What is this component?

[pchilson]

Hey guys,

I've been slowly working through this amp sketching out a schematic as there is none to be found anywhere.
The tube compliment is:
6SC7
6SF5
6V6GT x 2
5Y3GT

On the 6SF5 there is this little component attached from pin5 (plate) directly to ground, can't be more than 3/8" long, looks like a grease zerk. Black in color with a dab of violet and yellow. Leads come out of the side of the body kinda like an orange drop but definitely not out of the ends.

I can not measure any resistance across it.
Anyone know what this is and its intended purpose?

Thanks

[Robert M. Martinelli]

Hi,
I think it's a small ( = low capacitance, 4,7 or 47 pf, can't tell for sure from the pics ) capacitor sending the highest frequencies from the plate to GND ( low Xc at high frequencies ) to keep the thing from oscillating.

Hope this helps

Best regards

Bob

[guitician]

Yep, it's a capacitor. A rare tubular type, and by the Yellow/Violet markings it's value is 47pf.

[pchilson]

So, on a recap should this be an included replacement or safe to leave in place?
Any guess to the type? Mica?

Thanks

[gbono]

Yes it's a "dog bone" tubular ceramic capacitor. I suggest leaving it alone unless you are sure it is defective.

Read about them here - http://www.vishay.com/docs/22019/cergenin.pdf

[Mars Amp Repair]

'Tubular Dogbone' ? Sounds like a farrrrout hip old blues cat from the '70s man ;-] g

[dai h.]

basically an obsolete style from what I've seen. Some have less coating and you can see through the tube, while for others the whole body is coated over. There were some new I've seen that had the same shape but they were apparently huge. Some old resistors have the same "dog bone" structure also.

[gbono]

Okay, if your going to use Tubular Dogbone for your stage name I get 15%

I have only encountered these in high frequency designs (way above the audio band)..........

[tboy]

Originally Posted by pchilson

I've been slowly working through this amp sketching out a schematic as there is none to be found anywhere.

What kind of amp is it (make & model)?

[pchilson]

Originally Posted by tboy

What kind of amp is it (make & model)?

Its this Newcomb G-12

[Robert M. Martinelli]

Originally Posted by gbono

I have only encountered these in high frequency designs (way above the audio band)..........

Me too, as I was saying, its Capacitive Reactance Xc is very low at higher frequencies and very high at audio frequencies: Xc= 1/(2Pi*F*C) so, at 20 KHz its capacitive Reactance is about 170 KOhms, at 20 Hz we have 170 MOhms, while at 20 MHz it's 170 Ohms ( and we're waaaay into the RF "realm" ).

This is why my best guess is that capacitor is there to keep that stage from unwanted oscillation, as it's a route to GND for all the frequencies above the audible range.

Hope this helps

Best regards

Bob

[Mars Amp Repair]

[QUOTE=gbono;91441]Okay, if your going to use Tubular Dogbone for your stage name I get 15%