Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
Tube rectifier filament connection




 
The AM Forum
May 24, 2020, 06:36:36 PM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: Tube rectifier filament connection  (Read 574 times)
0 Members and 1 Guest are viewing this topic.
kb3ouk
Member

Offline Offline

Posts: 1610

The Voice of Fulton County


« on: April 26, 2020, 01:59:05 PM »

If you have a directly heated rectifier tube, what is the best or correct way to hook up the filament transformer to the filter? Obviously with a non tapped transformer you just go to one leg, but with a center tapped transformer I've seen the center tap be used to pull off the rectified voltage, but I've also seen schematics with a center tapped transformer float the tap and use one of the legs.
Logged

Clarke's Second Law: The only way of discovering the limits of the possible is by venturing a little past them into the impossible
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Online Online

Posts: 1484


« Reply #1 on: April 26, 2020, 07:11:38 PM »

I dont see any problem and since it is center-tapped there should be fairly good DC current balance to both filament sides.


Phil

* Filament Transformer with CT.pdf (20.54 KB - downloaded 31 times.)
Logged

Charlie Eppes: Dad would be so happy if we married a doctor.
Don Eppes: Yeah, well, Dad would be happy if I married someone with a pulse.NUMB3RS   Smiley
W2PFY
Contributing
Member
*
Online Online

Posts: 13148



« Reply #2 on: April 27, 2020, 10:30:23 PM »

Quote
but I've also seen schematics with a center tapped transformer float the tap and use one of the legs.

I think you need to re-word what your trying to say? But if I read the above correctly, that sounds like a half wave rectifier. R U talking about the filament transformer center tap that is not used or the plate transformer?

Not trying to be a dick, just trying to understand?

Terry
Logged

The secrecy of my job prevents me from knowing what I am doing.
Opcom
Patrick J. / KD5OEI
Contributing
Member
*
Offline Offline

Posts: 7527



WWW
« Reply #3 on: April 29, 2020, 10:50:08 AM »

There are many factors, many 'rules', as whether or not a center tap is needed or even wanted, and also how filaments should be connected.

The CT is the cadillac, the untapped winding is the chevrolet.

The entire thing relates to either the voltage drop across the filamentary cathode as a percentage of output voltage, or, the way the current is distributed from the filament, or added internal features like a shield (as seen in gas and MV rectifiers).

CT is more important for durability and getting the full ratings in low-forward-drop, high voltage/high current rectifiers (usually the unshileded gas/vapor types).

CT is more important for hum reduction in lower-voltage vacuum rectifiers from the smaller ones to the 5R4/5U4 sizes. Compare the filament voltage to the DC output voltage, percentage wise. Obviously a better filter will remove more hum therefore a CT is not always used.

CT is important in all rectifiers when they are operated close to their current rating or when a cut-cost filter is used.

A center tap should be used for vacuum rectifiers and for unshielded gas and vapor rectifiers, but is not always done.

In case of rectifiers having a shield which surrounds the filament and is connected to one side of it, a center tapped filament winding  makes no difference because the shield side of the filament is to be used for DC output.

For filament-type gas and mercury-vapor rectifiers having a shield which surrounds the filament and is connected to one side of it, the DC output should be taken from the terminal that is connected to the shield-side of the filament.

When multiple rectifiers having filament type cathodes are used, and the filaments are in parallel, like filament pins should be connected together.

See RCA TT-4 or TT-5 transmitting tube manual, and, the RCA RC-xx receiving tube manuals for many paragraphs on the topic of rectifier connections and center taps. 

Logged

Radio Candelstein - Flagship Station of the NRK Radio Network.
kb3ouk
Member

Offline Offline

Posts: 1610

The Voice of Fulton County


« Reply #4 on: May 16, 2020, 09:56:47 PM »

Quote
but I've also seen schematics with a center tapped transformer float the tap and use one of the legs.

I think you need to re-word what your trying to say? But if I read the above correctly, that sounds like a half wave rectifier. R U talking about the filament transformer center tap that is not used or the plate transformer?

Not trying to be a dick, just trying to understand?

Terry

The filament transformer for the rectifiers had a center tap but instead of it being used as the take off point for the rectified high voltage, the center tap was shown as not being connected to anything, and the filters being hooked to one of the legs of the secondary. This was a full wave rectifier.
Logged

Clarke's Second Law: The only way of discovering the limits of the possible is by venturing a little past them into the impossible
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Online Online

Posts: 1484


« Reply #5 on: May 17, 2020, 12:53:14 PM »

Below is a circuit in which the Power transformer has a Center-Tapped secondary HV winding and a Center-Tapped secondary for the filament winding, with the current flow indicated.

Let's say the secondary voltage of the HV winding is 1,000 volts. Each plate will see a positive and negative 500 volt sine wave because the transformer is center-tapped. Of course, each plate will only conduct on the positive portion of the sine wave and thus a series of 120 Hz positive "bumps" will appear on the left side of the choke.The peak voltage at this point will be Vfil = 1.414 X 500Vac - Rplate/Current. The final voltage will be Vfinal =~ Vfil - Rchoke/Current.

Now the filament transformer. Obviously it is supplying filament power with a voltage of 5Vac, 6.3vac, or whatever. This secondary is simply acting as a conduit for DC current flow from ground, up through the filtering, then to the filaments, and then current flow via the tube's internal space charge. Current returns to ground via the Center Tap of the HV secondary.

It doesn't matter if the return DC current is connected to one side of the filament transformer or the center tap. The filament transformer is simply acting as a DC current return to the filaments.

Connecting the filament transformer's center tap to the choke is a personal preference because it offers a more balanced DC current flow through the filament transformer.

In either connection, the filament transformer and the HV transformer are conducting both AC and DC currents at the same point in time.

Phil


* Filament CT Transformer Non-Cathode PS.pdf (30.09 KB - downloaded 27 times.)
Logged

Charlie Eppes: Dad would be so happy if we married a doctor.
Don Eppes: Yeah, well, Dad would be happy if I married someone with a pulse.NUMB3RS   Smiley
kc2we
Member

Offline Offline

Posts: 101


« Reply #6 on: May 22, 2020, 08:32:21 AM »

Another strategy used by the "old timers" is to use a resistor across the filament transformer with a resistor center tap. use two resistors of equal value or an Ohmite "divide-ohm" with tap in middle. Two 10 ohm 5 watters and tap the middle. of course depending on filament voltage, the transformer needs to handle some extra current. At 5 volts 20 ohms draws about 1/4 amp filament current which is insignificant. Also, DC IR drop is minimal. ST
Logged

Seth Taylor
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Online Online

Posts: 1484


« Reply #7 on: May 22, 2020, 04:11:47 PM »

Another strategy used by the "old timers" is to use a resistor across the filament transformer with a resistor center tap. use two resistors of equal value or an Ohmite "divide-ohm" with tap in middle. Two 10 ohm 5 watters and tap the middle. of course depending on filament voltage, the transformer needs to handle some extra current. At 5 volts 20 ohms draws about 1/4 amp filament current which is insignificant. Also, DC IR drop is minimal. ST

But that was mainly used for hum reduction in audio circuits and is not needed in rectifier circuits.


Phil
Logged

Charlie Eppes: Dad would be so happy if we married a doctor.
Don Eppes: Yeah, well, Dad would be happy if I married someone with a pulse.NUMB3RS   Smiley
kc2we
Member

Offline Offline

Posts: 101


« Reply #8 on: May 22, 2020, 04:20:25 PM »

If hum (ac ripple) is on the DC depending on filament imbalance will cause hum on a class C plate modulated amp or audio stages. The AC ripple will be introduced into the B+. The filtering past the rectification might take some hum out if present. But most power supply filters are designed for 120 Hz ripple elimination, not 60 Hz. So results /performance may be unpredictable. If it's for CW or RTTY, then doesn't matter. Although FCC part 97 regs specifies purity of emissions below 144 MHz. Don't recall the limits. ST
Logged

Seth Taylor
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.061 seconds with 18 queries.