Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

New project 300B
New project 300B
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 13th April 2018, 09:09 PM   #21
RajkoM is offline RajkoM  Bosnia and Herzegovina
diyAudio Member
 
RajkoM's Avatar
 
Join Date: Feb 2009
Finally,
well said 6A3sUMMER
  Reply With Quote
Old 13th April 2018, 09:45 PM   #22
6A3sUMMER is offline 6A3sUMMER  United States
diyAudio Member
 
Join Date: Jun 2016
If you want to use the essential design of schematic in post #1, I suggest the following:

Be careful when designing 300B amplifiers.
IMHO, 300Bs are expensive, they have finesse, and they should be used gently, not at their maximum ratings.
Need more power? Then use parallel single ended (learn how to implement this properly, another discussion), or use push pull.
And this applies to all 300B amps, not just post #1.

300A Maximum specs:
450V plate to filament
100mA with self bias
70mA with fixed bias
(I recommend using the 300B specs for grid resistors, as per below)
40 Watts plate

300B Maximum specs:
400V plate to filament
100mA with self bias
70mA with fixed bias
36 Watts plate
grid resistor with fixed bias 50k Ohms
grid resistor with self bias 250k Ohms

Post # 1 schematic: 1.414 x 350VAC = 495VDC - 50V rectifier drop = 445V B+, that is plenty.
And it is too much B+ for fixed bias, only a self bias circuit will get the plate to filament voltage back below 400V.

It is possible that the fixed bias tap was mis labeled at 250VAC, and could have been 50V.
But 1.414 x 50VAC is only -70V fixed bias to work with, not enough for a 300B with 445V B+
So do not use the fixed bias circuit.

Choose self bias because you have 445V B+. Remove the fixed bias circuit.
Connect a 1 k Ohm grid stopper resistor directly at the socket grid connection, and the other end to the grid leak resistor and coupling cap (As follows). Use a 180k, 150k, or 100 k Ohm grid leak resistor from the non-grid end of the grid stopper resistor, and connect the other end of the grid leak resistor to ground. Connect the coupling cap to the junction of the grid stopper resistor and the grid leak resistor.

Now, change the self bias resistor from 820 Ohms to 1k Ohms.
80mA will give 80V self bias. 445V - 80V gives 365V plate to filament, and 29 Watt plate dissipation. The current and voltage will vary some from the estimated numbers above, but will be within maximum specs; and this is a good place to start.

Be sure to use an output transformer that is rated for 80mA DC or more (100ma to be safe for up to the maximum 300B current rating), so there will not be bass distortion (and saturation which will affect mids and highs too).
  Reply With Quote
Old 14th April 2018, 06:13 AM   #23
hooman is offline hooman  Iran
diyAudio Member
 
hooman's Avatar
 
Join Date: Nov 2008
Location: tehran
if can use ecl82 from mullard or tel this project is best .ecl82 work in tv vertical .vertical need to linear stuff like ecl82.
__________________
iranaudiotube.blogfa.com
1979.....i lost everything
  Reply With Quote
Old 14th April 2018, 05:50 PM   #24
artosalo is offline artosalo  Finland
diyAudio Member
 
artosalo's Avatar
 
Join Date: Feb 2010
No bad.
Attached Images
File Type: png 300B_ECL82.png (38.8 KB, 212 views)
  Reply With Quote
Old 14th April 2018, 05:54 PM   #25
xcicc is offline xcicc  Italy
diyAudio Member
 
xcicc's Avatar
 
Join Date: Aug 2008
Location: World
Wooooooowww........
  Reply With Quote
Old 14th April 2018, 06:05 PM   #26
artosalo is offline artosalo  Finland
diyAudio Member
 
artosalo's Avatar
 
Join Date: Feb 2010
At 1 W THD = 0.31 %.
  Reply With Quote
Old 14th April 2018, 06:08 PM   #27
xcicc is offline xcicc  Italy
diyAudio Member
 
xcicc's Avatar
 
Join Date: Aug 2008
Location: World
Can you post the file?
Thanks
  Reply With Quote
Old 14th April 2018, 06:19 PM   #28
artosalo is offline artosalo  Finland
diyAudio Member
 
artosalo's Avatar
 
Join Date: Feb 2010
Here you go:
Attached Files
File Type: asc 300B_SE_ECL82_1.asc (5.3 KB, 34 views)
  Reply With Quote
Old 14th April 2018, 06:20 PM   #29
xcicc is offline xcicc  Italy
diyAudio Member
 
xcicc's Avatar
 
Join Date: Aug 2008
Location: World
Thanks a lot
  Reply With Quote
Old 14th April 2018, 07:33 PM   #30
6A3sUMMER is offline 6A3sUMMER  United States
diyAudio Member
 
Join Date: Jun 2016
artosalo,

Your schematic looks pretty good.
It is a nice implementation of fixed bias.

The bias circuit will need to be modified versus the original schematic in the early post (and probably a 55, 60V tap to get the -74V with enough filtering and efficiency; 250V tap is wasteful of dropping resistor power loss).

And the B+ will need to be modified versus the original schematic in the early post (with the power transformer proposed, it will need to be modified to choke input, and an additional RC filter at the end to get enough voltage drop and enough filtering)

With a 25:1 transformer, and 3,200 Ohm primary, as shown, the output tap is 5.12 Ohms.
(The right edge of the schematic was cut off).

I did not check the 300B tube curves, but I guess if is true that 375V plate to filament (or a little less due to the OPT primary DCR and 17 Ohm sense resistor drops), and -74V fixed bias gives 79.8 mA in the 300B. That is 29.9W or slightly less plate dissipation with the aforementioned voltage drops, a pretty good combination for output power and tube life.

What output transformer or transformers do you use that is stable with the negative feedback values you have chosen?
Apparently you did not need to use a dominant pole, because there is none in the schematic.

Last edited by 6A3sUMMER; 14th April 2018 at 07:43 PM.
  Reply With Quote

Reply


New project 300BHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
300B amp project Hun7er Tubes / Valves 26 20th September 2015 10:57 AM
which 300B project? help please Mach_Y Tubes / Valves 165 23rd January 2014 04:56 AM
My 300B project... widowmaker Tubes / Valves 10 9th January 2007 10:35 PM


New To Site? Need Help?

All times are GMT. The time now is 07:53 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki