Power Scaling Schems?

[withnail_jet]

Does anyone have schematics for the Power Scaling circuit as employed by Kevin O'Connor/London Power? It looks interesting, and I'm wanting to try it in a cathode-biased amp.

I found a schematic which was supposedly for the Power Scaling circuit as used in another amp, but it lacked the 'Drive Compensation' control, which I understand is crucial for getting a non-altered sound at lower volumes...

[loverocker]

Drive Comp is just a voltage dividing volume control. You put it between section of the amp that *isn't* PSed (usually the preamp) and the bit that is PSed. So it can reduce the drive to the PSed section when you dial down the B+ in the section that is.

So it's almost always a PrePIMV or PostPIMV (depending on how you do the PSing)

If you PS the whole amp, you don't need Drive Comp.

[jaysg]

KOC gave his blessing to posting this at 18watt. He doesn't want the Sag or fixed bias follower circuits published I guess.

[withnail_jet]

KOC gave his blessing to posting this at 18watt. He doesn't want the Sag or fixed bias follower circuits published I guess.

Thanks, that was the one I was referring to. So, it's basically the same as the PSK-2 kit?

Thanks for the details, loverocker!

[UR12]

You might want to check out this thread regarding Power Scaling. There is also a hugh thread at 18watt.com on scaling if you interested along with a posted schematic.

http://ampgarage.com/forum/viewtopic.ph ... er+scaling

Here is a link to Kevin's schematic

http://www.soultoneamps.com/SchematicAr ... caling.pdf

[withnail_jet]

Thanks a lot, all great info sources! This particular project is cathode-biased, so I think this "public domain" circuit may do what I need. If I ever need it for a fixed-biased amp, I'll buy the appropriate kit from London Power.

[Kersey]

Greetings and thanks for the great forum!

Seems like this link is dead - does anyone have this schematic?

Thanks!
KK

[withnail_jet]

Just PM'd it to you...

[LeeMo]

http://www.powerscaling.com/

Mickey C has started a forum.

[dehughes]

http://www.powerscaling.com/

Mickey C has started a forum.

Man, what's up with that site? I cannot use my Gmail account to register...and that's my only e-mail account. PLUS, I cannot seem to find a link to e-mail the site admins and grovel for access. Does anyone know who I'd contact to see about joining that forum? I'm pretty sure I'm not a spammer...yet...

[LeeMo]

http://ampgarage.com/forum/viewtopic.php?t=3005
The first post is from Mickey_C. His email addy is in his profile. His web addy is Soultoneamps.com if I recall. The schemo is on that site.It's in the FAQ section.
http://www.soultoneamps.com/schematics/ ... caling.pdf

LeeMo

[dehughes]

Thanks. I'm trying to get setup on that forum, but it is SUPER TIGHT. Geez....I understand, though....

[PRR]

> I cannot use my Gmail account to register...

Right. And once you tried that, you have a cookie that is hard to get rid of.

Try an EDU address and you fall in a black hole. (If they really intend to discriminate against poor students, they are rejecting a few VERY brilliant folks; also full-time tech staff like me.)

> and that's my only e-mail account.

How do you connect? Most ISPs "do" give you a mailbox. Accessible with any modern mail client and often via a Web interface also. It may be incredibly hard to find. On Verizon I go round and round in circles through their "help" system every time I forget my account setup. Someone else used the old SBC as their ISP, I went to find their setup, and it sure is hid good. LAST resort is to get the CD and run it; but it corrupts Windows and often won't do Mac (never unix).

I got in with a "real ISP" which is not even my ISP any more. Usta be, in days of 2400-dialup. They let me slide $5/month to keep a very old email address someone in my family clings to. They incidently let me register 5 more addresses (one for each of my dogs) on the same $5.

> it is SUPER TIGHT. Geez....I understand, though....

I fully understand their GOAL. I've seen well-run forums tarnished, and neglected forums trashed, by mass spam attacks. But I've also seen too-tight forums wither because the same three guys run out of things to say to each other. It's a balancing act. [hr]
Back on track....

> This particular project is cathode-biased

Then yes, you just reduce supply voltage, probably just on the power stage.

The Kevin-plan posted above will do the deed. Note that the MOSFET gets HOT. On the other thread referenced here it has a 486-CPU heatsink with fan. Note also that you break the power stage apart from lower level stages and feed the low-level stages normal juice. So you have to know and understand the amp: good building or fixing skills may not be enough.

I'm a simple man, and distrust transistors (I know them too well). Cathode-biased power stages can be power-reduced with a large resistor in the supply. Rectifier, cap, (choke and) resistor, cap, then to PT and screens. For full power use a small resistor, like 100 ohms. For half voltage, half loudness (quarter power), use a resistor equal to the power stage demand. A 300V 100mA power stage is 300V/100mA= 3K effective resistance load on the power supply. Put a 3K resistor in there, half of the 300V appears across the resistor, half across the power stage. Resistor dissipates 7.5 Watts, a 10W part will smell hot, use a 20W part. A 9K resistor will split voltage about 9:3, or about 1/4 voltage or 75V across the power stage. Seven 10K 10W resistors on three switches can give 10K, 5K... 1.4K, and a fourth switch can give "Full".

[UR12]

I fully understand their GOAL. I've seen well-run forums tarnished, and neglected forums trashed, by mass spam attacks. But I've also seen too-tight forums wither because the same three guys run out of things to say to each other. It's a balancing act. [hr]

Then you are doing better than I am. I thought I understood the goals until I read this last post on THIS forum. Now I have to ask myself why he is even here advertising his site. Maybe to find manufacturers on this site to recruit into the Power Scaling group to make money?

Back on track....

> This particular project is cathode-biased

Then yes, you just reduce supply voltage, probably just on the power stage.

Personally I like to reduce voltage to the whole amp in a cathode biased amp. Some like this approach and some don’t. This eliminates the need for another attenuator down stream to limit the drive signal to the power section.

The Kevin-plan posted above will do the deed. Note that the MOSFET gets HOT. On the other thread referenced here it has a 486-CPU heatsink with fan.

The picture is of my 18 watt amp. I installed the heatsink/fan assly just to see how well it would work. It really is overkill for an 18 watt amp but when you get to 50 watts and above you definitely need to look at some AMD (air movement device) for cooling.

I'm a simple man, and distrust transistors (I know them too well). Cathode-biased power stages can be power-reduced with a large resistor in the supply. Rectifier, cap, (choke and) resistor, cap, then to PT and screens. For full power use a small resistor, like 100 ohms. For half voltage, half loudness (quarter power), use a resistor equal to the power stage demand. A 300V 100mA power stage is 300V/100mA= 3K effective resistance load on the power supply. Put a 3K resistor in there, half of the 300V appears across the resistor, half across the power stage. Resistor dissipates 7.5 Watts, a 10W part will smell hot, use a 20W part. A 9K resistor will split voltage about 9:3, or about 1/4 voltage or 75V across the power stage. Seven 10K 10W resistors on three switches can give 10K, 5K... 1.4K, and a fourth switch can give "Full".

There is any number of ways to reduce voltage/power in an amp but most, if not all options will require the use of some device that is going to get hot, including your power resistors. I don’t think that any of us tube guys are extremely trustful of SS devices but this application is what these big high voltage power mosfets were made for. My only rule with SS devices in tube amps is to keep them out of the signal path and using mosfets to reduce voltage in an amp complies with that rule. The smaller ones also make great switching circuits to replace relays. It’s funny though that most guys think nothing of throwing a SS pedal in front of their favorite tube amp.

I often wonder why someone hasn’t come up with a switching supply for tube amps. Set the switching freq to say 40khz (or somewhere out of hearing range) and you have no 60/120hz hum to deal with, the filter caps can be a lot smaller values and the physical size of the HV power tranny can also be smaller. This arrangement could also allow for using triacs instead of Mosfets and vary the AC component before it is rectified thus allowing you to have a Variable Voltage power supply where you are varying the AC and not the DC. There are lot’s of cool things your poor students could play around with.

Getting back to the seven resistors…… The last amp I did with a variable voltage regulator only used 6 parts in the regulator circuit. 1) mosfet, 3) resistors, 1) Zener and 1)pot. It puts out 1 – 30 watts and works great with a small heatsink and no fan!

[dehughes]

Well, I'm up on the Power Scaling forum. Thanks Mickey!

Now, I'll try to re-read all that nice, informative, over-my-head posting above....