Monoblock 300B Uncompromised design

No compromise monoblock 300B/2A3 single ended design
Introduction , design philosophy.
It's always the same and mostly the reason why I'm in this business: I am not happy with commercial amplifiers. Most do-it-yourself designs tend to be better, however they are full of economic compromises (for different reasons than commercial designs) and so things aren't done the way they ought to be. (Except maybe the Ongaku and even that project has a visible flaw; at least the one that I heard). In this design the only concession I have made
is in the size of the chassis , which is a little small for my taste ,but is cute nonetheless.
I imagined that the perfect direct heated triode design should be one that lets you choose between a 300b or 2a3, since both are great tubes
and can share the same output transformer and same base connections .The only thing that changes are the supply voltages .
How do you obtain that? Well, there is a trick that's even beneficial for the overall sound quality ( so it will certainly not hurt the design ).
It's called choke loading the rectifier . It reduces ripple and makes life easier for the rectifier .
I also wanted a tube rectifier for best quality and of course the best components . The resistors can be improved upon with caddocks ms-244series,but that's optional . I don't think it makes a big difference ,but I decided to use blackgate capacitors in the power supply.
Also used is my famous negative power supply for grid bias : every time I read an article and the author uses this, he mentions its superiority over fixed bias (so by now you should all know ).
Then there is the issue over the non potted output transformer and the toroid potted power transformer…. very heavily choked filtered power supply.
To limit ripple to almost immeasurable limits ,one of the very finest power high voltage capacitors from Hitachi is used( latest generation electrolytic. Big difference over older generation ).
And above all a simple circuit and a mono block approach so as not to have one channel reacting with the other .
How does it sound ?
Well it's a big improvement compared to my stereo 300b/300b interstage design
Is it due to the mono block arrangement? Is it the blackgates? Is it the Hitachi power capacitor or maybe the power supply? Who knows.
Anyway from now on I'm convinced that uncompromised designs ought to be built like that. There will be similar projects over the next few months.
New improvements ?
As I am becoming a distributor for Jensen ,I will try a paper in oil capacitor as a signal capacitor and maybe a few in the power supply ,
perhaps another stabilizer capacitor in the bias supply.

BOM :
Power toroid M8x100 + nut + washer
Toroid 2x6vac 3 rubbers for mounting
LL1623/90mA 4 M4 hex screws
Black metal pot 4 M4 hex screws
LL1660 4 M3 hex screws
2 chokes 20H/80mA 4 self threading screws 3,5x9,5
choke 30H/20mA 2 screws 2,9x6,5
switch 10A/250v on-off
switch 10A/250v 2 x on-off ( S1)
fuse 6,35mm
euro connector 2 M3x10 screws
bridge rs606 black M3x16 hex
Hitachi 1200µF/500 volt with mounting ring & 3 M4 screws black
Black gate 2x47µF/500 volt with mounting ring & 2 M3x10 black screws
Speaker terminal 2 hex M3x16
Ux4 tube socket 2 hex black M4x10 & rubber o ring
Octal tube socket 2 M3x10 hex black
Noval tube socket 2 M3x6 hex black
300B tube , 274B tube , 2C51 or 5670W
RCA input jack
hook up wires ( silver optional )
4 rubber feet 4 M3x16
mA meter 0- 150 ma (not on prototype)
volt meter 0-500 volts ( not on prototype)
12 bolts M3
5 bolts M4
bolt M8
16 M3 self threading x 13
chassis ( top + bottom plate + wooden frame )
components : resistors and capacitors from schematic
Construction :
Use antislip washers between bolt and chassis/component
Mount power transformer , the other side mount toroid and put rubbers between toroid & chassis , tighten M8x100 so that both transformers
are tightly mounted
Mount chokes , tube sockets ( be care full not to over tighten & don't forget rubber o ring on ux4 tube socket ) , ring for capacitors , switches ,
euro connector, fuse , mA & volt meter, RCA & speaker terminal
Mount bridge rs606
Twist all transformer wires that belong together ( black-black ,red-yellow , pink -pink , blue-grey )
Prepare LL1623 : see alternative C
Prepare inter stage if you want to use it alternative T
Prepare wires :
red ( 9 , 15 , 9 , 15 cm)
black ( 25,8,10,14,12,9,12,8,10,20 cm)

The electronic part :
For 2 identical amps use wires of equal length in both amplifiers
When stripping a wire make sure that you do not damage the wires because they might break
Strip at least 0,5 cm
leave 1 cm between body and twisted part. Afterwards tin twisted part and cut off excess.
Use cutter to scrape oxidation off wires and solder lugs to prevent bad contact. Sometimes there might be lacquer on a conductor.
When soldering let the solder flow and don't be tempted to blow to let it cool down
Pre tin all the tube pins that will be soldered on and chokes
When soldering components to tube socket cut off excess length so that the component is as close as possible (0,5 cm = perfect)
Take diodes by228 and twist anodes together
>From power transformer:
Twist yellow wire to anode d1 by228
Twist red wire to anode d2 by228
Solder yellow to pin 4 and red to pin 6 of octal tube socket
diodes other end should point upwards
connect fuse and euro connector and power switch see diagram 1
twist on off the black with pink and the other with the other pink wire
solder one pair to the fuse and the other pair to the euro connector ( not the middle pin)
from the prepared twisted wire ( can be any combination of colour) cut of the necessary length from the fuse to the powers witch ( leave 5 cm margin)
connect one end to the fuse and the other end to the euro connector
and the other side to the power switch ( twist and solder the 10nF/630v cap on) screw tightly

take 0,11 ohm/5 watt , leave 1 cm of wire on one side and cut off excess
take 6 vac transformers , the 2 primaries in parallel ( red-blue , and yellow-grey)
twist each wire the other side resistor 0,11ohm tin it
prepare red wire and red wire to switch S1
twist 2 red wires to any one resistor ( one red wire goes to choke 1 and the other to S1)
the other side of resistor solder to pin 2 & 8 octal tube socket

next we prepare the filament wires (pre twisted 2 x 20 cm )
take the 2 18 ohm resistors and twist one side together , on the other side twist the filament wires ( diagram 3 )
Pre tin pin 11 and 4 of ux4 tube socket, solder the resistors and filament wires on
Solder blue grey on the bridge rs606 ac connections
Take capacitor 4700µF/16v and twist and solder on each side resistor of 0,47ohm/5watt
and one pair of the twisted filament wires from noval pin 1 & 9(solder them on to pin 1 & 9 )
Solder this on the + and - of the bridge (watch out for capacitor polarity)
Take 2 6800µF/16v twist together (diagram 4) the negative side
take resistor 1 ohm & 1,5 ohm twist together prepare twisted wire
and twist on, (other side goes to switch S1)
and solder to between the positive lugs of capacitors
Take 4,7µF/400v and 0,47/630v and twist together (use pliers to bend legs)
twist filament wires of ux4 tube socket on the ends and solder
solder on the negative side of 6800µF/16v
Prepare wire 1,5mm ofc
see diagram 5
Take 0,47/630v and bend legs with pliers so they will rotate in same direction as screws of
the capacitor 1200µf/500v twist resistor on the legs
see diagram 6
mount capacitor so that negative points to the back
Take the other 4,7µF/400 and 0,47/630 volt and twist together
take resistor 390 ohm and capacitor 220µF/6v and twist together
then twist both these groups together,
take 100 ohm resistor and twist one side to the positive side of the 220µF/6v
the other side solder to noval pin 2 and 8
Take the 39k resistors and twist one side together solder the other to pin
4 & 6, the other side of 39k connect (red wire) to blackgate see diagram 2
Take 150k and 390ohm resistor and twist together, solder 390 ohm to pin 3 & 7
the other side of the 390 ohm solder to RCA pin directly, the other side of 150k solder to negative of RCA input, make sure that the RCA
Is isolated from the chassis
use black wire to connect ground RCA plug to ground 390ohm (cathode 2c51) from there go to ground blackgate see diagram 2
take 2k2 & 82k & 0,47mkp and twist together see diagram 7
prepare 20cm of solid core green wire solder to pin 2 ux4 other side goes to LL1623
solder 2k2 to pin 3 ux4
negative bias : diagram 8,9,10
take the 4 1000µf/100 volt capacitors
twist plus & minus of 1000µf/100v of one pair together
twist plus and plus together of the other pair
take 2 diodes by228 twist anode and cathode together
twist cathode to plus of capacitors group1
twist anode to negative of the other capacitor of the same group
connect transformer black & white to these see diagram 9
solder these connections so they stay fixed together
take the 2 1k5 twist together and connect to capacitors see diagram 10
take multi turn potentiometer & 56k resistor and twist on to pin 2 pin 1 connect to negative capacitor ( the last one) and the wiper connect
to the 82k resistor use pink cable
see diagram 11
take the plastic mmp005 self adhesive and glue on under the capacitors you want to attach using a nylon string
one last connection to make solder black wire to the diodes of the high voltage power supply
and connect that next to the euro connector via a 10 ohm resistor , (twist it around the thread of the m3x16 screw also connect the middle pin
of the euro connector to the screw twist around and but additional bolt and washer on
check everything , power up if sure ( add fuse )
measure filament voltages no tubes , this should be higher then normal , then insert 300b and 2c51 and measure again , it should be within 5%
of specified voltages (max 10%)
adjust bias on 300b until you measure -80 volts on the grid
plug in 274b and measure again , be care full off lethal voltages measure plate of 300b ( 380 volts) , plate 2c51 ( around 200 volts )
turn off and mount in wooden frame , add bottom plate and enjoy
options :
version with ampere meter and volt meter
to measure it's the internal terminals
the outer terminal are for the internal light
on the negative side of the voltmeter hook up 1M ohm resistor solder other side through black wire to negative of black gate , positive side
to positive of black gate ( together with wire of ampere meter)
on the negative side of the ampere meter connect solder 68k resistor and the other side of the resistor solder to positive of black gate ,
positive solder to the other side of the choke ( or 1200µF/500volt plus terminal)
if you choose to try the LL1660 inter stage transformer a few things change ,
disconnect the 39K resistors and 0,47µf coupling capacitor
and connect the transformer as the schematic shows , do not forget that the bias first goes through the transformer and the 82k now goes to ground
( 18 ohm ground)
also add 12200 ohm 5 watt in series with the choke of 30H/20mA to reduce plate voltage
added feature , the meters have internal lighting 12volt 50mA ac or dc
so connect the m in series and add a resistor 270 ohm in series and connect this on the black and white wire of the power transformer
( use twisted wire supplied )
the other solution is to connect to primary of power transformer and the series resistor becomes 4700 ohm 25 watt so not so preferable
and enjoy the best power amplifier you ever build
( it is now time to make a new pre amp and the special speakers for tube single ended amplifiers 93 db/w/m )
thanks for choosing AquaBlue and this design

Benny Glass head of technical department