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 10th June 2013, 09:32 PM   #11
diyAudio Member
 
dirkwright's Avatar
 
Join Date: May 2011
Location: Virginia
I think you'd be better off building a Williamson clone. But, that's just my opinion.
  Reply With Quote
Old 11th June 2013, 01:02 AM   #12
john65b is offline john65b  United States
diyAudio Member
 
john65b's Avatar
 
Join Date: Jan 2005
Location: Chicago
Goat: Use the 12AX7 and the 12AU7 as phase splitter? According to the original schematic, nothing needs to be changed, correct (shows 12BH7 OR 12AU7)?
__________________
All right, but apart from the sanitation, medicine, education, wine, public order, irrigation, roads, the fresh water system and public health, what have the Romans ever done for us? Life of Brian
  Reply With Quote
Old 11th June 2013, 01:13 AM   #13
GoatGuy is offline GoatGuy  United States
diyAudio Member
 
Join Date: Dec 2012
Location: SF Bay Area
Yep that works too.

The reason that 300K resistor is hanging off the first-stage 12AX7 is to maximize gain to the near-limit. Choosing a pair of finals with lower mu ... increases the need for gain yet more. You could however, drive them at higher current draw ... with a little math work, and a slightly different transformer. So... use the KT88's. They're really quite nice.

GoatGuy
  Reply With Quote
Old 11th June 2013, 02:07 AM   #14
john65b is offline john65b  United States
diyAudio Member
 
john65b's Avatar
 
Join Date: Jan 2005
Location: Chicago
OK, so it looks like: keep the 12AX7, 12AU7 instead of the 12BH7, and the KT88. OK...
__________________
All right, but apart from the sanitation, medicine, education, wine, public order, irrigation, roads, the fresh water system and public health, what have the Romans ever done for us? Life of Brian
  Reply With Quote
Old 12th June 2013, 07:46 AM   #15
diyAudio Member
 
artosalo's Avatar
 
Join Date: Feb 2010
Quote:
Originally Posted by GoatGuy View Post
...The reason that 300K resistor is hanging off the first-stage 12AX7 is to maximize gain to the near-limit....

GoatGuy
Actually, no it isn't.

The reason for such high resistance is to bias the direct connected phase splitter to near optimum operating point. This kind of circuit is a compromise.
Both stages should be biased individually, as I have said earlier.

If the gain of 12AX7 stage wanted to be increased or maximized, it can simply be done by bypassing the 820 ohms cathode resistor with some 100 uF electrolyte.
This increases the gain some 2 dB.

Last edited by artosalo; 12th June 2013 at 07:48 AM.
  Reply With Quote
Old 12th June 2013, 01:31 PM   #16
diyAudio Member
 
dirkwright's Avatar
 
Join Date: May 2011
Location: Virginia
Quote:
Originally Posted by artosalo View Post
Actually, no it isn't.

The reason for such high resistance is to bias the direct connected phase splitter to near optimum operating point. This kind of circuit is a compromise.
Both stages should be biased individually, as I have said earlier.

If the gain of 12AX7 stage wanted to be increased or maximized, it can simply be done by bypassing the 820 ohms cathode resistor with some 100 uF electrolyte.
This increases the gain some 2 dB.
Actually, it's both. The resistance is chosen to achieve the correct bias point for the following stage, but the supply voltage is made high enough to make that resistance very high to provide a high gain for the first stage. Bypassing the cathode resistor is just a small trick to get a little more gain in exchange for a lot more distortion. Like I said before, the Williamson circuit is superior to this thing anyway.
  Reply With Quote
Old 12th June 2013, 01:51 PM   #17
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by dirkwright
Actually, it's both.
Yes.

Quote:
Bypassing the cathode resistor is just a small trick to get a little more gain in exchange for a lot more distortion.
Not necessarily. You would get a little more distortion in the input stage, but enough extra gain to reduce the much greater distortion from the output stage. There could be an effect on common-mode distortion in the first stage; bypassing might make this worse. The exact balance of advantage depends on details. Mullard and Leak always bypassed the first stage cathode resistor.
  Reply With Quote
Old 12th June 2013, 02:21 PM   #18
diyAudio Member
 
artosalo's Avatar
 
Join Date: Feb 2010
The key point seems to be forgotten.
Direct connected voltage amplifier and cathodyne phase splitter is a compromise.
It is working well only when a pentode amplifier is used, since this operates at optimum point with sufficiently low anode voltage.
Therefore both should be biased and optimised separately.
  Reply With Quote
Old 12th June 2013, 02:26 PM   #19
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
In this circuit adding a coupling cap would add an LF pole and possibly compromise LF stability. However, a DC potential divider with the upper resistor bypassed by a cap should be OK. That would give some separation of stages, at the expense of adding loading to the first stage. Once again, it all depends on details.
  Reply With Quote
Old 12th June 2013, 02:31 PM   #20
45 is offline 45  Italy
Sin Bin
 
Join Date: Dec 2008
Location: United Kingdom
Quote:
Originally Posted by dirkwright View Post
I think you'd be better off building a Williamson clone. But, that's just my opinion.
That's mine too. Quicksilver amps are among the worst commercial amps I have listened to. In my opinion they gained some popularity just because they came out in a time where tube amps only represented a niche and there were not too many around.
I have a friend who had the version with KT88's and he had to change the rectifiers every year! Really bad design. From the schematics it is easy to see why his rectifiers had short life. They were expensive and didn't sound good at all to me. The other models are not so much better....
Attached Images
File Type: gif QS_KT88.gif (33.6 KB, 154 views)
  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
Quicksilver 8417 question john65b Tubes / Valves 1 24th October 2008 07:40 PM
quicksilver 8417 biasing ozdiyer Tubes / Valves 2 26th August 2007 07:54 PM
Quicksilver 8417 Monoblocks dshortt9 Swap Meet 0 5th December 2003 03:57 PM
Quicksilver 8417 dreaded 42 Tubes / Valves 4 4th September 2003 11:14 PM
Quicksilver 8417 model changes Original Burnedfingers Tubes / Valves 3 12th April 2003 02:45 PM


New To Site? Need Help?

All times are GMT. The time now is 09:19 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