DYNACO ST-70 BIAS

[XaudiomanX]

Hi to all.
After years of playing with Dyanco ST-70's and MK III's I have fianlly decided
to bias the amps using current draw instead of the old resistor to ground
method(conventional way), which makes you dependent on the actual value of the
resistor.Usually you put your volt/ohm meter on volts and put one lead(black)
on chassis ground and the other lead(red) on the tube socket where it says
1.56V. Well if you bias an ST-70 to 1.56 volts you are running EL-34's into
class A which is very hot for the tubes using todays B+ voltages. These amps
were designed when the wall outlets AC voltage was 117VAC. Now they are as high
as 125VAC at the outlet making the amps run at a higher B+ DC voltage, thus
making the tubes run hotter at the standard 1.56VDC. I decided to bias the tube
using an ammeter instead, biasing the tubes at 45MA(milliamps) per tube or 90MA
combined. The tubes run best at that DC voltage and will last a lot longer.
When I put the amps back to checking the bias across the 15.6 ohm resistor, the
bias voltage read 1.42VDC. So from the tube pin that says 1.56 VDC and the
other lead on ground, set the bias voltage to about 1.41VDC-1.43VDC and you
will keep the tubes for a long time. At the usual 1.56VDC the tubes are drawing
approx 65MA a piece, which is even more than KT-88's(6550's) should handle at
the B+ voltage the amps produce today.
I know I will be set up for a verbal beating and disagreements, but the numbers
don't lie. I didn't make this stuff up. It was taken right from a tube manual.
HAPPY LISTENING!

[Jon Yaeger]

Paul,

I dealt with the line voltage problem by installing a 100 ohm 20W resistor
between the XMFR center-tapped (B+ side) and ground. This brought B+
voltage back to design ranges. The drawback is that it produces heat inside
of the chassis.

- Jon

Organization: AOL http://www.aol.com
Newsgroups: rec.audio.tubes
Date: 17 Feb 2004 03:50:06 GMT
Subject: DYNACO ST-70 BIAS
Hi to all.
After years of playing with Dyanco ST-70's and MK III's I have fianlly decided
to bias the amps using current draw instead of the old resistor to ground
method(conventional way), which makes you dependent on the actual value of the
resistor.Usually you put your volt/ohm meter on volts and put one lead(black)
on chassis ground and the other lead(red) on the tube socket where it says
1.56V. Well if you bias an ST-70 to 1.56 volts you are running EL-34's into
class A which is very hot for the tubes using todays B+ voltages. These amps
were designed when the wall outlets AC voltage was 117VAC. Now they are as high
as 125VAC at the outlet making the amps run at a higher B+ DC voltage, thus
making the tubes run hotter at the standard 1.56VDC. I decided to bias the tube
using an ammeter instead, biasing the tubes at 45MA(milliamps) per tube or 90MA
combined. The tubes run best at that DC voltage and will last a lot longer.
When I put the amps back to checking the bias across the 15.6 ohm resistor, the
bias voltage read 1.42VDC. So from the tube pin that says 1.56 VDC and the
other lead on ground, set the bias voltage to about 1.41VDC-1.43VDC and you
will keep the tubes for a long time. At the usual 1.56VDC the tubes are drawing
approx 65MA a piece, which is even more than KT-88's(6550's) should handle at
the B+ voltage the amps produce today.
I know I will be set up for a verbal beating and disagreements, but the numbers
don't lie. I didn't make this stuff up. It was taken right from a tube manual.
HAPPY LISTENING!

[Steve O'Neill]

Hi:

No verbal beatings, however...something is wrong with the numbers you cite
at the end of your post: "At the usual 1.56VDC the tubes are drawing approx
65MA a piece..." If the common cathode resistor is truly 15.6 ohms, the
voltage drop @ 65mA/tube should be 2 x .065 x 15.6 ~2V not the 1.56 you
mention. What I find especially curious is that earlier in you r msg you
note that 90mA tot across the 15.6 ohm resistor produces ~1.4V; pretty much
the expected value . Is 65mA the current thru one tube of the pair and 35mA
thru the other (indicating a fairly serious imbalance in bias current)? Or
is there something I've missed?

Also, while today's higher mains voltages do indeed result in higher B+
voltages, biasing to lower currents will increase the B+ even more which may
be stressful on an original filter cap.

[TheSchwrtz]

You are probably correct mathematically but who knows what actually is
happening when the power is turned on? Ohms Law is what it is but so are
meters. I am only going by what my meter was showing when I was reading the
actual current draw(total almost 130MA with 1.56VDC bias), although I never
actually took the value reading of the cathode resistor. When I rebiased the
tubes down to a total of 90MA and took the voltage reading it showed 1.416VDC,
so the resistor is probably out of tolerance. I will check it later.

[Bob Hedberg]

When I first got my st-70, I spent a considerable amount of time
messing with it, like listening to different bias points.
I found that the recommended bias did actually sound the best to my
ears. IMHO.
Lower bias tended to make the bass flabby, and the music lost focus
and dynamics. Higher bias produced tighter focus and dynamics, but
also sounded cold and harsh. Again, IMHO.
I also ended up with a feedback resistance almost exactly that of the
original one while in UL mode, by listening to varous types of music
and using a pot for the feedback.

Go figure.
Bob Hedberg.

Post by XaudiomanX
Hi to all.
After years of playing with Dyanco ST-70's and MK III's I have fianlly decided
to bias the amps using current draw instead of the old resistor to ground
method(conventional way), which makes you dependent on the actual value of the
resistor.Usually you put your volt/ohm meter on volts and put one lead(black)
on chassis ground and the other lead(red) on the tube socket where it says
1.56V. Well if you bias an ST-70 to 1.56 volts you are running EL-34's into
class A which is very hot for the tubes using todays B+ voltages. These amps
were designed when the wall outlets AC voltage was 117VAC. Now they are as high
as 125VAC at the outlet making the amps run at a higher B+ DC voltage, thus
making the tubes run hotter at the standard 1.56VDC. I decided to bias the tube
using an ammeter instead, biasing the tubes at 45MA(milliamps) per tube or 90MA
combined. The tubes run best at that DC voltage and will last a lot longer.
When I put the amps back to checking the bias across the 15.6 ohm resistor, the
bias voltage read 1.42VDC. So from the tube pin that says 1.56 VDC and the
other lead on ground, set the bias voltage to about 1.41VDC-1.43VDC and you
will keep the tubes for a long time. At the usual 1.56VDC the tubes are drawing
approx 65MA a piece, which is even more than KT-88's(6550's) should handle at
the B+ voltage the amps produce today.
I know I will be set up for a verbal beating and disagreements, but the numbers
don't lie. I didn't make this stuff up. It was taken right from a tube manual.
HAPPY LISTENING!

Bob H.

Just grab that plate in one hand, the chassis in the other,
and FEEL the power of tube audio!!!
(not literally, of course, just kidding. DON'T DO THAT!)

[TheSchwrtz]

Bob,
We all have our likes and dislikes about things in audio. What you experienced
with higher bias is exactly what Audio Research did to the tube world. They
keep themselves in the unique art of audio by offering tubes in their solid
state amps, if you know what I mean. They too, bias everything right to the
bare limit of explosion with high B+ regulated power supplies to get that "High
Definition" sound they are so famous for. Nothing to hard to do. Tight,
thunderous bass, very high end detail, not very linear sound. All the things
solid state is known for coming through tubes. So I do know what you mean. Some
days I bias high and then other days I bias where they should be. Who knows?
You're right! I'm right! He's right! I think you get my drift. I am only making
a post message on my findings and it is also a way of saying HI to everyone
without starting a war.