Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Dec Sat 16, 2017 2:42 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 14 posts ] 
Author Message
 Post subject: load line calculation questions
PostPosted: Nov Mon 06, 2017 5:41 pm 
Member

Joined: Sep Sat 17, 2011 7:03 pm
Posts: 247
Location: Sheridan,Wy., 82801
I have a 12v6gt audio output tube tied to a 5K ohm to 8 ohm output xfmr. I am trying to calculate what value cathode resistor to use with my tube. I see, in the RCA tube manual, that a Class A amplifier, operating at 180 volts, should use a -8.5 volts as the grid bias voltage. The max output for this particular setup is 2 watts. I cannot come up with these numbers! My power xfmr, which is an isolation xfmr, followed by a bridge rectifier, a 22 mf electrolytic cap., a 30 henry choke, another 22 mf electrolytic cap, and a 1 meg ohm bleed down resistor, shows + 186 volts. I have arrived at the values below, but my output wattage is 8 watts. The norm for this particular setup, using the prescribed 180 volts,is 2 watts. With my 186 volts, I came up with this: 186+ volts from power xfmr, -9 volts for screen grid voltage bias = 177 working volts, 11.2 ma current + 35 ma current rise from my 177/5k ohm/8 ohm xfmr, which totals 46 ma , and 117 volts x .046 amps = 8.14 watts The 8.14 watts is well below the max of 12 watts for the tube setup with 250 volts on the plate, and my -9 volts on the control grid is well below the -13volts on the 250 plate volt version. So. The thing should not red plate on me. In the opinion of a learned elmer, is this setup ok. I should have no problem with the bypass cap calculation! Thanks Lenny


Top
 Profile  
 
 Post subject: Re: load line calculation questions
PostPosted: Nov Mon 06, 2017 6:15 pm 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 36072
Location: Livermore, CA
Hi Lenny

This reminds me of school where extra information is supplied not needed to solve a problem.

Tube manual shows for 180 volt operation a 12V6 will draw 32 ma, plate & screen. This requires a cathode resistor around 270 ohms to give -8.5 volts grid bias.

_________________
Norm


Top
 Profile  
 
 Post subject: Re: load line calculation questions
PostPosted: Nov Mon 06, 2017 10:39 pm 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 20678
Location: Annapolis, MD
I can't find any tube specs that show typical cathode-bias numbers. <<EDIT: I mistakenly had P-P here>>

If you pick an operating point based on the fixed-bias specs, and then implement it with cathode bias, then the behavior will be different from the published data---unless you bypass the cathode resistor.

_________________
-Mark http://pixellany.com

"It's always something". --Gilda Radner (1946 - 1989)


Last edited by pixellany on Nov Tue 07, 2017 7:02 am, edited 1 time in total.

Top
 Profile  
 
 Post subject: Re: load line calculation questions
PostPosted: Nov Tue 07, 2017 2:56 am 
Moderator
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 22134
Location: Somers, CT
AC signals are 180 degrees out of phase between P-P cathodes. What would the bypass cap add?

Pete

_________________
Just because it can be done doesn't mean it should be done.


Top
 Profile  
 
 Post subject: Re: load line calculation questions
PostPosted: Nov Tue 07, 2017 3:43 am 
Member
User avatar

Joined: Sep Mon 16, 2013 2:42 am
Posts: 2414
Location: Tucson, Arizona U.S.A.
Where did push-pull operation come from? The OP sounds like he has a single ended output stage.

I don't see where the numbers he gives are coming from. But what it looks like is that he is calculating plate dissipation rather than output power. Also, the amount of output power available depends on how much distortion can be tolerated so that needs to be part of the calculations too.

_________________
Jim Mueller


Top
 Profile  
 
 Post subject: Re: load line calculation questions
PostPosted: Nov Tue 07, 2017 3:49 am 
Moderator
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 22134
Location: Somers, CT
I read as he was trying to figure how to set the bias voltage based on what cathode resistance was needed, but there was a lot of other information that sort of confused me as to what was being asked.

_________________
Just because it can be done doesn't mean it should be done.


Top
 Profile  
 
 Post subject: Re: load line calculation questions
PostPosted: Nov Tue 07, 2017 6:59 am 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 20678
Location: Annapolis, MD
Sigh......Yes---single-ended!!
editing my post

_________________
-Mark http://pixellany.com

"It's always something". --Gilda Radner (1946 - 1989)


Top
 Profile  
 
 Post subject: Re: load line calculation questions
PostPosted: Nov Tue 07, 2017 7:11 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 31406
Location: Maryland 20709, USA
pixellany wrote:
IIf you pick an operating point based on the fixed-bias specs, and then implement it with cathode bias, then the behavior will be different from the published data---unless you bypass the cathode resistor.
I've never seen a cathode bias resistor without a bypass cap.

If the bypass cap is properly sized, the bias voltage is created by the zero-signal idle current.

- Leigh

_________________
73 de Leigh W3NLB
http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


Top
 Profile  
 
 Post subject: Re: load line calculation questions
PostPosted: Nov Tue 07, 2017 7:20 am 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 20678
Location: Annapolis, MD
lenny wrote:
I have a 12v6gt audio output tube tied to a 5K ohm to 8 ohm output xfmr. I am trying to calculate what value cathode resistor to use with my tube. I see, in the RCA tube manual, that a Class A amplifier, operating at 180 volts, should use a -8.5 volts as the grid bias voltage. The max output for this particular setup is 2 watts. I cannot come up with these numbers!


You are mixing apples and cucumbers.....

For the stated conditions, the 2 watts is the maximum AC power that gets delivered. The suggested load is the speaker impedance times the impedance ratio of the transformer. With the transformer you stated, your speaker would have to be 8.8 ohms to exactly match the specified 5500 ohm load. This is a compromise between power and distortion (look at the curves for 250-volt operation)

None of the above has anything to do with DC power. For the DC analysis, use the plate and screen currents and voltages, plus the DC power lost in the OPT primary---all of this is for the zero-signal case. the data sheets also give you the currents for maximum signal. regardless, it is power dissipated in the tube, which has nothing to do with the AC power delivered to the speaker.

_________________
-Mark http://pixellany.com

"It's always something". --Gilda Radner (1946 - 1989)


Top
 Profile  
 
 Post subject: Re: load line calculation questions
PostPosted: Nov Tue 07, 2017 7:22 am 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 20678
Location: Annapolis, MD
Leigh wrote:
pixellany wrote:
IIf you pick an operating point based on the fixed-bias specs, and then implement it with cathode bias, then the behavior will be different from the published data---unless you bypass the cathode resistor.
I've never seen a cathode bias resistor without a bypass cap.

If the bypass cap is properly sized, the bias voltage is created by the zero-signal idle current.

- Leigh

I have seen quite a few, but the (profound?) point was simply that cathode bias does not give you the expected AC behavior unless it is bypassed.

_________________
-Mark http://pixellany.com

"It's always something". --Gilda Radner (1946 - 1989)


Top
 Profile  
 
 Post subject: Re: load line calculation questions
PostPosted: Nov Tue 07, 2017 7:27 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 31406
Location: Maryland 20709, USA
pixellany wrote:
Leigh wrote:
pixellany wrote:
IIf you pick an operating point based on the fixed-bias specs, and then implement it with cathode bias, then the behavior will be different from the published data---unless you bypass the cathode resistor.
I've never seen a cathode bias resistor without a bypass cap.

If the bypass cap is properly sized, the bias voltage is created by the zero-signal idle current.

- Leigh
...the (profound?) point was simply that cathode bias does not give you the expected AC behavior unless it is bypassed.
Of course.

That's why they're all bypassed. :eek:

- Leigh

_________________
73 de Leigh W3NLB
http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


Top
 Profile  
 
 Post subject: Re: load line calculation questions
PostPosted: Nov Tue 07, 2017 7:34 am 
Member
User avatar

Joined: Jul Mon 26, 2010 8:30 pm
Posts: 20678
Location: Annapolis, MD
I'm not going looking for exceptions at this hour, but at least we agree on the effect of bypassing.....:)

_________________
-Mark http://pixellany.com

"It's always something". --Gilda Radner (1946 - 1989)


Top
 Profile  
 
 Post subject: Re: load line calculation questions
PostPosted: Nov Tue 07, 2017 7:36 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 31406
Location: Maryland 20709, USA
pixellany wrote:
...at least we agree on the effect of bypassing.....:)
Yep.

And the effect of a good night's sleep. Nighty night.

- Leigh

_________________
73 de Leigh W3NLB
http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


Top
 Profile  
 
 Post subject: Re: load line calculation questions
PostPosted: Nov Tue 07, 2017 9:04 pm 
Moderator
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 22134
Location: Somers, CT
I've single ended audio stages that were not bypassed in an effort to apply negative feedback to improve linearity at the expense of gain and other parameters. Not common, but it was done.

I have seen a few schematics in this forum that show cathodes bypassed in push pull amplifier stage designs but I have never seen a good justification for that practice.

Pete

_________________
Just because it can be done doesn't mean it should be done.


Top
 Profile  
 
Post New Topic Post Reply  [ 14 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 9 guests



Search for:
Jump to:  


















Privacy Policy :: Powered by phpBB