Tubes & "ground" potential/continuity

[J Moose]

Might be a stupid question... but tubes, and specifically (for this case) preamp tubes... I assume that one of those pins would be tied to ground right?

One of my amps has had this really really odd problem which is related to ground... I randomly get 60 cycle hum at full volume. Sounds great! Talked to a few techs and followed their advice, can't find the issue so I sorta shelved the amp and pulled the preamp tubes for other things... And since I did that last week at least two or three of those 12ax7's have completely failed in other amps... one of them today...

So now I'm thinking...

Thanks!

 

[kmcguitars]

Re: Tubes & "ground" potential/continuity

I Imagine pin 9 would be a common ground via the heater wiring ?

 

[J Moose]

Re: Tubes & "ground" potential/continuity

That's what I'm trying to confirm! lol

I wonder if I can check the circuit continuity with a meter... from guitar ground, to audio ground... to tube ground at the sockets. How are they all related? Is there any relation? I know there's a relation between the amplifiers circuit & the guitar... does that relate to the tubes at all?

hmmmm...

 

[J Moose]

Re: Tubes & "ground" potential/continuity

ehhh?

Asked a bunch of people this question and I'm getting conflicting reports...

Seems like nobody really knows if the heater center is shared with ground.

 

[GoldenVulture]

Re: Tubes & "ground" potential/continuity

One of my amps has had this really really odd problem which is related to ground... I randomly get 60 cycle hum at full volume.

Sounds like a faulty cap,maybe in the pre amp section.

J Moose Re: Tubes & "ground" potential/continuity
ehhh?

Asked a bunch of people this question and I'm getting conflicting reports...

Seems like nobody really knows if the heater center is shared with ground.

Depends on how your amp is wired.As a lot of tubes are run from an AC tap on your power transformer,they are directly connected with no grounding point.EG:Typical 12AX7 ; with this,4&5 are tied together,9 is your other pole.[for 6 volt operation]

In some designs,valve grids maybe grounded[where they have multiple grids]This is more likely in output valves.

This is a circuit for a Marshall Major.You'll see the cathodes in the pre amp valves are connected by components to ground.At the output valves they are tied together with grids and directly grounded.Also the heater supply is a "center tapped" ,separate winding on the power transformer.It i grounded via its "tap".Some amps will have no center tap and ground connection for the heaters,connected to the rest of the circuit

You could do a signal path check,but you have to be aware that at points in the circuit,high voltages are present.

I've recently replaced my valve sockets and other components,over a period of time.When i put in the new valve sockets i started having very bad gain problems.It turned out to be dying pre tubes.Why the new valve sockets seemed to precipitate their demise,i can't quite think why.Anyway the way i isolated it as a valve problem;was by measuring the plate voltages with "valve" out and in.Out there was a couple of hundred volts available -with valve in,it died to about 50 volts.Put in a good valve and there is not a lot of difference of a couple of hundred volts available.
..........I don't know the explanation for why it happens,but on going through the rest of my pre tubes;the duds sucked my high voltage.If you look at the major circuit,you'll notice a resistor on the plate voltage feed.These measurements were done to the valve side of that resistor.Measurements from the other side do not show the draw very much ,if at all.