Home » Audio » Group Build » SE 300B Project, Part 11 - The Amplifier Schematic
SE 300B Project, Part 11 - The Amplifier Schematic [message #31860] Thu, 30 March 2006 05:38 Go to next message
Damir is currently offline  Damir
Messages: 1005
Registered: May 2009
Illuminati (2nd Degree)
Finally, here`s a complete schematic of SE 300B amplifier, one channel shown:

The driver is described in part 9, and active, CCS load in various «chapters», and «Guinevere» preamp project. Again, links for PCB and current settings (10-11mA) are here:

http://audioroundtable.com/GroupBuild/messages/1079.html

http://audioroundtable.com/Tubes/messages/929.html

The output stage is known from parts 1, 2 and 4 – there`re some little changes, for example, cathode resistor R12 is enlarged to 910 Ohms. We can use 820 R / 25W resistor, and 91 Ohms / 1W resistor in series. It can «work» like slow-blow fuse – in normal operation, 80mA cathode current produces 0,08^2*91 = 0,58W dissipation. If for some reason current through the output tube rises more then 105 mA, this, 91R resistor would overheat and burn out, (hopefully) protecting other (expensive) amplifier`s parts.
Negative secondary / speaker terminal can be grounded (for safety reasons), but I left it to «float».
We can use the «star ground» variant, where all the driver grounds are put together in one point near the tube socket, and then grounded to the one and only «star ground» point on the chasis. The same is with output tube, see the schematic. The «minus» pole of PS decoupling capacitors can be grounded to the same «common» points, C3 (and C7 other channel) to the power amp`s common ground point, and C4 (and C8 other channel) to the driver`s common ground point.

And here`re the wiring details of E180F driver:

Wiring details for the output stage are in Part 4 – there`re some little changes (Ci, Rk and Rgs – their values are changed a bit), but the main change is a grid choke in the place of grid resistor, Rg.
In the next, (probably) last chapter, I`ll have photos of the finished amp, and some more construction details.

Bravo, great work! [message #31862 is a reply to message #31860] Thu, 30 March 2006 10:13 Go to previous messageGo to next message
Norris Wilson is currently offline  Norris Wilson
Messages: 361
Registered: May 2009
Grand Master
Thanks Damir for your amplifier design information.
Obviously, you have put in allot of time and hard work to bring this to us. It looks great, and is very much appreciated.

I was wondering if I over looked the power supply information?

I am looking forward to your completed information on this design.

And I hope you will have time in the future to include substituting a 2A3 for the 300B? And also, will be so kind as to share this information as you have the detailed information on this 300B project.

Bravo

Norris Wilson

Excellent! [message #31863 is a reply to message #31860] Thu, 30 March 2006 10:28 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 17571
Registered: January 2001
Illuminati (33rd Degree)

Thanks for your hard work!


Re: Bravo, great work! [message #31864 is a reply to message #31862] Thu, 30 March 2006 10:41 Go to previous messageGo to next message
Damir is currently offline  Damir
Messages: 1005
Registered: May 2009
Illuminati (2nd Degree)
Thank you for your kind words. Power supply schematic is in the Part 10. The easiest way to implement 2A3 is to use the same PS schematic, and not use C1 and C5 (first PS caps). Then you`d have a choke input PS, with about B+=312V we need (depends of the rectifiers). `Cos of that I specified "larger" chokes then we really need (250mA and 150mA) - cost difference is negligible.
The only problem is filament voltage - probably it is the best to ask for "modified Centurion" trafo with two 5Vct/2,5A (2,5-0-2,5V) filaments windings. The original has two 5V/1,5A windings, and one 14V/1A we`re not using and it can be ommited (we`d have two ct terminals needed that way).
Cathode resistor of output tube can be lowered with switch across 91R resistor, leaving only 820R or so by shorting 91R.
Driver Ia must be lowered to 10mA, we`d lowered Ua that way, maybe Rk must be changed also (`cos of lower B1+).
Huh, it isn`t too easy to build "universal" 2A3/300B amp, but it is possible...

Re: Excellent! [message #31865 is a reply to message #31863] Thu, 30 March 2006 12:25 Go to previous messageGo to next message
Damir is currently offline  Damir
Messages: 1005
Registered: May 2009
Illuminati (2nd Degree)
Thank you - I hope that the final part will be soon, then we can put amp and PS schematics into "Projects" section...
Maybe someone would be interested to actually build it ?

Re: Excellent! [message #31867 is a reply to message #31865] Thu, 30 March 2006 21:17 Go to previous messageGo to next message
2wo is currently offline  2wo
Messages: 44
Registered: May 2009
Baron
Hi Damir,

I have been following this series with great interest And as it happens a wile back I acquitted a few D3a’s for another project.
Bas Horneman is working with them in his LD 91 and I see your positive reports. I am going to give them a go in my WE91’s. The question remains weather to load them with anode chokes or CCS.
This amp need a kick in the ass. It sounds good, very good, but they don’t have the dynamic drive of my puny little DRD 45’s on my 97db speakers.

I have more to say but out of time…John


Re: Excellent! [message #31868 is a reply to message #31867] Fri, 31 March 2006 04:54 Go to previous messageGo to next message
Damir is currently offline  Damir
Messages: 1005
Registered: May 2009
Illuminati (2nd Degree)
Hi, I tried D3a only triode connected, with resistive and CCS loads. Good sound and high amplification, but large input capacitance (Part 9).
I planned pentode connection too, basically P. Millett`s OP (in his large pentodes test) - Ua=Ug2 = 150V, Ia/Ig2 = 20/6mA. Then Ra=8k2/10W and Rg2=27k/2W, bypassed with 4µ7/450V and referenced to cathode, Rk=51R, unbypassed; B+ around 310V. But, I found trioded E180F/CCS combination and stopped with experiments... Especially, IMO - high impedance out drivers and grid choke load aren`t that reccomendable.
Hey, you can always "borrow" CCS from "Guinevere" and try it...

Re: Excellent! [message #32175 is a reply to message #31865] Wed, 01 August 2007 03:06 Go to previous message
red is currently offline  red
Messages: 2
Registered: May 2009
Esquire
Hi Damir - I have built the Legacy amp and I am tempted to convert to your schematic as I have all the parts (also have a few C3g's).

A quick question - do you think that the grid stopper are essential in this amp design - I currently have grid stopper at 39ohms - and am experiencing some courseness at higher frequencies - do you think that this may be the issue?

Red

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