Power Scaling Schems?

[mooreamps]

Their intent is to find manufacturers, but I am not sure some of them want to deal with the heat problem, at least the two of them I have spoken with.

-g

[UR12]

There is no heat problem until you get above 50 watts. Even after that it is something you have to spend money on but you can deal with the heat.

[PRR]

> deal with the heat problem

Total heat in chassis falls. (Unless you done something wrong.)

Heat in power tubes falls. But we have a new heat source, the MOSFET/resistor voltage-dropper. In a tight "authentic" chassis, finding a place for this may indeed be a problem.

> no heat problem until you get above 50 watts.

You did end up with a finny-thing hanging off the end. You do point out that it is overkill for an 18W.

Take an "18W". Cathode-biased, it'll idle around 36 Watts heat. It may run 300V or 350V, but take 300V and 120mA as an example.

18W out - 300V 120ma = 36W tube dissipation - 0V 120ma = 0W regulator diss - 36W total
9W out - 210V 84ma = 18W tube dissipation - 90V 84ma = 7.5W regulator diss - 26W total
4W out - 150V 60ma = 9W tube dissipation - 150V 60ma = 9W regulator diss - 18W total
2W out - 105V 42ma = 4.4W tube dissipation - 195V 42ma = 8W regulator diss - 13W total
1W out - 75V 30ma = 2.3W tube dissipation - 225V 30ma = 6.8W regulator diss - 9W total

The maximum heat in the regulator is one quarter the power in the output stage when working full-up. For a cathode-biased amp, this is pretty-near the idle heat; round-up about 20% because we let self-bias amps slide a bit into Class AB. For a fix-bias amp, look-up the operating conditions for a similar application. 2*6L6GC ala 5F6A will idle near 400V 100mA but roar to 400V 220mA, so at half-voltage there could be up to 200 110mA or 22 Watts heat in the regulator.

The maximum heat happens at half-voltage, which will give something less than quarter power output.

The heat in the regulator is fairly constant for all outputs from 50% power to 5% power.

9W heat is not a big problem for modern MOSFETs on a good chassis or aluminum flange. At 22W regulator heat you do have to think where to put the large surface area (or fan) needed.

But the total heat in the box falls. Adding power reduction will not blister your Tolex any more than before, and generally less if you actually use the power reduction. Unless you are "forced" to put the heatsink too close to the cabinet.

> a switching supply for tube amps. ...have a Variable Voltage power supply where you are varying the AC and not the DC.

It's an interesting thought. The standard PC power supply has the Watts we need. And it is "variable", though wired to vary to keep itself at a multiple of the 2.5V reference. Fiddle a resistor you can swing it several volts. To get 400V instead of 5V you would have to re-wind the HF transformer. Copying the input winding as a new secondary winding would put you in a ballpark. (That's where I get stuck: there is no way my older eyes and fat fingers can re-wind those dinky cores.) You'd need fast high-voltage rectifiers, but they can be had. You have to figure out the regulation resistor ratios for your new voltage. And for variable power, it may get tricky. Some of these switchers won't turn-down very far without motorboating. Also it seems sweet to take your heater power from a jazzed 5V output since this is so much lighter than a 50/60Hz heater transformer, but one regulator runs all the outputs so we can't hold 6.3V on one while varying 400V-40V on the other. (However surplus 12VDC switchers are dirt-cheap.)

[UR12]

>
But the total heat in the box falls. Adding power reduction will not blister your Tolex any more than before, and generally less if you actually use the power reduction. Unless you are "forced" to put the heatsink too close to the cabinet.

All true and dialing down the voltage has other benifits as compared to power attenuaters that just dump off power into a hugh resistor and send a little to the speaker. I'm talking power soaks or power brake type devices. In that configuration the tubes are still running at full power. By turning down the B+ your actually extending the life of the tubes instead of pushing then harder.

> a switching supply for tube amps. ...have a Variable Voltage power supply where you are varying the AC and not the DC.

It's an interesting thought. The standard PC power supply has the Watts we need. And it is "variable", though wired to vary to keep itself at a multiple of the 2.5V reference. Fiddle a resistor you can swing it several volts. To get 400V instead of 5V you would have to re-wind the HF transformer. Copying the input winding as a new secondary winding would put you in a ballpark. (That's where I get stuck: there is no way my older eyes and fat fingers can re-wind those dinky cores.) You'd need fast high-voltage rectifiers, but they can be had. You have to figure out the regulation resistor ratios for your new voltage. And for variable power, it may get tricky. Some of these switchers won't turn-down very far without motorboating. Also it seems sweet to take your heater power from a jazzed 5V output since this is so much lighter than a 50/60Hz heater transformer, but one regulator runs all the outputs so we can't hold 6.3V on one while varying 400V-40V on the other. (However surplus 12VDC switchers are dirt-cheap.)

Oh yea now were talkin! I had a supply design more like some of the car stereo amps in mind. Only a high voltage design instead of a high current. A couple of big mosfets running in a power oscillator mode feeding a nice toroidal tranny. (Let some young lady with nimble fingers and good eyes wind the tranny) Take the HV AC component coming out of the sec of the tranny and run that through a triac dimmer type circuit and then rectify the output of the triac to feed the B+ to the amp circuitry. That's a simple block diagram and I know it is a little more complicated in real life. Kind of takes you back to some of the old vibrator suplies in the old Delco tube radios from back in the 40s and 50s. (Ok now i'm showing my age) There are lots of good applications for solid state devices in guitar amps that can be used in areas where no audio passes through them.

I did a regulator board about 6 months ago that had 6 mosfets on it with trimmer pots that allowed me to adjust any voltage I wanted on any of the preamp tubes in the amp. It worked great and you could actually adjust the gain of that preamp stage by setting the B+ on the tube anywhere you wanted. They ran very cool and probably put out less heat than a resistive voltage divider setup.

[mooreamps]

Take the HV AC component coming out of the sec of the tranny and run that through a triac dimmer type circuit and then rectify the output of the triac to feed the B+ to the amp circuitry. That's a simple block diagram and I know it is a little more complicated in real life.

For some reason, the triac's will not fire on the secondary side of the power transformer, but they will fire on the primary side of the power transformer. However, to my ear, they are as noisey as hell in the loud speaker. I use this set-up to test another build I am doing, running the plates at 1.4 KV, as a safe way to bring up high voltage. I can filter out most of the switching noise, but to a point where it no longer becomes cost effective to do it this way.

-g

[PRR]

> triacs will not fire on the secondary side of the power transformer

If you are using the standard lamp-dimmer scheme.... no, of course not. That ploy depends on phase-shifting a gentle sine wave. The spike-wave on the rectifier circuit is not at all going to work with standard phase-shift SCR/Triac control.

In general, triacs need complicated controllers to do DC voltage control. You can't turn them half-on, and you can't turn them off. (I also have doubts about them recovering in 40KHz systems, but it sure has been a long time since I looked into that. I do know the early ones were hardly any faster than the sluggish gas-tubes they replaced.)

> a safe way to bring up high voltage

Ah, be brave. Wire-up, step back, plug-in. It may work. If not, it may be exciting, a good story to tell after/if your heart starts working again.

[mooreamps]

Ah, be brave. Wire-up, step back, plug-in. It may work. If not, it may be exciting, a good story to tell after/if your heart starts working again.

Actually, this High Voltage circuit works fine. It would only be if I had to build a very high voltage circuit would I have to inplement other design options.

> deal with the heat problem

Total heat in chassis falls. (Unless you done something wrong.)

You don't understand. The concern is the long term relieablity with respect to part failure. The heat problem has been mitigated. However, I was asked to look at running a power scaling circuit for a fixed bias circuit, which is what I am addressing now.

-g

[Jack]

It amaze me to see anyone concerned about the reliability of a mosfet when tube amps are concerned

[mooreamps]

Not when you have nominal production runs of up to 800 units a month and you'r on the hook for honoring a one year parts and labor warentee.
-g

[drz400]

Not when you have nominal production runs of up to 800 units a month and you'r on the hook for honoring a one year parts and labor warentee.
-g

Plenty of people are using power scaling with no problems, some even have a lifetime waranty, I own a Badger which uses it and is lifetime warantee, granted it is cathode biased but it works great.

[mooreamps]

I'm sure they are fine amplifiers.
Looking forward, however, I'll see if my next 18 watt mixed mode prototype amplifier will get off the ground. It will be a power scaled amplifier with two power tubes running self biased class A and two power tubes running class A/B fixed biased.

-g

[drz400]

I'm sure they are fine amplifiers.
Looking forward, however, I'll see if my next 18 watt mixed mode prototype amplifier will get off the ground. It will be a power scaled amplifier with two power tubes running self biased class A and two power tubes running class A/B fixed biased.

-g

What tonal advantage do you think this will offer?

[mooreamps]

With a low signal level, the tone should be mostly class A. At higher signal levels the class A/B tubes kick in for an extra punch.

[drz400]

With a low signal level, the tone should be mostly class A. At higher signal levels the class A/B tubes kick in for an extra punch.

A/B is class A at low levels anyway isnt it?

[mooreamps]

With a low signal level, the tone should be mostly class A. At higher signal levels the class A/B tubes kick in for an extra punch.

A/B is class A at low levels anyway isnt it?

Depends on how it's biased. It's so variable; either close to class A, or in the middle somewhere, or very close to class B. I'm not so sure there is a cut and dry spec that calls it out to be exactly at the 25% bias point.

-g