Correct Bias Voltage for ST-70?

Postby JojoD » Wed Mar 08, 2006 12:09 pm

arnoldc wrote:jojo, what confused me was your previous post to get the plate current, which was 50mA, and then you followed up with taking away the screen current from it.

i guess we agree on the same thing though ;)


aprub! :D
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Postby m_shoe_maker » Wed Mar 08, 2006 12:22 pm

Naku po :!: Nataranta na tong ST70 user. :shock:

Anyway, simple lang ang solution ko.........

Master Hyper, what reading from my volt meter will I bias my ST70 :?:

Simpleng tanong sa Tube God, and his answer will be my Gospel Truth. 8)
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Correct Bias voltage for ST-70?

Postby john » Wed Mar 08, 2006 12:29 pm

One more thing that add to this biasing dilemma,They used 15.6 ohm carbon type resistors which are commomly off values more than 5%.

Stock Dyna ST-70 were equipped with Allen Bradley carbon type resistors which are normally off values by more than 5%. This can arose to erroneous voltages within the circuit.

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Postby arnoldc » Wed Mar 08, 2006 12:41 pm

john, in my experience Allen-Bradleys are off more than 5%!!!
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Postby garp » Wed Mar 08, 2006 1:16 pm

Here are some numbers for comparison:

Assuming 400V plate voltage, class AB, NEGLECTING SCREEN CURRENT:

el34 70% dissipation or 17.5W= 1.36V across 15.6R or about 43ma per tube

el34 80% dissipation or 20w=1.56v across 15.6R or about 50ma per tube

Most el34 amps are biased even below 70% in class AB.

If Jojo's 7ma screen current is the norm for the ST-70. That puts plate current at about 43 ma. In this case 1.56 is really not that high.
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Postby arnoldc » Wed Mar 08, 2006 3:03 pm

again, the plate current PLUS the screen current. you don't take the screen current away from the plate current. the 15.6 ohm resistor is biasing the plate current, not plate + screen.

an EL34 at 400 volts can swing from 50mA to 200mA, plate current is from 8.8mA to 50mA.
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Postby cia » Wed Mar 08, 2006 4:18 pm

Mozilla,
What is your take on this. This thread has really gotten me confused on what the correct bias should be on an ST70.
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Postby JojoD » Wed Mar 08, 2006 4:52 pm

the current flowing through the cathode resistor is the sum of the plate current and screen current yes?

anyway, what john mentioned regarding the drifting of the cathode resistor's value is a grim reality. I am in agreement that this would lead to false biasing indeed.

:D
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Postby arnoldc » Wed Mar 08, 2006 5:06 pm

The ST-70 is using a wimpy power supply by today's standard (don't you feel the heat?) but the trick is to bias the tubes at Class A operation, meaning it always draws the same current regardless of the material, or lack of signal going thru it.

Now, if you bias the ST-70 so low that it enters Class AB region, then the current draw fluctuation will be greater. See my post above about EL34 in Class AB push pull.

And you should have no signal (shorted input) mean measuing the bias.
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Postby arnoldc » Wed Mar 08, 2006 5:11 pm

jojo, you're right. the current that you're getting off the cathode is actually the plate + screen current. The screen current, which we can assume that depending on the tube used, will be anywhere from 7mA (as you posted) to 8.8mA (as I posted). no big deal ;)

so we can say that when biasing the ST-70 the screen current was never considered, but plays a factor.
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Correct Bias Voltage for ST-70?

Postby john » Wed Mar 08, 2006 5:39 pm

Sad and expensive realization, There is always a need to modify the stock ST- 70 for it to perform best on the listening room.

Modern production EL34 tubes draws more filament current than those NOS ones. Thus, making the stock power tranny hotter.

Biasing will always be a problem since depletion of the power tubes will never be equall after they were plugged on the tube sockets. Modifying to independent biasing is a MUST.

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Postby hypertriode » Wed Mar 08, 2006 9:18 pm

m_shoe_maker wrote:Naku po :!: Nataranta na tong ST70 user. :shock:

Anyway, simple lang ang solution ko.........

Master Hyper, what reading from my volt meter will I bias my ST70 :?:

Simpleng tanong sa Tube God, and his answer will be my Gospel Truth. 8)


just my opinion fritz: i agree with john that NOS EL-34's can take the punishment inflicted by high or bias settings while new production ones can't. also take into account the stock stereo 70 transformer's size - it gets really HOT; reason why i overrate my stereo 70's trannies by at least 50% more. having done that, for me the stereo 70 sounds it best at 1.56v bias voltage or 50 ma per tube; set the bias to 1.4v or lower you lose a lot of dynamics (and dynamic-sounding isn't one of the stereo 70's traits to start with). i'd rather listen to a quad of EL-34's on a stereo 70 for one year at it's optimum (50 ma/tube) than spend two years (scrimping on tube replacement cost) with so-so sound...but this is just me :)
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Postby marty_e » Wed Mar 08, 2006 10:06 pm

hypertriode wrote:
m_shoe_maker wrote:Naku po :!: Nataranta na tong ST70 user. :shock:

Anyway, simple lang ang solution ko.........

Master Hyper, what reading from my volt meter will I bias my ST70 :?:

Simpleng tanong sa Tube God, and his answer will be my Gospel Truth. 8)


just my opinion fritz: i agree with john that NOS EL-34's can take the punishment inflicted by high or bias settings while new production ones can't. also take into account the stock stereo 70 transformer's size - it gets really HOT; reason why i overrate my stereo 70's trannies by at least 50% more. having done that, for me the stereo 70 sounds it best at 1.56v bias voltage or 50 ma per tube; set the bias to 1.4v or lower you lose a lot of dynamics (and dynamic-sounding isn't one of the stereo 70's traits to start with). i'd rather listen to a quad of EL-34's on a stereo 70 for one year at it's optimum (50 ma/tube) than spend two years (scrimping on tube replacement cost) with so-so sound...but this is just me :)


Amen Brother. Thank God I can't afford Mullards :wink: :wink:
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Postby JojoD » Thu Mar 09, 2006 12:31 am

arnoldc wrote:jojo, you're right. the current that you're getting off the cathode is actually the plate + screen current. The screen current, which we can assume that depending on the tube used, will be anywhere from 7mA (as you posted) to 8.8mA (as I posted). no big deal ;)

so we can say that when biasing the ST-70 the screen current was never considered, but plays a factor.



therefore running the ST70 at 50mA per tube would just be fine and as you said, bias can be lowered as long as it does not approach the tubes cutoff region.


Biasing will always be a problem since depletion of the power tubes will never be equall after they were plugged on the tube sockets. Modifying to independent biasing is a MUST.



that is why it is also a must for ST users to use matched power tubes and assume that the pair will die at the same time. I wish Dynaco had incorporated at least an adjustable DC balance and separate cathode resistors or your proposed independent biasing to acommodate not so matched tubes.

but I think it is understandable since they also offer the amp in kit form so the simpler the better for them and kit builders. :(


:)
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Postby vintage_dog » Thu Mar 09, 2006 12:34 am

vintage_dog wrote:.... my recollection of the st70 being biased "correctly" (below 45ma), is that it looses a bit of dynamics. will try that tonight.


i biased my st70 down to about 1.35, and blah, didn't quite like it - slow and lacks dynamics, so it's back to 1.56 as recommended by the designers. luckily, the power transformers have been upgraded but they still get very hot...
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Postby JojoD » Thu Mar 09, 2006 12:43 am

vintage_dog wrote:
vintage_dog wrote:.... my recollection of the st70 being biased "correctly" (below 45ma), is that it looses a bit of dynamics. will try that tonight.


i biased my st70 down to about 1.35, and blah, didn't quite like it - slow and lacks dynamics, so it's back to 1.56 as recommended by the designers. luckily, the power transformers have been upgraded but they still get very hot...


the tubes would surely love the low bias and prolong their life - but the ears won't like it. :lol:
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Postby minicooper » Thu Mar 09, 2006 1:17 am

I have the VTA mod implemented on my ST70 and I can't complain. Part of the mod actually addressed the issue on biasing the EL34's and the move to eliminate the rare 7199 driver tubes.

for what it's worth this may be the solution for those who are worried about the short tube life issue on the stock ST70.

description taken from Roy Mottram's site (tubes4hifi.com)

Uses three 6201 (mil-spec 12AT7) tubes, for totally neutral uncolored sound. One half of the first tube is used for each channel as a voltage amplifier. One each of the other tubes is used per channel as a combined phase splitter/driver in a long-tailed pair configuration. 13 db total loop feedback is used. Parts quality is top notch, with mil-spec tubes, metal film resistors, and propylene caps.
There is an individual bias pot for each output tube, as well as a signal balance pot for the phase splitters.
The bias is set for 40 ma, about 20% less than stock to reduce heat and conserve tube life.
Open loop response of this design is incredible. It has flat frequency response from
7 Hz to 70 KHz, with good square waves from 7 Hz to 30 KHz. Open loop gain is 40.
The PCB is 8.0" x 3.3". Max power output is 40 wpc.
:)
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Postby m_shoe_maker » Thu Mar 09, 2006 12:06 pm

hypertriode wrote:
m_shoe_maker wrote:Naku po :!: Nataranta na tong ST70 user. :shock:

Anyway, simple lang ang solution ko.........

Master Hyper, what reading from my volt meter will I bias my ST70 :?:

Simpleng tanong sa Tube God, and his answer will be my Gospel Truth. 8)


just my opinion fritz: i agree with john that NOS EL-34's can take the punishment inflicted by high or bias settings while new production ones can't. also take into account the stock stereo 70 transformer's size - it gets really HOT; reason why i overrate my stereo 70's trannies by at least 50% more. having done that, for me the stereo 70 sounds it best at 1.56v bias voltage or 50 ma per tube; set the bias to 1.4v or lower you lose a lot of dynamics (and dynamic-sounding isn't one of the stereo 70's traits to start with). i'd rather listen to a quad of EL-34's on a stereo 70 for one year at it's optimum (50 ma/tube) than spend two years (scrimping on tube replacement cost) with so-so sound...but this is just me :)


That's it :!: The Master has spoken. :D
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Postby camotecue » Thu Mar 09, 2006 1:39 pm

m_shoe_maker wrote:
hypertriode wrote:
m_shoe_maker wrote:Naku po :!: Nataranta na tong ST70 user. :shock:

Anyway, simple lang ang solution ko.........

Master Hyper, what reading from my volt meter will I bias my ST70 :?:

Simpleng tanong sa Tube God, and his answer will be my Gospel Truth. 8)


just my opinion fritz: i agree with john that NOS EL-34's can take the punishment inflicted by high or bias settings while new production ones can't. also take into account the stock stereo 70 transformer's size - it gets really HOT; reason why i overrate my stereo 70's trannies by at least 50% more. having done that, for me the stereo 70 sounds it best at 1.56v bias voltage or 50 ma per tube; set the bias to 1.4v or lower you lose a lot of dynamics (and dynamic-sounding isn't one of the stereo 70's traits to start with). i'd rather listen to a quad of EL-34's on a stereo 70 for one year at it's optimum (50 ma/tube) than spend two years (scrimping on tube replacement cost) with so-so sound...but this is just me :)


That's it :!: The Master has spoken. :D


Now i'm more confused. I thought that new production tubes are sturdier and more robust than NOS tubes so we must be careful that we bias the latter according to specs or it depends on what type of tube. pasensya na po sa ignorance!
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Postby Tubedude » Thu Mar 09, 2006 1:49 pm

[/quote]
i biased my st70 down to about 1.35, and blah, didn't quite like it - slow and lacks dynamics, so it's back to 1.56 as recommended by the designers. luckily, the power transformers have been upgraded but they still get very hot...[/quote]

Sir VD,
What PS trasformer are you using now if may I ask?
One of my MK IV's has a nosiy PST. The 120/220 Hammond I'm looking at is large and expensive.
Thanks Dan
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