy of the instructions do you? No biggie if not...

I'm planning a from scratch (as opposed to a kit from say mojo or metro) 1959 build, and thinking that I'll probably include and account for power scaling there as well.

One other question and I'm done... Can you shed some light on drive compensation? From what I've read, it's basically just a pre-PI MV to avoid overdriving the PI. Is that correct?

[gingertube]

Power Scaling adjusts both the anode and screen voltage on the output tubes and the bias via a tracking regulator. At lower power settings you can very easily overdrive the output tube grids. The fix is "Drive Compensation".

Where you put the drive compensation will depend upon if you include the phase inverter in the scaled power supplies or just the output tube anode, screen and bias supplies. If scaling the phase inverter supply as well, then drive compensation becomes less important but it would be done as you suggest with a pre PI "Master Volume" control. If you do the more usual thing and just scale the output tube supplies then the drive compensation is usually done with a dual gang pot between the PI and the output tube grids. This last option is what I used whan I built a full powerscaled "Standard" from TUT5. The only change I made to Kevins published schemo was to use a quad of 6V6 for the outputs with a corresponding change of output tranny.

For your possible interest here are my power amp set to work notes:

Set to Work of the Power Amp.

Recall that I am using Hammond 370FX Power Tranny, 4 x 6V6G (ST Shape) Output Tubes and a 50W Marshall Output Transformer(Raa = 3K4).

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

Set Power Scale Control to 10
Set Compression (SAG) Control to 0
Set Fixed Bias
Set pentode Mode

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

Fit the 4 x 6V6G Output Tubes and selected 12AX7 PI tube (selected for matched triode sections). Actually the best I had was a CV4004 so used that.

Set the 4 bias contrls 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
24.0mA @ -27.1V
24.0mA @ -26.9V
24.0mA @ -26.6V
24.0mA @ -26.7V

Switched to Triode Mode - each tube idle current drops about 0.3 to 0.4 mA
Switch back to Pentode Mode

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

Set Power Scale Control back to 10 => B+ back to +345 Volts

Wind Compression Control from 0 to 10 => B+ drops to +105V
This looks a bit "drastic".

Set Compression back to 0 and switch to Cathode Bias.

Checks:
B+ = 343V
Bias Currents / Voltages
Pair#1 Cathode Volts = +23.9V
25.9mA , 25.3mA tube currents
Pair #2 Cathode Volts = + 23.7V
25.2mA, 25.4mA tube currents

Cheers,
Ian

[John_P_WI]

Sorry I don't have the instructions handy...

The drive comp can be pre PI master, an Improved Bootstrapped Master (search this site), or a Post PI or a mix of. The pre PI master and the improved bootstrapped Master does not take advantage of the PI gain where a lot of the "Marshall grind" happens in the 1959, here a post PI (Type 2 or "Lar Mar" or similar) would work. You can also scale the whole amp, and in a sense that is self limiting. I usually scale Va and Vs and then clamp the voltage on the PI and preamp to keep it constant during scaling.

I think Mickey, who owns Powerscaling, is the guy who approves membership and is a busy guy. You may have to email him directly. Once approved, there was a build a few years ago, the 5100 plexi, that you can check out and a lot of your questions will be answered.

Good luck

[John_P_WI]

Ian, it is good to see you post over here too. Hope all is well.

For clarification on Ian's notes, he mentions "compression" or SAG, this was an "extension effect" of power scaling built into the design of the "London Power Standard". In that design the power scaling and compression was built from scratch on a tag board. It is not part of the SB kits, but can be added as an additional kit.

[gingertube]

Yes the SAG control is just a variable current limit which is added to the B+ regulator. It sort of emulates a tube rectifier influence.

John_P_WI,
Yup I'm new over here. Joined here and Ampage for Guitar Amp stuff. I'm doing a bit more of that these days thou' still mostly into tube HiFi Amps.

Here is a pic of the Power Scaling Board taken during the build of the Standard. A bit hard to see but the power MOSFET is mounted to that bit of C Section Aluminium Extrusion in which the PS Board is sitting. I bought that piece of ally from the off-cuts bin of the local aluminium supplier for a few cents. That gave plenty of heatsinking.

Cheers,
Ian