MV, Powerscaling in 50W, 100W Amps

[bob p]

Wow, those connector legends are a headache.

[Gregg]

I have two identical 2x6L6 fixed bias power amps where I implemented the two most simple types of power scaling. One amp has both plate and screen voltages variable, the other one only the screen voltage. Now I'll have to do some listening tests.

[bob p]

A question for you guys who have used either the VVR board kit or the London Power kits: Do they put B+ voltage directly on the control pots?

II've never seen a schematic for either a VVR kit or a Power Scalling kit, but looking at the VVR board, it's evident to me that the VVR board places the full B+ DC voltage on the pot that you're going to mount on a front panel, and grab with one hand while you might have the other hand grounded through your guitar strings. That's just a recipe for disaster.

Can anyone tell me if the Power Scaling kits do the same thing?

[Tom Phillips]

The current generation of London Power power scaling kits do not put HV on the control pot.

[bob p]

Thanks for clarifying that, Tom. When you refer to "current generation" not doing that, does that mean that earlier incarnations of Power Scaling *did* put HV on the control pot? Just wondering how much safety people are putting into their designs.

Last night I looked at the VVR kits on the seller's site. Why? Just because I thought it might be fun to put a simple B+ voltage reduction circuit into a low-powered cathode based amp. When I saw how cheaply I could buy the VVR kit, I thought it might be worthwhile to go that route rather than designing my own system, building my own board, etc. I looked for a VVR schematic but couldn't find one. But the photos of the PCB that were available on the site made it easy enough to trace the circuit and draw a schematic.

Once I saw the schematic, I realized that VVC really looked like a kludge method for voltage scaling. It isn't very elegant, and it's sort of a ham-fisted approach. Worst of all, It puts the full B+ on the front panel control pot. Having direct control of the HV through a pot like that really scares me. Aren't these guys smart enough to design a circuit that uses an isolated pot to provide indirect control of the HV supply, rather than dealing with it directly? I think that VVR implementation is inherently dangerous, and is just a bad idea from a safety standpoint. I hope that the guy who sells it has good liability insurance in case VVR kills someone.

For the record, I'm not shilling for the other guy, and I've never bought his books or his kits.

[Tom Phillips]

...When you refer to "current generation" not doing that, does that mean that earlier incarnations of Power Scaling *did* put HV on the control pot? ...

Yes. The first generation kits. (Over 5 years old. Maybe more) Did put HV on the Pot. The supplied pot was a very high quality mil style unit. The design was changed in later years and the changes were more extensive than just getting the HV off the pot. IMO the London Power circuits are well designed using good engineering principles. I'm not familiar with the other kits you mentioned.

[bob p]

I'm not familiar with the other kits you mentioned.

This is the seller's site:

http://www.hallamplification.com/main.html?src=%2F#2,2

I had seen the kits mentioned in threads at one of the guitar sites... can't remember exactly where it was. Maybe the amp garage. A user on one of the forums claimed to be the designer, and posted images of one of his boards to generate interest in his product. I downloaded the images, though I can't remember the URL where I had seen them. Here they are:



It's a pretty simple schematic to trace. I decided to stay away from the product once I realized there was the potential (pardon the pun) for B+ exposure.

[bob p]

I apologize for not being able to link to the threads I was reading last night. I didn't bother to bookmark them while I was surfing.

Here's a local thread that discusses the circuit in question:

http://music-electronics-forum.com/t32497/

Here's another local thread. Post 5 links to a schem drawn by KOC that shows a London Power circuit that is very similar. It puts HV on the control pot, and it might be one of the older circuits that you described.

http://luisamark.com/mark/files/Powe...018%20watt.gif

Here's yet another vendor who is selling something that looks similar:

http://skipzcircuits.com/VariWatt.html

I don't like these circuits. I've decided that it's safer to stick with a ladder attenuator.

[exclamationmark]

Although most pots should have an earth connection via the washer, I still think it's a terrible idea having HV on a pot. I'm betting the thing would catch on fire if it actually shorted to the chassis and the wiper resistance was low enough. That aside, most VVR's are just a simple source follower, which kind of acts as a ghetto regulator - it's essentially operating open loop, but the variation in current vs voltage is quite small, so it works well enough... I guess. I think the main attraction is that it's ridiculously simple.

In order to not have HV on your pot, you'd need another gain stage to feed the source follower, and it's somewhat fiddly (bias and cutoff voltage and all that). At that point, you're pretty much 90% of the way to a closed loop regulator, minus an op-amp and a few capacitors. You can still get the sag thing happening with a closed loop regulator by hooking the referenece input to B+, but since it's not regulating anymore, there'd be little point in building one! So in a convoluted sort of way, that's probably why you see so many of these B+ on pot design types - there's simply no other good way to do it without getting rather complex.

[bob p]

When you need a user to control something that's really nasty and dangerous, responsible engineering is to isolate the user from the nasty & dangerous entity so that they are not placed at risk. This often dictates the need for indirect control, rather than direct control, of nasty things.

At the risk of sounding redundant: Responsible engineering dictates that you never place the user at risk. You NEVER place something nasty and dangerous directly on the user interface. That's just negligent. You want to isolate the nasty thing from the end user so that the end user isn't harmed when something fails. This is why things like optos were invented. The fact that people who design these circuits aren't using indirect methods of control indicates that: A) they are not safety conscious, B) they are not very good circuit designers, or C) they are willing to trade away user safety to maintain low cost and profitability.

I''m willing to be that none of these circuit designers have every submitted their work to UL for evaluation!

[jpfamps]

Although most pots should have an earth connection via the washer, I still think it's a terrible idea having HV on a pot. I'm betting the thing would catch on fire if it actually shorted to the chassis and the wiper resistance was low enough. That aside, most VVR's are just a simple source follower, which kind of acts as a ghetto regulator - it's essentially operating open loop, but the variation in current vs voltage is quite small, so it works well enough... I guess. I think the main attraction is that it's ridiculously simple.

In order to not have HV on your pot, you'd need another gain stage to feed the source follower, and it's somewhat fiddly (bias and cutoff voltage and all that). At that point, you're pretty much 90% of the way to a closed loop regulator, minus an op-amp and a few capacitors. You can still get the sag thing happening with a closed loop regulator by hooking the referenece input to B+, but since it's not regulating anymore, there'd be little point in building one! So in a convoluted sort of way, that's probably why you see so many of these B+ on pot design types - there's simply no other good way to do it without getting rather complex.

I would not be happy about having the HT on a pot on the front of an amp.

Notwithstanding the fact that the pot is unlikely to be rated for such high DC voltages, it's unlikely that this arrangement will conform to any safety standards.

It's quite simple to vary the control voltage on the pass MOSFET whilst keeping the voltage on the pot low (I've prototyped a circuit that does this), but I suppose it is a bit more complicated than the VVR-style circuits.

[bob p]

It's quite simple to vary the control voltage on the pass MOSFET whilst keeping the voltage on the pot low (I've prototyped a circuit that does this), but I suppose it is a bit more complicated than the VVR-style circuits.

What you're describing isn't at all hard to do. I think the reason that people aren't doing what you've described is because they haven't thought things all the way through. It seems that they stopped working on their designs as soon as they developed something that "worked," and they never thought very much about what could go wrong, or better/safer ways to achieve the same goal.

Using a low voltage control mechanism isn't rocket science. And there's already a readily available low voltage source in the amp. The fact that people aren't using it tells me that they aren't trying very hard to come up with a better solution. They're content to sell something simple and cheap because that maximizes their return on investment. The obvious trade off that they're willing to make is in user safety. That's just a bad decision.

[GainFreak]

These "dangerous" kits are selling from a long time and I haven't heard about anyone getting shocked or something. Or hanged because was selling kits that killed somebody. By the way weren't Kevin's first kits the "dangerous" type?
I agree it doesn't sound like a good idea to have HV on the front panel but almost always the pot is grounded directly to the chassis and you can also use a plastic knob. If you have a fuse where necessary nothing will catch on fire because the fuse will blow before that.

[Tom Phillips]

What you're describing isn't at all hard to do. I think the reason that people aren't doing what you've described is because they haven't thought things all the way through. It seems that they stopped working on their designs as soon as they developed something that "worked," and they never thought very much about what could go wrong, or better/safer ways to achieve the same goal.

Using a low voltage control mechanism isn't rocket science. And there's already a readily available low voltage source in the amp. The fact that people aren't using it tells me that they aren't trying very hard to come up with a better solution. They're content to sell something simple and cheap because that maximizes their return on investment. The obvious trade off that they're willing to make is in user safety. That's just a bad decision.

The solution to this concern has already been stated.

I'm not affiliated with London Power but I do know that Kevin O'Connor has alreadty done the engineering to address your concern. It's not that hard and has also been done in many laboratory HV power supplies, HiPot testes and oscilloscopes to name a few items. The kit prices are reasonable given all the development time that went into them and could be a good solution for your project.

Now what are you going to do about the high voltage inside that standby switch you flip with your fingers?

[bob p]

The solution to this concern has already been stated....
I'm not affiliated with London Power but I do know that Kevin O'Connor has alreadty done the engineering to address your concern.

It's good to hear that somebody is supposedly doing things the right way. I guess we'll all have to trust you on your recommendation that someone should buy the LP kit instead of the others because the LP kits are responsibly designed. I've done a responsible person's due-diligence in researching this problem, but I haven't found any useful, concrete information that can be relied upon. I looked for a schematic of what KOC is selling, but they aren't available. In fact, there is a purposeful avoidance of disclosure on his part. Someone can't figure out exactly what he's actually selling without buying it first, and he has an "all sales final" no return policy. That creates sort of a Catch-22 doesn't it? You can't find out what his solution provides unless you buy the kit or the book, and once you buy the kit or the book you have no option to return the item if it turns out not to be something you want. Your only option is to blindly send him money and hope that you like what you get in exchange. I refuse to support such a sales model.

It's not that hard and has also been done in many laboratory HV power supplies, HiPot testes and oscilloscopes to name a few items.

I already understand the problem and the solution. The problem (at least as I see it) is that people are selling dangerous kits to end users who might not fully understand the hazards that they present. That's why I've been participating in this thread -- to remove the veil that's been covering up this information so that end users can make informed decisions.

Based on your post it sounds like we're in agreement that it's not conscionable that people would be selling unsafe kits, given that the technology to make things safe has been around for a long time. The problem is that such technology just isn't being utilized by everyone selling B+ modification kits, when it should be.

Now what are you going to do about the high voltage inside that standby switch you flip with your fingers?

What are you talking about Tom? I don't own an amp that has B+ on a user interface. That's one of the first bad design features that I eliminated from my amps.