Monoblok KT88 PP amp

Beautifull hi end PP KT88 design

Introduction , design philosophy

As usual I try to do things as good as I think can be , and this design includes lots of experience and very sound solutions .

It is a fact that I am not happy with commercial amplifiers , and most do it yourself designs tend to be better ,but are full of economic compromises

( not for the same reasons as the commercial designs ) and so nothing is done the way it should be .But we keep trying !

Here I wanted a good proven design high quality tubes and a power output of 50 watts in class B ,in class AB2 still plenty available and in class A

probably 20 watts left .
I used the long tailed phase splitter because I have very good experience with it and from there on its Williamson style .
The output transformers are from LundahL as they make an excellent double C cored tranny, and since PP design is not really critical ,

the LL1663 5K Raa will do perfect , with taps at 33% we are also ok for ultra lineair operation mode .

I also wanted a tube rectifier for best quality , and off course the best components , the resistors can still be improved with Caddocks ms-244 series

which is optional , I do not think that this is a big difference though , I decided to use Mkp capacitors in the power supply , and since a PP design

doesn't need very tightly filtered high voltage lines I just opted for a few very high quality mkp caps , this is then followed by a 30H choke so the

preamp stages are perfectly filtered and hum free .
My all most 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 ).
The bias is shunt regulated so its as stable as a rock .
A mono block approach as not to have one channel reacting with the other , for power stages this is always a bonus .

How does it sound ?

Well it's a big sound coming from such a small amp , there is a lot of muscle like a V8 engine and the presence you get is awesome .

New improvements ?

You can try LL1620PP
As a splitter use transformer , 7901
Or after a classic CK gain stage a transformer as load to stage using 2 secondaries to split
Jensen paper in oil capacitors as a signal capacitor and maybe a few in the power supply
Maybe more capacitance in the powersupply or a choke input filter .

Bill of Materials :

Power toroid type 2 240va compatible for 50-60Hz 115Vac/230Vac
LL1663 PP or LL1620PP
Black/silver metal pot
choke 30H/20mA
switch 10A/250v on-off
fuseholder 6,35mm
female euro connector
bridge rs606
Speaker terminal 24/55AU
Volume pot (optional )Panasonic stereo log 50k
3 x Octal tube socket
2 x Noval tube socket
2 x KT88 or SV6550C
ECC81 or 12BH7 or 5814A or 6189W depending on you're preference
rca input jack
hook up wires ( silver optional )
4 rubber feet
mA meter 0- 150 ma
volt meter 0-500 volts

1 bag of plastic mmp self adhesive bases
1 bag of nylon straps
chassis ( top + bottom plate + wooden frame )
6 male female M3 spacers
2 pcb stripes

electrical components :

resistors :

3,9 ohm & 4,7 ohm 1/4 watt
10 ohm 1/4 watt 4
0,47 ohm/5 watt 2
33K 1/4 watt 2

Dale CMF65 1watt
1M 1
470 1
1K2 1
68K 1
200K 2
680K 1
22K 3
3K9 2
82K 2
12K2/5 watt 1
10K 1
2K2 1
100 2
39K 1
390ohm 1

pot 470ohm
bias pot 50K multiturn
vol 50K log Panasonic / cermet mono
4K7 3watt pot

capacitors :

10µF/630 mkp 4
10µF/400 mkp 1
4700µF/16 2
470µF/100 2
100µF/160 1
100µF/100 1
47µF/63 1
47µF/100 2
5,6µF/400 mkp 1
0,47µF/630 mkp 2
0,47µF/630 mpp 1

semiconductors :

RS606 1
BYV26E 4
Zener diodes 43v 2
BF871 + cooling fin
Zener 5,1volt
Blue led optional (diam 3,1 mm )

Construction :

Use antislip washers between bolt and chassis/component

Before mounting 110volt transformer a painfull task awaits you , use the 1,0mm2 laquered solid core wire and wind 27 turns on it ,

keep the windings as tight as possible on the core.
This is for the 5 volt winding of the 274b rectifier tube
Once its wound use self adhedsive tape to secure it , the windings have to be pulled as tightly
As Possible .

Mount power transformer , the other side mount toroid 120va and put rubbers between toroid & chassis , tighten M8x100 so that

both transformers are tightly mounted

Mount choke , tube sockets ( be care full not to over tighten ) , switches , euro connector , fuse , mA & volt meter
Then mount isolated from chassis , rca input connector ( audio in )& speaker terminal
Check with ohm meter

Mount bridge rs606
Mount PCB strips between tubes as showed on diagram use 15 mm spacers

Twist all transformer wires that belong together ( black-black ,red-yellow , pink -pink , blue-grey )

Use glue yellow contact glue to make sure that nothing gets loose , but still possible to loosen
Prepare LL1663PP : see drawing

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 ( use a stripper ) because they might break Strip at least 0,5 cm

leave 1 cm between body and twisting and after the wire has been
twisted on tin it and cut off excess

use cutter to scrape oxidation off wires and solder lugs if you doubt contact sometimes there might be a lacquer on 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 :
Connect in series 110 volt with 320 vac ( red - yellow ) , measure ac voltage and make sure its higher (440 vac and not less, if less then

disconnect 110 volt winding and reverse its polarity )
Twist one wire of high voltage winding to anode d1 by228
Twist other wire of high voltage 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
twist each wire from 120 va transformer for 5 volt to the other side resistor 0,11ohm tin it
prepare black wire and black wire to switch S1 ( on off)
twist 2 red wires to any one resistor ( one red wire goes to choke 1 and the other to capacitors)
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 wire 0,75mm2)
solder on pin 2 and pin 7 for the power tubes ( these must be twisted see diagram )
prepare twisted wire short length 0,75 mm2 ( 2 x 5 cm )
twist both together and solder on to blue grey , the short ones continue to the bridge rs606 ac connections

take capacitors 4700µF/16v and twist and solder each side of resistor of 2,2ohm/5watt to a positive side of capacitor ,

one side goes to positive of rs606 , other side feeds tubes , pair of the twisted filament wires from noval pin 4 and 5 together & 9

the negatives all together

*** watch out for capacitor polarity

negative bias : diagram 8,9,10

is positioned under the outputtransformer

take the 4 470µf/100 volt capacitors
twist plus & minus of 470µf/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

twist and solder 10K on white or black wire
connect transformer black & white to these see diagram 9

solder these connections so they stay fixed together

take the 6k2 and connect to capacitors see diagram 10

take multi turn potentiometer & 47k resistor and twist on to pin 2 pin 1 connect to negative capacitor

( the last one) and the wiper connect to the 100k 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

actual schematic :

prepare following groups of components = twist together parallell as on diagram

0,47µF/630v with 4,7µF/400v with 150K
3,3M & 100nF
3,3M & 100nF
10k & 390pF
47k & 10µF/400v & (on positive side 100k)

twist together in series , just one side of each component

1M & 200K
150k & 27k
390ohm & 100k
250k & 250k then add 3,3M & 100nF to each check schematic
470K & 470 K
4K7 & 100K & 0,47µF/630v
47k & 47k
other side of 47k add (4,7pF & other side of 0,47µF/630v)
other side of 47k add (4,7pF & other side of 0,47µF/630v)
10µF/630v & 2k7

capacitor 4,7pF solder straight to tube pins

start soldering to tube pins , be carefull so that you don't short circuit other components
so keep components wires at least 5 mm away from each other
use pcb for help or use diagram for help

powersupply wiring diagram : check it out for correct ground layout

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 ,

( I prefer to hook ground powersupply to chassis and not to use earth connection of euroconnector

Countdown sequence :
check everything , power up if sure ( add fuse ) , without tubes except 274b

measure filament voltages no tubes , this should be higher then normal , then insert tubesand measure again , it should be within 5%

of specified voltages (max 10%)
adjust bias on kt88 until you measure -55 volts on the grid , check mA meter should measure about

turn off and mount in wooden frame , add bottom plate and enjoy

ampere meter and volt meter
to measure it's the internal terminals
volt meter should have 1M & 200K in series ( scale =500volt but actual range = 600volt)
ampmeter should have 2,2ohm in parallel
the outer terminal are for the internal light ( 12,6v 50mA)
to use lightning connect :
so connect the lights 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 )

( it is now time to make a new pre amp and the special speakers for tube PP amplifiers 93 db/w/m or any decent 90db/w/m)

thanks for choosing AquaBlue and this design

Benny Glass head of technical department

copyright webdesign 2002 aquablue Benny Glass