Cascode gain. Did I do something wrong? - 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 27th January 2008, 10:35 PM   #1
Serge66 is offline Serge66  Switzerland
diyAudio Member
 
Serge66's Avatar
 
Join Date: Nov 2007
Location: Canton of Jura
Question Cacode gain. Did I do something wrong?

Greetings All,

Last year, I was playing with the attached cascode circuit. The tubes are 6DJ8 / 6922. According to the theory, the gain is high. However, I could not get a gain higher than 30, which the mu of these tubes. Have I missed something? Theoretically, the gain can reach mu squared (900).
Thanks for your input and help.

Serge
Attached Files
File Type: pdf cascode test results.pdf (45.9 KB, 122 views)
__________________
"I have a bag, therefore I like am, like I exist, like..." says the bimbo with the Crouton bag.
  Reply With Quote
Old 27th January 2008, 11:29 PM   #2
Serge66 is offline Serge66  Switzerland
diyAudio Member
 
Serge66's Avatar
 
Join Date: Nov 2007
Location: Canton of Jura
Hello!!!
__________________
"I have a bag, therefore I like am, like I exist, like..." says the bimbo with the Crouton bag.
  Reply With Quote
Old 27th January 2008, 11:43 PM   #3
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
You'll never see anything like 900. The gain is the product of the gain of the bottom tube times the gain of the top tube. The bottom tube is working into a very low resistance load, so its gain is pretty low. The top tube's load is none too big, either.

Bypassing the cathode resistor (or using LED bias) will pump the gain up a bit, but an ECC88 cascode is more likely to have a gain in the 80-100 range.
__________________
The more you pay for it, the less inclined you are to doubt it.- George Smiley
  Reply With Quote
Old 27th January 2008, 11:55 PM   #4
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Let's look at the last example in your table. You've got 4mA of current. The cathode resistor is 810R. Plate resistor is 33k. At that current, the plate resistance is about 5k. But the effective plate resistance of the bottom tube is 5k + (mu + 1)0.810k, or nearly 30k.

Now, the cathode impedance of the upper tube is the total plate resistance divided by mu, or about 1k. The bottom tube is running less than unity gain!
__________________
The more you pay for it, the less inclined you are to doubt it.- George Smiley
  Reply With Quote
Old 28th January 2008, 12:00 AM   #5
diyAudio Member
 
Join Date: Mar 2003
Location: Zürich
Quote:
Hello!!!
Hi Serge

I see that lots of questions asked here are of the 'broken record' type: every now and then they appear again, and well, most of the times the answers are the same. If not a straight answer, a refference where you can get it. In the case of the cascode with a E88CC your best bet is Valve amplifiers by Morgan Jones. Maybe you have it, so you can check pages 94-100. Now I did just a fast scan, and the example gives a gain of about 200 with 1.34mA and 100k plate resistor.

(It seems quite plausible that, although the cathode of the bottom valve is at 2.5V and therefore accepts a signal of 1VRMS without clipping, the output of the stage - which is fed from 285V - won't produce a 200VRMS swing.)

Hope this helps!

Erik
__________________
my surname is indeed 'de Best': neither misspelling nor snobbism! Ask SY!
  Reply With Quote
Old 28th January 2008, 12:32 AM   #6
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Erik, do you get 200 from gm (lower) times RL (upper)? If so, did you consider the effect of the cathode resistor on gm? I think that 200 is correct if it's bypassed.
__________________
The more you pay for it, the less inclined you are to doubt it.- George Smiley
  Reply With Quote
Old 28th January 2008, 12:38 AM   #7
Serge66 is offline Serge66  Switzerland
diyAudio Member
 
Serge66's Avatar
 
Join Date: Nov 2007
Location: Canton of Jura
Thanks Guys,

I'll buy the Morgan Jones book. Sounds like a good investment.
I'll try the LED bias. I do not really like the idea of bypass caps.

Serge
__________________
"I have a bag, therefore I like am, like I exist, like..." says the bimbo with the Crouton bag.
  Reply With Quote
Old 28th January 2008, 12:41 AM   #8
diyAudio Member
 
Brett's Avatar
 
Join Date: Jan 2002
Quote:
Originally posted by Serge66
I'll buy the Morgan Jones book. Sounds like a good investment.
Do. I can't recommend it enough. It's $A90 from Evatco but should be less from Amazon or the like.
  Reply With Quote
Old 28th January 2008, 12:50 AM   #9
diyAudio Member
 
Join Date: Mar 2003
Location: Zürich
Hi Stuart

You are right, the cathode is bypassed! That makes quite a difference, of course!

Erik
__________________
my surname is indeed 'de Best': neither misspelling nor snobbism! Ask SY!
  Reply With Quote
Old 28th January 2008, 07:15 PM   #10
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Default Re: Cacode gain. Did I do something wrong?

Quote:
Originally posted by Serge66
Last year, I was playing with the attached cascode circuit. The tubes are 6DJ8 / 6922. According to the theory, the gain is high. However, I could not get a gain higher than 30, which the mu of these tubes. Have I missed something? Theoretically, the gain can reach mu squared (900).
Thanks for your input and help.
A gain of u^2 is highly theoretical, and basically a rough estimate. You'll never see anything like that. It's like saying that the u-factor of a 12BY7A is 1100. No way will you ever get close to a gain that high with a single 12BY7A stage. Not gonna happen.

Still, a gain of 30 is pretty pathetic. There are a number of problems with that schemo. V1a operates as a grounded grid stage. You need to lose that R10. You also need to bypass its grid to AC ground (and make sure you're generous with the bypass capacitor) and make sure that you keep the wiring in that part of the circuit as short as possible. The cascode is quite capable of very high gains, and that always means a possibility of instability. Adding impedance to the grid will compromise the gain, and make the thing very liable to RF instability. This is why most UHF triodes designed for grounded grid service (the only way the thing will work at all at extreme frequencies) come in nine-pin miniature pinouts with four or five connections to the grid; these are supposed to be parallel connected to the ground plane to minimize as much as possible any grid-to-ground impedance.

Given that, are you sure it isn't oscillating? An overlooked RF oscillation will also rob AF gain.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  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
What about FETs for Cellular Base Stations in Audio Cascode Voltage Gain Stage (line tiefbassuebertr Solid State 8 23rd August 2009 02:53 AM
Wrong Voltage / current on BoZ... or Wrong Measuring ? gionag Pass Labs 8 14th May 2008 11:24 AM
Heretical Unity Gain line stage PLUS a little gain ? yagas Tubes / Valves 7 24th January 2006 01:30 PM
Cascode gain stage info????? aletheian Tubes / Valves 22 30th August 2005 01:23 AM


New To Site? Need Help?

All times are GMT. The time now is 11:14 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