bias test points

[Photios]

what's the part number for the external bias points? you know, these things:

[IMG:500:280]http://i145.photobucket.com/albums/r227 ... points.jpg[/img]

[Bob-I]

mouser 530-105-0802-1 for RED 530-105-0803-1 for BLACK. Don't forget the 1ohm 1% 2 watt resistors.

[David Root]

2W rating is overkill, I think. Even if you were drawing 250mA per tube, (unlikely) that's only .0625W per 1 ohm resistor. These resistors need to fit between two tip jacks, I set mine at 0.75" centers and use 1/2 watt Speer metal films, which fit nicely. If you have 2W 1 ohms lying around, no harm in using 'em, though.

[Alexo]

.5W's can also act as a handy fuse if you get a short in a power tube.

Back when I started tinkering, my first project was to restore an old Fisher stereo, all the old hi-fi guys strongly recommended 1/4 watt 10 ohm resistors between the 7868's cathodes and ground, iirc, for this reason. Funny that I've never seen a similar rec. in the instrument amp world.

[Photios]

do you have to use metal film resistors? I'm ordering from Allied and they don't have a 1 ohm 1% metal film resistor...would a wire wound resisitor work?

[David Root]

It's a matter of size, really. If you can fit in a wirewound, it'll work fine, but you're likely paying too much. Mouser has Speer 1% 1/2W metal films for $0.17 ea. if you buy 10. Just don't use cheap carbon films, or metal oxide power resistors.

And yes, we don't think about fusing power tube cathodes in the MI amp community. We should, the Ancients had that right, and the higher the B+, the spikier your rectifier and the higher the cathode current, i.e. how close to the edge are you?, the righter they were. I still use 1/2W metal films, even though they're not flameproof.

[Allynmey]

Photios, I use 3W Ohmite 1 ohm 1% Resistors there. Hoffman has those bias points also.

[drz400]

.5W's can also act as a handy fuse if you get a short in a power tube.

Back when I started tinkering, my first project was to restore an old Fisher stereo, all the old hi-fi guys strongly recommended 1/4 watt 10 ohm resistors between the 7868's cathodes and ground, iirc, for this reason. Funny that I've never seen a similar rec. in the instrument amp world.

I like fusing cathodes with a real fuse that is accessable from the outside.
I did the sensing resistor mod for a guy who wound up cursing me from Europe when he popped a 1/2W and they couldnt figure out what was wrong on the road, replaced tubes and he was still dead. Learned my lesson from that one, If you use a HT fuse then make the sensing resistor 1% 1ohm 3W, that is what Humble uses RS-28 DALE from Mouser

[David Root]

Now I see the argument for 3W. You don't want two fuses in the same line, makes sense.

[lastwinj]

1ohm 1watt carbon film. close enough for our purposes, .15 each.

germ