6J7 preamp

[disco]

It's a bad idea because you want to have low output impedance connecting to high input impedance.

Put a 50Kohm or 100Kohm pot (log or audiotaper) in front of your 6J7 tube and use shielded cables. Connect the input ground to the pot (left finger -not the wiper), and also connect the shield of the cable running to the grid here. Take a seperate run of wire from this point to your starground. Don't connect the other end of the shield (near the grid) to anything.

[Original Burnedfingers]

Why is the plate voltage fused? Would different value chokes be better for this application?

[6J7]

Thanks Disco and Joe Roberts about pot advices, I'll do that.

Plate is fused in case something goes wrong with a tube, I thought it was a good idea.. but I did not find a good value (fast 10-15mA ?).

About chokes you're right, probably not a very good choice but it seems to do the job, I don't hear any 100 Hz noise.
First I tried with IGPW chokes (4H 100mA) but the sound was unpleasant, flat.. so returned to the Hammond 156R and sound opened. I wonder why..

[Shoog]

People report that chokes can have a significant effect on the sound of an amp. There are all sorts of parasitics which could account for this.

Shoog

[Joe Roberts]

Thanks Disco and Joe Roberts about pot advices, I'll do that.

Ignore what I said and go with Disco's last suggestions, now that I have seen pics. It will get you to the same place.

[Shoog]

Following on from your initial success with the earthy bar adjustment - can I suggest you try this easily reversed experiment.
-Snip the wire between your small signal ground and your power supply ground.
-Attach a small gauge wire to the mid point of your small signal ground.
-Attach the other end to the PSU ground bar, then try it attached to the safety earth - see if either configuration gives a reduction in the hum.
If not rejoin the bus bar at the snip point.

Shoog

[Original Burnedfingers]

Originally Posted by 6J7

Thanks Disco and Joe Roberts about pot advices, I'll do that.

Plate is fused in case something goes wrong with a tube, I thought it was a good idea.. but I did not find a good value (fast 10-15mA ?).

About chokes you're right, probably not a very good choice but it seems to do the job, I don't hear any 100 Hz noise.
First I tried with IGPW chokes (4H 100mA) but the sound was unpleasant, flat.. so returned to the Hammond 156R and sound opened. I wonder why..

I've never used a plate fuse before and to be honest here I wonder if it is of any value. I have seen it in output tubes but then again I think it would be better to use a cathode resistor of a small wattage that would simply go open. I mentioned the choke size because I still do not know how to properly size a choke for an application. I have built some line stages that had specified a choke of 30H @ 40mA so I have a few of them on hand.

[6J7]

As my english is not good, I am not sure to understand 100% of your suggestions.

So I drew a ground schematic, could you please tell me if it's ok ?

A few more questions

- do I have to connect shield between pot ground and bus PS ground wire ?
- same question for output ground wire..
- is it better to connect the grid leak resistor (300K) directly between the wiper and pot ground ?

About plate fuse, why not remove it, in this case a 300R 1/4W should be enough (2V on cathode, 6-7mA).

Thanks

[disco]

Here you go: a starground.



It's in its most extensive version. Often the input side collapses to one feed (to starground) from the orange dot. Also, normally the tube has one feed to starground. If the output is located at >15cm distance, run an extra feed to starground.

The last capacitor of the PSU connects to starground, as does your case (make 1 connection only). The heaters connect to the starground, either through two resistors, a humpot or through a capacitor. The mains safety ground goes to your case and connects to everything one can touch on the outside.
So, there's one central hub we call ground, which defines the only 0,000 volt reference in your amplifier. All voltages are either above or below this potential.

Your groundbar may also be used but it takes practise to implement as disrupting large currents can interfere with small audio signals and multiple ground points may introduce alternative paths to ground.

[disco]