Wardsweb Custom ST-70 Modification and Build

This page documents the build of my custom
ST-70 from a stock Dynaco ST-70 stereo amplifier.
I used some of the
original parts and fabricated a new chassis and cabinet.

The chassis started with a sheet of .080 5052 aluminum cut
14" wide and 12.5" deep from Westbrook
Metals

The stock tube sockets are ceramic w/gold pins Part #
055-534 from Parts
Express

The stock hardware is stainless button head socket screws
from Fastener
Express

The stock power transformer is a new PA060
from Dynakit Parts

The stock choke will be replaced with a new
C-354 from Dynakit Parts

The driver board is from Shannon
Park’s new ST-70 driver board

The driver board parts are from Mouser Electronics - BOM
purchase link

The driver board tube sockets are ceramic w/gold pins Part
# 90100
from Angela

The two .1uF coupling caps between the 12AX7 input tube and
the 12AU7 phase inverter are .22uF Auricaps Part #
C-AUD22-400 from tubesandmore.com

The four .047uF headed to the output tubes off the phase
inverter are .1uF Auricaps Part # C-AUD1-400

The big bias pots are 25K ohm 10-15 turn pots and the 10K
ohm resistor is a 5K ohm

The 47uf capacitors in the bias supply are 100uF @ 100v
Sprague Atom Part # C-SA100-100

The selenium
rectifier was replaced with a 1N4004
diode

The multi section capacitor was replaced with separate
capacitors:

The 1st filter stage directly on rectifier will use a 20uF
@ 500V Sprague Atom Part # C-SA20-500

The 2nd filter stage on the center tap of output
transformers will use a 250uF @ 500V JJ filter can Part #
C-EC250-500

The 3rd and 4th filter stages (front end) use a 100/100uF
@ 500V JJ Dual section can Part # C-EC100X2-500

A stock ST-70 unmodified and with
original cloth transformers. The output transformers are the first to come
out.

The power transformer comes out and
then all the rest. Tired chassis and after some elbow grease and metal
polish.

The beginnings of a new layout. The
pcb modded slightly for additional clearance to tube sockets.

Only the tube sockets will be mounted on the top of the pcb. The stock vs.
Dynakit power supplies.

The back of the chassis will bend
down at the line. The miniature Honeywell (2TW1-3) switch is for 4/8 ohm
taps.
The power entry module is a Corcom (PS0SSSSXB) IEC, fuse,
filtering and on/off switch.

The driver board was stuffed with
most parts on the bottom side and only the sockets and pots on the
top.
Note much time and care was taken to get the sockets
soldered in exactly to match the mounting height of the bias pots.

I only want these top items to come through the chassis.

The chassis has been punched,
drilled and cut for all mounts.
The next step was a bit of trial
and error, as I tried different layouts and routings to determine an
optimal design.

Time for a side track. I decided I
wanted the chassis chromed, so I pulled all the parts off and sent it off
to the chrome shop.
Then time to start cutting some Baltic
birch to complete the new cabinet. I air nailed two sheets and try some
different arches.
Once I determine the arch I want, a few
passes on the table saw to trim it down.

A few more passes to remove excess
wood and then on to the table sander.
The front starts with
a build up of four layers of Baltic birch and back to the sander.

The metal chassis slides into the
wood cabinet.
I still need to veneer the cabinet here and then
it was time to get back to wiring.

Here are a few competed pics with
and without tubes.

I want to acknowledge Craig Ostby
with NOSValves.com
His help with this build has
been invaluable.