Pots at B+ Voltage

[Steve Conner]

That one has the full B+ across the pot, though.

[redelephant]

You guys are funny, ok maybe it is not the best circuit for doing this, but it ( or variations of it) have been used for years on 1000’s of amps with very few failures….and its dead simple and is hardly new anymore

Of course the pot you use has to be rated for the voltage it subjected too… so you have to shop about to get one that is

Would it pass CE safety testing .. I don’t know , but for most hobbyist that doesn’t even come into consideration.

Even if you used a metal shaft pot it should have a big ass plastic knob on it and the body should be connected directly to safety ground

Sure it works, and the likelyhood af anyone getting harmed is small, but it is bad engineering, if I dare use that term. In old books as in new, pots are known to be prone to failures. It's simply piss.

Here's a circuit I've used and feel is a little better. The extra baggage u get along this is a soft start function, the current into the capacitor is limited, so you get a slowly rising output voltage. The 'con' with the mirror is a slight waste in current, but we're talking 1-2mA, so no worry. Output as shown here is from 175V to 380V, but some adjustments of the resistor values can give a larger range if needed. A constant current source also has another plus, it isolates the ripple from the b+ pretty well.

[Steve Conner]

That sums up my personal feelings on the matter

[redelephant]

The pot in my previous post admittedly has a large voltage on the top end, about half of b+, so here is another option I just scribbled, havent tested it, but have done similar types, so it should do fine.

[Steve Conner]

Also, the MPSA42 is only rated at 300V, IIRC.

[Chuck H]

Ahh ha ha. I just grabbed the first one I saw. Great. (Pee Wee Herman, "I meant to do that, guys.")

Still, I do know there's power scaling circuits that DON'T have the full B+ on the pot though.

[greekie]

Steve's doesn't. But it kinda takes the "simple" part out of it, even though it's not a whole lot of extra components, and I fully get what's going on.

However, the Alpha linear 24mm pots are all rated for 500V AC for 1 min, and have an insulation resistance of 100M ohm @ 500V. Their lifetime isn't as high as you'd like for some things, but I think they work well, and their dual linear pots for bias control as well are hard to beat at price and availability.

Jake

[d95err]

Kevin O'Connor uses low control voltages on his modern DC-Powerscaling circuits. This also has many other advantages, but the complexity of the circuit goes up. Lots of circuits like this are described in TUT6 (and briefly in TUT4).

In the older powerscaling circuits that have full B+ on the pot, Kevin recommends using mil-spec 2W pots with a high voltage rating.

[redelephant]

Your right! Good to have inputs on designs...Scetched in a hurry last nite. Here's a cascoded version, and also, since it is getting complex I think the first circtuit chould be a mix between the old and new.

There's a bazillion ways to do this, the challenge I think is keeping it simple, safe, and trying to get the most out of it in terms of PSRR and stability.

Btw. I tend to post circuits that are in the ballpark, and I assume if anyone intends to apply those ideas, they tweak it to suit their circuit. For instance, when designing I often use a certain value for b+ which isn't correct, but simplifies the math, and in real life some component values need adjusting...I assume most circuit ideas other posts are the same.

[jpfamps]

And even if you were an EE designing a commercial amp product, you have to think of the techs who'll be repairing it 10 years down the line.

Hmmm......

My experience is that, sadly, not every manufacturer thinks like this.

[jpfamps]

"In the face of this I think the proper response is to strive for Zen-like simplicity. MWJB, what do you say to a bunch of zeners selected by a rotary switch?"

I've considered this option; indeed it is used by one UK manufacturer.

However, getting a rotary switch that is up to the task isn't cheap.

You still have the problem of the B+ being connected to a switch that can work loose (and it does....).

[guitarmike2107]

The 2watt PEC pots are rated with a working voltage of 500v. problem solved on 95% of amps

[J M Fahey]

The brute force approach is indeed applying full 500V to the pot.
Simple, but stressing.
A still very simple addition of just 1 HV MosFet and a couple resistors in a , say, 10X DC amplification stage, can turn 0 to 50V supplied by the pot into 500 to 50V available at the source resistor.

[kg]

just use a variac.

[MWJB]

...or move to different continents? ;-) ;-)