I've posted this question on ARF as well but for those of you here who don't visit there I'll post it here as well in case any of you have an answer.

I'm working on recapping this standard AA5, ( a Clarion C-100 ( made by Warwick)) ... and it's been re-worked already a lot in the past so I'm trying to piece it back together properly.

Someone has replaced the output transformer with what seems like a reasonable substitute and for the most part it's working nicely except for what seems like an excessive AC hum level.

I'll be checking lead dress and looking for tubes with problems that may be a cause.

Recently I've been successful in killing hum problems in AA5s that used to have a hum-buck tap on the original output transformer but where it's been replaced with one that doesn't have one. So in that case I've upped the filter-cap values or added a 3rd stage of filtering with great success.

So I can certainly do the same here as well, if need be.

But my question about this particular radio's schematic relates to the 10k resistor R9 from B++ to the cathode of the 50L6.

What is the design purpose of this? I haven't seen this employed before on other AA5s.

Is it there to help cancel some of the hum by feeding a small amount of B+ ripple into the cathode out of phase?

It doesn't seem to make any noticeable difference in this radio. As I clip it out or put it back in.

Any ideas?
http://www.nostalgiaair.org/Resources/228/M0023228.htm

:I've posted this question on ARF as well but for those of you here who don't visit there I'll post it here as well in case any of you have an answer.
:
:I'm working on recapping this standard AA5, ( a Clarion C-100 ( made by Warwick)) ... and it's been re-worked already a lot in the past so I'm trying to piece it back together properly.
:
:Someone has replaced the output transformer with what seems like a reasonable substitute and for the most part it's working nicely except for what seems like an excessive AC hum level.
:
:I'll be checking lead dress and looking for tubes with problems that may be a cause.
:
: Recently I've been successful in killing hum problems in AA5s that used to have a hum-buck tap on the original output transformer but where it's been replaced with one that doesn't have one. So in that case I've upped the filter-cap values or added a 3rd stage of filtering with great success.
:
: So I can certainly do the same here as well, if need be.
:
:But my question about this particular radio's schematic relates to the 10k resistor R9 from B++ to the cathode of the 50L6.
:
:What is the design purpose of this? I haven't seen this employed before on other AA5s.
:
:Is it there to help cancel some of the hum by feeding a small amount of B+ ripple into the cathode out of phase?
:
:It doesn't seem to make any noticeable difference in this radio. As I clip it out or put it back in.
:
:Any ideas?
:http://www.nostalgiaair.org/Resources/228/M0023228.htm

Could it be noise on your electrical circuit. Try it on a different circuit...Neal

:Could it be noise on your electrical circuit. Try it on a different circuit...Neal

Hi Neal:

Thanks... good idea... however it turns out that it was a defective 12SQ7GT ... As soon i I replased it with the metal type. (pin 1 grounded) the AC hum shut up.

I started by grounding the input grid to the 50L6 and it got instantly quiet... so I knew it was prior to that.

I tried redressing the wires around the 12SQ7 and re dressing the cap to it from the volume control. Nothing helped... so I pulled it and the metal one replacing it did the job.
So thanks.

But my question still remains:
What function does the 10k resistor from B++ to the cathode of the 50L6 (R9) serve?

oops.. I meant Nom.. not Neal... sorry.

Peter,

It would seem to be a form of fixed bias or possibly a combination of fixed and self bias. I too am not sure why this was done. It may have something to do with lowering distortion. You can bias a tube by placing a negative voltge on the grid or a positive voltage on the cathode (cathode resistor(s))

Radiodoc
******************

:oops.. I meant Nom.. not Neal... sorry.

:Peter,
:
:It would seem to be a form of fixed bias or possibly a combination of fixed and self bias. I too am not sure why this was done. It may have something to do with lowering distortion. You can bias a tube by placing a negative voltge on the grid or a positive voltage on the cathode (cathode resistor(s))
:
:Radiodoc
:******************
:
:

Hi Radiodoc:
Yes.. that seems to be it.. I also came to that conclusion avter cogitating on it a while.

Seems like an early attempt to do the same as a cathode bypass cap by keeping the bias state steadier... no?