DIY CAT SL1 preamp - Page 5 - 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 8th October 2012, 11:44 AM   #41
diyAudio Member
 
merlin el mago's Avatar
 
Join Date: Sep 2009
Location: Catalonia - Europe
Quote:
Originally Posted by jonusgrumby View Post
I bought one of the blank circuit boards on ebay a few years ago. This is the circuit that I used on it. It worked well and sounds fine, but I agree it is very complicated.

I really didn't think the original circuit was biased optimally, so I made several changes. I used the 6DT8 since they are similar to the 12AT7 but very avaialble and very inexpensive. I didn't need all the gain of the 12AX7.

The board required a trace to be cut or you'll but too much voltage on the heater-cathode. I used two heater supplies and boosted the top tube heaters.

I'd really only recommend this board if you want to experiment with the mu follower in a feedback loop.
Have I to mod any resistor or cap value to use 12AU7 / ECC82 & 12AX7 / ECC83?

Could you post heater wiring?
  Reply With Quote
Old 13th October 2012, 10:58 PM   #42
Asen is offline Asen  Bulgaria
diyAudio Member
 
Join Date: Jan 2001
Location: Sofia
Send a message via MSN to Asen
Hi,

The Stereophile reviews and measurements on CAT SL-1 have drawn my attention, so Iíve spent some time simulating different variants of the circuits shown in this thread. I think the AnalogMetric circuit works fine. Only the SRPP cathode cap should be 150pF, not 100n, as it creates non-linearities around 1kHz.
Iíve tried to replace the input ECC82 tube with ECC88, as in the newer versions of CAT SL-1. According to my simulations all the current/voltage values are very close to the ECC82 version. Iíve replaced the cathode resistor with 630 Ohms, and the 150pF in the SRPP stage with 330pF to keep the HF behaviour stable. The ECC88 version has lower distortion but much higher gain. To lower the gain the only thing I could think of was to put an input divider. All the other things Iíve tried have changed the overall ballance of the circuit.
The simulated distortion is negligible - for an 18V pulse at the output on 47k dummy load the second harmonic is only 150uV.
BTW some 6moon review pics show thereís a 0,22uF cap at the input of the original preamp - it is connected directly to the attenuator circuitry. So maybe some observations in this thread are correct.
Iíd be glad if SO has any ideas how to improve the ECC88 version and lower the gain without changing the balance the circuit has.

Asen
Attached Files
File Type: pdf CAT_SL1_ECC88.pdf (274.1 KB, 352 views)
  Reply With Quote
Old 14th October 2012, 02:43 PM   #43
diyAudio Member
 
Join Date: Feb 2011
other then the gain is too high, the hiss from the original schematic is another problem. I once inserted a JJ ECC802S and all hiss gone... never i can never get there from all the NOS tube that I have (Mullard, Telefunken & Seimens etc)... However, the sound from the JJ is sooo...
  Reply With Quote
Old 14th October 2012, 04:11 PM   #44
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Well regarded design, but there are many here that would do a better job frankly. A couple such that immediately come to mind are the Salas 6V6 or the 26 DHT..
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 11th February 2013, 10:50 PM   #45
diyAudio Member
 
Join Date: Feb 2008
Hi reading this thread, I get a quite good understanding. BUT: there is a resistor in the upper valve of the u-follower: R6, that is 1K. Discussion said, it must be 1M. That is my understanding. BUT: Guys here built it with 1K. So the upper valve is not modulated the correct way... Please clarify, why not 1M, but 1K should be used...
  Reply With Quote
Old 11th February 2013, 11:48 PM   #46
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
In the circuit linked from post 42 it is R2 which should be much greater than 1k. At 1k the upper valve does not operate quite as intended as an active load. For that the grid needs to be AC coupled to the lower anode but DC coupled to the bottom of the 3k cathode bias ressitor. 1k is almost a short circuit in this position so the anode load for the lower triode is an active load with lower impedance than intended (although still highish) in series with a parallel CR of 0.22uF and 47k. Note that this is not a mu-follower; that would have the signal taken from the upper cathode not the lower anode.

The gain of the second stage will be a bit lower than intended and distortion will be a bit higher than intended. Fortunately the high amount of global negative feedback will largely hide this problem. Better performance (with the 1k value for R2) might be obtained by simply removing the capacitor and 47k so the two 12AX7 triodes have a bit more voltage so can run more linearly.

This circuit shows how forgiving valves can be. The circuit contains several mistakes, yet it still (sort of) works.
  Reply With Quote
Old 12th February 2013, 12:03 AM   #47
diyAudio Member
 
Join Date: Nov 2011
Location: Athens
Quote:
Originally Posted by DF96 View Post
In the circuit linked from post 42 ..........
Better performance (with the 1k value for R2) might be obtained by simply removing the capacitor and 47k so the two 12AX7 triodes have a bit more voltage so can run more linearly.

This circuit shows how forgiving valves can be. The circuit contains several mistakes, yet it still (sort of) works.
If you remove the capacitor and 47k then the voltage in the cathode of V4A will be higher and consequently the bias of V3A will be lower , so I do no consider it a good idea .
  Reply With Quote
Old 12th February 2013, 12:52 AM   #48
45 is offline 45  Italy
diyAudio Member
 
45's Avatar
 
Join Date: Dec 2008
Location: UK
Quote:
Originally Posted by Dimitris AR View Post
If you remove the capacitor and 47k then the voltage in the cathode of V4A will be higher and consequently the bias of V3A will be lower , so I do no consider it a good idea .
Removing the 47K and the cap means that you change the mu-follower into a SRPP (if you take the output from the upper cathode).

In principle I agree with the main point of DF96: the anode currents of the input 12AU7 and the 12AX7 mu-follower are a bit too low. Maybe, thanks to the feedback it measures well but I would not be sure about the sound....

I would change R3 and R13 from 3K to 1.5K. This way the 12AX7 should work more or less at 0.65 mA and anode of the lower valve should be around 112V instead of 120V. Not a big deal, really!
I would also tweak R14 and R8 to get a more linear behavior from the input valve.

Last edited by 45; 12th February 2013 at 12:54 AM.
  Reply With Quote
Old 12th February 2013, 01:29 AM   #49
diyAudio Member
 
Join Date: Nov 2011
Location: Athens
Quote:
Originally Posted by 45 View Post
Removing the 47K and the cap means that you change the mu-follower into a SRPP (if you take the output from the upper cathode).

In principle I agree with the main point of DF96: the anode currents of the input 12AU7 and the 12AX7 mu-follower are a bit too low. Maybe, thanks to the feedback it measures well but I would not be sure about the sound....

I would change R3 and R13 from 3K to 1.5K. This way the 12AX7 should work more or less at 0.65 mA and anode of the lower valve should be around 112V instead of 120V. Not a big deal, really!
I would also tweak R14 and R8 to get a more linear behavior from the input valve.
Well it's not a good idea as formulated , but with further changes it will work fine , of course the circuit will turned to SRPP , I know it , and instead of changing R3 and R13 from 3K to 1.5K , you can change the value of R10 such a way so the voltage at the anode of V2A remain 120V or close ( and you can add a decoupling capacitor for more filtering ) then you have to change the value of R7 too , as you see there is a place for a lot of improvements .

Last edited by Dimitris AR; 12th February 2013 at 01:37 AM.
  Reply With Quote
Old 12th February 2013, 04:38 AM   #50
diyAudio Member
 
Join Date: Sep 2005
Gentlemen.

Sorry, but there seems to be a lot of "voodoo" embedded in this thread. Many years ago, 1950 or so, I learned from my father, a high end Audio "guru" from the 1930s, that the way you design wonderfully sounding audio amplifiers is to design a good sounding amplifier with about 15dB more gain than you need and then put about 15dB of negative feedback around the amplifier to reduce the gain to what you really need.

This superior technique has always worked well for me.

For instance my College Senior EE power amplifier using transistors was designed using that technique. The Mackintosh Amplifier Clinic came on Campus to test our amplifiers and declared my design as the first "straight wire with gain" amplifier they had ever measured.

I am "preaching" this way because the various circuit configurations that you adhere to
have all sorts of internal negative feedback loops. "Cathode Ray" writing in the Wireless World in the 1950s did articles on feedback loops within feedback loops.

If you don't watch what you are doing carefully you may wind up with an amplifier that perform worse than if you used a simpler configuration with limited feedback.

I look at the various forms of tube hookups as an introduction of internal loops of positive and negative feedback. The tube has no idea which circuit it is connecter into, it just does what the potentials between the electrodes tell it to do. If the voltage conditions tell it to distort the signal it will, if not it won't.

Hans J Weedon.
  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
Convergent Audio Tech SL1 preamp mobyd Tubes / Valves 6 17th September 2013 09:08 PM
CAT SL1 SIGNATURE schematik phono vog Tubes / Valves 1 16th September 2013 02:49 PM
Greetings and request for info on Cat SL1 preamp somi electronic Parts 2 16th September 2013 02:46 PM
CAT preamp marknoir Swap Meet 1 24th January 2007 03:55 AM
A Noisey CAT SL1 Reference hifiscratch Tubes / Valves 2 23rd February 2004 06:00 PM


New To Site? Need Help?

All times are GMT. The time now is 11:05 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