Hi,
relatively new to tubes DIY, I built my first line preamplifier (capacitor output from plate) with 6J7.
It works well, then tried to connect transformers (LL1681) 13:1 to lower output impedance, I don't notice any improvement.
Actually the preamplifer is connected to a small trends audio class D amp (50K input), later it will be my 807 PP.
My question is:
Is there any interest to add a CF after the first stage ?
By the way, I have hum (50 Hz) problems with 6J7.. is this tube normally quiet when AC heated ?
Thanks
relatively new to tubes DIY, I built my first line preamplifier (capacitor output from plate) with 6J7.
It works well, then tried to connect transformers (LL1681) 13:1 to lower output impedance, I don't notice any improvement.
Actually the preamplifer is connected to a small trends audio class D amp (50K input), later it will be my 807 PP.
My question is:
Is there any interest to add a CF after the first stage ?
By the way, I have hum (50 Hz) problems with 6J7.. is this tube normally quiet when AC heated ?
Thanks
An externally hosted image should be here but it was not working when we last tested it.
With input shorted, the hum is always at the same level.
With source (V-DAC) connected, the hum increase when turning up the volume..
Thanks
Move your psu ground from the 100uF cap to the C3 minusses (left and right tied together). Drop the 100 ohm to chassis. Any better?
Next, move the heater grounds there too. The way you had connected grounds made the heater potential run through your signal ground.
It's best to keep groundruns low ohmic by choosing thick solid copper wire or thinner braided copper wire or thinnest silver wire. Be aware for the witch hunt when using silver as it's only done for AF by audiophools (people depending on their ears only).
Next, move the heater grounds there too. The way you had connected grounds made the heater potential run through your signal ground.
It's best to keep groundruns low ohmic by choosing thick solid copper wire or thinner braided copper wire or thinnest silver wire. Be aware for the witch hunt when using silver as it's only done for AF by audiophools (people depending on their ears only).
Last edited:
Ground the grid lead shield to the chassis. Do not connect shield to hot anywhere.
If your pot has a metal back, ground that to chassis also--or ground the shield to the metal back then ground both to chassis.
I have had better luck with chassis grounds for this sort of thing than grounding to the signal ground. Is that chassis powder coated inside also? Make sure you scrape off some of the coating to get to bare metal.
It sounds to me as though this is an input wiring issue. A pic of actual wiring would be more helpful than the schematic.
Last edited:
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%20Preamp/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
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%20Preamp/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
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...
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...
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
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
Last edited:
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.
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.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.