6J7 preamp - Page 3 - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 1st September 2012, 08:33 PM   #21
disco is offline disco  Netherlands
diyAudio Member
 
disco's Avatar
 
Join Date: Apr 2006
Location: Holland
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.
__________________
jaap
  Reply With Quote
Old 1st September 2012, 09:42 PM   #22
diyAudio Member
 
Join Date: Nov 2002
Why is the plate voltage fused? Would different value chokes be better for this application?

Last edited by Original Burnedfingers; 1st September 2012 at 09:52 PM.
  Reply With Quote
Old 2nd September 2012, 06:38 AM   #23
6J7 is offline 6J7
diyAudio Member
 
6J7's Avatar
 
Join Date: Apr 2009
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..
  Reply With Quote
Old 2nd September 2012, 06:54 AM   #24
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
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
  Reply With Quote
Old 2nd September 2012, 08:16 AM   #25
diyAudio Member
 
Joe Roberts's Avatar
 
Join Date: Feb 2006
Location: VA
Quote:
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.
  Reply With Quote
Old 2nd September 2012, 10:45 AM   #26
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
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
  Reply With Quote
Old 2nd September 2012, 11:13 AM   #27
diyAudio Member
 
Join Date: Nov 2002
Quote:
Originally Posted by 6J7 View Post
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.
  Reply With Quote
Old 2nd September 2012, 05:34 PM   #28
6J7 is offline 6J7
diyAudio Member
 
6J7's Avatar
 
Join Date: Apr 2009
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
Attached Images
File Type: jpg Ground.jpg (80.1 KB, 204 views)
  Reply With Quote
Old 2nd September 2012, 06:36 PM   #29
disco is offline disco  Netherlands
diyAudio Member
 
disco's Avatar
 
Join Date: Apr 2006
Location: Holland
Here you go: a starground.

Click the image to open in full size.

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.
__________________
jaap

Last edited by disco; 2nd September 2012 at 07:02 PM.
  Reply With Quote
Old 2nd September 2012, 07:35 PM   #30
disco is offline disco  Netherlands
diyAudio Member
 
disco's Avatar
 
Join Date: Apr 2006
Location: Holland
Click the image to open in full size.
__________________
jaap
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
6J7 driver in WE91 like amplifier bobluis Tubes / Valves 9 10th February 2011 06:09 PM
Tube Preamp Popping on Source Input Change (?) / 6DJ8 Preamp On Chip Amps MartyM Tubes / Valves 3 16th October 2008 04:52 AM
Guitar Preamp = MIC Preamp = other preamp ,,are they all same? Paswa Solid State 1 12th March 2008 05:16 PM
twisted pear Preamp/other suggestions for Preamp kit LukeDuke Chip Amps 1 1st February 2008 09:57 PM
Electret mic preamp - will phono preamp work? darthmullet Analogue Source 6 12th October 2007 07:32 AM


New To Site? Need Help?

All times are GMT. The time now is 08:15 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2