Screen Voltage on Canadain RCA 97K-1

2/7/2010 8:35:41 PMBrian

I measured the plates and screen voltages on the 6K8 and 6K7 tubes and get 300 and 70 volts respectively.
Can someone point me in the right direction (using Ohm's Law) as to how to calculate what the voltage should be given the resistors involved?

2/7/2010 11:55:41 PMThomas Dermody

The voltages are actually given in the schematic on page 5.

http://www.nostalgiaair.org/PagesByModel/325/M0015325.pdf

If you want to figure out the voltages on your own, consult your tube manual for the plate and screen grid impedances for the the tubes you mentioned. The RF screen grids are in parallel with eachother and the oscillator plate, and R17. They are all in series with R14. This will tell you approximately what the voltage drop should be from the main B+ circuit to the other side of R14. The RF plates are all tied into the main B+ line through their respective coils, which have rather low resistances. To be more exact, you must consider all of the loads within the radio (or the resistance of the entire radio, including the rectifier), and this gets complicated, but it is possible to calculate approximately. Things are further complicated by the presence of signals, as they increase the impedance of coils, though if you wish to calculate all of the voltages on your own, you should get a good approximation without this extra information.

I am guessing that you just want the information on page 5, but if you really want to calculate all of this out yourself, it is possible.

2/8/2010 12:01:57 AMThomas Dermody

...Sorry. You said Canadian 97K-1, which is not the schematic I looked at here (U.S. 97K). Hopefully the circuitry is similar. If not, though, the information I gave will help you calculate approximately what the voltages should be throughout the set. Draw a schematic of pure resistances, representing all tubes, resistors, and coils in the set, and break it down from the mass unit to all of the separate units. For resistors in series, you just add them. For resistances in parallel, it's 1/(1/R1+1/R2...).

2/8/2010 7:32:50 AMBrian

Thank you Thomas. Given that I am using a digital meter and measuring 300 and 70 volts respectively, do you think this is reasonable compared to 275 and 95 as indicated on the schematic?

:...Sorry. You said Canadian 97K-1, which is not the schematic I looked at here (U.S. 97K). Hopefully the circuitry is similar. If not, though, the information I gave will help you calculate approximately what the voltages should be throughout the set. Draw a schematic of pure resistances, representing all tubes, resistors, and coils in the set, and break it down from the mass unit to all of the separate units. For resistors in series, you just add them. For resistances in parallel, it's 1/(1/R1+1/R2...).
:
:T.
:

2/8/2010 8:33:23 PMEdd

Sir Brian. . . . . .

On that set of your's, there is basically just the full B+ buss and the sub B+ buss.

If you will consult the attached marked up schema, one sees the RED MARKUP B+ moving to the right from its derivation at the rctifier toobie. Then upwards, to feed that full supply level to the AF output tube supply.

Further tracking of that buss, transversuing to the left, has a minor tap off to further drop the B+ to the early stage AF tube. Next stop to the left is a full supply B+ level to the target anode of the eye tube.

To the left from that last area is the feed to the plate of your 6K7 if amp tube along with a further path to the left to the plate of the hexode portion of the 6K8.

Now, in going back to the sub B+ buss, it is derived from a voltage divider bridge of R14-R17 [YELLOW]that ties into the main B+ buss.

The sub B+ power flow is to the left via the FUSCIA MARKUP it branches apart to feed the screen grids of the 6K7 and the 6K8 on one run.

The other run feeds over to the left and then up to the plate of the oscillator section of the 6K8.

The sub B+ level would be dependent upon the main B+ level, the ratio of the R14-17 divider resistors or the current consumption of the tubes tied in to it.

Seems like a slight fault with the sub B+ level as it now stands, are R14 and R17 to spec ?

On the other aspect, of the 300VDC level on the main B+ buss , a wee bit high, but I do consult with the RCA Radiotron Tube and Design manual and see the 6K8 being used up to the mid to high 200V levels.

Matter of fact, in its compared performance with 100 VDC on the plate of the hexode , the conversion efficiency amd overall performance is appreciably better when using the higher voltage.

An aside thought, since I am uncertain about WHICH post, recently, and WHO it is / was . . . in which case was the mentioning of the use of a 1 K value power resistor in place of a field coil on a radio.

Should this happen to be the set involved, the reason for the higher derived B+ voltage just might be due to the
use of a 1K value of power series dropping / filter resistor. The value should be 1,500 ohms instead.

73's de Edd

RCA 97K . . . series . . . SCHEMATIC:

2/9/2010 11:35:32 AMBrian

Thanks Edd,

Would C38 (if leaky) be a possibility? It is well buried in the chassis and I have not replaced yet.
Brian

:Sir Brian. . . . . .
:
:
:On that set of your's, there is basically just the full B+ buss and the sub B+ buss.
:
:
:If you will consult the attached marked up schema, one sees the RED MARKUP B+ moving to the right from its derivation at the rctifier toobie. Then upwards, to feed that full supply level to the AF output tube supply.
:
:
:Further tracking of that buss, transversuing to the left, has a minor tap off to further drop the B+ to the early stage AF tube. Next stop to the left is a full supply B+ level to the target anode of the eye tube.
:
:
:To the left from that last area is the feed to the plate of your 6K7 if amp tube along with a further path to the left to the plate of the hexode portion of the 6K8.
:
:
:Now, in going back to the sub B+ buss, it is derived from a voltage divider bridge of R14-R17 [YELLOW]that ties into the main B+ buss.
:
:
:The sub B+ power flow is to the left via the FUSCIA MARKUP it branches apart to feed the screen grids of the 6K7 and the 6K8 on one run.
:
:
:The other run feeds over to the left and then up to the plate of the oscillator section of the 6K8.
:
:
:The sub B+ level would be dependent upon the main B+ level, the ratio of the R14-17 divider resistors or the current consumption of the tubes tied in to it.
:
:
:Seems like a slight fault with the sub B+ level as it now stands, are R14 and R17 to spec ?
:
:
:On the other aspect, of the 300VDC level on the main B+ buss , a wee bit high, but I do consult with the RCA Radiotron Tube and Design manual and see the 6K8 being used up to the mid to high 200V levels.
:
:
:Matter of fact, in its compared performance with 100 VDC on the plate of the hexode , the conversion efficiency amd overall performance is appreciably better when using the higher voltage.
:
:
:
:An aside thought, since I am uncertain about WHICH post, recently, and WHO it is / was . . . in which case was the mentioning of the use of a 1 K value power resistor in place of a field coil on a radio.
:
:
:Should this happen to be the set involved, the reason for the higher derived B+ voltage just might be due to the
:use of a 1K value of power series dropping / filter resistor. The value should be 1,500 ohms instead.
:
:
:
:
:
:
:73's de Edd
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:RCA 97K . . . series . . . SCHEMATIC:
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2/10/2010 12:47:35 PMEdd(83995:83981)

Sir Brian. . . . . .
Ab-so -rootley, on the possibility of C38 leaking and pulling down the Sub B+ level a bit.

What you might try is to monitor the Sub B+ at the origin of its tap off at voltage divider bridge of R14-R17.

Then you unplug 6K8 and 6K7 tubes to see what the sub B+ rises up to without loading . . . Then plug in one tube at a time and recheck the voltage level. One of those individual tubes will have loaded and pulled the voltage down a wee bit more than the other.

I am thinking that it will be ~ 12 ma consumption for the 6K8 and ~11 ma for the 6K7. Seems like C38 is over to the side of the chassis and down under a lip.

That C38 would have to be seriously leaky down in the k's of resistance to be effecting us on that degree of a voltage drop.

The potential other thing aparrent on that supply line, is leakage within the the C41---5 ufd filter , which we hope that you have already replaced with a good unit.

You did not make any reference to the 1k power res sub for a speaker field coil, so I guess that situation was on somebody elses set. And, you also still have the original speaker in your unit, and no one has subbed in a speaker with a lower value of field coil reisistance / inductance.

BTW have you ever heard stations picked on this radio ? That initially being even before you started changing parts, considering that you made an initial g r a d u a l power up, thru a Variac, to see if there were initially any power electrolytic filter issues in the unit.

2 questions included.

Standing by . . .

73's de Edd

RCA 97K . . . series . . . SCHEMATIC:

2/10/2010 1:49:08 PMBrian(83998:83995)

Thanks again.
It is still the original speaker intact, and all electrolytics are new. Stations come in good with lots of volume, nice tone, even a few shortwave stations too. That capacitor is deeply buried so I will measure resistors first.
:Sir Brian. . . . . .
:
:
:
:Ab-so -rootley, on the possibility of C38 leaking and pulling down the Sub B+ level a bit.
:
:
:What you might try is to monitor the Sub B+ at the origin of its tap off at voltage divider bridge of R14-R17.
:
:
:Then you unplug 6K8 and 6K7 tubes to see what the sub B+ rises up to without loading . . . Then plug in one tube at a time and recheck the voltage level. One of those individual tubes will have loaded and pulled the voltage down a wee bit more than the other.
:
:
:I am thinking that it will be ~ 12 ma consumption for the 6K8 and ~11 ma for the 6K7. Seems like C38 is over to the side of the chassis and down under a lip.
:
:
:That C38 would have to be seriously leaky down in the k's of resistance to be effecting us on that degree of a voltage drop.
:
:
:The potential other thing aparrent on that supply line, is leakage within the the C41---5 ufd filter , which we hope that you have already replaced with a good unit.
:
:
:You did not make any reference to the 1k power res sub for a speaker field coil, so I guess that situation was on somebody elses set. And, you also still have the original speaker in your unit, and no one has subbed in a speaker with a lower value of field coil reisistance / inductance.
:
:
:BTW have you ever heard stations picked on this radio ? That initially being even before you started changing parts, considering that you made an initial g r a d u a l power up, thru a Variac, to see if there were initially any power electrolytic filter issues in the unit.
:
:
:2 questions included.
:
:
:Standing by . . .
:
:
:
:
:73's de Edd
:
:
:

:
:
:
:
:
:
:

:
:RCA 97K . . . series . . . SCHEMATIC:
:
:
:
:

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2/14/2010 8:46:23 PMBrian(84115:83998)

:Thanks again.
:It is still the original speaker intact, and all electrolytics are new. Stations come in good with lots of volume, nice tone, even a few shortwave stations too. That capacitor is deeply buried so I will measure resistors first.
:
::Sir Brian. . . . . .
::
::
::
::Ab-so -rootley, on the possibility of C38 leaking and pulling down the Sub B+ level a bit.
::
::
::What you might try is to monitor the Sub B+ at the origin of its tap off at voltage divider bridge of R14-R17.
::
::
::Then you unplug 6K8 and 6K7 tubes to see what the sub B+ rises up to without loading . . . Then plug in one tube at a time and recheck the voltage level. One of those individual tubes will have loaded and pulled the voltage down a wee bit more than the other.
::
::
::I am thinking that it will be ~ 12 ma consumption for the 6K8 and ~11 ma for the 6K7. Seems like C38 is over to the side of the chassis and down under a lip.
::
::
::That C38 would have to be seriously leaky down in the k's of resistance to be effecting us on that degree of a voltage drop.
::
::
::The potential other thing aparrent on that supply line, is leakage within the the C41---5 ufd filter , which we hope that you have already replaced with a good unit.
::
::
::You did not make any reference to the 1k power res sub for a speaker field coil, so I guess that situation was on somebody elses set. And, you also still have the original speaker in your unit, and no one has subbed in a speaker with a lower value of field coil reisistance / inductance.
::
::
::BTW have you ever heard stations picked on this radio ? That initially being even before you started changing parts, considering that you made an initial g r a d u a l power up, thru a Variac, to see if there were initially any power electrolytic filter issues in the unit.
::
::
::2 questions included.
::
::
::Standing by . . .
::
::
::
::
::73's de Edd
::
::
::

::
::
::
::
::
::
::

::
::RCA 97K . . . series . . . SCHEMATIC:
::
::
::
::

::
::
::
::
::
:

2/14/2010 8:50:28 PMBrian(84116:84115)

I changed R14 with a 14K resistor, which had at some point previous been replaced with 20K wire wound resisitor. Now reading 287 volts on plates and about 81 volts on screen of 6K7. This seems more reasonable. Likely a 12K resistor as called for, will do the trick.
Brian
::Thanks again.
::It is still the original speaker intact, and all electrolytics are new. Stations come in good with lots of volume, nice tone, even a few shortwave stations too. That capacitor is deeply buried so I will measure resistors first.
::
:::Sir Brian. . . . . .
:::
:::
:::
:::Ab-so -rootley, on the possibility of C38 leaking and pulling down the Sub B+ level a bit.
:::
:::
:::What you might try is to monitor the Sub B+ at the origin of its tap off at voltage divider bridge of R14-R17.
:::
:::
:::Then you unplug 6K8 and 6K7 tubes to see what the sub B+ rises up to without loading . . . Then plug in one tube at a time and recheck the voltage level. One of those individual tubes will have loaded and pulled the voltage down a wee bit more than the other.
:::
:::
:::I am thinking that it will be ~ 12 ma consumption for the 6K8 and ~11 ma for the 6K7. Seems like C38 is over to the side of the chassis and down under a lip.
:::
:::
:::That C38 would have to be seriously leaky down in the k's of resistance to be effecting us on that degree of a voltage drop.
:::
:::
:::The potential other thing aparrent on that supply line, is leakage within the the C41---5 ufd filter , which we hope that you have already replaced with a good unit.
:::
:::
:::You did not make any reference to the 1k power res sub for a speaker field coil, so I guess that situation was on somebody elses set. And, you also still have the original speaker in your unit, and no one has subbed in a speaker with a lower value of field coil reisistance / inductance.
:::
:::
:::BTW have you ever heard stations picked on this radio ? That initially being even before you started changing parts, considering that you made an initial g r a d u a l power up, thru a Variac, to see if there were initially any power electrolytic filter issues in the unit.
:::
:::
:::2 questions included.
:::
:::
:::Standing by . . .
:::
:::
:::
:::
:::73's de Edd
:::
:::
:::

:::
:::
:::
:::
:::
:::
:::

:::
:::RCA 97K . . . series . . . SCHEMATIC:
:::
:::
:::
:::

:::
:::
:::
:::
:::
::
:

2/15/2010 6:33:01 AMEdd(84133:84116)

Sir Brian. . . . . .
AAAAh hahhhh . . . With that 12K being transposed in the past with a 20 k that disrupted the situation in two ways.

The series combination was presenting LESS of a loading across the B+ proper , thus letting the B+ rise higher
than was desired.

The second aspect was the upsetting of the voltage divisional ratio and having the tap off being LOWER down from the main B+ than was desired.

If you are now happy with your physical selection attributes and power rating of the 14 K power resistor you have selected, you may still utilize it by merely shunting across it with a 2 watt metal film resistor of a 82K ohms value.

That would trim in the parallel combination in to 11,958k = = = 12k and should raise your sub B+ yet a bit higher.

The lower power rating of the shunting resistor is permissible by virtue of their resultant power division being proportional, with the 14 K still sloughing off the bulk of the power dissipation.

73's de Edd

I changed R14 with a 14K resistor, which had at some point previous been replaced with 20K wire wound resisitor. Now reading 287 volts on plates and about 81 volts on screen of 6K7. This seems more reasonable. Likely a 12K resistor as called for, will do the trick.
Brian
Thanks again.
It is still the original speaker intact, and all electrolytics are new. Stations come in good with lots of volume, nice tone, even a few shortwave stations too. That capacitor is deeply buried so I will measure resistors first.
Sir Brian. . . . . .
Ab-so -rootley, on the possibility of C38 leaking and pulling down the Sub B+ level a bit.

What you might try is to monitor the Sub B+ at the origin of its tap off at voltage divider bridge of R14-R17.

Then you unplug 6K8 and 6K7 tubes to see what the sub B+ rises up to without loading . . . Then plug in one tube at a time and recheck the voltage level. One of those individual tubes will have loaded and pulled the voltage down a wee bit more than the other.

I am thinking that it will be ~ 12 ma consumption for the 6K8 and ~11 ma for the 6K7. Seems like C38 is over to the side of the chassis and down under a lip.

That C38 would have to be seriously leaky down in the k's of resistance to be effecting us on that degree of a voltage drop.

The potential other thing aparrent on that supply line, is leakage within the the C41---5 ufd filter , which we hope that you have already replaced with a good unit.

You did not make any reference to the 1k power res sub for a speaker field coil, so I guess that situation was on somebody elses set. And, you also still have the original speaker in your unit, and no one has subbed in a speaker with a lower value of field coil reisistance / inductance.

BTW have you ever heard stations picked on this radio ? That initially being even before you started changing parts, considering that you made an initial g r a d u a l power up, thru a Variac, to see if there were initially any power electrolytic filter issues in the unit.

2 questions included.

Standing by . . .

73's de Edd

RCA 97K . . . series . . . SCHEMATIC: