Low output impedance pre - 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 22nd July 2003, 03:07 PM   #1
miguel2 is offline miguel2  Portugal
diyAudio Member
 
Join Date: Jun 2002
Location: Portugal
Default Low output impedance pre

Hi,

I have a EF86 preamp that I use with a ZenV4 and a GC. Since I moved to passive I/V conversion (TDA1543) I am having a very low sound, so I need more gain in the pre.
So I decided to add a E88CC in cathode follower mode after the EF86s. My problem is that simulation results show 150 V on the anode of the E88CC and I am in doubt if this is too much.

I tried the setup last night but for some unknown reason the heater of the E88CC refused to work. I am using a 12VDC supply and put a 12AX7 heater in series just to get the voltage drop. But the heater of the E88CC passed the cutrrent with only 1.5V across it, being around 10V across the heater of the 12AX7. Then I used a 6V psu and the E88CC worked fine

Miguel
Attached Images
File Type: jpg sem título.jpg (15.0 KB, 580 views)
  Reply With Quote
Old 22nd July 2003, 05:16 PM   #2
dhaen is offline dhaen  Europe
diyAudio Moderator Emeritus
 
dhaen's Avatar
 
Join Date: Jun 2002
Location: U.K.
The problem is:

The 12AX7 needs to have it's heaters in series, ie connect pins 4 and 5 for 300mA operation. It then expects to drop 12.6 volts.

The E88CC has only one heater option: 6.3 volts at 300mA, between pins 4and 5.

Clear as mud? Check the datasheets @
http://www.mif.pg.gda.pl/homepages/frank/vs.html

Cheers,
  Reply With Quote
Old 22nd July 2003, 07:28 PM   #3
diyAudio Member
 
Join Date: Feb 2003
Location: Birmingham, UK
Uno momento John...if the 12AX7 (ECC83) has it's heater connected in series then the current is 150ma at 12.6 volts, but connecting them in parallel will give you a consumption of 300ma at 6.3 volts.

Sooo...if Miguel connects the 12AX7 with pins 4 & 5 connected together as one terminal, and using pin 9 as the other, it can then be connected in series with the E88CC across the 12 volt DC supply.

Why the use of a 12AX7 just as a voltage dropper for a E88CC is another question...
  Reply With Quote
Old 22nd July 2003, 07:56 PM   #4
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
Hi,

Quote:
The 12AX7 needs to have it's heaters in series, ie connect pins 4 and 5 for 300mA operation.
As is the 12AX7/ECC83 has the heater in series for 12.6 V operation.

Quote:
Why the use of a 12AX7 just as a voltage dropper for a E88CC is another question...
Maybe Miguel is using a 25V supply ? Weird arrangement...

If you start out from a 12V6 supply you only need to wire the heaters of the EF86 and ECC88 in series.
Both use pins 4 & 5 for heaters.


Cheers,
__________________
Frank
  Reply With Quote
Old 22nd July 2003, 09:53 PM   #5
PRR is offline PRR  United States
diyAudio Member
 
PRR's Avatar
 
Join Date: Jun 2003
Location: USA
> miguel2 has attached this image:

Aside from your heater-power troubles, the 330Ω cathode resistor is really too low for a line-level buffer.

A good value of load resistor for a cathode follower is much higher than the usual bias resistor.

There are two popular ways to do this:
Click the image to open in full size.

The one on the left will self-bias, and can be adjusted without regard for the stage before it.

The one on the right saves several parts and a bass roll-off, but now you have to consider both stages together.

You see both ways in fancy designs.

(You can add a grid-stopper if you wish, though even for hot tubes like the 99 they are seen more often than they are needed.)

Any good high-output cathode follower tends to have large heater-cathode voltage. For line-level work you can usually hold this well below 100V, maybe around 50V, which is safe for most tubes. The left design can easily work that way, the right design requires tube X1 to sit with low plate voltage which may conflict with other goals.
  Reply With Quote
Old 23rd July 2003, 12:35 AM   #6
dhaen is offline dhaen  Europe
diyAudio Moderator Emeritus
 
dhaen's Avatar
 
Join Date: Jun 2002
Location: U.K.
Quote:
Originally posted by bournville
Uno momento John...if the 12AX7 (ECC83) has it's heater connected in series then the current is 150ma at 12.6 volts, but connecting them in parallel will give you a consumption of 300ma at 6.3 volts.

snip..
Oops. Yes, this properly explains the results.

Cheers,
  Reply With Quote
Old 23rd July 2003, 01:19 AM   #7
diyAudio Member
 
Join Date: Dec 2001
Location: Earth
miguel
u mention that you needed more gain from your pre-amp adding a cathode follower would not increase your gain. Cathode follower is like a current amp where it lower your output impedance by a factor of rp/mu+1 and your gain is
muRk/(rp+rk(mu+1)) where rp=tube plate resistance, rk= cathode resistor. Normally the gain is less than 1. How about running your first stage ef86 in pentode mode followed by a cathode follower. The layout you have drawn could be improve. In your cathode follower there is resistor at the anode. Normally it should not be used layout similar to PRR has drawn. When you use r2(1000K) in that configuration instead of PRR drawings the input impedance is normally of the resistor only but when connected in drawing one increases the input impedance by a factor of 7-8 times of r2. This helps in using a pentode first stage with high output impedance. Hope this helps.
  Reply With Quote
Old 23rd July 2003, 01:36 AM   #8
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
Hi,

Quote:
This helps in using a pentode first stage with high output impedance. Hope this helps.
Let's not forget a few things,Nick:

Using a penthode as a voltage amplifier will have a noise penalty, not to mention the fact that's far less linear than a triode and the gain would be sky-high, quite likely overloading the ECC88 following it.

In triode mode the EF86 has roughly the same gain as the ECC88, which makes me wonder why he'd use it in the first place since a single ECC88 could easily take care of things.

Good EF86s ain't exactly cheap either...

Cheers,
__________________
Frank
  Reply With Quote
Old 23rd July 2003, 06:21 AM   #9
diyAudio Member
 
Sch3mat1c's Avatar
 
Join Date: Jan 2003
Location: Milwaukee, WI
Send a message via ICQ to Sch3mat1c Send a message via AIM to Sch3mat1c
For a preamp it depends on the gain needed and whatever attenuation, tone or volume control circuits for instance, have to be pushed afterward; but anyway, Frankenhouse suffers no ill effects from using a pentode, and incidentially, a seemingly dirty type which used to be used in TVs (a 6U8 or 6GH8).

Tim "Ending anti-pentode and anti-12AU7 criticicism since 1898!" (to tie in with my sig)
__________________
Seven Transistor Labs
Projects and Resources
  Reply With Quote
Old 23rd July 2003, 06:59 AM   #10
diyAudio Member
 
Brett's Avatar
 
Join Date: Jan 2002
Quote:
Originally posted by fdegrove
Using a penthode as a voltage amplifier will have a noise penalty, not to mention the fact that's far less linear than a triode and the gain would be sky-high, quite likely overloading the ECC88 following it.
Um, nope, depends how you use them. The last version of my GM70 amp was running pentodes for drivers, as was the version before last of the KT88/6550 amp. The 6550 version was changed because it had seperate driver and output stage supplies and I needed some of the parts elsewhere. But it sounded awesome, and when I'm done with the other PSU, I'll put them back.

http://www.pmillett.addr.com/pentodes.htm

Basically using something very similar to the 12HG7 setup.

Oh, and I won't mention that my 'main' power amp is all pentode, operated as pentodes and it's by far the best amp I've ever heard.

Egads, I've agreed with Tim.......but 12AU7's still suck.
  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
3C24 in a SE output stage - output transformer impedance recommendation !!! aldovan Tubes / Valves 15 6th September 2008 11:18 AM
Output impedance overdrajv Tubes / Valves 8 5th March 2008 06:54 PM
Can An Output Transformer Change A Voltage Amp's Output Impedance From 0.1 To 47 Ohms kelticwizard Everything Else 11 25th March 2007 06:17 AM
BOZ output impedance haenisch Pass Labs 2 30th August 2006 11:12 PM
Output Impedance? ble0t Parts 4 19th January 2006 06:45 PM


New To Site? Need Help?

All times are GMT. The time now is 04:06 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