power scaling on Carvin x100B ?

[jamme61]

can someone explain how the RMS POWER switch works on the Carvin X100?
I had an old carvin and liked this option a lot I like to use on my Plexi Retro rockitt. Thanks for any help here's the schematic

http://www.carvinservice.com/crg/schema ... 9apr08.pdf

attached older version of the switch schematic

[sluckey]

It reduces the B+ supply to the PI plates which simply decreases the drive to the output tubes. The new one has a two step reduction and the old one has a one step reduction.

[jamme61]

thanks switch question- if I wanted to put this in another amp what type switch would I need?

[jamme61]

Will this work http://www.amazon.com/Heavy-Duty-Toggle ... B001FRE1E8

[teemuk]

^Looks a bit flimsy.

Do note that the switch must be able to reliably break several hundreds of volts of DC while most of those little switches tend to be rated only for something like 125 volts at AC current.

You'll be looking for a switch that is rated at least for 500VDC.

You'll know the real meaning of "heavy duty" when you see one.

In addition, user touchable metal levers in anything that carries several hundreds of volts seem a bit scary. If I remember right, there are even safety regulations against using such switches in said applications.

[d95err]

Remember that the switch will see the difference between the selectable PI voltages, not the full PI voltage. E.g. 400V on one side and 200V on the other, meaning the voltage across would be just 200V.

A big heavy-duty switch would be overkill. I would use something like this:
http://www.tube-town.net/ttstore/Switch ... :4053.html

(Then again, I just build amps for myself. I would be more careful with ratings if I'd sell them...)

[jamme61]

Remember that the switch will see the difference between the selectable PI voltages, not the full PI voltage. E.g. 400V on one side and 200V on the other, meaning the voltage across would be just 200V.

A big heavy-duty switch would be overkill. I would use something like this:
http://www.tube-town.net/ttstore/Switch ... :4053.html

(Then again, I just build amps for myself. I would be more careful with ratings if I'd sell them...)

Yea I have this type switch- should I isolate the switch from the dc with capacitors? I like this mod but messing with this voltage worries me thanks for the time on this stuff

[rp]

Anyone want to comment on how come this method of attenuation isn't used more often? Is it a Carvin patent? I've never seen it mentioned in the DIY world and everyone is always looking to knock some volume down.

The OP likes it, anyone else?

[teemuk]

Anyone want to comment on how come this method of attenuation isn't used more often? Is it a Carvin patent?

Not a Carvin patent (it would have been prior art anyway) and I would disagree about this scheme not being used often.

Given that there are at least about a dozen different methods to make output power variable I'd say this is one of the most common ones. This particular scheme has been used at least by Egnater and Mesa/Boogie in certain amps (both also employ other methods as well).

It has also been around for a few decades already so if you weren't familiar with it yet it just means you haven't researched this stuff enough.

[John_P_WI]

rp,

Honestly this thread should be renamed as is truly has nothing to do with London Power's Power Scaling.

To answer your question why has it not been implemented by others, the answer is that it is not that impressive. What it does do is add a little compression and darken the tone slightly to an otherwise sterile, bright and stiff output section. I does take the edge off slightly.

It pales in comparison to true power scaling, and in fact, IMHO the OP would be better off experimenting with a Mosfet Pass Regulator for the PI / preamp section instead of the limited approach that Carvin took. Ultimately, true power scaling would be the best approach.

Just so ya know I'm not talking out of my ass, here is my 1983 X100B Boogie wanna be that I bought new back then.

[img:640:360]http://i1148.photobucket.com/albums/o57 ... 0mcovg.jpg[/img]

[teemuk]

To answer your question why has it not been implemented by others, the answer is that it is not that impressive.

It's a decent idea.

But it introduces phase inverter distortion, and although there is a "push-pull" mechanism in it, it isn't exactly the same as true output stage distortion.

- As clipping takes place in low current stages this scheme will not introduce dynamic current draw/sag and effects associated to it. These effects play a major part in "overdriven power amp -tone".
- The triodes in a voltage starved LTP phase inverter clip differenty to real power pentodes/beam tetrodes
- This scheme will be devoid of effects of power tube grid clipping and associated bias shifts. These effects also play a major part in "overdriven power amp -tone".
- Transformer coupled output tubes interact with the varying load impedance. A phase-inverter stage will not.
- Different dial settings will alter open loop gain and response; in addition to distortion, tone won't remain uniform at different dial settings.

There are probably other issues too.

The scheme starves PI voltage causing reduction in headroom and earlier clipping in PI. Since the idea is to sum two asymmetrically clipped signals (from the PI) it is actually a rather decent attempt to mimic distortion similar to that of ovedriven push-pull power stage, which uses transformers to sum two asymmetrically clipped signals from the power tubes.

However, being practically akin to a low current "preamp stage" the circuit behaviour will lack all the dynamics and fine detais of the real deal. It works to lower headroom and introduce distortion at lower volume levels; the resulting distortion, however, won't sound like authentic power amp distortion.

[John_P_WI]

However, being practically akin to a low current "preamp stage" the circuit behaviour will lack all the dynamics and fine detais of the real deal. It works to lower headroom and introduce distortion at lower volume levels; the resulting distortion, however, won't sound like authentic power amp distortion.

^^ Yes