Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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

Phono stage design considerations part 1: choosing 1st stage tube
Phono stage design considerations part 1: choosing 1st stage tube
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 December 2013, 07:44 PM   #1
Joshua_G is offline Joshua_G  Israel
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
Default Phono stage design considerations part 1: choosing 1st stage tube

1st stage tube considerations

The phono stage is intended for both MM and MC, probably with SUT (step-up transformer).

It looks to me that the most important characteristic for that tube should be very low noise and microphonic.

On top of that, reasonably low miller capacitance is necessary (for MM cartridges).

Tough signal levels at this stage are low, good linearity is preferred, especially low 3rd and higher odd harmonics distortion levels.

Long life, reasonably low plate resistance, availability and reasonable cost are preferred.

I hope I didn't miss anything here.

All the above relate to anode resistor loading, capacitor coupling to the next stage (including the RIAA equalization network).

What are your recommendations?

Replies based on experience, especially listening evaluations between different tubes, will be appreciated.
  Reply With Quote
Old 1st December 2013, 08:47 PM   #2
Eli Duttman is offline Eli Duttman  United States
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
High gain is essential to deal with the 20 dB. roll off in the EQ network.

Low CMiller, while highly desirable, is not absolutely essential. All sorts of 12AX7/ECC83 based circuits are available and the majority of MM carts. and SUTs work well with them.

Low noise definitely is important. Because of the partition factor, it's difficult to find a pentode that's quiet enough. The 6AC7 could work out. Definitely use silicon rubber "O" rings on the metal envelope to tame microphonics.

Because of its μ of 33, things can get "dicey" with the 6922, as gain without CCS loading is definitely marginal. All in all for a project where you will experiment to "nail" RIAA EQ down, the 6GK5 seems hard to beat, with its high μ, low noise, high gm, and low RP.
__________________
Eli D.
  Reply With Quote
Old 1st December 2013, 09:15 PM   #3
Joshua_G is offline Joshua_G  Israel
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
Hi,
Thank you.

Quote:
Originally Posted by Eli Duttman View Post
High gain is essential to deal with the 20 dB. roll off in the EQ network.

Low CMiller, while highly desirable, is not absolutely essential.
Indeed.

Quote:
Originally Posted by Eli Duttman View Post
All sorts of 12AX7/ECC83 based circuits are available and the majority of MM carts. and SUTs work well with them.
Tubes with about 1mA anode current are outside my list.

Quote:
Originally Posted by Eli Duttman View Post
Low noise definitely is important.
Low noise with high gain is probably the most important parameter for phono 1st stage.

Quote:
Originally Posted by Eli Duttman View Post
Because of the partition factor, it's difficult to find a pentode that's quiet enough. The 6AC7 could work out.
I've seen phono stages with D3a, C3g and E810F pentodes on the 1st stage. I wonder how they compare.

Quote:
Originally Posted by Eli Duttman View Post
Definitely use silicon rubber "O" rings on the metal envelope to tame microphonics.
That goes without saying.
Yet, the tubes, especially the one in the 1st stage, should be of very low noise and microphonic.

Quote:
Originally Posted by Eli Duttman View Post
Because of its μ of 33, things can get "dicey" with the 6922, as gain without CCS loading is definitely marginal.
Indeed, it looks like it is so.

Quote:
Originally Posted by Eli Duttman View Post
All in all for a project where you will experiment to "nail" RIAA EQ down, the 6GK5 seems hard to beat, with its high μ, low noise, high gm, and low RP.
The datasheet statess noise figure for RF, not for audio frequencies, so I wonder what is the tube's noise in audio frequencies.

Also, I'd love to hear from people who have experience with this tube about how its' microphonic it is.
  Reply With Quote
Old 1st December 2013, 09:28 PM   #4
Johan Potgieter is offline Johan Potgieter  South Africa
diyAudio Member
 
Join Date: May 2005
Location: Pretoria, South Africa
If I may chime in; I was wondering if Eli would reply, and he did. He made me aware of the 6GK5 a few years ago, and it is a quite good choice (also not that expensive - but don't go buying up all of them now!).

It may be somewhat more microphonic than the ECC83, but that is not the most important factor. It has a frame grid, which makes it comparatively stable to drift. You seem to know that noise is invertedly proportional to gm, thus working at a high plate current has benefits. I doubt that one will find a more suitable input tube for a phono preamp. I have also used an E88CC if one needs to be economical of tube count, but found the 6GK5 superior noise-wise.
  Reply With Quote
Old 1st December 2013, 09:29 PM   #5
Merlinb is offline Merlinb  United Kingdom
diyAudio Member
 
Merlinb's Avatar
 
Join Date: May 2006
Location: Lancashire
The gain does not have to be that high. Assuming passive EQ with only half of it being performed after the first stage, you only have 20dB of loss to contend with. A gain of a few dBs more than 20dB will therefore be enough to overcome the noise contribution of the second stage (particularly since flicker noise dominates), and most triodes can manage this easily. A mu of about 30 is probably as low as you would want to go, so the ECC88 does hover at that end of the spectrum.

CCS loading is a universally poor choice, as the noise current generated by any CCS will be similar to or even greater than that of the triode itself, and will only degrade the SNR. You really can't beat passive loading for best SNR.

Last edited by Merlinb; 1st December 2013 at 09:37 PM.
  Reply With Quote
Old 1st December 2013, 09:41 PM   #6
Eli Duttman is offline Eli Duttman  United States
diyAudio Member
 
Join Date: Apr 2004
Location: Monroe Township, NJ
Quote:
Tubes with about 1mA anode current are outside my list.
Yes, the 'X7 triode is a wimp. That's easily dealt with. Enter the ZVN0545A. A fair number of people have built the tweaked RCA setup that I've uploaded. Comments have been favorable and continue to be so. Are better designs available? Certainly. However, the tweaked version of RCA's seminal passive EQ design gets the job done quite well, indeed.
Attached Images
File Type: jpg Full Tweaked RCA Phono Stage.jpg (45.4 KB, 1324 views)
__________________
Eli D.
  Reply With Quote
Old 1st December 2013, 10:19 PM   #7
Joshua_G is offline Joshua_G  Israel
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
Hi Johan,
Thank you.
  Reply With Quote
Old 1st December 2013, 10:22 PM   #8
Joshua_G is offline Joshua_G  Israel
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
Hi,
Thank you.

Quote:
Originally Posted by Merlinb View Post

CCS loading is a universally poor choice, as the noise current generated by any CCS will be similar to or even greater than that of the triode itself, and will only degrade the SNR. You really can't beat passive loading for best SNR.
Indeed.
On top of that, CCS loading increases odd and high order harmonics.
  Reply With Quote
Old 1st December 2013, 10:42 PM   #9
Joshua_G is offline Joshua_G  Israel
diyAudio Member
 
Joshua_G's Avatar
 
Join Date: Nov 2008
Location: Small village, Israel
Hi Eli,
Thank you.

Quote:
Originally Posted by Eli Duttman View Post
Yes, the 'X7 triode is a wimp. That's easily dealt with. Enter the ZVN0545A.
I'm not going to use SS devices, save for rectifying the heaters voltage, nor am I going to use questionable tubes.

Quote:
Originally Posted by Eli Duttman View Post
A fair number of people have built the tweaked RCA setup that I've uploaded. Comments have been favorable and continue to be so. Are better designs available? Certainly. However, the tweaked version of RCA's seminal passive EQ design gets the job done quite well, indeed.
I'm looking for the best possible design. Getting the job done well isn't what I'm looking for.

So, please, let's go back to square one. What tube(s) is/are the best for a phono 1st stage? Are there any contenders to the 6GK5?
  Reply With Quote
Old 1st December 2013, 10:57 PM   #10
P.Lacombe is offline P.Lacombe  France
diyAudio Member
 
Join Date: Jul 2001
Location: Orleans, France
Quote:
Originally Posted by Joshua_G View Post


Tubes with about 1mA anode current are outside my list.

Please.... Why so much hate ?
  Reply With Quote

Reply


Phono stage design considerations part 1: choosing 1st stage tubeHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Paradise phono stage part GB Birger Group Buys 301 22nd June 2013 06:33 PM
help with phono stage circuit design lightmaster Solid State 3 3rd October 2012 06:54 PM
Adding a 3rd gain stage to an MM phono stage, to make it suitable for LOMC? andyr Analogue Source 10 16th August 2012 04:10 AM
Output stage madness - inductances and other design considerations ostripper Solid State 0 20th December 2009 08:34 PM
Should I replace Scott 222C phono stage with Claret phono stage? Bing Yang Analogue Source 0 22nd August 2005 06:41 AM


New To Site? Need Help?

All times are GMT. The time now is 07:17 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki