91a 300b conversion to 2a3 - 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 18th March 2005, 11:33 PM   #1
diyAudio Member
 
Join Date: Feb 2005
Location: u.s.a
Default 91a 300b conversion to 2a3

Can anyone tell me if I'm operating this converted 300b-2a3 amp at the right operating point. It sounds a bit dull but that might be my low Eff speakers, never heard another 2a3 amp so no baseline. I didnt change all components 880R to 750R for example. Measured voltages attached on the current schem as it stands now, thx
The circuit attached is the best the software can do.
Attached Images
File Type: jpg weco_2a3_circuit.jpg (38.5 KB, 528 views)
__________________
wig
  Reply With Quote
Old 19th March 2005, 12:06 PM   #2
kmtang is offline kmtang  Canada
diyAudio Member
 
kmtang's Avatar
 
Join Date: Nov 2004
Location: Vancouver
Hi,

You can reduce the value of the 1K dropping resistor to around 250 ohm. Also, the 1uF capacitor to 15-22uF for reducing of hum ripple on the B+. By doing so, the B+ should now read +330-340Vdc which will give you more brightly fast sound.

The max plate volt of 2A3 is 300Vdc. Therefore, you are still operating it rather safely.

Johnny
  Reply With Quote
Old 19th March 2005, 12:55 PM   #3
Previously: Kuei Yang Wang
 
Join Date: Nov 2002
Default Re: 91a 300b conversion to 2a3

Konnichiwa,

Quote:
Originally posted by gtmcgar1
Can anyone tell me if I'm operating this converted 300b-2a3 amp at the right operating point.
You do not.

1) Bump up the Supply voltage for your driver stage.
2) Bump up the anode voltage of your driver valve. You want around 1/2 of the supply voltage.

Quote:
Originally posted by gtmcgar1
I didnt change all components 880R to 750R for example.
This makes not such a great deal of impact, but I'd recomemnd you adjust things.

Sayonara
  Reply With Quote
Old 20th March 2005, 07:22 AM   #4
diyAudio Member
 
Join Date: Feb 2005
Location: u.s.a
1) Bump up the Supply voltage for your driver stage.
2) Bump up the anode voltage of your driver valve. You want around 1/2 of the supply voltage


I read your information on pentode drivers and was hoping you'd give me some pointers thx

Please confirm if I have this correct,..

change r4 to 750ohm for 2a3
change r1 to 250ohm to inc supply v
change c1 to 15-22uf for better ripple, dont know if i have room.
change r6 to 50-60k for increased anode driver v

thx for the help this is my first diy amp build,
__________________
wig
  Reply With Quote
Old 20th March 2005, 09:24 AM   #5
Previously: Kuei Yang Wang
 
Join Date: Nov 2002
Konnichiwa,

Quote:
Originally posted by gtmcgar1
change r4 to 750ohm for 2a3
Yes.

Quote:
Originally posted by gtmcgar1
change r1 to 250ohm to inc supply v
No, simply adjust it to give 295V on the 2A3 Anode.

Quote:
Originally posted by gtmcgar1
change c1 to 15-22uf for better ripple,
That will increase the supply voltage too. Only use this if you have problems with hum.

Quote:
Originally posted by gtmcgar1
change r6 to 50-60k for increased anode driver v
No, absolutely not.

Reduce R2 to 3k3 or therabouts.

That should give around 290-300V for the supply to the driver stage.

Change R7 to 75k
Change R8 to 30K
Change C6 to at least 2.2uF

Adjust R9 in circuit for an anode voltage around 60V higher than where the screengrid settles, which I'd expect to be in the 70V region.

The result should work a lot better.

Ciao T
  Reply With Quote
Old 21st March 2005, 07:23 PM   #6
diyAudio Member
 
Join Date: Feb 2005
Location: u.s.a
Default thanks

Thanks for all the help guys. I'll let you know what she sounds like once I get the voltages sorted out, thx Again.
__________________
wig
  Reply With Quote
Old 29th March 2005, 11:06 PM   #7
diyAudio Member
 
Join Date: Feb 2005
Location: u.s.a
Default changes were made

Updated as you suggetsed and much much better, thx
driver supply settled at 295v
anode 140v with screengrid at 75v (r9 1.2k)

Thanks again, photo is pre upgrade.
Attached Images
File Type: jpg 2a3_rebuild_compressed.jpg (80.3 KB, 260 views)
__________________
wig
  Reply With Quote
Old 31st March 2005, 12:59 PM   #8
kmtang is offline kmtang  Canada
diyAudio Member
 
kmtang's Avatar
 
Join Date: Nov 2004
Location: Vancouver
Really interested to know how it sounds for the 2A3 with 6SJ7 driver.

I have built the 2A3 monoblocks based on the design of JE Lab's 2001 edition. That is with use first triode of 6SL7 as gain stage and the 2nd one as cathode follower. I used all Hammond's power transformer, choke and 1627SE OPT.


Johnny
  Reply With Quote
Old 1st April 2005, 08:02 PM   #9
diyAudio Member
 
Join Date: Feb 2005
Location: u.s.a
Default reply

This was originally my first 300b amp, I converted because I picked the wrong Hammond PT which wasn't really up to the job. I also built a 300b version with Angela PT/1627 output which sounds great. The plan now is to try biamping (since i have 2 sets of amps) but I can't decide on what speakers I need or the best way to do the xover. I've been listening to the 2A3 amp for about a week on b&W602's and its surprisingly detailed, I haven't felt the need to go back to the bigger amp yet. I'm pleased with the result so far, now that I have it operating in the sweet spot.
__________________
wig
  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
FS: for 300B (JE Labs 300B SE) builders cartman Swap Meet 2 29th October 2008 01:13 PM
Quicksilver 300b monoblock (KT88 conversion) eleakist Tubes / Valves 4 18th June 2007 11:50 AM
* 35 watt * 300B SE = super 300B + Class A2 rick57 Tubes / Valves 4 13th July 2005 04:18 PM
mp3 conversion ??? onis_uk Parts 8 20th October 2004 10:31 PM


New To Site? Need Help?

All times are GMT. The time now is 07:14 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