6J7 preamp

[Osvaldo de Banfield]

Pin 1 MUST be to ground, not to the cathode!!!

[Joe Roberts]

Originally Posted by Osvaldo de Banfield

Pin 1 MUST be to ground, not to the cathode!!!

That is for sure.

[disco]

for starters....

[Osvaldo de Banfield]

Originally Posted by Joe Roberts

That is for sure.

Appears for the guy, not to be so.

[6J7]

Thank you all for your answers, tomorrow I'll take a photo and try what you said.

About pin 1, when looking at the datasheet:

"with shell and internal shield connected to cathode"

I tought it was the way to connect it..

[Joe Roberts]

Appears for the guy, not to be so.

But he is off to a great start. Nice looking project!

Time to learn a few basic tube DIY lessons the hard way, the best way to learn.

[6J7]

Hi,

This morning I connected the grid wire shield to ground, connected the 6J7 pin 1 to ground and completely disconnected the pot (maybe easier to find the problem)

Not yet connected to my system to see if there is an improvement but I tried this:

- input shorted
- preamp output to line-in computer (playthrough option + headphones), laptop on battery only.

As soon as I switch on the main preamp switch (not the B+), there is HUM.. even when the 6J7 are removed from the sockets..

Here is a pic:

https://dl.dropbox.com/u/425350/6J7%...p/IMG_1631.jpg

- C1 si common to L & R
- chassis connected to earth (also green/yellow wire of the transformer)
- C1 ground connected to earth through a 100R (test without earth connection, it does not really change something)

Transformer is a toroidal..

Do you see some things totally wrong on the pic ? ( Yes, I am a beginner..)

Thank you

[6J7]

Following the same procedure (laptop + headphones), I moved the preamplifier to the living room on its table, I clearly hear a hum even when it's not plugged in the mains..

When switching off the 1KVA isolation transformer who is just under the table, hum disappears.

When moving the preamplifier 1 meter away from the 1KVA transformer, hum disappears.

When connecting the preamp to the mains (but switched off) while it's away, no hum.

When I switch it on (no B+), hum appears...

[Shoog]

Single star ground for all grounding points has never failed to kill hum for me. Bus type grounds often produce issues with hum. Pick a central point on your case and run earth leads with tags to that point. Run a bolt through the case, attach all tags and bolt down.

PS - The connection between signal earth and safety earth has been cut, reinstate it heavy gauge wire without the resistor. Think of current as water flowing down hill with the safety earth as a sink hole. It encourages the high currents in the power supply section to bleed off to earth rather than up to the small signal ground. If you try this it may allow you to avoid using star grounding.

Shoog

[6J7]

I removed the resistor and installed a big wire, there is definitively a big improvement ! hum not completely gone but far better than before, thanks.

This preamplifier has too much gain, time to try again with the stepdown transformers.

I have to reinstall the volume pot, is it a bad idea to connect it to output instead of input ? it's a 10K.