Correct Bias Voltage for ST-70?

Correct Bias Voltage for ST-70?

Postby john » Tue Mar 07, 2006 8:36 pm

Attention ST-70 Users!

You are biasing theEL34 tubes too high! 1.56 volts devided by 15 ohms will result in 104 milliampere. devide that by 2 (104 ma for two tubes) will result in 52 milliampere for each tube. This biasing will results in operating the tubes outside the reccomemded 36 ma - 45 ma allowed for an EL34 tubes. This will shorten the tubes' life considerably.

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Postby cia » Tue Mar 07, 2006 9:00 pm

What then do you suggest is the proper/correct bias. Thanks.
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Correct Bias Voltage for ST-70?

Postby john » Tue Mar 07, 2006 9:22 pm

1.35 volts is the maximum Bias voltage at the test points of each channel. This corresponds to 45 ma of bias current for each tube.

Use Ohm's Law to calculate.


Your welcome Sir,


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Postby iceman90a » Tue Mar 07, 2006 11:01 pm

Sir,

isn't 1.56v written on the ST70? is this being misunderstood as the bias?

thanks
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Postby KD » Tue Mar 07, 2006 11:03 pm

Considering the ST-70 was introduced in 1959, why is this "mistake" being unearthed only now?
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Correct Bias Voltage for ST-70?

Postby john » Wed Mar 08, 2006 7:02 am

Hi KD and Iceman,

The 1.56V test point voltage, is the voltage drop at the 15 Ohm resistor for each channel. This is not the Bias Voltage ( Negative Grid Voltage) applied to the grid but as a measuring Test Point to calculate for the bias current.

During the introduction of the Dyna ST-70, EL 34 tubes are abundant and can withstand the above 45ma maximun limit. Nowadays, NOS EL34s' are rare and very scarce and thus having a match quad is very difficult.

You can very my claim via looking at the charred bases of those EL34 tubes that had been set-upped with this 1.56 volts at the test points. And plenty of NOS EL34 tubes had been destroyed through the application of Excessive bias current by ST-70 owners.


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Postby arnoldc » Wed Mar 08, 2006 8:15 am

Tama si john on the formula, it's simple Ohm's Law.

KD, it may not be a mistake. Maybe tama ang Dynaco, but it's the "interpretation" that is in question here.

Teka KD, Iceman, di ba nag attend kayo ng Tubes 101 class? How could you have missed that? :twisted:

Anyway, look at the tube datasheets lagi-

The EL34 at Class A can take a maximum of 100mA Plate Current

At Class AB Push Pull it says max of 2 x 95mA

Go figure.
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Postby iceman90a » Wed Mar 08, 2006 8:42 am

arnoldc wrote:Tama si john on the formula, it's simple Ohm's Law.

KD, it may not be a mistake. Maybe tama ang Dynaco, but it's the "interpretation" that is in question here.


so it was being misunderstood

arnoldc wrote:Teka KD, Iceman, di ba nag attend kayo ng Tubes 101 class? How could you have missed that? :twisted:


Sir naman - nanonood din ako ng live concert - pero di pa rin ako magaling kumanta! :D

btw - thanks for explaining john
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Postby arnoldc » Wed Mar 08, 2006 8:44 am

ice, tama ka dyan! ako alam ko pag sintunado yung singer/band, pero sintunado pa din ako :lol:
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Postby JojoD » Wed Mar 08, 2006 9:14 am

Tama po yun 1.56V na bias.

Kung yun cathode resistor is 15.6 ohms and 1.56V bias (voltage across the 15.6 ohms resistor).

V/R=I

then;

1.56/15.6=0.1A or 100mA ---> this is for 2 tubes. divide it by 2 and you get 50mA per EL34.


cheers
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Postby vintage_dog » Wed Mar 08, 2006 9:15 am

thanks for the education john...yes, the st70 drives those tubes pretty hard and hot. my recollection of the st70 being biased "correctly" (below 45ma), is that it looses a bit of dynamics. will try that tonight.

if folks have a hang-up on the 1.56v, they can still maintain that setting, but change the resistors to 20ohms. :D
Last edited by vintage_dog on Wed Mar 08, 2006 9:16 am, edited 1 time in total.
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Postby zetroce » Wed Mar 08, 2006 9:16 am

arnoldc wrote:ice, tama ka dyan! ako alam ko pag sintunado yung singer/band, pero sintunado pa din ako :lol:


Kumakanta ka ba sa K_V. :wink: Ako kasi, pinababayaan ko na lang yung katabi ko ang kumanta, minamasahe ko na lang siya para ganahan... :wink: :lol: :twisted:
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Postby JojoD » Wed Mar 08, 2006 9:18 am

also, 50mA is not purely plate current, you must subtract the screen current to accurately get the plate current. this would roughly be around 42mA-45mA per tube.
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Postby iceman90a » Wed Mar 08, 2006 9:25 am

zetroce wrote:
arnoldc wrote:ice, tama ka dyan! ako alam ko pag sintunado yung singer/band, pero sintunado pa din ako :lol:


Kumakanta ka ba sa K_V. :wink: Ako kasi, pinababayaan ko na lang yung katabi ko ang kumanta, minamasahe ko na lang siya para ganahan... :wink: :lol: :twisted:


sarap ka pala kasama sa kantahan - libre masahe! :P
aaayyy type! :D
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Postby vintage_dog » Wed Mar 08, 2006 9:30 am

JojoD wrote:also, 50mA is not purely plate current, you must subtract the screen current to accurately get the plate current. this would roughly be around 42mA-45mA per tube.


am catching up...now how does one measure that (screen current)? :D

thanks for the education guys...keep 'em coming :wink:
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Postby zetroce » Wed Mar 08, 2006 9:30 am

iceman90a wrote:
zetroce wrote:
arnoldc wrote:ice, tama ka dyan! ako alam ko pag sintunado yung singer/band, pero sintunado pa din ako :lol:


Kumakanta ka ba sa K_V. :wink: Ako kasi, pinababayaan ko na lang yung katabi ko ang kumanta, minamasahe ko na lang siya para ganahan... :wink: :lol: :twisted:


sarap ka pala kasama sa kantahan - libre masahe! :P
aaayyy type! :D


Yung katabi ko kasi., mahilig sa wireless mic, at magaling mag-entertain. :lol: Ikaw ba, ganun din??? :?: :twisted:
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Postby arnoldc » Wed Mar 08, 2006 10:33 am

jojo, the screen current is IN ADDITION to the plate current, not taken away from the plate current.

even if the EL34 can take up to 100mA, just think what is the rating of that power transformer? did you notice how hot the power transformer is? yes, bias it high, and draw more current = generate more heat and shorten life of your power transformer. it will also affect the DC current rating of the output transformer, can it take more than 100mA??
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Postby JojoD » Wed Mar 08, 2006 11:57 am

arnoldc wrote:jojo, the screen current is IN ADDITION to the plate current, not taken away from the plate current.

even if the EL34 can take up to 100mA, just think what is the rating of that power transformer? did you notice how hot the power transformer is? yes, bias it high, and draw more current = generate more heat and shorten life of your power transformer. it will also affect the DC current rating of the output transformer, can it take more than 100mA??




i know that already prof, maybe you didn't "caught" what I was thinking probably due to my poor communication skills.

anyway, what I was saying is that if we get 100mA for the 2 tubes (EL34) and divide that in 2 we would have 50mA per tube - this is not just plate current - in fact, this is plate current plus screen current.

i once accidentally run them in 100mA per tube, that's 400mA for 4 tubes and yes they still worked. it was a good thing that my secondary for the HT was rated at 600mA.
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Postby arnoldc » Wed Mar 08, 2006 12:01 pm

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 ;)
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Postby JojoD » Wed Mar 08, 2006 12:07 pm

vintage_dog wrote:
JojoD wrote:also, 50mA is not purely plate current, you must subtract the screen current to accurately get the plate current. this would roughly be around 42mA-45mA per tube.


am catching up...now how does one measure that (screen current)? :D

thanks for the education guys...keep 'em coming :wink:



good day sir, if you have a stock st70, you can disconnect the screen grid(pin 4 of EL34) and insert an ammeter. the current that you measured here is your screen current. if you have the mod with triode/UL switch that uses a screen resistor, just measure the voltage across this resistor to get your screen current again using Ohm's law.

since the cathodes of the EL34 are tied together and connected to a 15.6 ohms resistor to ground, Kirchoff law says the current flowing through this resistor would be the sum of the 2 branches (EL34). using Ohm's law we can compute for the current and we then divide the current by 2 (assuming the EL34 are well matched) to get the current per tube.

however, the result has almost always been misunderstood to be the plate current when in fact it is the plate current + screen current.

my EL34 screen current averages at 7mA.
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