Download Report

Vacuum Tube Audio
PAS3-Z modification
assembly and installation instructions
this is a partial SAMPLE of the full manual
Overview
This kit has been configured by Tubes4HiFi to duplicate our popular SP6 kit onto
3 PCBs made specifically to fit into the Dynaco PAS2/3 preamplifier, and thus the
parts list and jumpers used are specific to this version.
Introduction
The PAS2/3 was the most popular tube preamplifier ever made.
It was produced from 1960 until around 1985. Two features that made it so popular were
it's low price and it's simple construction. The circuitry was far from state-of-the-art, but
in the early years of it's production it was considered a decent performer and excellent value.
Unfortunately Dynaco went out of business in the mid 80's and although other budget
preamps were available by that time the cost was double or triple so the PAS2/3 series
remained very popular in the aftermarket as used units.
During the late 80's and early 90's a few aftermarket modifications became available and the
most popular of those remain today. Unfortunately for most users, the modifications
themselves were more expensive than a newer preamp and thus a poor value.
The Curcio Audio mod is still available for around $660 for a kit, and the Frank VanAlstine
(AVA) is still available for $449 for a kit.
Thus Vacuum Tube Audio (www.tubes4hifi.com) has decided to offer this modification which
is identical to our very popular SP6 kit for the low price of $189. This is a VERY good value.
We do like to point out to our potential customers though that for this price they can also buy
the SP6 kit itself or the improved SP8 kit ($209) which potentially is a better value because by
the time you add additional components to finish a preamp project such as volume and
balance controls, input selector, and input/output jacks, you could wind up with a totally NEW
preamp for just slightly more than the cost of fully modifying an older PAS2/3 and working
within the limitations of it's small chassis and transformer.
On the other hand, we offer this kit because we've had so many requests from customers who
want to keep their original classic PAS2/3 chassis and replace the inner circuits with the best
possible circuits available at the best possible price, which gives them excellent value.
The Circuit Design
Our PAS3-Z modification replaces the original circuit design with a modern tube preamp
circuit which includes a new phono stage, line stage, and power supply.
The PCBs are sized to fit perfectly into the existing PAS3 chassis without any modification
of the chassis, and they mount in the same way using the original hardware.
The circuit has a hi-gain phono preamp using a 12AX7 gain stage followed by a highly
accurate passive EQ, followed by another 12AX7 gain stage, for a total gain of 45 db.
The line stage uses a 12AU7 in a cascode SRPP gain stage for high gain with excellent
linearity and low output impedance, for a gain of 15 db.
Power Supply
Vacuum Tube Audio prefers to use tube rectification in preamp circuits, rather than most
modifications which use solid-state rectifiers. We feel that solid-state rectifiers make an
obvious and not so great difference in the sound, and thus our new power supply uses the
original 12X4 tube rectifier for soft-start, and replaces the original quad cap with modern
higher quality capacitors with much higher storage and lower leakage.
Using a tube rectifier also provides an easy way to filter out AC power line spikes and
interference that not only pass thru solid-state diodes but also the switching noise and spikes
that are generated by solid-state diodes themselves.
We continue to use the original power transformer in this modification not only to save an
expensive replacement but also because our original SP6 preamp design was optimized to
use this transformer in it's design, thus no changes are needed for various resistance values.
However, although our power supply is similar to some other aftermarket supplies, in this
case the voltage and current requirements of our new design are specific to our parts values.
The raw B+ is double RC filtered and then individually supplies the phono amp PCB and the
line stage PCB with additional RC filtering using large value capacitors.
In addition, the phono amp and the line amp add additional power supply filtering for each
channel (left and right) directly on their individual circuit boards.
The original selenium rectifier is also replaced with modern high speed fast recovery silicon
diodes and better quality capacitors to supply the DC power for the tube filaments.
In this application they are ideal for the job at hand. Because the original Dynaco preamps
used a voltage doubler circuit to power the filaments, and since we continue to use the
original power transformer in this modification, we are also using the voltage doubler circuit
which provides about 22vdc to the PCBs rather than the more common 12vdc used with
these tubes in other circuits. The two tubes on each PCB share the 22v in series, and thus
actually run on 11vdc per tube. We do incorporate a more modern noise limiting feature that
references the heater power supply to a voltage divider on the high voltage B+ rather than
tying it to common ground.
Tube information
This circuit modification requires the user to buy a minimum of two new 12AU7 tubes that are
used in the line amplifier. The original circuits used 4 – 12AX7 tubes plus the 12X4 rectifier,
the new circuits use 2 – 12AX7 (hi-gain) and 2 – 12AU7 tubes (lo-gain).
It is highly recommended that ALL tubes in the preamp be replaced with NEW tubes.
Tube characteristics change with age and wear out. In other words, a fresh 12AX7 is
not the same as that old 12AX7 after 20 years of use.
No Negative Feedback Loop
The new circuit topology does NOT use any negative feedback.
In the old circuit this was especially part of the phono design, and also used in the tone
control section of the old line amplifier. This is one of the main reasons the original circuit
had major limitations on maintaining gain and bandwidth with accurate reproduction.
Unlike negative feedback that has to wait until something goes wrong before it can work to
undo the damage, the proper design of a modern circuit can eliminate negative feedback
and do the proper job in the first place without any band-aid type fixes, while reducing tube
noise by using less gain to accomplish the job with lower distortion and coloration of the
sound.
Tube Selection
Fortunately both the 12AX7 and 12AU7 are very common tubes and there are many dozens
of available types and brands that can be used. We would like to comment though that on the
budget side, we prefer using the EH brand for the 12AX7 if you are using new tubes, as we've
found it has lower noise than most other new brands. Of course if you have some good
quality NOS or used Telefunkins or Mullards, use those!! For NOS replacements of the
12AU7s the RCA clear tops are highly recommended, or on the budget side mil-spec 5814,
5963, or 6189 tubes are also excellent.
PCB Assembly Cleanliness is essential. Before soldering, be sure to clean both sides of
the PCB with 90% to 99% isopropyl alcohol. When using all new parts and a fresh PCB,
you shouldn't need to clean any parts leads, but if the parts have sat in your closet for
a year or two, then expect a good amount of oxidation to have developed that should
also be cleaned off be any soldering.
First, solder all the resistors in place, and then solder the tube sockets.
Finally solder the smaller capacitors, and then the larger capacitors.
See the specific assembly directions below in a later section.
Be consistent in orienting the resistors; keep all the parts labels the same so they can all be
read from the same side when the PCB is finished. This will pay dividends later, if you need
to locate a resistor or capacitor in the wrong location.
Be sure to confirm all the electrolytic capacitor orientations, as a reversed polarized
capacitor can easily vent (or even explode) when presented with high-voltage.
Confirm twice, solder once.
Testing Before testing, visually inspect the PCB for proper parts placement and soldering
quality connections. Wear safety eye goggles, as an exploding power-supply capacitor
will spray hot caustic chemicals. Make a habit of using only one hand, with the other hand
behind your back, while attaching probes or handling high-voltage gear, as a current flow
across your chest can result in death. In addition, wear rubber-soled shoes and work in
a dry environment.
Remember, safety first, second, and last.
1 - If possible, use a variac and slowly bring up the AC voltage, while looking for smoke or
part discoloration or bulging.
2 - Measure the voltage between ground and the B+ outputs for both PC-5 and PC-6.
The voltages should be within about 5% of the values marked on the schematics.
Only after you are sure that both heater and B-plus power supplies are working well,
should you attach the line-stage amplifier to a power amplifier.
this is a partial SAMPLE of the full manual
PCB ASSEMBLY DIRECTIONS
refer to the PCB layout diagrams above and to the attached parts list below.
1 - start by inserting the resistors onto the PCBs and solder them in place.
2 – next insert the tube sockets and solder them in place
3 – next insert the smaller capacitors and solder them in place
4 – then insert the larger capacitors and solder them in place
5- if you haven't already done this, remove the original PCBs from the chassis.
6- cut off all the wires that attached the tone controls to the original PCBs,
the tone controls will not be used with this modification
7- cut off the wires that attached the FILTER switch to the original PCBs,
the FILTER switch will not be used with this modification
mount the PC6 phono board and the PC5 line board oriented as shown on the diagram.
They mount identically to the original PCBs (from the bottom side of the chassis)
POWER SUPPLY WIRING
before installing the power supply PCB into the chassis, refer to the diagram above
and connect the following wires to the BOTTOM side of the PCB:
- a pair of wires about 10” long connected to each side of the fuse (AC IN and AC OUT)
- a pair of wires about 8” long connected to B+L and GND
- a pair of wires about 7” long connected to XF
- a pair of wires about 7” long connected to F+ and F- a pair of wires about 10” long connected to F+ and F- a pair of wires about 8” long connected to B+IN and GND
on the TOP side of the PCB, connect a pair of wires about 4” long to B+P and GND
From the upper left rear side of the power transformer, connect the two green leads to XF on
the power supply PCB.
Mount a 1/2” hex spacer to support the power supply PCB, using a 4-40 1/4” screw thru the bottom of the
chassis into the hex spacer.
Route all the loose wires from the bottom side thru the 1/2” hole between the power supply PCB and the
transformer.
Then use another 4-40 1/4” screw thru the PCB and connect it to the hex spacer.
Refer to the chassis layout below for correct orientation of the PCB.
Pull the loose wires thru the hole again so they aren't bunched up under the PCB.
Now you can mount the PC6 phono board and the PC5 line board to the chassis, from the bottom side,
using the original hardware. Refer to the chassis layout below for correct orientation of the PCBs.
POWER SUPPLY WIRING (continued)
From the bottom side of the chassis, de-solder the wire on the left side of the AC outlet
(number 2A in the diagram below) and also de-solder the other end of that wire where
it connects to the power switch.
Take the two wires coming from the power supply board AC fuse and connect one to the AC outlet where you
just removed the wire, and connect the other wire coming from the AC fuse and connect it to the power switch.
This now puts a fuse in series with the power switch.
Take the pair of wires about 8” long connected to B+L and GND and connect them to B+1 and GND
on the PC5 line board.
Take the pair of wires about 7” long connected to XF and connect them to the 12X4 tube pins 3 and 4.
Take the pair of wires about 7” long connected to F+ and F- and connect them to PC5 F+ and FTake the pair of wires about 10” long connected to F+ and F- and connect them to PC6 F+ and FTake the pair of wires about 8” long connected to B+IN and GND, connect the B+IN to 12X4 tube pin 7,
connect the GND wire to the chassis ground lug next to 12X4 tube
From the top side of the chassis, take the pair of wires about 4” long from B+P and GND
on the power supply PCB to B+ and GND on the PC6 phono board (marked B+2)
PC6 phono PCB wiring
** from bottom side of the chassis, we need a jumper to connect B+ from one half of the PCB
to the other half (**see photo)
from the top side of the chassis, connect a pair of wires from the RIGHT PHONO INPUT jacks to
the left side phono input on the PCB (**see photo),
then connect a pair of wires from the LEFT PHONO INPUT jacks to the right side phono input on the PCB
Connect a pair of wires from the left side phono output on the PCB to the input of the selector switch
Connect a pair of wires from the right side phono output on the PCB to the input of the selector switch
PC5 line PCB wiring
from bottom side of the chassis, connect a pair of wires from the LEFT input of the PCB to the left channel
of the volume control, and connect a pair of wires from the RIGHT input of the PCB to the right channel
of the volume control
From the top side of the chassis, connect a pair of wires from the LEFT channel PCB output to the LEFT
channel RCA jacks on the rear panel
Connect a pair of wires from the RIGHT channel PCB output to the right channel RCA jacks on the rear panel
this is a partial SAMPLE of the full manual
the full manual included with the kit includes additional
instructions, PCB layout diagrams, full parts list, and full set of
schematics, as well as links to a website showing full photos
sequences of assembly and installation