line stage - Classic Valve Design

PAS Purist Line Amplifier
Designed by Norman Koren
(aka PAS-K Line)
for the
Dynaco® PAS2, PAS3, PAS3X
Classic Valve Design
Classic Valve Design assumes no responsibility for circuit or user damage
from the use or misuse of these boards or any other product. We simply provide
these on an AS-IS basis with workmanship quality as the only thing guaranteed
at this time.
This product is designed for and use around LETHAL VOLTAGES. We
assume the user has a reasonably competent grasp of line operated electronics
at the time of sale.
* Dynaco is a registered trademark of Panor Corporation *
* Board design is © Norman Koren. Classic Valve Design is an authorized dealer *
The Dynaco PAS series of preamplifiers has been one of the most
successful and popular preamplifiers in history. They are reliable, hold their
resale value and are incredibly popular “blank slates” to modify.
The PAS-K line board is an upgrade for the original PAS series of pre
amplifiers designed by Norman Koren. It has in our opinion, beautiful sonics and
is flexible (keep an eye out on our site for “Application Notes” regarding these
** The stock Dynaco tone controls are not compatible with this unit **
Due to the fact this board uses three instead of two tubes, it is also highly
recommended that if you use your 12X4 rectifier, that you install a secondary
(12.6V, 500mA - Hammond 166G12) transformer to power its heater and the
pilot lamp, if you do not convert to a LED power indicator. If you convert to an
LED one (order CK-57), the recommended auxiliary transformer is 12.6V @
300mA (Hammond 166F12 or 166F12C/D is fine).
The 166F12 transformer can be mounted across the hole left by the can
capacitor if you use our PAS Regulator PCB (order CK-54), or one of the many
capacitor boards available on the internet. If you wish to keep the stock can
capacitor, mount it on the back near the existing PA-211 power transformer (but
do not block the vent holes). The 166G12 must be mounted externally (unless
you're good at making fancy brackets).
The old selenium rectifier and tubular capacitor heater power supply can
NOT handle the extra tubes of these board. You will need to at least upgrade the
rectifiers to modern silicon rectifiers for that. The 3A rated 1N540x series (like
1N5408) are commonly available and lend themselves to that duty well (the 1A,
1N400x series (like 1N4007) can't handle the cold-heater surge of a 3+3
arrangement and would soon fail, perhaps damaging the PA-211 power
We will discuss more on heater arrangements later on in this manual, as
well as some options for powering an odd-number of tube heaters.
Parts List
(numbering follows the original Koren schematic)
(quantities for resistors and capacitors are for one channel only)
R3B: 12K, ½W
R3C: 560 ohms, ½W
R3G, R3GJ: 470K, ½W
R3GS: 27K, ½W
R3P: 220K, ½W
R4B: 22K, ½W
R4C: 750 ohms, ½W
R4G: 1Meg, ½W
R4P, R6C: 120K, ½W
Rout: 150 ohms, ½W
RIL: 47K, ½W
C3B: 1uF, 100V film
C3C: 15pF, 500V silver mica
C3G, C3P: 100n, 400V polypropylene film
C3M: 8pF, 500V silver mica*
Cout: 0.47uF, 400V polypropylene film
(capacitors with higher voltage ratings here are OK)
* = In our tests, 7pF is just fine (board stray capacitance makes up the rest)
CON4, CON5: Terminal blocks, 5.08mm, 2 position PCB.
CON1, 2, 3: Terminal blocks, 5.08mm, 3 position PCB.
(can be same as above [2 pos.] unless you are powering with 6.3V from another
Heater Balancing Resistor: 68 ohm, 3W resistor if your PAS has an oddnumber of signal tubes (like 2 on the phono board, 3 on this one = 5).
Installation/use described in “Heater Wiring”.
Assembly is straight forward, though there is a couple items of note to
make your life easier:
1) Take a clipped lead from installing one of the other components, bend it
and install the “FB” jumper using it. It is a frequently forgot item and the
unit won't work without it.
2) If you are powering the unit from the standard PAS doubler, you can use a
2 position terminal block for CON1, 2 and 3 by placing them on positions
“4” and “5”, ignoring “9” completely.
3) A large physical size of Cout can hamper installation. If you are using large
capacitors for Cout, install them after you have mounted the line board on
your PAS chassis.
Heater Wiring
The Dynaco PAS uses a voltage doubler circuit to turn the 12V heater
winding of the PA-211 into 24V. Dynaco then runs the heaters of the 12AX7's on
each board in series and powers them with higher voltage and lower current.
The reason for this is simple – the PAS has limited space and the PA-211
power transformer had to fit in. To have made the 12V winding for the current
required by all the tube heaters, would have required a core larger than would
have fit in the PAS.
Since the 12AX7 draws 150mA from 12V, putting two in series will also
draw only 150mA, but require 24V. It is far easier to wind a small power
transformer for higher voltage than it is for higher current.
This works well for an even number of tubes, but what about an odd
number? We need to “simulate” another tube heater with a resistor. That's where
the 68 ohm, 3W heater balancing resistor comes in handy.
If you are using the Koren or other 3rd party phono board, the even number
of signal tubes is maintained. But if you are using a two-tube phono board, you
will need the balancing resistor.
Heater Wiring for Two 3-Tube Boards
Connector designations are for two of our Koren boards, but will vary for
your type.
Heater Wiring for Mixed 2 and 3-Tube Boards
Use of the heater balancing resistor when mixing 2 and 3 tube boards in
your upgraded PAS.
Why all this fuss over heaters? Why the need for an auxiliary transformer if
you want to keep your stock pilot lamp and 12X4? Let's do some math...
PA-211 12V winding rating: 800mA
Stock Dynaco:
12AX7 x 4
12X4 x 1
#57 bulb
= 300mA (two groups of two in series)
= 300mA
= 240mA
= 840mA
This does not include the I2R losses of the heater voltage doubler. As we can
see, the heater winding is taxed as it is.
Let us take the 12X4 and pilot lamp and give them their own transformer. Now,
we can add more signal tubes.
12AX7 x 6 = 450mA (three groups of two in series)
That's it!
So, why not add the 12X4 again and use a LED pilot? You can, but with
even further reduced heater voltage to your 12AX7's. With 450mA being drawn
from the doubler, it's typical each heater sees .7 to 1V less than its rated 12.6V.
It's just the nature of doubling and losses.
Even if you mix a 3 and a 2 tube board, current draw will remain the same
(the extra "tube" power is just burned in the heater balancing resistor)
Documentation written by Gregg van der Sluys, Classic
Document Revision 1 / 3-23-2011
Valve Design