parks audio Tube st35 User manual

st35 power amplifier manual
the diytube series

2
Disclaimer
Under no circumstances does Parks Audio LLC assume liability or responsibility for injury or
damages sustained in the assembly, test or operation of this kit or for damages to any other
equipment connected to it. As this is a partial kit, proper assembly is buyers responsibility.
Parks Audio LLC reserves the right to make design changes or improvements without the
obligation to revise prior versions. All specifications are subject to change without notice.
WARNING: Lethal voltages (greater than 400VDC) are present in this project.
Use a Variac or isolation transformer while working on and testing the unit.
Use a rubber mat to stand on while working on and testing the unit.
Keep one hand in your back pocket if probing voltages with a DMM.
Wrap a small piece of electrical tape around the test lead probe shaft to expose just the
tip.
Do not connect or disconnect wires to the terminal blocks when unit is powered or
plugged in.
Lethal voltages exist in the capacitors even after unit is powered down, so wait at least
one hour to after unplugging to allow charge to dissipate.
Warranty Information
All goods purchased from Parks Audio LLC have a thirty (30) day warranty from the date of
purchase against defects.

3
Preparation
Before assembly, some preparation needs to done. Parts must be purchased, a work area needs
set up and chassis decisions should be made. The axiom haste makes waste has never been more
true. Make sure your chosen transformers and the board will fit comfortably using the chassis of
your choice. The PCB itself makes a perfect template, as shown on the website, for socket and
mounting holes. Be sure to plan where RCA connectors, binding posts, standby & ON/OFF
switches and the AC cord will be oriented on the chassis. Physically lay out the parts in a space
equal to your chassis to assure everything fits. Also, note that the vacuum tube sockets can
only be mounted on one side, which is designated in the silkscreen.
Overview of the Schematic and Design
The design closely resembles the Dynaco Stereo 35. The use of one 12DW7 per channel has
been replaced by ½ a 12AX7A and ½ a 12AU7. The 12AX7A is a voltage amplifier that directly
drives the 12AU7, a cathodyne phase splitter. The only other meaningful change is the
replacement of a single cathode resistor and capacitor with four individual resistors and
capacitors. A larger value resistor in series with a variable resistor parallels the main cathode
resistor, providing a range of 320-450 ohms per cathode. This circuit is then in series with a 10
ohm resistor to calculate the current very easily via a test point - set conservatively for 350mV,
thus 35mA.
Using the Assembly Drawing
The assembly drawing is a quick reference to what component is where and the numbering
scheme for the connectors. It is also a good place to make any notes.
Soldering
This PCB is a double-sided, plated through-hole design on .094 FR4 material with LPI (liquid
photo-imagable) solder mask and 2 ounce copper per side. When soldering these components, let
the solder flow through the hole to form a teepee on both sides of the board for an optimum
connection. Ground plane connections sink a lot of heat from the soldering iron, so take care to
do these well - you may have to touch up from the opposite side.
TIP: PCB Stuffing
Use the flat end of the 10-pole terminal block as a lead bending tool. This works for most
of the parts. Do take care to not damage the meniscus of the components. Insert all resistors
before soldering, bending the leads down to hold them in place. This allows one to fix any
placing errors that might occur very easily. Use a good pair of snips to remove the excess leads.
The top side of the PCB is designated as the side with the ground plane and with the sockets
marked as THIS SIDE ONLY.
1. Stuff all fixed value resistors into the PCB. Mount R36 and R34 on the opposite side of
the PCB (i.e. opposite of R37 and R35). This is to maximize heat dissipation.
2. Stuff variable resistors. You may choose to put them on the top side of the PCB and drill
extra holes in your chassis for external adjustments. There are guides hole in the PCB for
this. Also, be sure you will can easily insert a DMM probe between the resistors.
3. Kink leads on the silver mica capacitors and stuff.

4
4. Stuff D1 and D2. Mind the orientation of the cathode, which is represented by a stripe on
both the silkscreen and the diodes body.
5. Stuff C5,C6,C7 & C8. Mind the polarity. The board has + symbols to show where the
positive terminal from the capacitor should be placed. Note that many modern caps have
only the - terminal marked, usually with a stripe running down one side. If you choose
to substitute, the diameter must be no more than 10mm. Use a smaller value, for example
100uF, if necessary.
6. Stuff all the polyprop capacitors.
7. Place and solder all the tube sockets  make sure they are mounted on the top side only. It
is recommended to use an old miniature nine pin tube to break in the sockets. I have
found a very small nail works well for me  but dont break the ceramic!
8. Place and solder the terminal blocks. Do not over solder the connectors as solder could
then spill under the connector causing a short.
9. Stuff R42, the IRCL, making sure it doesnt lean on or touch the J1 terminal block as it
gets warm.
10. Stuff the 4 filter capacitors. Mind the polarity. If these are substituted, the diameter must
be 25mm or less. The height must be 30mm or less to fit in a 2 high chassis.
11. Carefully go over your work, looking for:
a. any cold solder joints (these will look dull)
b. solder splashes and shorts between socket pins
c. any connections that solder didnt flow to the other side (like ground plane
connections)
d. snip excess leads
e. compare your board to photos from the website  a good check for capacitor
orientation
Initial Impedances: When it Doubt, Ohm it Out
These are without connections to the terminal blocks. When connected, those readings that will
differ will be in brackets. Different transformers will have different readings  this one is a PA-
774.
J1
1. >2M [94]
2. >2M [94]
3. >200K
4. >200K
5. GND
6. GND
7. 100 [50]
8. 100 [50]
9. >10M
10. >10M
J2
1. >10M
2. >10M
3. GND

5
J3
1. >10M
2. >10M
3. 28.3K [1 ohm]
J4
1. >10M
2. >10M
3. 28.3K [1 ohm]
J5
1. >10M
2. >10M
3. GND
J6
1. >10M
2. GND
J7
1. >10M
2. GND
Initial Settings: Feedback and Bias
WARNING: Do these adjustments with the unit off and unplugged, as well as having had
one hour to bleed voltage from the filter caps.
Note: Most amps will just use a fixed resistor here, but the diytube circuit uses a pot for
experimentations sake - thus this extra step.
1) Don't have your negative feedback connected yet, that is your output secondary or else it
will be a real low impedance.
2) Probe your DMM from J4 pin 3 (right channel) to ground and adjust R30 until the
resistance is 20.3K (set to 28.3K if using the 16 ohm tap as feedback, as in the original
Z565).
3) Probe your DMM from J3 pin 3 (left channel) to ground and adjust R31 until the
resistance is 20.3K (set to 28.3K if using the 16 ohm tap as feedback, as in the original
Z565).
4) Your feedback is set.
Adjusting Bias
1) With the unit off, probe your DMM from V3 pin 3 (the cathode) to ground. Adjust R22
until you measure 400 ohms to ground.
2) Then probe your DMM from V4 pin 3 (the cathode) to ground. Adjust R19 until you
measure 400 ohms to ground.
3) Then probe your DMM from V5 pin 3 (the cathode) to ground. Adjust R28 until you

6
measure 400 ohms to ground.
4) Then probe your DMM from V6 pin 3 (the cathode) to ground. Adjust R25 until you
measure 400 ohms to ground.
WARNING: The following is done while the unit is ON and IDLE. Take care and follow
proper high voltage safety rules.
5) With all your connections in place, power up. Your initial bias has been set - this makes
sure that you don't accidentally have a tube aggressively biased, and they should all end up
near the same range on power up.
6) Probe your DMM (set to DC volts) from the test point which is sandwiched between the
R19 & R22 to ground. V3 is adjusted with the 'inside' test point. Adjust R22 until the voltage
is .350V or 350mV.
7) Probe your DMM (set to DC volts) from the test point which is sandwiched between the
R19 & R22 to ground. V4 is adjusted with the 'outside' test point. Adjust R19 until the
voltage is .350V or 350mV.
8 ) Probe your DMM (set to DC volts) from the test point which is sandwiched between the
R25 & R28 to ground. V5 is adjusted with the 'inside' test point. Adjust R28 until the voltage
is .350V or 350mV.
9) Probe your DMM (set to DC volts) from the test point which is sandwiched between the
R25 & R28 to ground. V6 is adjusted with the 'outside' test point. Adjust R25 until the
voltage is .350V or 350mV.
Voltages
These are zero-signal, nominal measurements in VDC, unless otherwise noted. They are all
referenced to ground.
PIN V1 V2 V3 V4 V5 V6
1 116 207 - - - -
2 0 116 0 0 0 0
3 780mV 120 13 13 13 13
4 3.1VAC 3.1VAC 3.1VAC 3.1VAC 3.1VAC 3.1VAC
5 3.1VAC 3.1VAC 3.1VAC 3.1VAC 3.1VAC 3.1VAC
6 115 208 - - - -
7 0 114 360 360 360 360
8 780mV 118 - - - -
9 3.1VAC 3.1VAC 364 364 364 364
Modifications
If volume controls are desired, use a 100k or better audio taper pot. A linear pot
will not work correctly. You can salvage the 250k pot off of your Dynaco SCA-

7
35. Radio Shack has a 100k Alps stereo pot that many people use. Each channel
must have its own control, so either a stereo pot or two single pots must be used.
The pot connections will be:
-pin 1 to RCA audio IN
-pin 2 (the wiper) to J6-1 or J7-1 (depending on channel)
-pin 3 to J6-2 (GND) or J7-2 (GND)
To determine which pin is which, use a DMM and crank the volume pot fully
counter-clockwise (lowest volume setting). Resistance between pins 1 & 2 will be
nearly the full range of the pot. Pins 2 & 3 will be very low resistance. Rotating
the pot fully clockwise (highest volume setting) will yield opposite results, i.e.
pins 1 & 2 will be very low resistance and pins 2 & 3 will be nearly the full range
of the pot. If using a stereo pot, follow the same process with pins 4, 5 & 6.
A 100 ohm 1W resistor (Mouser# 281-100) can be soldered on the underside of
the board between pins 1 & 2 on J2,J3,J4 and J5 for triode operation. Do not
attach any UL taps to pin 2 on these connector blocks if you do this. Do not
populate C20 & C21.
If you are using a filament supply with a center tap and choose to use the center
tap, do not populate R40 and 41. These are for non-CT filament supplies, such as
the Dynaco PA-774.
SW2 can be omitted and the secondary CT wired straight to ground if a standby
switch is not desired.
R42, the IRCL, is very important for proper operation of this unit. It is possible to
wire a switch in parallel with it in order take it out of the circuit once the unit has
warmed up, but this is not recommended- especially for Hammonds, as they are
rated at 115VAC. Note: This will increase the B+ by around 12V and will slightly
increase heater voltage. Switch would need to be reset as well.
A .05uF, 1kV ceramic cap may strap the ON/OFF switch (pins 1 and 2). This can
prevent pops on turnoff.
A footprint for a two-pole connector is located over R34 & R35 which is for those
wishing to use a choke, for example the Hammond 1H choke (Mouser pn# 546-
158SA).

8
diytube Stereo 35 Rev E PCB Parts Revised: 8/12/2013
Tip: Put a dot in the checkbox if you have the part already as a quick
reference when ordering parts. X out the checkbox when you have installed
the part on the PCB.
Item QTY Reference Part Mouser Part Cost
______________________________________________________________________
2 R1,R2 47.5k, 1/2W 71-RN60D-F-47.5K 0.11
6 R3,R4,R13,R14, 475k, 1/2W 71-RN60D-F-475K 0.11
R15,R16
6 R5,R6,R18, 1k, 1/2W 71-RN60D-F-1.0K 0.11
R21,R24,R27
2 R7,R8 300k, 2W 594-5083NW300K0J 0.10
2 R9,R10 27k, 1W 281-27K-RC 0.13
2 R11,R12 33k, 1W 281-33K-RC 0.13
4 R17,R20,R23,R26 470, 2W 594-5083NW470R0J 0.10
4 R19,R22,R25,R28 10K Pot 72-T93YB-10K 1.58
1 R29 360k, 2W 594-5083NW360K0J 0.10
2 R30,R31 100K Pot 72-T93YB-100K 1.33
1 R33 6.8k, 2W 594-5083NW6K800J 0.10
4 R34,R35,R40,R41 100, 2W 594-5083NW100R0J 0.10
2 R38,R39 150k, 1/2W 71-RN60D-F-150K 0.11
1 R42 Inrush Limiter 527-CL90 2.21
4 R43,R44,R45,R46 10 Ohm, 1/4W 71-RN55D-F-10 0.10
4 R47,R48,R49,R50 1k, 1/4W 71-RN55D-F-1.0K 0.11
4 R51,R52,R53,R54 100, 1/4W 71-RN55D-F-100 0.11
4 C1,C2,C3,C4 0.1uF,400V 75-715P400V0.1 3.61
4 C5,C6,C7,C8 470uF,35V 5mmLS 647-UVR1V471MPD 0.69
4 C9,C10,C11,C12 120uF 5985-380-450V121 3.57
2 C16,C17 27pF 140-500N5-270J-RC 0.10
2 C18,C19 33pF 140-500N5-330J-RC 0.14
2 C20,C21 18pF 140-500N5-180J-RC 0.11
2 C22,C23 0.22uF 5989-250V.22-F 0.68
2 D1,D2 UF4007 Diode 625-UF4007-E3 0.19
1 J1 10pin .375 Block 571-114376644 5.23
4 J2,J3,J4,J5 3pin .375 Block 571-14376645 1.47
2 J6,J7 2pin 5.08MM conn 571-282837-2 0.40

9
Zener Mod:
Omit R17, R20, R23 & R26. Sub the following:
4 R18,R21,R24,R27 10V zener 2% 78-BZX85B10-TAP 0.10
4 R19,R22,R25,R28 100 ohm Pot 72-T93YB-100 1.58
Notes:
- C14 & C15 are input coupling capacitor footprints that are PCB
jumpered. These can be cut open and used if DC blocking is needed.
- R36 & R37 can be cut open and used with low value power resistors
if you have high filament voltage.
- R32 & C13 omitted.
Go to www.mouser.com and log in to your account. Use Service & Tools > Parts
List Importer. Paste list into window and Import to My Current Order. CRTL-C
to copy. CTRL-V to paste.
/* start cut-n-paste */
71-RN60D-F-47.5K 2
71-RN60D-F-475K 6
71-RN60D-F-1.0K 6
594-5083NW300K0J 2
281-27K-RC 2
281-33K-RC 2
594-5083NW470R0J 4
72-T93YB-10K 4
594-5083NW360K0J 1
72-T93YB-100K 2
594-5083NW6K800J 1
594-5083NW100R0J 4
71-RN60D-F-150K 2
527-CL90 1
71-RN55D-F-10 4
71-RN55D-F-1.0K 4
71-RN55D-F-100 4
75-715P400V0.1 4
647-UVR1V471MPD 4
5985-380-450V121 4
140-500N5-270J-RC 2
140-500N5-330J-RC 2
140-500N5-180J-RC 2
5989-250V.22-F 2
UF4007-E3/54 2
571-114376644 1
571-14376645 4
571-282837-2 2
78-BZX85B10-TAP 4
72-T93YB-100 4
/* end cut-n-paste */

10
diytube Stereo 35 Rev E Chassis Parts Revised: 8/12/2013
Item QTY Reference Part Mouser Part Cost
_______________________________________________________________________
1 SW1 Neon Power Switch 112-R13-66B-G 1.38
1 IEC Power w/ Fuse 161-R3014-E 2.19
1 2A Midget Fuse 504-GMA-2 0.48
1 3-Prong AC cord 545-P004-006 3.99
1 Stereo Volume Pot 313-2420F-100K 2.73
1 12X10X2 Chassis 546-1444-29 24.00
1 (omit if FPE) 12X10 Bottom 546-1434-29 10.63
1 RCA White Jack 568-NYS367-9 1.88
1 RCA Red Jack 568-NYS367-2 1.88
2 Dual binding post 164-19B2-EX 3.95
4 Red Test Points 530-105-0802-1 0.72
2 Black Test Points 530-105-0803-1 0.72
6 Standoffs 1/4X1/2 534-2210 0.33
6 .625 Bushings 836-2073 0.16
/* start cut-n-paste */
112-R13-66B-G 1
161-R3014-E 1
504-GMA-2 1
545-P004-006 1
313-2420F-100K 1
546-1444-29 1
546-1434-29 1
568-NYS367-9 1
568-NYS367-2 1
164-19B2-EX 2
530-105-0802-1 4
530-105-0803-1 2
534-2210 6
836-2073 6
/* end cut-n-paste */
I source these locally at Ace Hardware, or use boltdepot.com.
8 #6 x 1/2 sheet metal screw 18-8 SS 2300
12 #6-32 x 1/4 machine screw 18-8 SS 5316
12 #8-32 x 1/2 machine screw 18-8 SS 1346
12 #8 flat washers 18-8 SS 2943
12 #8 K-Lock nuts 18-8 SS 12020
2 #6-32 x 1/2 machine screw 18-8 SS 1336
2 #6 K-Lock nuts 18-8 SS 12019