preamp question - 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 25th December 2008, 02:29 PM   #1
diyAudio Member
 
Join Date: Dec 2007
Question preamp question

Hello!

Im going to build something similar to this amp:

http://www.ax84.com/static/sel/AX84_SEL_080729.pdf

and I dont understand why would someone amplify a signal, and
then halve it with two 220K voltage divider.

Any ideas?

Merry Xmas,

Miha
  Reply With Quote
Old 25th December 2008, 02:36 PM   #2
diyAudio Member
 
Frank Berry's Avatar
 
Join Date: Sep 2003
Location: Midland, Michigan
It's not a voltage divider ... It's a high frequency boost circuit.
That's the circuit after the first volume control. The second pair of resisters is a voltage divider. Only the designer knows why it's there.
__________________
Frank
  Reply With Quote
Old 25th December 2008, 02:37 PM   #3
flg is offline flg  United States
diyAudio Member
 
flg's Avatar
 
Join Date: Jul 2005
Location: North East
There is also a cap there Mids and Highs are not attenuatted but the bottom end is cut 6db.
__________________
"It was the perfect high end audio product: Exotic, inefficient, expensive, unavailable, and toxic." N.P.
  Reply With Quote
Old 25th December 2008, 02:46 PM   #4
lineup is offline lineup  Sweden
diyAudio Member
 
lineup's Avatar
 
Join Date: Dec 2005
Location: the north
This is not unusual for Tube amplifiers.

When using 1 tube and signle end configuration. Same for amplifiers using one single JFET transistor.
In these cases there are reasons to try to find the optimal working condition for the single transistor/tube.

With biploar transistors, it is very common to use emitter resistors to reduce gain.


In the shematic is used no less than 3 tubes gain stages!
No wonder there is too much gain.

You are basically right.
As any voltage amplification is bound to degrade signal
we should not amplify more than we need + a bit extra margin.
Because more amplification will only add more distortion than what we want
__________________
lineup
  Reply With Quote
Old 25th December 2008, 04:32 PM   #5
diyAudio Member
 
Join Date: Dec 2007
Wouldnt it be more logical to reduce the gain of the stage by lowering the plate resistance? Maybe there would be less distortion operating the tube with less plate swing, thus greater linearity, than first generating too much signal and then atteunating it..

Anyway this is a guitar amp, so more or less distortion is what one needs.

What about the loop around the cathode follower with the diode?
Whats that for?

Thanks!

Miha
  Reply With Quote
Old 25th December 2008, 08:59 PM   #6
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
Originally posted by overdrajv
What about the loop around the cathode follower with the diode?
Whats that for?
Since this has a SS power supply, the positive rail will come up very fast, much faster than the heaters can warm up. They included that to prevent overvolting the grid of the second 12AX7, which can't stand positive grid voltages and grid current. Once the heaters are hot, the diode is reverse biased and drops out of the circuit (except for its junction capacitance, of course, but who cares? It's a gee-tah amp).

I took care of the problem of SS power supplies and DC coupled stages by making separate heater and HV supplies. Power up the heaters first, then turn on the HV, and you don't have to worry about it.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 28th December 2008, 10:59 AM   #7
diyAudio Member
 
Join Date: Dec 2007
But this amp does include separate switches for heaters and B+ ..
  Reply With Quote
Old 28th December 2008, 06:49 PM   #8
Merlinb is offline Merlinb  United Kingdom
diyAudio Member
 
Merlinb's Avatar
 
Join Date: May 2006
Location: Lancashire
Default Re: preamp question

Quote:
Originally posted by overdrajv
Hello!

Im going to build something similar to this amp:

http://www.ax84.com/static/sel/AX84_SEL_080729.pdf

and I dont understand why would someone amplify a signal, and
then halve it with two 220K voltage divider.

Any ideas?

Merry Xmas,

Miha
The quality of overdrive generated in a valve is partly related the anode voltage. For a 'lead sound' it is desirable to use a large anode resistor, so the anode voltage swings low, which helps generate a 'hard' sound.
However, the output signal is too large to deliver directly to the next stage (particularly at low frequencies), so it's usual to include some sort of attenuation, which can also include treble boosting too.

The diode/resistor protects the cathode follower from arcing, which might occur if the standby switch is not used.
  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
Uno preamp question john blackburn Chip Amps 14 15th February 2010 11:26 PM
preamp question east electronics Solid State 4 18th July 2009 12:28 AM
Preamp Question! Lab_Rodent Solid State 9 6th December 2007 12:14 AM
X0 preamp question Tyimo Pass Labs 5 25th March 2007 09:32 AM
Preamp Question mapiao Tubes / Valves 0 6th February 2007 03:41 AM


New To Site? Need Help?

All times are GMT. The time now is 02:37 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