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20th September 2014, 01:00 PM

#1

seadoo

diyAudio Member

Join Date: Sep 2014

Location: Michigan

Dynaco ST-70

I bought a ST70 off fleabay that was original
and have done a few mods to it. The first thing I did was replace the
selenium rectifier with a diode , I didnt have to add or change any
resistors to get the bias to adjust to the proper voltage. I also added
two diodes from the power transformer to the 5AR4 tube. I replaced the
choke and multicap. I went with a larger 80,40,30,20 version. I also
placed a 100 ohm 1/4 watt resistor between pin 4 (screen ?) and the output
transformer on each of the output tubes. On the drive board the only thing
I changed were the coupling caps from the original (1960) to modern
russian k40 PIO caps, I kept the values the same as original. I also added
a .047 mfd cap to each input to block any DC input. Is there anything else
that should be done, the amp is driving a pair of Magnepan 1.6 speakers
ands sounds better than my Yamaha A-1020.

20th September 2014, 02:47 PM

#2

JonSnell Electronic

diyAudio Member

Join Date: Dec 2012

Location: The Jurassic Coast, England. GB

Replace the screen grid resistors with 2Watt
as the EL34 can draw screen current when driven hard!
A good basic
Ultra Linear Amp.

__________________
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support for Flying Mole, Class D, Refurbishment and Repairs. www.jonsnell.co.uk

20th September 2014, 02:49 PM

#3

kevinkr

diyAudio Moderator

Join Date: Sep 2004

Location: Boston, Massachusetts

Blog Entries: 6

The ST-70 does not use screen grid resistors,
and hasn't really demonstrated a need to have them.

http://www.dynakitparts.com/image/data/Dyna-ST70.pdf
(This is the original version of the assembly manual)

__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan

20th September 2014, 03:14 PM

#4

seadoo

diyAudio Member

Join Date: Sep 2014

Location: Michigan

The screen resistors are not part of the
original design and were added in case of tube melt down the resistor is
supposed to blow before damage occures to the output trans. If there is no
chance of tube failure causing damage to the output trans I will remove
them as I dont want to add parts that have no purpose.

20th September 2014, 03:17 PM

#5

artosalo

diyAudio Member

Join Date: Feb 2010

Quote:

Originally Posted by seadoo

...Is there anything else that
should be done...

I would add bias
balance adjustment and separate 1...10 ohms cathode resistors to to output
tubes to make proper bias adjustment easier.

20th September 2014, 03:33 PM

#6

bear

expert in tautology

diyAudio Member

Join Date: Apr 2002

Location: New York State USA

Actually, I recommend reducing the value of
the cathode resistors by a factor of 10. Note this on the outside of the
amp, assuming the usual octal connector is used for bias adjustment. Think
you will find the bass to magically become more "solid".

I also
disable the mono/stereo switch by removing the wires, go with a short
twisted pair to the board.

I have never seen a blown output
transformer on an ST-70, never even heard on one. I suppose it could
happen.

Strapping the tubes for triode is an option that some
like, although the output power drops somewhat.

_-_-

__________________
_-_-bear
http://www.bearlabs.com/
-- Btw, I don't actually know anything, FYI -- [...2SJ74
Toshiba bogus asian parts - beware!
]

20th September 2014, 04:09 PM

#7

seadoo

diyAudio Member

Join Date: Sep 2014

Location: Michigan

Looks like the screen resistors I added are
coming out. I have allready bypassed the mono/stereo switch. I made a
bucking trans to reduce the input voltage to 114 vac. I also added a 25
ohm thermister on the input side of the power trans. Think Ill try the
cathode resistor mod to see if it tightens up the lower end.

20th September 2014, 05:11 PM

#8

rayma

diyAudio Member

Join Date: Apr 2011

Location: Upper midwest

Quote:

Originally Posted by kevinkr

The ST-70 does not use screen grid
resistors, and hasn't really demonstrated a need to have
them.

http://www.dynakitparts.com/image/data/Dyna-ST70.pdf
(This is the original version of the assembly
manual)

On occasion, a bad tube will
use these as a fuse.

20th September 2014, 05:14 PM

#9

rayma

diyAudio Member

Join Date: Apr 2011

Location: Upper midwest

Quote:

Originally Posted by seadoo

I bought a ST70 off fleabay that was
original and have done a few mods to it. Is there anything else that
should be done, the amp is driving a pair of Magnepan 1.6 speakers
ands sounds better than my Yamaha
A-1020.

Partial cathode coupling on
the output stage works well for this amp, see an Audio Research schematic.
It does require somewhat more drive.
You can improve the 7199 output's
swing symmetry by tuning the 7199 pentode's 1.5M screen resistor for
110VDC at the 7199 triode's pin 8 (cathode).
I used a 1M cermet pot for
this, with a 499k series resistor, in place of the original 1.5M resistor.

Last edited by rayma; 20th September 2014 at 05:31
PM.

20th September 2014, 07:33 PM

#10

seadoo

diyAudio Member

Join Date: Sep 2014

Location: Michigan

By reducing the cathode resistors are you
saying replacing the 15.6 ohm resistor between the cathode and ground with
a 1.56 ohm resistor ?

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