Proper type of switch for choke/standby/resistor setup?

[dehughes]

Hello there,

So, I want to use my standby switch for changing from standby to either a resistor or choke. I've seen this done on a Dumble-ish schematic or two. What type of switch would I need? DPDT? Something different? I have a hard time getting my head around switches for some reason...

Thanks in advance...

[brownnote]

You could do it with a SPDT....center off

[M Fowler]

My thinking was backwards of yours, I was thinking a DPST to shunt the resistor with the switch?

Mark

[dehughes]

You could do it with a SPDT....center off

This sounds correct....center is "standby", up is the choke, down is the resistor (or something like that).

Would this switch do it?

http://www.mojomusicalsupply.com/item.a ... id=1400008

[tubeswell]

Hi dehughes

I did this with a DPDT in a 5E3 build last year

Scheme attached

I recommend you use a good quality switch with good contacts

(PS I got a brother in Wilsonville - in the wops. He says its a lot like NZ in that neck of the woods)

[dehughes]

Thanks tubeswell. Wilsonville is a nice place...I have a pastor friend who lives in a nice little corner out there. Very cool.

As for your schematic...it's not quite what I had in mind, but it's a neat idea. I was just thinking of having the choke/resistor option handled by a standby switch, instead of your setup where you have a power, standby, and choke/resistor switch.

A question about your schematic: Why do you switch in a 4k7 after the choke, but not otherwise? Does the voltage stay too high with the choke as opposed to the resistor?

[brownnote]

I found this diagram (attached), is this what you had in mind?

[dehughes]

EXACTLY it, Brownnote!

Maybe use a switch like this one, the long bat Carling SPDT?

http://www.hoffmanamps.com/MyStore/perl ... =271540435

[brownnote]

Sure, that's good one.
I like the NKK S303 switches:
http://www.alliedelec.com/Search/Produc ... N=S303T-RO

If you can find the S303 it has solder lugs as opposed to the S303T which has the blade connectors, although they do have a hole and are easily soldered to.

[dehughes]

GREAT. Thanks so much!

Something like this?

http://www.mouser.com/Search/ProductDet ... TEiW8XU%3d

[brownnote]

That's the one!
I just like the action and feel on those.

[tubeswell]

A question about your schematic: Why do you switch in a 4k7 after the choke, but not otherwise? Does the voltage stay too high with the choke as opposed to the resistor?

Yep I found the 4k7 helped keep the screens about 20V under the plates with the choke running, and gave a good sound. Without any resistor there, the screens were about the same as the plates (which were higher than I like) or maybe the screens were even about 4V higher? (can't exactly remember). Anyway the DPST gives the ability to do that, but any DP switch would allow the same

And being unbypassed, this resistor adds a bit of screen current feedback as well, which makes it a bit more linear.

[dehughes]

Interesting...I just put a 1k screen resistor on the grids of my 6V6s (Mercury Magnetics Brown Deluxe tranny...cathode biased, though) and it does seem to be a bit more "giving" now (no resistors before). The amp squishes a bit more in a pleasing way....kinda like being hit by a futon as opposed to a dense rubber mat. I think I'll keep it. I put 3w resistors in there...would that be enough? Should I put some 5w ones in?

[tubeswell]

Interesting...I just put a 1k screen resistor on the grids of my 6V6s (Mercury Magnetics Brown Deluxe tranny...cathode biased, though) and it does seem to be a bit more "giving" now (no resistors before). The amp squishes a bit more in a pleasing way....kinda like being hit by a futon as opposed to a dense rubber mat. I think I'll keep it. I put 3w resistors in there...would that be enough? Should I put some 5w ones in?

Measure the (quiescent) voltage drop across each resistor, then divide that by the actual DC resistance of each resistor, and you will get the screen current at idle (its likely to be somewhere between 2mA - 4mA or in that ballpark) Multiply this screen current by the voltage between each screen and the cathode and you will get the dissipation of the resistor at idle. Double this dissipation value to get a 'safely' rated resistor.

[dehughes]

Interesting...I just put a 1k screen resistor on the grids of my 6V6s (Mercury Magnetics Brown Deluxe tranny...cathode biased, though) and it does seem to be a bit more "giving" now (no resistors before). The amp squishes a bit more in a pleasing way....kinda like being hit by a futon as opposed to a dense rubber mat. I think I'll keep it. I put 3w resistors in there...would that be enough? Should I put some 5w ones in?

Measure the (quiescent) voltage drop across each resistor, then divide that by the actual DC resistance of each resistor, and you will get the screen current at idle (its likely to be somewhere between 2mA - 4mA or in that ballpark) Multiply this screen current by the voltage between each screen and the cathode and you will get the dissipation of the resistor at idle. Double this dissipation value to get a 'safely' rated resistor.

Will do. Thanks!

Okay...each 1k/3w screen resistor is seeing about .95w total, so I think I'm safe with these 3w ones. They don't get hot at all. Next time, I'll use RS-5 Dales just for the sake of overkill/overbuilding.

Oh, and I picked up a DPDT switch which works just dandy for my bright cap (another thing...different from the standby switch, obviously). Thanks all!