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Power Scaling vs. Seymour Duncan Convertible

9/28/1999 6:24 AM
Power Scaling vs. Seymour Duncan Convertible
Does anyone know how the Power Scaling on the London Power Amps work? Also, is this similar to the Seymour Duncan Convertible power attenuator?  
Anyone ever try a London Power amp?  
9/28/1999 1:48 PM

From what I've heard, K.O. has a constant heater voltage (separate transformer likely) and then has some silicone based voltage reduction working on the HT supply. (Probably switchable zeners on the CT, but could be fancier than that)  
He also has some sillycone circuitry that scales the bias voltages alongside the B+ scaling. Not the kind of thing one could set up without a few years experience, I suspect.  
Not familiar with the Duncan setup, so I couldn't compare, but the K.O. approach is not power attenuation (dumping heat after amplification) but rather lowering wattage at the source.  
I haven't heard these amps yet, likely 'cause I wouldn't/couldn't spend that much on an amp anyways, but I only live about 10 minutes from K.O., so I really should drop in for a test drive sometime...  
... especially his big-*ssed Russian transmitting tube bass amps, way cool!  
... apologies to K.O. if I've lied or spoken out of turn...  
... Whit
9/30/1999 11:02 AM
Mark Abbott
Whit, How much are London power amps?
How much are they and how do they compare to the majors? thanks Mark
9/30/1999 1:39 PM

The prices are listed at the bottom of this page on the London power website:  
As I said in my first post, I haven't had a chance to go over for a test drive yet, but it's becoming a priority now that I'm curious.  
I would suspect the sound is there, and assume the build quality is excellent. Compared to the majors? Well, if I had unlimited dough, I'd much rather have an amp built by a guy I could talk with than an amp that got packaged by "anonymous inspector #321" at the end of the conveyor belt.  
... Whit
9/30/1999 4:40 AM
Re: Power Scaling vs. Seymour Duncan Convertible
The SD amp used different bias splitter resistors at the outupt tubes and had them on a rotary switch.. a master volume really. It does control output power, just not the way most people think! :-)
9/30/1999 10:04 AM

I have a question about this. Apparently the Seymour Convertible has the equivalent of a line out. Now if you max out the amp into a resistor load (bypassing the speaker) and then take the line out, are you getting any power amp distortion at all? I was thinking it wouldn't but my friend is convinced it works. I was thinking he was getting preamp distortion...  
9/30/1999 12:10 PM
Andy Fuchs

The DUncans I've seen with 'wattage' pots, used a second 'dummy' driver tube, which 'sunk' the driver tube voltage downward, by biasing the extra (dual triode)so it lowered the plate voltage at the driver tube, and somewhat reduced the drive and therefore wattage. I like the SUndown methode (300 ohm 5 watt pot on the cathodes), but Dennis and I disagree over the changes in bias current, the way he has it set up. The bias supply (positive/ground end) should really be connected to the cathodes of the power tubes. The cathodes go about 20 or 30 volts 'in the air' when the pots fully turned to low power setting. The tube bias shafts alot that way. The Carvin, B+ reduction the the driver is cool too.
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