vvr

[bigsmitty]

What are the pros and cons of using a vvr to regulate just the power amp vs. the whole amp? I'm going to install it in a silverface champ that's been converted to 5f1 specs.

[Enzo]

Kinda the same difference it would make to lower the voltage to your entire house to dim the livingroom lights instead of just dimming the table lamp.

Your preamp circuits, while certainly functional over a wide range of voltages, are probably optimal as they sit. The point of these mods, as I see it, is to reduce the power amp to a lower power output. But while doing that, you don;t want to reduce headroom and increase unwanted distortion.

Now I am all for experimentation, within limits it might generate some intersting results.

[olddawg]

Kinda the same difference it would make to lower the voltage to your entire house to dim the livingroom lights instead of just dimming the table lamp.

Your preamp circuits, while certainly functional over a wide range of voltages, are probably optimal as they sit. The point of these mods, as I see it, is to reduce the power amp to a lower power output. But while doing that, you don;t want to reduce headroom and increase unwanted distortion.

Now I am all for experimentation, within limits it might generate some intersting results.

You know, the VVR is all the rage over at the 18 watt forum. (That and EF86 preamps) For the life of me I can't understand why a simple L pad would not work better. Especially on an amp that is only 18 watts. I also never understand why people want to use those big expensive power soaks on huge amplifiers. It's usually just as economical to buy a smaller amp. I guess some people like the look of their stacks no matter what. I actually built a hotrodded 18 watt into a standard red Marshall headbox and use with a matching 1965A cab. It looks like a JCM800 halfstack but has club volume. If I need less volume than that I use an Epi VJ. IMHO it's always better to choose the amp for the venue and usually better on your back .

[fyl]

A basic L-pad isn't as transparent as a proper voltage regulator: it adds series resistance, thus modifying the speakers' behavior as well as tonal response. A VRM allows to dial just the perfect loudness, keeps the same tone for most settings and greatly prolongs tube life.

[Albert Kreuzer]

You know, the VVR is all the rage over at the 18 watt forum. (That and EF86 preamps) For the life of me I can't understand why a simple L pad would not work better. Especially on an amp that is only 18 watts. I also never understand why people want to use those big expensive power soaks on huge amplifiers. It's usually just as economical to buy a smaller amp. I guess some people like the look of their stacks no matter what. I actually built a hotrodded 18 watt into a standard red Marshall headbox and use with a matching 1965A cab. It looks like a JCM800 halfstack but has club volume. If I need less volume than that I use an Epi VJ. IMHO it's always better to choose the amp for the venue and usually better on your back .

Buy a smaller amp? The OP is going to use the VVR on a champ

[Steve Conner]

A basic L-pad isn't as transparent as a proper voltage regulator: it adds series resistance, thus modifying the speakers' behavior as well as tonal response. A VRM allows to dial just the perfect loudness, keeps the same tone for most settings and greatly prolongs tube life.

A L-pad and a voltage regulator both affect the tone in different ways. (My latest amp build has both, so that is IMO.) Neither is necessarily bad, see Van Halen's alleged use of a variac to make his amp sound better.

Even if neither device changed the tone in real terms, it would subjectively seem as if they did, because of all that Fletcher-Munson nonsense. This is why I argue that "transparent" power scaling is meaningless marketing-speak and you should just use whichever method you like the effect of.

On my build, I just regulated the power tube screen voltage. The tonal effect of that is different to what you'd get if you also scaled the preamp. Scaling the plate voltage too would make another difference, but would need a much bigger VVR.

An L-pad adds series resistance, but I found that that series resistance actually helps. So I design the L-pad backwards, for example 8 ohms across the amp output and 47 in series with the speaker. It makes the speaker give more bass and treble, which goes some way to compensating the Fletcher-Munson loudness thing. It's not really enough of a compensation, but it works pretty well considering the low cost and component count.

[mooreamps]

On my build, I just regulated the power tube screen voltage. The tonal effect of that is different to what you'd get if you also scaled the preamp.

If that's true, I notice the "old timers" don't try to run you through the meat grinder, as compared to when I write about it... They must like you for some reason....

-g

[Steve Conner]

I am one of the old timers, I've been around here since the 90s.

I actually got the idea of scaling the screen voltage from you.

If you scale the preamp supply voltage too, then its gain and headroom drops along with the power stage's signal handling capability.

But if you scale the screen voltage alone, the preamp voltage actually increases at low screen voltage settings, at the same time as the power tube's cathode bias is decreasing, and the grid voltage it can accept before clipping is also decreasing.

Result, the gain of the amp seems to increase as the voltage is turned down. At low voltage settings the power stage gets viciously overdriven and makes some serious grind. Eventually it disintegrates in a splattering, farting mess, but I found some good tones just before that.

[mooreamps]

That's because you are not using combination bias inside your class A power amp.... :}


Don't worry, someone else will someday write a fng book about it someday and take all the credit for it...



-g