100ohm on Mission amp

[angelodp]

Hi, i am building a Mission amp 5E3 and there is a suggestion to put the 100ohm resistors on pins 2 - 8 and 7-8 instead of the way its shown on the orig drawing. I am new at this so i have re-drawn it and wondering if it makes sense the way i have it.

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[tubeswell]

Hi angelodp

Bruce just means to use pin 8 as a handy anchor point for the ground side of the 100R artificial heater 'ground-reference' resistors (albeit that these are elevated to the 6V6 cathode voltage). Pin 8 isn't connected to anything on a 6V6, so that socket pin is available as a spare terminal lug.

The way you have drawn it will work also. It's just that Bruce's idea is probably tidier-looking once its done

[angelodp]

I was simply trying to establish a layout for the pic that Bruce sent. He includes this pic which does not match the layout, so I went ahead and redrew the layout to check things. I know there are many ways to do circuits and so i am just double checking that I have it correct.

[scottb1966]

I am currently building a Mission kit and am wondering what these 2 resistors do. They are not in the original 5E3 layout. Do they change the tone, decrease hum, etc?

[tubeswell]

I am currently building a Mission kit and am wondering what these 2 resistors do. They are not in the original 5E3 layout. Do they change the tone, decrease hum, etc?

They are necessary when wiring up a heater winding that does not have a centre tap. They act as an artificial centre tap/ground reference point for the heater voltage, and they ensure that both sides of the heater winding are swinging at more-or-less precisely opposite VAC swings with respect to each other, so that you cancel-out any possible heater AC hum in the tubes (hum which would otherwise be induced into the signal path either through the tubes themselves, and/or through stray EMF from the heater wires into other signal wires)

[JoeM]

... Pin 8 isn't connected to anything on a 6V6, so that socket pin is available as a spare terminal lug.

Pin 8 is the cathode, hence the connection for the heaters being elevated. (Cathode bias on the 5E3 of course).

[scottb1966]

They are necessary when wiring up a heater winding that does not have a centre tap. They act as an artificial centre tap/ground reference point for the heater voltage, and they ensure that both sides of the heater winding are swinging at more-or-less precisely opposite VAC swings with respect to each other, so that you cancel-out any possible heater AC hum in the tubes (hum which would otherwise be induced into the signal path either through the tubes themselves, and/or through stray EMF from the heater wires into other signal wires)

Thanks, I will solder them in. I am about 1/2 done. Finished the turret board last night. Should have it running by the weekend.

[tubeswell]

Pin 8 is the cathode, hence the connection for the heaters being elevated. (Cathode bias on the 5E3 of course).

Right you are - crikey my lapses are getting awful. I was thinking of Pin 1 when I wrote that.

[angelodp]

I soldered it in as shown and I am all done.... amp sounds great. Dead quiet on HB's with an open chassis. Should be even quieter once in the cabinet. I was thinking of putting a shield on the inside on the panel that screws into the cab in front of the chassis. I guess a copper or aluminum foil will work. Do the vintage 5E3's have that shielding??

ange

[Bruce / Mission Amps]

I soldered it in as shown and I am all done.... amp sounds great. Dead quiet on HB's with an open chassis. Should be even quieter once in the cabinet. I was thinking of putting a shield on the inside on the panel that screws into the cab in front of the chassis. I guess a copper or aluminum foil will work. Do the vintage 5E3's have that shielding??

ange

There isn't a huge amount of gain in these amps that which would require an abnormal amount of special shielding.
However, I do use some Al foil, HVAC tape on the back upper valance panel when I build my own kits.
I don't remember seeing any foil or Faraday shielding on old amps like this but many old-ass tweed amps had a strip of, "gasp" , asbestos sheeting under that rear upper panel!
Mostly to keep the hot running power tubes from cooking the cheap 1/4" thick plywood panel.