Power Scaling in Morgan amps?

[Diablo]

I built two amps, a Fender 5E8A and a Mathcless DC30 clone with the Variwatt from Skipz Circuits. The 5E8A doesn't have a master volume, but the DC30 does. The Variwatt adds some interesting overdrive possibilities, but it isn't the same tone as a cranked up amp. On the DC30, I never even use the Variwatt because the PPIMV allows for all the overdrive tone at lower volume you could ever need. The Variwatt on that amp doesn't add to the tone flavors. The 5E8A is fixed bias, and the Variwatt circuit adjusts the bias automatically as you turn the B+ up or down. If you scale just the power amp, you can get much more overdrive as you turn the B+ down. You can also scale the whole amp, if you like, and reduce the amount of overdrive available. I don't know how similar the Variwatt is to the London Power Scaling circuit, but I've read there are some differences.

[teemuk]

I can't remember which model it was, but I'm sure I remember seeing the circuitry on a factory schematic, so it wasn't a mod. I'll look in the industrial espionage folder when I get back to work.

I'm also interested in seeing this one.

[Amp Kat]

London Power is the only one I've seen using that actual MOSFET circuit which does btw track bias from 0-maximum voltage. The other Co's use a high low power switch or a pot that limits the signal to the peak inverter like on ampegs or that other crazy amp with the plug-in modules. The switch for low to high power is nothing more than switching the center tap to ground in and out of the circuit for the HV supply.


The power scaling used by London Power really has no protection circuitry and a shorted tube really presents no more of a problem to the MOSFET as a regular power supply because it's just passing the voltage on and limiting it so there are other components that will fry before it does like screen resistors and fuses so if it does fry then the whole circuit was going down anyway and all you would have to change is the MOSFET and tracking transistors at the most which aren't an arm and a leg by any means. Did you guys ever find out which circuit the Morgan used or did I miss it in a thread post ?

[teemuk]

London Power is the only one I've seen using that actual MOSFET circuit which does btw track bias from 0-maximum voltage.

Not the only one I've seen. The idea of continuously variable (via VVR) B+ or screen supply has been around from at least late 1970's, though it kinda took to early 1990's for the idea to really break through. Implementing variable bias control to the thing is nothing new.

The most ingenious ideas on this realm have been, IMO
- Maven Peal: Who added a simulated voltage sagging effect to an otherwise "stiff" regulator output
- DV Mark: That used switching power supply instead of a linear regulator and also implemented an autobias feature that supported using a wide variety of different output tubes plus pentode/triode mode switching and selection for class of operation.

[Amp Kat]

Not the only one I've seen. The idea of continuously variable (via VVR) B+ or screen supply has been around from at least late 1970's, though it kinda took to early 1990's for the idea to really break through. Implementing variable bias control to the thing is nothing new.

The most ingenious ideas on this realm have been, IMO
- Maven Peal: Who added a simulated voltage sagging effect to an otherwise "stiff" regulator output
- DV Mark: That used switching power supply instead of a linear regulator and also implemented an autobias feature that supported using a wide variety of different output tubes plus pentode/triode mode switching and selection for class of operation.

Maybe that's where K.O. got all his ideas from . The amp I had (Studio) did have all of those features plus some more and lots of frequency shaping circuits tied to switches but the power supply wasn't switching however you could use any tube combination you wanted and all of his components were very good quality not elite but very good. The Solen Fast caps respond very well to the Mosfet and do a nice job of filtering and power scaling. I was going by Amps I've seen since the 80's to now mostly and there really aren't a lot of this going on as far as that scale.

[jpfamps]

I built two amps, a Fender 5E8A and a Mathcless DC30 clone with the Variwatt from Skipz Circuits. The 5E8A doesn't have a master volume, but the DC30 does. The Variwatt adds some interesting overdrive possibilities, but it isn't the same tone as a cranked up amp. On the DC30, I never even use the Variwatt because the PPIMV allows for all the overdrive tone at lower volume you could ever need. The Variwatt on that amp doesn't add to the tone flavors. The 5E8A is fixed bias, and the Variwatt circuit adjusts the bias automatically as you turn the B+ up or down. If you scale just the power amp, you can get much more overdrive as you turn the B+ down. You can also scale the whole amp, if you like, and reduce the amount of overdrive available. I don't know how similar the Variwatt is to the London Power Scaling circuit, but I've read there are some differences.

I've retro fitted some power reduction circuitry to amps, and it seems to me that the amps tone darkens somewhat as you reduce the power.

I expect that the Fletcher-Munson curves explain this.

Further, without as you say some from of master volume, you can drive a massive signal into the power valves, in fact way beyond the HT voltage. This gives an frequency doubling/ fuzz type effect, probably due to you effectively getting 1/2 wave signal rectification.

As ever you seem to get better results when any power reduction features are an integral part of the design rather than retro fitting to an existing design.

Oddly, some of my first amp building experiments had a variable power supply; not I hasten to add due to any great foresight or understanding of electronics on my behalf, but because I changed jobs and found my new work place had a load of Heathkit IP-2717 bench power supplies, which had variable 400 VDC, -150 VDC and 6.3 and 12.6V AC supplies, which was ideal for messing around with valve amps.

[mooreamps]

I've had no issues with my power brake ; for either self biased or fixed bias power valves....and I didn't need the FET to do it..."just saying".......

[Alan0354]

Anyone compare the sound of power scaling between scaling the B+ and the screen voltage?

[Gingertube]

London Powerscaling

I built a "Standard" from Tut 5 for a friend of mine.
A quad of 1960's 6V6G for the outputs, each output tube pair switchable between triode mode and pentode mode, and each pair switchable between cathode bias and fixed bias. I included the powerscaling. The power scaling adjusted anode and screen voltages to the output tubes ONLY, phase splitter/driver and preamp supplies were not "scaled". I included the "Compression" or "Sag" control which is just a variable current limit and the Voltage control MOSFET.

The following set to work measurements may help you understand the operation.

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 for the 4 output tubes
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

EDIT: I actually had these backward, what I thought was triode mode was Pentode Mode and vv.

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 +105 Volts

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


Note in fixed bias how the bias tracks the B+ adjustment via the tracking bias regulator.

Cheers,
Ian