Best Place For Heater CT?

[Colossal]

I like to elevate heaters to DC via the center tap and, in conjunction with good lead dress, I think it results in very quiet amps. But if I didn't elevate, I'd ground the center tap at V1 for the aforementioned reasons.

[morcey2]

Remember that if you have a cathode follower or some other stage that has one of the cathodes at an elevated potential _and_ you use tubes that are sensitive to heater-to-cathode voltage, you must use an elevated heater reference. Examples are the Tung-Sol reissue 12AX7 and sovtek 12AX7LPS. I had 2 tung-sols fail in the V2 position on my HO before I put in an elevated reference that was at a higher potential than the power tube cathode resistor.

I've also built two amps that I just tied one of the heater legs to the power-tube cathode and called it good. Neither were real high-gainers, but they were both dead quiet.

Matt

[Gaz]

This is how I do the grounding in my amps (except bias caps, of course).

[IMG1275]http://i52.photobucket.com/albums/g39/joeyvoltage/groundscheme.jpg[/img]

What do you do with the bias caps? I do grounding exactly like that diagram, except I elevate the heaters.

[roberto]

I reverse the polarity.

[pablogt]

I use the same grounding scheme as Roberto and Gaz. See the attached schematic for my Express. All groundings are labeled with the local grounding star to which they belong (s1,s2...s5). In this case, heaters are elevated. If they were not, they would be grounded to s1, the PT's grounding.

Pablo

[rp]

Hate to stir this up again, the overview of the replies here seems to be many ways and whatever works, but and I’m going w/ the consensus connecting the heater CT to the input jack /V1/chassis ground end of my bus bar. Only thing I don’t like is how it runs across the length of the board.

And, I keep asking myself as the point here is to dump any heater noise as soon as possible into the chassis and then to the mains ground, why not just connect the heater CT at the chassis mains ground? I understand that it risks ground loops as who knows where the heater current will travel trough the chassis but as the heater is common to all stages why does it matter?

Also, as everything in this amp is isolated and everything grounds to the bus bar with just one chassis connection at the input, the heater reference, even if at the chassis mains end, completes the circuit at V1 / input anyway. In other words it all seems like the same thing to me, only at the mains end it dumps any noise out to the wall quicker.

The thing I like about grounding the CT to the bus is that then there is no potential on the chassis.

Any more thoughts?

[Phil_S]

Hate to stir this up again...why not just connect the heater CT at the chassis mains ground?

The mains ground (in US, the green wire from the wall socket) is required to be isolated.

I'm an amateur, and there is much I don't quite understand, but that isn't going to stop me from writing this.

As I understand it, there can be a lot of hash riding on the mains ground. Remember, this goes all the way back to the breaker panel in your house. That is one reason you don't want to use it, as it is not a quiet spot.

A better reason is that the mains ground is really the redundant neutral -- a safety feature. (I'm not sure about EU, are both of the other legs hot -- 110 each -- or is one carrying 220 and the other a return?) In either case, by connecting the heater secondary to the mains ground, you are cross connecting the PT primary and secondary. This in my view is a big no-no. If you want the mains ground to do it's job, leave it by itself.

[pdf64]

I just happened to read Merlin's analysis of grounding http://www.valvewizard.co.uk/Grounding.pdf
He mentions heater reference grounding, advising that if a grounded reference is to be used, it can be beneficial to try various different points.

As it's purely a reference, no current flows.

I just tied one of the heater legs to the power-tube cathode and called it good. Neither were real high-gainers, but they were both dead quiet

That agrees with my thinking and experience; as long as the lowest potential of the heater is above the highest potential of the pre-amp cathodes, then the heater-cathode junction should stay reverse biased and therefore minimise leakage.
No need for a center tap.

Phil, please have a read of the above article, it may help to give a good insight of the system, issues and required safe practice.
Pete