Sound of the 6SN7? - Page 6 - 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 2nd September 2010, 02:01 AM   #51
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
Quote:
Originally Posted by tinitus View Post
actually strange, because TriodeDick's first KT88 PP from 2001 had cathode bypass caps, on KT88
and the later 3 versions have none
is it cost saving ?
I think he switched it to fixed bias, but it's not very clear from the schematic.

--
  Reply With Quote
Old 2nd September 2010, 07:24 AM   #52
Green77 is offline Green77  Sweden
diyAudio Member
 
Join Date: May 2007
Location: MJOLBY
Hi"

tomtt: I run the tube for around 2 hours. And it wasn't new, i had it in a old el34 amp for maybe 300 hours.

rongon: I think i have started to understand it at last In the old design there was around 205 v at the ecc99 pin 1 and 6. In your design there will be 300 v. That's correct? Better?
And the optimal resistor value at R5 is above 33K..will 31 K be alright?

Thanks //Daniel
  Reply With Quote
Old 3rd September 2010, 02:25 PM   #53
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
Quote:
Originally Posted by Green77 View Post
Hi"

rongon: I think i have started to understand it at last In the old design there was around 205 v at the ecc99 pin 1 and 6. In your design there will be 300 v. That's correct? Better?
I think it *might* sound better. Not because it's a higher voltage or anything like that. I tried to find a "better" operating point for the 6SN7 and then find a matching operating point for the ECC99 that follows. (The 6SN7 anode being DC-coupled to the ECC99 grids makes this more complicated than if the two stages were RC-coupled.)

However, now that I know that the ECC99 had 205V on its anodes, I can see that the ECC99 was biased kind of "cold," which could sound a bit "dry."

ECC99 with 205V at the anode and 130V at the cathode, and a B+ of 440V with anode load resistor of 30k ohms:

Va = 75V
Ia = 8mA
Vg = -10V

I'm sorry, that just does not compute. There's no way the ECC99 would bias up like that. There must be some mistake. I checked this against the ECC99 characteristic curves from the JJ Electronic ECC99 data sheet. I'm pretty sure I'm doing this right...


As I wrote earlier, I've built some stuff using 5687, which is very similar to ECC99 but with a different pinout. My experience is that the 5687 sounds thin and a touch "harsh" at currents lower than 10mA, and the original schematic you posted had the ECC99 biased to have much less current going through each of its two triodes. I suspect that that might sound thin and/or "dry." Since that was your complaint about the sound of your amp, I thought it might be fun to try to fix that...

With the current set at about 10mA per ECC99 triode, the ECC99 should have an internal resistance of about 3.5k ohms. That means an anode load resistor of 13k to 15k ohms would be desirable (3500 * 4 = 14,000). I found 13.5k ohms would work in this circuit, and that's within the desirable range (and happens to equal two 27k ohm resistors in parallel).

Quote:
Originally Posted by Green77 View Post
And the optimal resistor value at R5 is above 33K..will 31 K be alright?
31k ohms for the 6SN7 anode load is close enough to 33k that it should be fine. (8,000 * 4 = 32,000)

--
Choosing operating points along the tube's characteristic curves is where a lot of the fun of designing an amplifier lies -- or at least for me, at this point.

After experimenting with various triodes in a simple single-stage line amp (common cathode), I've heard for myself that the relationship between the triode's internal anode resistance and the value of anode load resistor is important to the sound you get out of that tube -- just like it says in the books. You really do want the anode load resistor to be at least four times the resistance value of the triode's internal resistance.

However, a triode's internal resistance will increase if you run it at a lower than optimal current. If you do that, you'll need to increase the value of the anode load resistor to compensate. However, a triode run at low current also changes in other ways -- its amplification factor (mu) may be a little lower, and its transconductance goes down (sometimes a lot).

I've found that there is a 'sweet spot' for each triode, a point at which the tube begins to get fully 'turned on.' Really low current operating points tend (to my ears at least) to yield anemic or "dry" sound, and that's what your complaint was. So I tried to address it by suggesting that you put a touch more current through your driver stage tubes.


I'm not an expert -- I'm a complete amateur at this -- but I've been trying to learn as best I can. I hope someone who knows more than I do will weigh in and correct any mistakes I've made.

--
  Reply With Quote
Old 3rd September 2010, 03:43 PM   #54
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
Another thing...

Since you don't have a bypass capacitor in parallel with the 6SN7 cathode resistor, the 6SN7's internal anode resistance will be higher. How to figure that out:

From Designing Common-Cathode Amplifiers

Rk' = (Rp+ra)/(mu+1)

for 6SN7 with 31k anode load and unbypassed 360 ohm cathode resistor:

the resistance seen looking into the plate when the cathode resistor is unbypassed is:
ra'(unbypassed Rk) = ra + (mu + 1)*Rk

8000 + (20 + 1) * 360 = 15,560

That means the ra of the 6SN7 is now about 15k ohms. You'll need a 60k ohm anode load resistor to meet the 4 * ra rule for the anode load resistor. I think you need to try bypassing the cathode of the 6SN7 with a 330uF 16v capacitor (low ESR electrolytic), at least to hear if that improves the sound for you. If you don't like the sound with the cathode bypass capacitor, then leave it off.

--
  Reply With Quote
Old 4th September 2010, 09:52 AM   #55
Green77 is offline Green77  Sweden
diyAudio Member
 
Join Date: May 2007
Location: MJOLBY
Hi!
Ok, think i have it now. I will send for parts and change it when they arrive.

Click the image to open in full size.

Is it nesesary to have so large resistor 6,8 K / 10W at the anode? Will 5 W work?

Thanks
  Reply With Quote
Old 4th September 2010, 11:00 AM   #56
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
You might want to think about the gain structure. If you're committed to not having a feedback loop from the transformer secondary to the cathode of the first stage tube (as usually done in a 5-20 circuit like this), and putting aside the distortion and source impedance performance, you'll have an amp with VERY high gain. Eyeballing it, you'll have something like 16-17 from the first stage, maybe 8-9 from the second. So at clipping, the input voltage is something like 300mV. Normal input sensitivities are 5-7 times that. That means that your preamp will almost always be running well under unity gain, and its noise performance will be 5-7 times worse than with a more conventional gain structure.

This can all be handled in system design, but if you have a preamp with gain (most common), you'll be blown out of the room with the volume control barely cracked open.
__________________
The more you pay for it, the less inclined you are to doubt it.- George Smiley
  Reply With Quote
Old 4th September 2010, 11:15 AM   #57
piano3 is offline piano3  United Kingdom
diyAudio Member
 
Join Date: Aug 2010
Location: London,UK
I have a power amp with 250 mV input sensitivity and believe me, it is a pain in the neck. It is going to be retired/modified soon.
  Reply With Quote
Old 4th September 2010, 11:37 AM   #58
Green77 is offline Green77  Sweden
diyAudio Member
 
Join Date: May 2007
Location: MJOLBY
Yes, i have consider that. Since i have a preamp maybe i should ad a NFB.. Will i have to change something else? Or could i just ad a resistor and a small cap? What value? 15K and 180pF maybe..
  Reply With Quote
Old 5th September 2010, 04:24 PM   #59
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
Quote:
Originally Posted by Green77 View Post
Yes, i have consider that. Since i have a preamp maybe i should ad a NFB.. Will i have to change something else? Or could i just ad a resistor and a small cap? What value? 15K and 180pF maybe..
From what I've seen, there are two ways to go about this.

1) Add NFB to reduce gain (and reduce harmonic distortion).

2) Take a stage away (the 6SN7 input stage).


Option 1) is the more common, and would probably be the more desirable, especially with your UL-wired KT88 output stage.

I tried option 2) in a triode power amp and found that it works for me. I figured the first voltage amplifier stage (your 1/2 6SN7 first stage) is an unnecessary duplication of the line stage amp. So why not get rid of the power amp's first stage?

I can think of one reason. Putting the phase splitter/driver all in one stage would not perform as well as a two-stage driver where the first stage does the phase splitting and the second does the driving of the output stage. But ignoring that...

Here's a two-stage EL34 (6CA7) power amp I made on a Dyna ST70 chassis, as an experiment. It turned out better than I expected. It's not optimal at all, but maybe somebody would like to comment on it?
Attached Images
File Type: jpg EL34_triode_pp_pwramp.jpg (84.7 KB, 348 views)

Last edited by rongon; 5th September 2010 at 04:26 PM.
  Reply With Quote
Old 5th September 2010, 04:56 PM   #60
diyAudio Member
 
Join Date: Dec 2008
Location: North San Diego, CA
Default 6sn7 variables

A while back I sold about 40 6sn7 tubes of many makes

It turned into a interesting adventure as I had buyers, one who just left his programing job in the middle of the day, desparate to get versions of 6sny tubes I thought were lousy!

I prefer the GE gray plates as smooth and lush, and I kept those....

But there have been many versions of tubes made, and the guys with the expensive gear know what version of most common tubes gives them the sound they want.

The 6sn7 has to have enough idle to become "Lush" and have a deep sound stage. Cathode "R"s of around 330 to 510 ohms and plate values of around
33K work great.

It is not that easy to do some math and dertime values, real sonic tests should always do that, as tubes can be pretty sneaky

Local smal amounts of plate to grid feedback, a 2.2meg "R: from plate to grid on a 6sn7 will shape up that square wave, make the tube quiet.

Also, if the cathode is bypassed, put about 51 to 120 ohms from cathode to cap so some dry "R" for neg feedback

Also could be the resistors, I always uded 2-watt carbons for the Vintage sound, so remember, caps and "R"s have thier sonics

Building line stages and other gear with vintage tubes showed in sonic tests that a lot of tubes like round number values. 330K grid, 330 ohm kathode, and 33k plate sound and work well.

The SRPP stage can be dry and flat, so you have to decide on "Tone" or "Clean" when picking desings.

I prefer the classic stages as more "Tone" of the Vintage style

Another trick that Marantz and other great designers used was to have a small value "R" in series with the grid "R" to ground, and at that junction, add a small amount of + volts to push the idle up for a more lush sound.

I was re-building a Marantz preamp, noticed the grid "R"s were two "R"s in series with the junction point going to the power supply???

so what he was doing was to add about 3-5V + to the grids of the 12ax7s so they would idle high for more Lush.

Adjusting the voltage to the driver is important, as 6sn7s have a lot of current gain and do not need to run too hot, 300+ more that enough, and when you go over about 275+ the tube gets more harsh and "Hot" sound
  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
6SN7/6SN7 Aikido + CS4398 DAC = HEAVEN john65b Tubes / Valves 23 30th April 2014 07:28 PM
ECC88 vs 6SN7 kacernator Tubes / Valves 13 17th March 2008 09:55 PM
Help me.......What is the 6SN7 ??????? cd149dc Tubes / Valves 10 17th June 2007 04:09 PM
Is it the 6SN7 or me? arnoldc Tubes / Valves 4 12th October 2006 11:31 PM
6sn7 Buffer mod_evil Tubes / Valves 12 9th September 2006 09:38 PM


New To Site? Need Help?

All times are GMT. The time now is 07:45 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