HV CT Ground

C Moore · Post by **C Moore** » Sat Apr 16, 2011 9:15 pm

I have built a 2x6V6 cat bias amp inside of an old PA head. It all sounds good except for some extra hum on the volume and tone pot (increases and decreases with those two pots). I have fooled with the pot grounds and cannot get them any better than they are. Just looking for other possibilities.
In general, is it OK to ground the high voltage center tap at the same location as the power tube cat resistor/cap ground.?
Thanks

martin manning · Post by **martin manning** » Sat Apr 16, 2011 9:35 pm

I've seen it done with no ill effect. What frequency is the hum? If it's 60Hz look at filament wiring, 120 Hz look at HT.

C Moore · Post by **C Moore** » Sat Apr 16, 2011 9:49 pm

martin manning wrote:I've seen it done with no ill effect. What frequency is the hum? If it's 60Hz look at filament wiring, 120 Hz look at HT.

I see this question all the time, and I wish I really knew. I have tried to measure it, at the speaker, with my DMM; but i get nothing at that point.
To my ear it has more of a 60Hz hum than a 120Hz buzz-hum. The thing is, my heater wiring "looks" pretty good, and I cannot chop-stick the heaters to get any relief. I fooled with the Heater CT and 100 ohm resistors to no effect. What I ended up with is ONE, 100 ohm resistor from each power tube heater (just one tube) to ground of the power tube socket
I guess I can try re-locating the HV ground to its own bolt and see what happens.
Thanks
ps....this is my first try at an amp that uses octal preamp tubes. any extra noise concerns when going octal......?
thanks again

ToneMerc · Post by **ToneMerc** » Sat Apr 16, 2011 10:28 pm

Try this, connect only a single 100 ohm resistor to the filament center tap. Then connect the the other side of that resistor to the power tube cathode.

TM

martin manning · Post by **martin manning** » Sun Apr 17, 2011 1:39 am

Do you have a filament winding CT and two 100-ohm resistors? Use one or the other, but not both. Using the cathode voltage to elevate the filament as TM suggested is a good idea.

For a 120 Hz reference, the note B2, 5th string, second fret on a guitar is 123 Hz. For 60 Hz you need B1, 3rd string, second fret on a bass, which is 62 Hz.

C Moore · Post by **C Moore** » Sun Apr 17, 2011 1:48 am

martin manning wrote:Do you have a filament winding CT and two 100-ohm resistors? Use one or the other, but not both. Using the cathode voltage to elevate the filament as TM suggested is a good idea.

For a 120 Hz reference, the note B2, 5th string, second fret on a guitar is 123 Hz. For 60 Hz you need B1, 3rd string, second fret on a bass, which is 62 Hz.

I am not using both methods (although I did not think it was a problem). I am just using 2x100 ohm from power tube heaters to ground. So you guys thing I should try "elevating" the heater reference.?
OK I will do as TM suggests. Is that better/different than using a 100 ohm from each heater of a power tube to the power tube cathode.?

ToneMerc · Post by **ToneMerc** » Sun Apr 17, 2011 1:58 am

hired hand wrote:So you guys thing I should try "elevating" the heater reference.?
Is that better/different than using a 100 ohm from each heater of a power tube to the power tube cathode.?

Yes, but make sure you remove the two 100 ohm resistors at the power tubes to ground.

That filament CT/cathode wiring scheme biases the filament supply with a small amount of DC voltage, which can reduce hum slightly.

TM

C Moore · Post by **C Moore** » Sun Apr 17, 2011 2:09 am

Absolutely will remove my existing heater reference set-up.
Thanks

martin manning · Post by **martin manning** » Sun Apr 17, 2011 10:16 am

You can elevate either type of center tap (real or artificial via 2x100-ohm) by tying it to the cathode. Another option is to replace the 2x100-ohm with a 470-ohm pot, wiper to ground. This will let you balance the filament circuit for minimum hum.

I don't really know what the problem is with using both a CT and 2x100-ohm; in theory there would be no voltage difference between the two grounds. We know that won't be true in practice, and perhaps that only becomes a problem when the real CT and the artificial CT are grounded in two different places.

Phil_S · Post by **Phil_S** » Sun Apr 17, 2011 4:13 pm

hired hand wrote:In general, is it OK to ground the high voltage center tap at the same location as the power tube cat resistor/cap ground.?

You are getting lots of good advice here. I thought I'd just put in my 2 cents anyway. In general, I put all the high potential grounds on the same ground lug, a bolt through the chassis. This includes power tube cathodes, output transformer, power tube caps, HT center tap, and probably something else I've forgotten. It is OK? IMHO, most definitely.

tubeswell · Post by **tubeswell** » Sun Apr 17, 2011 6:52 pm

I would ground the HT centre tap with the first (reservoir) filter cap ground, whatever else its grounded with. (Why? See point 7 of R.G.'s attached article)

C Moore · Post by **C Moore** » Sun Apr 17, 2011 7:11 pm

Thanks again to everybody....elevated heaters did not solve my troubles.
I have been trying to get rid of this problem for several weeks now. I only have a limited knowledge of electronics, but I am thinking that:
1. Because I have installed a Ampeg Schematic in a old PA Head, I am having some kind of lead dress/lay-out issues. My pots are quite a distance from the rest of the amp, tubes, components. So I have some long ground leads, or short ground leads that are going to less than perfect locations. But I do not have enough experience/knowledge to recognize the exact problem.
2. Is it possible that the existing Power Transformer (circa 1950) is "noisy" and is causing this noise, or is the PT the least likely culprit at this point.?
Thanks
ps a lot of you guys have tried to help me with this amp already. It has come a long way, sounds good, and really only has this one last issue. i will post some detailed gut shots AGAIN.....thanks

C Moore · Post by **C Moore** » Sun Apr 17, 2011 7:37 pm

C Moore · Post by **C Moore** » Sun Apr 17, 2011 7:55 pm

tubeswell wrote:I would ground the HT centre tap with the first (reservoir) filter cap ground, whatever else its grounded with. (Why? See point 7 of R.G.'s attached article)

I have it grounded there, along with the cat bias for the power tubes......so HV CT, Reservoir cap, Power tube cats.

C Moore · Post by **C Moore** » Sun Apr 17, 2011 11:58 pm

Let me ask another question....
I have never converted a PA Head before. Maybe I am making a wrong assumption. The OT has taps for 0....4.....8. I am using the 0 and 8 taps. 0 goes to the sleeve of the speaker jack and also to ground via a un-used tube socket.
On the other side of the OT there are taps for 500...250...S....F.
In between the S and F taps is stamped either FB or PB....just the two letters, no tap. I do not have anything connected to any of that stuff. The amp originally had inputs for microphone and phonograph, and I assumed those taps were for that stuff. Do I need to be involved with any of those taps for some reason.?
Thank You

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