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    Stereo 70 SERIES II 15.6 ohm resistor

    MechEngVic
    MechEngVic

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    Stereo 70 SERIES II 15.6 ohm resistor Empty Stereo 70 SERIES II 15.6 ohm resistor

    Post by MechEngVic on Mon Feb 25, 2019 6:58 am

    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.

    Stereo 70 SERIES II 15.6 ohm resistor Vp3Lqf9
    peterh
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    Post by peterh on Mon Feb 25, 2019 7:45 am

    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
    MechEngVic
    MechEngVic

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    Post by MechEngVic on Mon Feb 25, 2019 9:57 am

    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
    peterh

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    Post by peterh on Mon Feb 25, 2019 10:18 am

    MechEngVic 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

    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!

    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.
    Peter W.
    Peter W.

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    Post by Peter W. on Mon Feb 25, 2019 11:26 am

    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.


    Last edited by Peter W. on Mon Feb 25, 2019 11:27 am; edited 1 time in total (Reason for editing : spelling)
    MechEngVic
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    Stereo 70 SERIES II 15.6 ohm resistor Empty Re: Stereo 70 SERIES II 15.6 ohm resistor

    Post by MechEngVic on Mon Feb 25, 2019 10:47 pm

    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?
    MechEngVic
    MechEngVic

    Posts : 10
    Join date : 2019-01-16

    Stereo 70 SERIES II 15.6 ohm resistor Empty Re: Stereo 70 SERIES II 15.6 ohm resistor

    Post by MechEngVic on Mon Feb 25, 2019 10:48 pm

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

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    Post by peterh on Tue Feb 26, 2019 1:23 am

    MechEngVic wrote:
    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?

    Yes. Buy a few extra, as this is on the limit, mount it clear of cables and other
    things as it will be warm.
    MechEngVic
    MechEngVic

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    Join date : 2019-01-16

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    Post by MechEngVic on Tue Feb 26, 2019 2:22 am

    I will do that! Thanks for the help.
    MechEngVic
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    Post by MechEngVic on Mon Mar 11, 2019 9:11 pm

    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!

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