Stereo 70 SERIES II 15.6 ohm resistor

I have a SERIES II and I have to replace the dreaded 15.6 ohm resistor.  The SERIES II has a 15.6 ohm 1 WATT 1% wirewound (I think the original st-70's was a 3 WATT...?).

Two Questions:

1. Does anyone know the wattage on the 15.6 ohm resistor that Dynakitparts sells? I sent them a message but haven't heard back yet.

2. Would a 15 ohm 1 WATT 1% wirewound work?

The 15 ohm is the closest I can find. Would a 15 ohm have an undesirable effect on the bias adjustment LED's? Based on the schematic, what are viable options to replace this non-available resistor (if dynakitparts resistor is a 3 WATT)?

If I can avoid paying almost 8$ per resistor from Dynakitparts I would sure like that also.

Hopefully the more amp-savvy can help.

peterh wrote:As this amp uses a built-in indicator circuit you will have changed bias if anything else but
a 15.6 ohm resistor is used.
Using a 15 ohm will increase bias from 50 to 51mA.
Using 2 parallelled 33 ohm resistors would be a better choice, resulting in 47.5mA / tube

Thanks for the info!
So let's see if I've got this straight:
15.6ohm=50mA to tube, 15ohm=51mA, and the 2 parallel 33ohm=47.5mA.
So does that mean lowering the bias current is better than raising it?
Also, how important is this resistor's wattage rating for this circuit? Is higher or lower better?

Thanks again!

peterh wrote:
The power used in the resistor is 0.15w, means that you could use just about
anything.  A smaller resistor ( 0.25) will protect your amp better from
tube failure as it will evaporate much faster then a fuse.
Lowering bias will make life a little easier for the tubes, some people says that
current manufacture tubes is less resilient for overload then NOS.

Take this advice to heart!  And if you did pop a 1-watt resistor in this application - please make sure that you have gone over _EVERYTHING_ before restarting.

And, I have been writing for years that pre-blight tubes are far more resilient and tolerant the post-blight tubes. One more bit of sage advice. I am running tubes at this moment that have many thousands of hours on them and in all likelihood are older than some of the members here.

Peter W. wrote:

peterh wrote:
The power used in the resistor is 0.15w, means that you could use just about
anything.  A smaller resistor ( 0.25) will protect your amp better from
tube failure as it will evaporate much faster then a fuse.
Lowering bias will make life a little easier for the tubes, some people says that
current manufacture tubes is less resilient for overload then NOS.

Take this advice to heart!  And if you did pop a 1-watt resistor in this application - please make sure that you have gone over _EVERYTHING_ before restarting.

And, I have been writing for years that pre-blight tubes are far more resilient and tolerant the post-blight tubes. One more bit of sage advice. I am running tubes at this moment that have many thousands of hours on them and in all likelihood are older than some of the members here.

Thanks for your input! OK then, using a lower watt resistor gives me more ohm choices. To stick as close to 15.6 ohm without going less than, I found a 15.8 ohm 1/4 watt. Will that do the trick?

FYI: The dynakitparts 15.6 ohm resistor is a 3 watt.

I will do that! Thanks for the help.

Took everyone's advice, used a 15.8 ohm 1/4 watt resistor and bought spares. Mounted them up high. Also replaced everything except the diodes and transformers. Transformers were all in great condition. Glad to report the amp is up and running and sounding sweet as ever. Thanks to all for the advice!

peterh wrote:As this amp uses a built-in indicator circuit you will have changed bias if anything else but
a 15.6 ohm resistor is used.
Using a 15 ohm will increase bias from 50 to 51mA.
Using 2 parallelled 33 ohm resistors would be a better choice, resulting in 47.5mA / tube

Correct me if I am wrong.
Two 33 ohm resistors in parallel = 16.5 ohm. So my 15.6 ohm resistors that have drifted to 16.2 will be just fine and in fact will lower the current slightly, and that is not a bad thing.
For the sake of learning. Is there simple formula for calculating the bias setting for a stock Stereo 70 with a resistor other than the 15.6?
Thanks,
Tim

chet6120 wrote:

peterh wrote:As this amp uses a built-in indicator circuit you will have changed bias if anything else but
a 15.6 ohm resistor is used.
Using a 15 ohm will increase bias from 50 to 51mA.
Using 2 parallelled 33 ohm resistors would be a better choice, resulting in 47.5mA / tube

Correct me if I am wrong.
Two 33 ohm resistors in parallel = 16.5 ohm. So my 15.6 ohm resistors that have drifted to 16.2 will be just fine and in fact will lower the current slightly, and that is not a bad thing.
For the sake of learning. Is there simple formula for calculating the bias setting for a stock Stereo 70 with a resistor other than the 15.6?
Thanks,
Tim

Yes, but a resistor that has drifted means it is old or has been subject to currents approaching it's maximum, it's weakened, I wouldn't count on it. I don't know if you've had that bias resistor blow on you but it is not fun, it makes a big mess and usually takes a tube or two with it and possibly other resistors in the way, not to mention what it might do to your speakers, it makes a horrible loud noise as it's going. It's a good idea to replace that resistor every several hundred hours, and for sure if you put in a new set of tubes. You can see the burn spots on my amp from R8 and R9.

Bob Latino says running the EL-34 on an ST-70 at 0.40mA is good, no loss in sound quality and extends tube life, plus less strain on that bias resistor. Because of the circuit design, the bias resistor can be subject to flashes of high current if a tube arcs.

RI=V, I=V/R, 100mA=1.56V/15.6ohms. R=V/I, 1.56V/80mA=19.5 ohm (80mA is for both tubes, 40mA each). I would run 2 x 39 ohm resistors, closely matched, you'll probably even notice an improvement in sound quality with the paralleled resistors. 2 x 39 ohm = 19.5 ohm.

My other recommendation to make your amp next to worry free is screen stability resistors. Tubes will no longer arc, no more big stress on bias resistor, tubes will last even longer, 10,000 hours some say. And lastly, tube arcing can mean coupling caps are wearing out, skinning dc current. (I learned most of this stuff from the guys here on this forum)

http://tronola.com/html/maximize_tube_life.html (read the article pdf)

chet6120 wrote:

For the sake of learning. Is there simple formula for calculating the bias setting for a stock Stereo 70 with a resistor other than the 15.6?
Thanks,
Tim

For the circuit shown (ST 70 Series 2) it is easy.

Assuming the voltage at point A3 is 1.56V, the formula for the current that keeps the LEDs balanced is

current_in_ma = 500 * 1.56/R

Where R is the resistor used in Ohms.

For the original ST70 there are no LEDs and constant reference voltage so there is no equivalent formula.
You can use any resistor (within reason) and still bias the amp to the same 50ma current by setting the voltage across it to be R/10. They picked 1.56 because it was the same as a fully charged D cell battery I believe.

MechEngVic wrote:
Yes, but a resistor that has drifted means it is old or has been subject to currents approaching it's maximum, it's weakened, I wouldn't count on it. I don't know if you've had that bias resistor blow on you but it is not fun, it makes a big mess and usually takes a tube or two with it and possibly other resistors in the way, not to mention what it might do to your speakers, it makes a horrible loud noise as it's going. It's a good idea to replace that resistor every several hundred hours, and for sure if you put in a new set of tubes. You can see the burn spots on my amp from R8 and R9.

Bob Latino says running the EL-34 on an ST-70 at 0.40mA is good, no loss in sound quality and extends tube life, plus less strain on that bias resistor. Because of the circuit design, the bias resistor can be subject to flashes of high current if a tube arcs.

RI=V, I=V/R, 100mA=1.56V/15.6ohms. R=V/I, 1.56V/80mA=19.5 ohm (80mA is for both tubes, 40mA each). I would run 2 x 39 ohm resistors, closely matched, you'll probably even notice an improvement in sound quality with the paralleled resistors. 2 x 39 ohm = 19.5 ohm.

My other recommendation to make your amp next to worry free is screen stability resistors. Tubes will no longer arc, no more big stress on bias resistor, tubes will last even longer, 10,000 hours some say. And lastly, tube arcing can mean coupling caps are wearing out, skinning dc current. (I learned most of this stuff from the guys here on this forum)

http://tronola.com/html/maximize_tube_life.html (read the article pdf)

They are 58 y/o so they have had a good life and your account of what could happen is scary.
Two 39 ohm sounds like a great way to go. I have a bias balance kit from DynakitParts but I have read others say that it ddn't sound as good and I want it as stock as possible.
I have downloaded the article you linked and will read.
Thanks for the formula and advice.