Screen Kill Standby

[imaradiostar]

Makes sense- I work at a lab where we have to think that way or our work goes away! I'm sure if you're using decent quality switches it won't be an issue. I've been using crappy radio shack switches that way on a few of "my" (as in not for customer) amps and I haven't had any failures or strangeness.

I'll also say it's not uncommon and it's a lot safer to switch the AC into a cap than it is to switch the DC between a cap and a load.

I used cathode lift standby on one of my old amps because I couldn't get it to switch to standby quietly otherwise. That was...12 years ago? I dunno. I was 19 or so and I was just happy to have people come to my college apartment and say "you built that? Cool!" I have no idea if it was a sound way of doing things but I'd but someone on Ampage suggested it.

jamie

[Merlinb]

You people do make one hell of a fuss over a bleedin' switch! How hard can it be? You're like a bunch of old women!
You're gonna have to take the training wheels of eventually Jon...
Heck if this thread goes on any longer you'll be patenting the damn switch like M3SA does!

[Steve Conner]

Lol! You tell them, Merlin.

Actually sometimes these are the hardest design problems. 100 ways to wire up a standby switch, 3 are obviously wrong, but the remaining 97 work the same... There's no practical difference between them, so how do you choose one?

[Wilder Amplification]

And that's the whole thing...too many ways to accomplish the end objective.

I've been an electronics guru for almost 20 years, with 13 of that being amp building experience (and no, I didn't learn via a "kit"...there were no kits available that I know of when I got started with amps). But most of my experience has either been building clones or modding Marshalls, restoring old Fenders, etc etc. This is my absolute first time ever engineering something from the ground up, so naturally there will be a multitude of questions.

Of course...everything in tube technology has already been done, so essentially anymore it seems like you're just putting together a hodge podge of tried and true circuits nowadays. On top of that, it seems that in recent times too many people have removed the science aspect of guitar amps all in the name of tone and "blind engineering" seems to be the "norm" when it comes to guitar amps.

[Steve Conner]

Well, then you have to ask yourself, what are you trying to achieve with your amp design?

Are you going to sell it? Then choose the cheapest standby switch implementation.

Are you designing it to satisfy your own inner sense of engineering beauty, and you don't really care if you could ever make a profit on it? Choose the most elegant standby switch. It seems that's what this thread is about, but elegant isn't always right for something you want to commercialize.

"An engineer is someone who can make for five dollars what any fool can make for fifty."

I remember having discussions with black_labb about the separate fusing issue. He built a dual rail bass amp and was worried about blowing the screens if the plate fuse went.

By the way, if you were making several amps the same, you can probably get a full custom PT wound: my local winder has a minimum order of 10 units for smaller transformers, and he absorbs the setup costs that way. But they're all different: Sowter will make you just one custom transformer, with any windings you want, but it'll cost you the best part of 200 bucks.

[GainFreak]

Screen (or Screen + Preamp) supply switching is perfectly safe, and a good idea -PROVIDED you don't have a choke in the amp, which applies to the Marshall circuit below, and most Peaveys (that I've had in).

The problem is that the choke sets up a mighty magnetic field (B) when some screen/preamp current is flowing in it. If you then go and open a switch in series with this choke, and the current has nowhere to go. This forces the magnetic field to collapse, precipitating an increase in voltage across the choke that keeps on rising until the energy stored in the choke can go somewhere! Usually the energy finds something to break down, like your switch contacts or the choke insulation. Boom!

I didn't quite get that. If you replace the resistor on that Marshall schematic with a choke it will still be connected to the preamp tubes. The standy switch then is going to the screens. I don't see how the choke will find itself in the situation you described.