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Beefing
up a ST70

knowledge base for the classic Dynaco
ST70

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13 posts • Page 1 of
1

Beefing
up a ST70

by
battradio
» Sun Nov 04, 2012 10:11 pm

I'm Rebuilding a friends a stereo 70 with one of Shannons
driver boards and he was wanting to use KT 120 in it , the new power transformer
is 450 MA and has two 6.3 volt windings at 6A . Will be using the 5 volt
winding rectifed to DC for the filaments on the driver board , so the power
supply should be up to it , but was looking at the specs of the KT120 and see
they sugest a 51 k ohm grid resistor which is 3 times lower than the 150 k ohm
grid resistors on the ST70 driver board.

Is it realy nescary to go that
low with the grid resistor , thermal run away is ithe only reason i can think of
is why they would go that low of resistance . What would be the best idle
current for the KT120's? 70 or 80 ma

I'm using two chokes in the power
supply on for each set of output tubes and 50 uf cap on each pair of outputs ,
then running the PCB off the right channel .The first cap is made up of two 470
uf 450 volt caps inseries with 100k bleader resistors acros them to equilize the
voltage acros them , the two diodes are MUR4100e's 4 amp 100 piv fast switching
, using a 25 anp bridge from my junk box to rectify the 5 volts for the pcb
filaments .

Mark

battradio

KT88

Posts: 840

Joined: Tue Mar 10, 2009 12:58 am

Location: near ST.Louis MO.

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by
battradio
» Mon Nov 05, 2012 3:55 am

i wired the new output transformers , so i wouldn't have to cross the wires
coming from the driver board .

stock amp

Mark

battradio

KT88

Posts: 840

Joined: Tue Mar 10, 2009 12:58 am

Location: near ST.Louis MO.

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Re:
Beefing up a ST70

by
kheper
» Mon Nov 05, 2012 6:07 am

battradio wrote:What would be the best idle current for the
KT120's? 70 or 80 ma

Nice rebuild.

What is
the B+? Is the power supply a 360V-0-360V?

Unless you have a higher B+
than a stock ST70, KT120s won't give much more output wattage. The gm of an EL34
is 11 and the gm of a KT120 is 12.5.

KT120s run at a low plate
dissipation should have long life, however. 70 or 80ma idle bias won't come
close to maxing out a KT120. Since the power supply can shell out the current,
I'd go high.

http://www.jogis-roehrenbude.de/Roehren-Geschichtliches/Roe-Neuvorstellungen/BTB/KT120/KT120TungSol.pdf

kheper

KT88

Posts: 1236

Joined: Wed Dec 21, 2005 10:14 pm

Location: Philly, PA

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by
battradio
» Mon Nov 05, 2012 6:17 am

The Power trnsformer is 410 -0- 410 at 450 ma .

Mark

battradio

KT88

Posts: 840

Joined: Tue Mar 10, 2009 12:58 am

Location: near ST.Louis MO.

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by
kheper
» Mon Nov 05, 2012 6:26 am

battradio wrote:The Power trnsformer is 410 -0- 410 at 450
ma .

580Vdc * .80A = 46W

Have you thought
about triode mode for the KT120s?

kheper

KT88

Posts: 1236

Joined: Wed Dec 21, 2005 10:14 pm

Location: Philly, PA

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by
battradio
» Fri Nov 09, 2012 8:48 am

It will be about 480 B+ to the KT120's .

The amps not for me , PP 6B4G is the next amp i'm putting together for
myself

Mark

battradio

KT88

Posts: 840

Joined: Tue Mar 10, 2009 12:58 am

Location: near ST.Louis MO.

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by
battradio
» Sat Nov 10, 2012 12:03 pm

It basicly done except for four 43k ohm resisistors , the
original used 1 watt and changed them to 2 watts so the correct ones are on
order .

There is a pair of wires left loose till i test out the bias
circuit . will run some resistance tests when the resistors come in .

A couple of bypass caps to add then see how much power it puts out and
what the frequincy responce is .

The jumper wires across the board are
for the DC filaments .

Mark

Mark

battradio

KT88

Posts: 840

Joined: Tue Mar 10, 2009 12:58 am

Location: near ST.Louis MO.

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by
battradio
» Fri Nov 16, 2012 10:34 am

Got it up and running , leting the bias stabilise before i
run some tests on it .

Had to make some resistor changes to the bias
supply and the resistors feeding the B+ to the ciruit board , before i could
apply full voltage to it .

Mark

battradio

KT88

Posts: 840

Joined: Tue Mar 10, 2009 12:58 am

Location: near ST.Louis MO.

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by
battradio
» Fri Nov 16, 2012 1:03 pm

Well i connected the signal generator up to it with 8 ohm
dummy loads on the 8 ohm outut on both channels and put A scope to the dummy
loads and a wide band volt meter , with both channels driven it puts out 27.5
volta AC at 1KHZ into the 8 ohm load . Clipping occured at 27.65 volts AC .

After the math will call it 90 watts a channel .

The B+ turned
out right at 550 volts with both channel set 70ma at idle , the B+ into the
board for the 12Bh7A driver tubes is 394 volts , and the B+ into the front end
section of the board is 362 volts .

Letting it cook with the signal
genterator at 5 watts a channel for a couple of hours before i give it a listen
and run more tests .

Mark

Mark

battradio

KT88

Posts: 840

Joined: Tue Mar 10, 2009 12:58 am

Location: near ST.Louis MO.

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by
battradio
» Fri Nov 16, 2012 7:52 pm

At 20 hz it puts out 27.74 volts RMS , at 1 khz 27.5 RMS
and at 20 khz 26.70 all in to 8 ohms , will have to dig my other HP 400F volt
meter out so i can see how flat the fequincy responce is at at 1, 10 and 50
watts .

It looks like the output transformer rolls off on the top end
at 90 watts out .

Readings with both channels driven .

Mark

Mark

battradio

KT88

Posts: 840

Joined: Tue Mar 10, 2009 12:58 am

Location: near ST.Louis MO.

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Re:
Beefing up a ST70

by
skidave
» Tue Dec 04, 2012 7:46 pm

Mark,

Great pictures and detail. A few questions
for you. I'll put them below in list form so you can respond easily.

1)
What output transformers did you use?
2) Did you use a turn on delay for your
B+?
3) Do you have a schematic drawn for your power
supply?

Thanks,

Dave

skidave

Posts: 129

Joined: Fri Jan 06, 2006 2:04 pm

Location: York, PA

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Re:
Beefing up a ST70

by
battradio
» Fri Dec 07, 2012 12:48 am

skidave wrote:Mark,

Great pictures and detail. A few
questions for you. I'll put them below in list form so you can respond
easily.

1) What output transformers did you use? Bob Latino's A-260-
120
2) Did you use a turn on delay for your B+? NO
3) Do you have a
schematic drawn for your power supply? NOT yet
.

Thanks,

Dave

Hi Dave ,

1) What
output transformers did you use? Bob Latino's A-260- 120
2) Did you use a
turn on delay for your B+? NO
3) Do you have a schematic drawn for your
power supply? NOT yet .

Will draw a a schematic for the power
supply and post it , was going to use the relay but ran out of room
.

Will draw up a schematic in the week or so .

Mark

Mark

battradio

KT88

Posts: 840

Joined: Tue Mar 10, 2009 12:58 am

Location: near ST.Louis MO.

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Re:
Beefing up a ST70

by
burnedfingers
» Sat Dec 08, 2012 3:21 pm

A real nice build

burnedfingers

KT88

Posts: 576

Joined: Mon Sep 05, 2005 6:38 am

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