27 & 6SN7 Preamp - Crossover - Page 2 - 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 24th October 2009, 05:00 PM   #11
diyAudio Member
 
Join Date: Mar 2003
Location: Zürich
Coldcathode

I see that you are using an indirectly heated tube for rectifier, so the voltages come up slowly and you won't have problems with stripping cathodes, etc. Still you may have a look at this article from glass-ware, specially item 6 (about the diode). glass-ware

If the circuit biases as you show, it may be possible to eliminate the 22nF and 2x 1M resistor at the input of the LP section. Another issue regards the 2x 710k resistors used to bias up the lowest triode. For AC they are actually in parallel, so you have a 305k load there. Therefore I would say you need 2x 1420k resistors... but, I would really like to have someone confirm this!

Erik
__________________
my surname is indeed 'de Best': neither misspelling nor snobbism! Ask SY!
  Reply With Quote
Old 24th October 2009, 10:56 PM   #12
diyAudio Member
 
coldcathode's Avatar
 
Join Date: Jan 2009
I modded a 6SL7 Phono (Bob Danielak) schematic I found to add a phono section.

I changed some values to better coincide with RIAA time constants (I think I did better not 100% sure) I also am using the output coupling cap and the parallel R of the volume pot and output R to form the 20hz pole.

Does this make sense?
Attached Images
File Type: gif dc_phono.gif (7.2 KB, 331 views)
File Type: jpg 27-6SN7 pre-xover.JPG (165.5 KB, 336 views)
File Type: jpg Phono octal.jpg (77.8 KB, 321 views)
  Reply With Quote
Old 24th October 2009, 11:02 PM   #13
diyAudio Member
 
coldcathode's Avatar
 
Join Date: Jan 2009
Quote:
Originally Posted by ErikdeBest View Post
Coldcathode

I see that you are using an indirectly heated tube for rectifier, so the voltages come up slowly and you won't have problems with stripping cathodes, etc. Still you may have a look at this article from glass-ware, specially item 6 (about the diode). glass-ware

If the circuit biases as you show, it may be possible to eliminate the 22nF and 2x 1M resistor at the input of the LP section. Another issue regards the 2x 710k resistors used to bias up the lowest triode. For AC they are actually in parallel, so you have a 305k load there. Therefore I would say you need 2x 1420k resistors... but, I would really like to have someone confirm this!

Erik
Erik,

Point 1 - could possible do that but I would have to raise the cathode on the LP follower another 5V's that would put me un-balanced gain wise LP and HP (which I suppose is OK)

Point 2 - Holy Cow I think you are correct! I will go back over the calcs, probably just change the cap size rather than go that big on the R's
  Reply With Quote
Old 24th October 2009, 11:15 PM   #14
diyAudio Member
 
coldcathode's Avatar
 
Join Date: Jan 2009
If indeed they are parallel then I do need to increase the load. But, I am not sure they operate that way with AC?

I could do as you suggested on point 1.
Then add one more section to the HP side.

I will look over the 6SN7 curves to see if I can run it at 140V on the plates with the cathode at 160 or so.

Thanks!
  Reply With Quote
Old 24th October 2009, 11:39 PM   #15
diyAudio Member
 
coldcathode's Avatar
 
Join Date: Jan 2009
Erik,
You are DE BEST!

Easy fix to run the top SN7 at 140VP 2.6ma 160 on the cathode so -5V bias relative to the cathode output of the previous stage.

Thought about the AC and I am pretty sure you are correct, it has two paths throught the 710K so they are in effect parallel. Increased to 1.43M (715 is close enough to 710 for Government work!)

attached is revised schematic,
Attached Images
File Type: jpg 27-6SN7 pre-xover.JPG (154.9 KB, 323 views)
  Reply With Quote
Old 25th October 2009, 10:00 PM   #16
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
I note that the response most definitely will not be butterworth, but otherwise the topology looks ok at a glance.
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 25th October 2009, 10:14 PM   #17
diyAudio Member
 
coldcathode's Avatar
 
Join Date: Jan 2009
Kevin,

Response was intended to be Linkwitz-Riley. This avoids the "hump" in the summed response @ crossover frequency of 100hz.

I am also revising my Phono section because I needed to "digest" all the information I have gathered relative to AC and R/C reactance. This is a STEEP learning curve once you get beyond "copying" someone elses circuits.
  Reply With Quote
Old 25th October 2009, 11:41 PM   #18
diyAudio Member
 
coldcathode's Avatar
 
Join Date: Jan 2009
Revised the two schematics, including a diode to protect the first 6SN7 from cathode stripping at turn on, but I can't come up with a way to protect the HP follower?

Also, made major changes to design of the Phono section but just now noticed that when in Phono mode the signal will be inverted relative to "normal" because of the odd number of gain stages. Any suggestions?

The only other idea I had was to bypass the Rk on the 2md phono gain stage (thus doubling gain) and recalc the RC values for that pole, then I could bring it's signal in at the junction of the 27 with the SN7 follower?
Attached Images
File Type: jpg New Phono.jpg (116.1 KB, 277 views)
File Type: jpg 27-6SN7 pre-xover.JPG (155.9 KB, 116 views)
  Reply With Quote
Old 26th October 2009, 12:17 AM   #19
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Quote:
Originally Posted by coldcathode View Post
Revised the two schematics, including a diode to protect the first 6SN7 from cathode stripping at turn on, but I can't come up with a way to protect the HP follower?

Also, made major changes to design of the Phono section but just now noticed that when in Phono mode the signal will be inverted relative to "normal" because of the odd number of gain stages. Any suggestions?

The only other idea I had was to bypass the Rk on the 2md phono gain stage (thus doubling gain) and recalc the RC values for that pole, then I could bring it's signal in at the junction of the 27 with the SN7 follower?
A couple of comments, wrt to cathode stripping I've never seen it in a 6SN7 under any circumstances. (And as proponent of the type I have used a lot of them in line stages and power amplifiers.) I really don't think the diode is necessary, although the small amount of capacitance it introduces isn't really an issue either. And holding the grid 0.7V positive relative to the cathode means it isn't really preventing unwanted cathode current from flowing anyway. You really don't need to protect anything (including the HF follower) and frankly if you are concerned I would just make sure that the B+ time constant is just long enough to address this concern. This is my preferred approach and can result in relatively benign warm up transients at the output which concerns me more.

The third stage in your phono stage is a non-inverting cathode follower so you only have the two prior gain stage inversions, resulting in an output that is in phase with the input, although somewhat delayed.
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan

Last edited by kevinkr; 26th October 2009 at 12:23 AM.
  Reply With Quote
Old 26th October 2009, 02:52 AM   #20
diyAudio Member
 
coldcathode's Avatar
 
Join Date: Jan 2009
Quote:
Originally Posted by kevinkr View Post

The third stage in your phono stage is a non-inverting cathode follower so you only have the two prior gain stage inversions, resulting in an output that is in phase with the input, although somewhat delayed.
Kevin,

I have used similar couplings without the diode, I might just leave it out.

I know the phono stage itself is non inverting, but running it into the 27 inversion stage then inverts it the rest of the way to the amps. Since the amps I plan to use will be non-inverting and I would rather not have to invert the speaker leads I think I will increase the gain on the second stage of the phono section and bring the signal in to the grid of the follower. I can use a switch arrangement that "shorts" the input to the 27 and switches in the Phono lineout. I can then just add another 2 section pot to adjust the level of the Phono.

I do not have much experience with Phono Cartridges but my guess is that I can expect about 4-5mV? So overall gain of 500+ should be ok?
  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
Which 6SN7 preamp should I build? chrisvw Tubes / Valves 48 12th August 2010 12:08 AM
The complete 6SN7 preamp Joel Tubes / Valves 103 14th March 2008 10:32 AM
Help with 6SN7 preamp tubedude63 Tubes / Valves 7 9th October 2006 12:31 AM
12AY7/6SN7 preamp PSU check porkchop61 Tubes / Valves 100 2nd October 2006 09:13 PM
Preamp with 6sn7 & 6sl7 SRPP ??? Gregouille Tubes / Valves 4 18th June 2002 10:41 PM


New To Site? Need Help?

All times are GMT. The time now is 07:18 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