Risk of DC coupled input with tube pre-amp ? - 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 30th May 2011, 08:44 AM   #1
diyAudio Member
 
Join Date: Dec 2006
Default Risk of DC coupled input with tube pre-amp ?

Hi all.

I recently build a simple cathode follower (B+ = 200V) to replace the solid state pre-amp (I did not need the extra gain) in my integrated stereo amplifier. The reson I choose a tube (6H30P-DR) is because in my opinion it sounds better. This is debatable of course, but please not in this thread ;-)

There is a 1M resistor from grid to ground and thus the input does not have a DC component. I still use a capacitor on the input though cause I am afraid to blow up my expensive sources when something goes wrong (as in high voltage DC at the input).

My question is, what is the risk involved with DC coupling the input here? Or can I safely omit the input capacitor ?

Regards,

Ronald
  Reply With Quote
Old 30th May 2011, 09:29 AM   #2
diyAudio Member
 
mastodon's Avatar
 
Join Date: Jan 2011
The risk is not so much blowing up the input equipment, as putting a dc voltage (coming from the input equipment) on the cathode follower's grid, resulting in bias problems in the tube. If you're absolutely 100% sure that there is never going to be a dc voltage on the input, you could remove the input cap. But I'd leave it in place personally.
  Reply With Quote
Old 30th May 2011, 09:30 AM   #3
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Close to zero danger.

DC outputs are a different story.
__________________
The more you pay for it, the less inclined you are to doubt it.- George Smiley
  Reply With Quote
Old 30th May 2011, 09:34 AM   #4
diyAudio Member
 
Join Date: Dec 2006
Quote:
Originally Posted by mastodon View Post
The risk is not so much blowing up the input equipment, as putting a dc voltage (coming from the input equipment) on the cathode follower's grid, resulting in bias problems in the tube. If you're absolutely 100% sure that there is never going to be a dc voltage on the input, you could remove the input cap. But I'd leave it in place personally.
Normally there should not be any dc at the input, but even if it does occur, there is a cap directly after my cathode follower, AC coupling it (since there is DC on the output) to solid state power amplifier circuit.

So I suppose I it will be safe to ommit the input cap then? No chance of getting DC at the input when the tube gets defective?

Regards,

Ronald
  Reply With Quote
Old 30th May 2011, 09:35 AM   #5
diyAudio Member
 
Join Date: Dec 2006
Quote:
Originally Posted by SY View Post
Close to zero danger.

DC outputs are a different story.
Yeah, I know, fried speakers and such ;-)
  Reply With Quote
Old 30th May 2011, 09:40 AM   #6
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
Originally Posted by tingtong5 View Post

So I suppose I it will be safe to ommit the input cap then? No chance of getting DC at the input when the tube gets defective?
Low enough probability that it's not something I'd worry about at all. I've never seen a tube go bad in such a way that plate voltage was sent to the grid. I have once seen a grid to cathode short, but in this case, you won't have more than a volt or two.

Keep in mind that most sources will have an output cap of their own.
__________________
The more you pay for it, the less inclined you are to doubt it.- George Smiley
  Reply With Quote
Old 30th May 2011, 09:40 AM   #7
diyAudio Member
 
mastodon's Avatar
 
Join Date: Jan 2011
Quote:
Originally Posted by tingtong5 View Post
No chance of getting DC at the input when the tube gets defective?
Not unless some freak mechanical failure occurs in which the grid shorts to the plate of the tube. Which is incredibly unlikely under any other circumstances than an earthquake or a plane crash.
  Reply With Quote
Old 30th May 2011, 09:49 AM   #8
diyAudio Member
 
Join Date: Dec 2006
Quote:
Originally Posted by SY View Post
Low enough probability that it's not something I'd worry about at all. I've never seen a tube go bad in such a way that plate voltage was sent to the grid. I have once seen a grid to cathode short, but in this case, you won't have more than a volt or two.

Keep in mind that most sources will have an output cap of their own.
I still got a high voltage on the cathode ;-) My main source (logitech transporter) has a DC coupled output ( since I built a new analog output stage in it), the other is a surround receiver which probably has output caps.

But if I understand correctly, risk is near zero and I can safely ommit the input caps, which I do prefer because of the small gain in transparancy of the sound.

Ronald.
  Reply With Quote
Old 30th May 2011, 09:54 AM   #9
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
If you have a high voltage on the cathode, how can you have a 1M resistor to ground at the grid? That makes no sense. Perhaps you should post a schematic.
__________________
The more you pay for it, the less inclined you are to doubt it.- George Smiley
  Reply With Quote
Old 30th May 2011, 01:43 PM   #10
diyAudio Member
 
Join Date: May 2007
With the grid at ground level a 6H30 should have ca 6V to ground. Unfortunately this leads to higher distortion as the cathoderesistor will have quite a small value. Better to use a negative voltage and a CCS.
  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
Balanced input all DC coupled RIAA preamp FdW Analogue Source 149 26th August 2014 07:25 AM
Help designing Class-A, tube, transformer coupled, studio grade pre-amp gibsonripper Tubes / Valves 9 6th September 2013 01:35 AM
D-amp for DC coupled applications lalalala Class D 8 20th December 2010 09:23 PM
A DC coupled headphones amp Gluca Tubes / Valves 0 27th January 2010 01:08 AM
A safe (?) DC coupled amp Fuling Tubes / Valves 4 8th July 2004 06:51 PM


New To Site? Need Help?

All times are GMT. The time now is 07:04 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