Lowering gain in 12AX7 SRPP preamp - 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 20th December 2003, 12:47 AM   #11
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
Hi,

Being no fan of the half hearted 12AU7, I'd rather look at the ECC99 for this job...

It never hurts to have some current drive from a pre I'd say...

Cheers,
__________________
Frank
  Reply With Quote
Old 20th December 2003, 02:56 AM   #12
ALBQ is offline ALBQ  United States
diyAudio Member
 
Join Date: Dec 2003
Location: Albuquerque
Default schematic did not make it w/previous post

The schematic did not make it on my previous post - trying again. This is the schematic for a 6SN7 fixed bias srpp on my bench I am playing with. Use R1 to adjust the gain of the circuit - decrease R1 = decreased gain. Food for thought
Attached Images
File Type: jpg srpp-fixedbias2-linestage.jpg (16.4 KB, 791 views)
  Reply With Quote
Old 20th December 2003, 11:15 AM   #13
Asen is offline Asen  Bulgaria
diyAudio Member
 
Join Date: Jan 2001
Location: Sofia
Send a message via MSN to Asen
fdegrove

May be it is not brilliant but it works. Actually my amp needs 750mV for full power. When I build a follower power amp I'll change the front end of the pre too.
  Reply With Quote
Old 21st December 2003, 12:24 AM   #14
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
Hi,

Quote:
Use R1 to adjust the gain of the circuit - decrease R1 = decreased gain. Food for thought
Decrease it any further and you have the entire B+ on the grid.

Quote:
May be it is not brilliant but it works. Actually my amp needs 750mV for full power.
Hell, yes..The beauty of tubes is that they can take a lot of abuse, but building a SRRP circuit and tieing all the gain down in a FB loop isn't making any sense to me. The FB loop may reduce the distortion and Zo of anything preceding it, it still doesn't make sense.

Get the iron out Asen, if you need help we're here.

Cheers,
__________________
Frank
  Reply With Quote
Old 21st December 2003, 08:22 AM   #15
Asen is offline Asen  Bulgaria
diyAudio Member
 
Join Date: Jan 2001
Location: Sofia
Send a message via MSN to Asen
fdegrove

Thanks for the help. If I get the iron out I'd rather build an Aleph 30 and an Aleph P. But I don't have much time now.
BTW, attenuating the input of the SRPP is quite a good solution as far as the circuit could give an 80V output.
I've been thinking of building a follower (and I have the parts) but wasn't sure it would sound as good as the SRPP does.

Regards

PS - I haven't applied FB, just have added more resistance to the pot I use. It a kind of devider. Maybe you got me wrong.
  Reply With Quote
Old 21st December 2003, 03:18 PM   #16
ALBQ is offline ALBQ  United States
diyAudio Member
 
Join Date: Dec 2003
Location: Albuquerque
Default Lower gain in 12AX7 SRPP

Frank/Asen - The fixed bias mod does increase the grid voltage but, the orginal circuit will have about half B+ on it already ~115V and it will allow you to dial down the gain while still biasing the lower triode - ie. should retain the circuit's orignial voicing. I also agree w/you Frank that should be careful or you can end up going to far with this circuit - Less than 5-10K for R1 (>10:1 R1/R5) is probably pushing it. As I said, I am playing with a 6SN7 version of this circuit on my bench - found a pair of Raytheon brown bases for $5 each that I really want to build something with. Anyway, I played around with several cathode follewers (VTV, AE3, Foreplay), w/wo C4S, etc.. and finally ended up with Bruce Bozenblitz's SRPP - which I think sounds the best. Unfortunately, it has to much gain. From here I tried the buffer circuit in the 12BH7 posts but, the bottom triode is barely on and the circuit did not retain the same voicing. So, in searching for alternatives I found the voltage divider or "fixed bias" circuit in TubeCad. I am still experimenting with the circuit - pulled the grid cap because, it seems to me anyway, that the fewer components I use more open/alive most circuits sound. FYI - circuit as drawn has about 190V on the grid of the top triode and 1V in = 6V out

Asen - another thought - have you considered swapping the 12AX7s with a 12BH7A? It is the same pin out as a 12AX7 and 12AU7, has lower mu than either and IMHO is one of the better sounding 9pin bottles. Should be plug and play in your preamp provided the power transformer can supply the extra filament current - would guess it will be fine. I have a pair of RCA 12BH7s in my Foreplay clone - IMO they sound better than my Amperex's - were hands down my all time favorites until I tried the 12BH7s
  Reply With Quote
Old 21st December 2003, 03:39 PM   #17
Asen is offline Asen  Bulgaria
diyAudio Member
 
Join Date: Jan 2001
Location: Sofia
Send a message via MSN to Asen
ALBQ

Thanks for your post. Actually I use E88CC tubes in my pre. I think they sound great. This is the best pre I've ever listend to. I built another pre with 12AX7 some years ago - a McIntosh C22 replica, but the the SRPP is deffinitely better. Anyway, I don't intend to modify the circuit. My future plans are to build an Aleph pre/power amp setup.

Regards
  Reply With Quote
Old 21st December 2003, 10:18 PM   #18
ALBQ is offline ALBQ  United States
diyAudio Member
 
Join Date: Dec 2003
Location: Albuquerque
Default E88CC preamp

Asen,
Can you post a the schematic and/or pictures of you 6922 preamp? Always interested in what other folks are listening to/building
  Reply With Quote
Old 22nd December 2003, 10:01 AM   #19
Asen is offline Asen  Bulgaria
diyAudio Member
 
Join Date: Jan 2001
Location: Sofia
Send a message via MSN to Asen
Default My SRPP

Sory but I don't have pictures of the inside... I hope this one will appear.
Attached Images
File Type: jpg srpp.jpg (63.6 KB, 528 views)
  Reply With Quote
Old 22nd December 2003, 10:06 AM   #20
Asen is offline Asen  Bulgaria
diyAudio Member
 
Join Date: Jan 2001
Location: Sofia
Send a message via MSN to Asen
Default The circuit

The pot's not shown but there's a 220k resistor and a 10k pot in series.

Regards
Attached Images
File Type: jpg srpp.jpg (13.8 KB, 575 views)
  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
12ax7 srpp dac output Klimon Tubes / Valves 26 30th April 2010 12:59 AM
Lowering Gain on an Aikido Preamp SRMcGee Tubes / Valves 12 11th March 2007 08:49 PM
high gain 12ax7 preamp from scratch? fingerwizards7 Tubes / Valves 2 30th December 2006 05:56 PM
lowering gain on Aleph 3 hancock Pass Labs 4 21st December 2005 05:11 PM
Is There Any Downside to Lowering Gain? Cappy Chip Amps 5 2nd September 2004 07:08 AM


New To Site? Need Help?

All times are GMT. The time now is 09:32 AM.


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