Using the Crowhurst articles and Claus Byrith - 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 16th February 2006, 02:54 PM   #1
eddog is offline eddog  United States
diyAudio Member
 
Join Date: Nov 2004
Location: Small Town in Minnesota
Default Using the Crowhurst articles and Claus Byrith

Along with a schematic of a HarmonKardon250 I have changed the input stage just ahead of the split load phase inverter. In the original schematic the first half of the 6CG7 derived it's plate voltage by conecting it's plate to the cathode of the phase inverter stage through a resistor. I also did away with the coupling cap between the first stage and the PI by choosing the new Q voltage on the first 1/2 of the 6CG7 to be B+/3 as suggested in many of the threads I have read. I think these changes are good for the overall operation of the amp, but I would very much appreciate some feedback especially concerning NFB. In the original schematic the screen grids of the output tubes were tied to thier own 310v source, but I would like to operate in Ultralinear(as shown in my poorly drawn schematic). By going UL, i understand that to be a form of NFB in itself,also the first stage is under current injected feedback(correct)before I even bring any output voltage back to the stage. Do you think there is enough NFB already or do I need to bring some back from the secondary(excuse the poorly drawn Transformer)? Also please point out any obvious mistakes or perhaps it should be canned and start all over. Thanks for newbiehelp
  Reply With Quote
Old 16th February 2006, 02:57 PM   #2
diyAudio Member
 
Join Date: Jun 2005
Location: Aberystwyth / Llanelli (wales)
Send a message via MSN to DoomPixie
you forgot to atach the schematic
  Reply With Quote
Old 16th February 2006, 03:19 PM   #3
eddog is offline eddog  United States
diyAudio Member
 
Join Date: Nov 2004
Location: Small Town in Minnesota
Default Rats....

I am having trouble getting the attachment to go through. It is too big(1.5M). Is thier a way to make it smaller in MS Paint?
  Reply With Quote
Old 16th February 2006, 03:24 PM   #4
poobah is offline poobah  United States
diyAudio Member
 
poobah's Avatar
 
Join Date: Nov 2005
You need to upload your image to a third party image server... then attach the link to your post...

google "image server", "photo server"
  Reply With Quote
Old 16th February 2006, 03:26 PM   #5
eddog is offline eddog  United States
diyAudio Member
 
Join Date: Nov 2004
Location: Small Town in Minnesota
Default Got it figured out

here it is
Attached Images
File Type: gif tubeamp.gif (9.7 KB, 383 views)
  Reply With Quote
Old 16th February 2006, 05:44 PM   #6
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
First obvious: lose that 1M resistor on the split load.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 16th February 2006, 05:46 PM   #7
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Second obvious- you're running the 6CG7 at much too low current for good linearity. You want more like 8 mA with a correspondingly smaller plate resistor.

Lose the 10pF on the split load or put another 10pF on top. You need plate and cathode loads to be absolutely symmetrical.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 16th February 2006, 07:56 PM   #8
eddog is offline eddog  United States
diyAudio Member
 
Join Date: Nov 2004
Location: Small Town in Minnesota
Thanks SY. When I plotted the load line for the 1st half of the 6CG7 tube i did see that it was almost completely horz. but I was using the values that the tubes data sheet said would work and figured "hey they know better than me". As far as gain for the amp, if I have this correct, becuase my output tubes need 33 volts(RMS) swing each for full ouput, the output from the 1st stage should be slightly greater than that value because the PI will lower the gain less than unity? Crowhurst mentions that the grid resistors for the stage following a Splitload PI shouldn't be more than 250K, yet the HK amp had values of 330K. Does disaster loom at a value of 330K? I see the obvious problem of the cap across the 22k in the PI. I was confused between two different designs so it will be dropped.
So 360ohms in the cathode with an anode value of 36K will get me to Q of 5.6ma at 100volts with the grid biased at -2 volts. I see(or at least think I see) that I am going to have trouble driving the output tubes to full output once NFB is introduced because at a 2 volt swing in grid voltage I only have a peak plate voltage change of approx 45 volts each way. I see now why Byrinth recomended using the "concertina"only on small 10-15 watt amps. I think Byrinth would call this design "Bad engineering"
Thanks, I needed to discover this.Back to the Crowhurst articles with a new understanding.
  Reply With Quote
Old 16th February 2006, 08:11 PM   #9
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Horizontal is good, but low voltages and currents on 6CG7 (and the sister version, 6SN7) aren't.

The resistors following the split-load (i.e., the grid resistors of the output tubes) are really sized more to accomodate the output tube requirements. From the driver's point of view, the bigger the better, but the output stage wants them small because of grid leakage currents. In cathode bias configuration, a 6L6GC allows up to 500K for that resistor, so you're completely safe.

There are some ways you can get more gain in that first stage: you can cascode (a FET on the bottom works well), you can use a higher gain tube (I'd look at 12AT7/ECC81), you can even use (gasp!) a pentode. But the limitation is sure to be the rather limited swing of the split-load.

For tubes like the 6L6 which need a lot of drive, I think cascaded differential amps are all in all the best solution. Crowhurst didn't have good, practical constant current sources/sinks at hand, but in 2006, we do, so that tilts the field of engineering choices.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  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
Claus Byrith 30W PP kmj Tubes / Valves 184 24th November 2012 04:31 PM
Crowhurst's Twin Coupled Amplifier terrydaniel Tubes / Valves 27 13th October 2010 03:52 AM
Help on PP EL 34 from Claus Byrith Maurizio Tubes / Valves 7 7th November 2009 05:28 PM
Claus Byrith 4-30 Questions Mike L. Tubes / Valves 2 24th August 2007 01:11 PM
Eico HF-20 & Claus Byrith PS voltage & power draw Valvomaniac Tubes / Valves 0 27th March 2007 07:53 AM


New To Site? Need Help?

All times are GMT. The time now is 06:09 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