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845 class A2??

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Guten tag.

I´ve read alot at AA about running 845´s 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???:D
I´m 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 811A´s are disassembled while building new chassies, so this might be a great opportunity to do some, hrm, minor changes in the construction:D

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 can´t decide whether this would be worth the money or not.
I have everything (except for some small caps and resistors) to put the 811A´s back together, but changing to 845´s would cost quite some money.
On the other hand, those 811A´s 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.
845´s should manage without it I think.

845: To be or not to be??
 
I guess 211´s 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 I´ll stick to the 811A´s for now. They may have their sonic flaws but I kind of like the way they light up my livingroom:D
 
Except for a lot of mechanical buzzing (which I´m 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... :eek:
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:devilr:
 
Paul Barker said:
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 :devilr:
Actually I didn't know there were A2 curves. I have been asked for those in the past.
 
Oh happy day!!

At the end of this month I´ll recieve a package with three matched pairs of NOS Philips 813´s, so screw ém 845´s:D
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, they´re 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??
 
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.
 
I read at AA about some guy who squeezed 22W out of one 813 at 800V , 100mA and 5k load.
22W is quite good for an SE DHT amp, especially when the power tubes costs less than those Sino 845´s and alot less than 211´s.

Considering my taste for music "raw power" feels slightly more attractive than "silky soft", I must say. I´m more into Slayer than Vivaldi:devilr:
 
hey i know i might get fried for this comment but given that 845 require 1k+ why not try a parallel EL34?

i have not built a tube amp. I am considering building one though. so far I looked at the 300B (they were too expensive), the 845 at 1K or there abouts might be too dangerous givne that i have an inquisitive 18-19 month old running around the house.

the speakers (87db 1w 1m) will require 25W+ in the room (325 sq. ft. with a 9'6" ceiling) they are in to make realistic levels (95db / 2m).

a friend told me to think about using SE EL34 using 3-4 EL34s per channel. Other than NOS they would cost about $10-12 each.

any other advise anyone?
 
diyAudio Senior Member
Joined 2002
Hi,

i figured i'd keep the SE sound if i used parallel pairs.

I've no idea what it is you're expecting from a SE amp but the general consensus is that it's better to use a single tube than to put multiples in //.

This is true for both SE and PP amps.

So if you need more power use a higher power tube, e.g. a 6550 or KT90.

Cheers,;)
 
I agree with Frank on parallel output tubes. I have tried it with 2A3s and 807s and they both sound better when you pull one of the parallel tubes (or two in the triple parallel amp I built :) )

BTW the 845 doesn't need 1KV. It's quite happy down to silly voltages like a 300V B+. And it's easier to drive into A2 at low Va:clown:

Actually 800V Va is a great point for an 845. Use bipolar psu and your never more than 400V from ground. Nice! Drive it hard into A2 and up to 30 Watts is available... That's 60 Watts rms from a proper amp (i.e. push pull!!!).

ciao

James
 
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