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

845 class A2??
845 class A2??
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 2nd November 2003, 12:51 PM   #1
Fuling is offline Fuling  Sweden
diyAudio Member
 
Join Date: Aug 2001
Location: Sweden (Mora)
Default 845 class A2??

Guten tag.

Ive read alot at AA about running 845s at low voltages, 400-500V. People who has tried it claims that it sounds very good but the power is in the range of a 2A3 SE amp, ie 3-5W.

Today I looked at some datasheets and found A2 curves for an 845 equivalent and it looked quite nice I must say.

http://frank.pocnet.net/sheets/120/t/T110-1.pdf

So, has anyone tried or do I have to be the first one???
Im thinking something like:

Vp 450-475V
Ip 70-80mA
-Vg 35-40V (input swing from -125V to +50V)
Ra 5,6kohm

The thing is that I already sort of have everything except for the power tubes IF i butcher my 811A SE monoblocks.

For the moment the 811As are disassembled while building new chassies, so this might be a great opportunity to do some, hrm, minor changes in the construction

The circuit could be pretty much the same as before:
Input stage (trioded E81L) cap coupled to an EL86 cathode follower which is direct coupled to the output tube grid.
Of course it would require a negative rail for the driver stage, but that should not be any problems.

Output power should be about the same, some 11 watts.

I cant decide whether this would be worth the money or not.
I have everything (except for some small caps and resistors) to put the 811As back together, but changing to 845s would cost quite some money.
On the other hand, those 811As are not particulary linear so they require some NFB to sound even half-descent and quite a lot of NFB to tame the output impedance.
845s should manage without it I think.

845: To be or not to be??
  Reply With Quote
Old 2nd November 2003, 11:13 PM   #2
fdegrove is offline fdegrove  Europe
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
Hi,

Quote:
845: To be or not to be??
A good (not Chinese junk) 845 or even the much neglected 211/VT4-C should be fine in A2 but I'd run those babes much hotter.

1kV is the minimum for me but be careful at those voltages, one mistake and it's your funeral...

Cheers,
__________________
Frank
  Reply With Quote
Old 3rd November 2003, 07:51 AM   #3
Paul Barker is offline Paul Barker  United Kingdom
diyAudio Member
 
Join Date: Aug 2003
Location: Scarborough
I totally agree with your sentiments. I far prefer the nos vt4c to the Chinease 845. Since nos 845's are unobtainium that's all I have to go on.

Dhaen has a class A2 845 amp, surprised he hasn't shared details on this thread.
  Reply With Quote
Old 3rd November 2003, 09:21 AM   #4
Fuling is offline Fuling  Sweden
diyAudio Member
 
Join Date: Aug 2001
Location: Sweden (Mora)
I guess 211s at ~450V and zero bias would be another, probably better, option.
But after all I guess these tubes does their jobs best at about 1kV, so maybe Ill stick to the 811As for now. They may have their sonic flaws but I kind of like the way they light up my livingroom
  Reply With Quote
Old 3rd November 2003, 09:36 AM   #5
fdegrove is offline fdegrove  Europe
diyAudio Senior Member
 
fdegrove's Avatar
 
Join Date: Aug 2002
Location: Belgium
Hi,

Quote:
They may have their sonic flaws but I kind of like the way they light up my livingroom
They sound their best under the X-mass tree...

Seriously, what do you feel is wrong with the 811?

Cheers,
__________________
Frank
  Reply With Quote
Old 3rd November 2003, 09:52 AM   #6
Fuling is offline Fuling  Sweden
diyAudio Member
 
Join Date: Aug 2001
Location: Sweden (Mora)
Except for a lot of mechanical buzzing (which Im now taking care of) the major problem is the low damping factor. Without NFB the bass response gets really "sloppy".

During the breadboarding process I tried cathode feedback by connecting the OPT secondary between the 811A cathode and ground but that only made it worse.
Thinking of it, I probably made some faulty connection causing positive feedback...
Currently I use "regular" feedback from output transformer to input tube cathode trough an resistive network and it works fine even though I have some issues with that kind of feedback...

Except for the bass "sloppines" (when not using NFB) the sound is quite good, with nice imaging and a lot of raw power
  Reply With Quote
Old 3rd November 2003, 09:56 AM   #7
dhaen is offline dhaen  Europe
diyAudio Moderator Emeritus
 
dhaen's Avatar
 
Join Date: Jun 2002
Location: U.K.
Quote:
Originally posted by Paul Barker
I totally agree with your sentiments. I far prefer the nos vt4c to the Chinease 845. Since nos 845's are unobtainium that's all I have to go on.

Dhaen has a class A2 845 amp, surprised he hasn't shared details on this thread.
Indeed I do I didn't join in because mine runs at 1250v, and so is Mainly A1; just sliding into A2 on peaks.
What I though Fuling wanted, was a pure A2 stage running at low voltage. A different beast
Actually I didn't know there were A2 curves. I have been asked for those in the past.
  Reply With Quote
Old 3rd November 2003, 02:45 PM   #8
Positron is offline Positron  United States
diyAudio Member
 
Join Date: May 2003
Location: Midwest
Default 845

Yes, Fuling, I see what you mean. The damping factor would be squat. Also, at such low plate voltages, your not really taking advantage of the tube, puny output, yet large plate dissapation capability.
  Reply With Quote
Old 3rd November 2003, 03:05 PM   #9
Fuling is offline Fuling  Sweden
diyAudio Member
 
Join Date: Aug 2001
Location: Sweden (Mora)
Oh happy day!!

At the end of this month Ill recieve a package with three matched pairs of NOS Philips 813s, so screw m 845s
I made the deal with a radio amateur just half an hour ago, he even promised to try and find a pair of sockets.

Since these tubes deserves proper plate voltage and good outpur iron this means the beginning of another project, big and costly of course... Maybe its time to try those Lundahls out, theyre manufactured locally and everything.
Very well, this belongs to another thread.

The 811A project will continue as planned before I had this 845 idea. After all I have all the parts and the new chassies are allmost finished.
With some degree of NFB the bass response is just great, and is ~11W out really that bad from ~40W plate dissipation??
  Reply With Quote
Old 3rd November 2003, 03:13 PM   #10
Paul Barker is offline Paul Barker  United Kingdom
diyAudio Member
 
Join Date: Aug 2003
Location: Scarborough
The Raw power you describe from the 811 is exactly the strength of the 813. I don't know a valve with as much authority. Base problem would also be solved.

211 hasn't the same gutsy feel but it is "better" overall because it has more linear and silky soft sound. Only 845 I have experience with is the worst kind, so I'm not qualified to comment on sonics.
  Reply With Quote

Reply


845 class A2??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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
1500W RMS Class D PCB - which Class D Driver Chip is it ? mosfetsound Class D 11 1st April 2011 11:17 AM
Can a Class AB PP amp be said to be operating in Class A at low signal levels? ray_moth Tubes / Valves 19 23rd January 2009 07:52 PM
How about a round-up of Class A kit power amps, or collectable vintage class A? Brisso57 Solid State 4 14th February 2007 10:30 AM


New To Site? Need Help?

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


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki