Hello gentlemen, I need to replece the 22.5 bias battery from my Elmac AF-68. This is an oddity on this units so I was adviced to place a Zener diode instead of the battery.
The problem here is that I have no experience with diodes and Im not sure how to place it.
I need 22 negative volt so I think to place it with the band side attached to the chassis/grond.
¿is this right?

Milton

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
That is correct and I hope you have computed out its dropping resistor properly also. A tad of negative polarized filtering is desirable also, in case the unit is feeding to grid circuit, as zeners introduce in a bit of shot noise.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

:Hello gentlemen, I need to replece the 22.5 bias battery from my Elmac AF-68. This is an oddity on this units so I was adviced to place a Zener diode instead of the battery.
:The problem here is that I have no experience with diodes and Im not sure how to place it.
:I need 22 negative volt so I think to place it with the band side attached to the chassis/grond.
:¿is this right?
:
:Milton

Zener diodes pass current when it rises above the zener voltage. So, that said, if you want to regulate a grid so that it's 22 volts negative with respect to the cathode, you should place a resistor in series with the cathode so that a little more than 22 volts is dropped across the resistor (the cathode will be positive with respect to B-). Then parallel the resistor with a 22 volt zener if you can find such a thing. You may need to take a slightly lower value zener and put a resistor in series with it. The zener will bleed off any voltage on the cathode that rises above 22 volts. Perhaps you can also put two 12 volt zeners in series to equal 24 volts? I don't know if such a thing is possible. I doubt it. someone else on here will know.

If your power supply is fairly well regulated, you can get by with just a resistor and no zener diode. Detectors are fussy, but amplifiers aren't that fussy. If you have a picky circuit, what you must worry about is a surge in your household voltage. If a refrigerator or other appliance kicks in or out, this will shift line voltage, which will also shift power supply voltage if it isn't well regulated. If you're using batteries, this isn't a problem. So....if you can get by with just a resistor, then go for it. I wish I knew what kind of circuit you were dealing with. If the circuit uses filamentary type tubes, biasing grids is different. Instead of biasing the cathode of the tube you need a negative grid bias at, you must place a resistor in series with the main B- wire that feeds the radio and obtain your negative voltage here. Using several resistors will allow you to obtain several negative voltages. Using zener diodes of the required voltages will allow you to regulate perfectly if perfect regulation is not otherwise possible. In this case you'd connect zener diodes of the required voltages from the various tap points to B-. The reason why you can't bias each cathode separately with filamentary tubes is that usually the filaments are all fed from the same A source, which means that they're all connected together. It'd be easier to explain on a schematic. Hope you get the picture.

Thomas

Guys, if you want to see the circuit here is the link to see the part of the circuit involved in this disscussion.

http://pg.photos.yahoo.com/ph/milton_valerio/detail?.dir=/fa61&.dnm=13bf.jpg&.src=ph

the tubes are 6L6GB
I have a 22 volt Zener diode ready to put in the circuit if its the best choice.
Hope this help.

Milton

:Zener diodes pass current when it rises above the zener voltage. So, that said, if you want to regulate a grid so that it's 22 volts negative with respect to the cathode, you should place a resistor in series with the cathode so that a little more than 22 volts is dropped across the resistor (the cathode will be positive with respect to B-). Then parallel the resistor with a 22 volt zener if you can find such a thing. You may need to take a slightly lower value zener and put a resistor in series with it. The zener will bleed off any voltage on the cathode that rises above 22 volts. Perhaps you can also put two 12 volt zeners in series to equal 24 volts? I don't know if such a thing is possible. I doubt it. someone else on here will know.
:
:If your power supply is fairly well regulated, you can get by with just a resistor and no zener diode. Detectors are fussy, but amplifiers aren't that fussy. If you have a picky circuit, what you must worry about is a surge in your household voltage. If a refrigerator or other appliance kicks in or out, this will shift line voltage, which will also shift power supply voltage if it isn't well regulated. If you're using batteries, this isn't a problem. So....if you can get by with just a resistor, then go for it. I wish I knew what kind of circuit you were dealing with. If the circuit uses filamentary type tubes, biasing grids is different. Instead of biasing the cathode of the tube you need a negative grid bias at, you must place a resistor in series with the main B- wire that feeds the radio and obtain your negative voltage here. Using several resistors will allow you to obtain several negative voltages. Using zener diodes of the required voltages will allow you to regulate perfectly if perfect regulation is not otherwise possible. In this case you'd connect zener diodes of the required voltages from the various tap points to B-. The reason why you can't bias each cathode separately with filamentary tubes is that usually the filaments are all fed from the same A source, which means that they're all connected together. It'd be easier to explain on a schematic. Hope you get the picture.
:
:Thomas

What you do is connect the center tap of the transformer to the chassis and then put a 5 or 10 watt resistor between the cathodes and the chassis (they no longer connect directly to the chassis). Choose a resistor that'll give you a little more than a 22 volt drop. Then place the zener across the resistor so that it maintains exactly 22 volts at the cathodes. The zener, of course, must be inserted properly with respect to polarity (as I recall, reverse of a normal diode).

Resistor biasing without a zener is actually ideal, as this will change with load change. The bias will automatically adjust. An amplifier operating at a lower voltage requires less of a bias than one operating at a higher voltage. If your power supply is unregulated, overall voltage shifts can occur when plugging into various locations, and with various other appliances on a circuit (toasters, refrigerators, etc.). Some prefer the perfect zener bias. Go for it if you like it. Also, a capacitor in parallel with the resistor-zener may improve fidelity.

You may use a zener all by itself without the resistor, but it may not be able to handle the full current. If it shorts, you may blow tubes depending upon the circuit.

Thomas

------------------------------------------------------------------------------------------------------------

I thought that model number sounded vaguely familiar from many-many years ago. That unit will be using 6L6’s as the modulator tubes along with the old common dry cell battery providing your 1st grid bias. I have long since forgotten if the unit used 6146(s) for the RF final(s)or could they have chosen the 6JE6 / 6LQ6 sweep tubes? Now since that old common /standard battery served you well for muuuuuultitudes of years as your bias source, what I might suggest is the mechanical mounting of hand fabbed aluminum “rectangular “C’ clips with an encircling loop of tape to hold two common rectangular 9 Vdc batteries and their clip on terminal connectors and leads along with a tandem holder for 2 AAA cells so the series string will add up to ~21Vdc. Since I am sure your old plain battery had been gradually drifting down in value and shifting the biasing operation , progressively heavier towards deep class A operation.
The structural mass of the cell construction of the multi cells in a premium alkaline 9 V units equates to a very-very-very close match to individual AA cells, if you have ever torn one down for electro / mechanical comparison and evaluation. As well, you can also see the liberal shelf life ratings in years that those premium alkalines profess on their side markings. Now, here you are , not going to be pulling more than sub ma consumption in their use.

I have followed the same suit in my Triplett 630 NA analog VOM situation wherein a 30 Vdc , like battery, is required for the higher range ohms scales. I used three 9V alkaline series connected batts to get 27 Vdc for replacing the original 30Vdc battery. No problems in the capability of zeroing the ohms function in those higher ranges whatsoever. That even being with the actual slight current loading of the battery on the lower ohm ranges. 4 years and counting on the set I have in there currently.Your situation of merely feeding grid bias is a piece of cake in comparison.

73’s de Edd

http://forums.teamphoenixrising.net/images/smilies/fart.gif

------------------------------------------------------------------------------------------------------------

:What you do is connect the center tap of the transformer to the chassis and then put a 5 or 10 watt resistor between the cathodes and the chassis (they no longer connect directly to the chassis). Choose a resistor that'll give you a little more than a 22 volt drop. Then place the zener across the resistor so that it maintains exactly 22 volts at the cathodes. The zener, of course, must be inserted properly with respect to polarity (as I recall, reverse of a normal diode).
:
:Resistor biasing without a zener is actually ideal, as this will change with load change. The bias will automatically adjust. An amplifier operating at a lower voltage requires less of a bias than one operating at a higher voltage. If your power supply is unregulated, overall voltage shifts can occur when plugging into various locations, and with various other appliances on a circuit (toasters, refrigerators, etc.). Some prefer the perfect zener bias. Go for it if you like it. Also, a capacitor in parallel with the resistor-zener may improve fidelity.
:
:You may use a zener all by itself without the resistor, but it may not be able to handle the full current. If it shorts, you may blow tubes depending upon the circuit.
:
:Thomas

Also...Radio Shack used to sell that 22 1/2 volt battery (looks like an AA battery). I bought the last one at one of my local stores. I guess they don't stock them anymore...maybe your store has one left. Furthermore, Batteries Plus DOES stock them, and they stock 67 1/2 volt batteries. They stock a lot of very interesting stuff. Not sure how many people buy 67 1/2 volt batteries.

t.

Thank you both guys for your kindly help.
Well Thomas, That probably would be valid for United States Radio Shack stores but I am in Costa Rica and Radio Shack stores here havent the big inventary that US shops, what I mean is that it is virtually impossible to find here some 22.5 battery.
Anyway AES have it and I dont feel it is an expensive one $7.00 but if I can modify the circuit and forget about batteries I prefer that.
thank you again for your valuable help.

Milton Valerio

:Also...Radio Shack used to sell that 22 1/2 volt battery (looks like an AA battery). I bought the last one at one of my local stores. I guess they don't stock them anymore...maybe your store has one left. Furthermore, Batteries Plus DOES stock them, and they stock 67 1/2 volt batteries. They stock a lot of very interesting stuff. Not sure how many people buy 67 1/2 volt batteries.
:
:t.

Do try to modify the circuit....it will perform well without the battery. The only problem with a zener and a dropping resistor is that there will be 22 volts less flowing through the tube in order to build up the 22 volt differential between the cathode and the grid.

T.

Thomas, I understanded well all the instructions, but I have a doubt. what I need is to bias the grids of the 6L6GB’s with the -22 volt.
Are this info right for achieve this?

Milton

:Do try to modify the circuit....it will perform well without the battery. The only problem with a zener and a dropping resistor is that there will be 22 volts less flowing through the tube in order to build up the 22 volt differential between the cathode and the grid.
:
:T.

Guys,

Assuming this is an AC operated rig, and room permitting, could possibly use a small 120/24 volt transformer and make a bias supply using a diode some resistors and a couple of caps. I have also seen bias supplies incorporated into equipment using a centertap grounded fullwave power supply by tapping off half of the power transformer secondary thru a capacitor and resistor to a diode and a pi type filter.

Radiodoc

:Thomas, I understanded well all the instructions, but I have a doubt. what I need is to bias the grids of the 6L6GB’s with the -22 volt.
:Are this info right for achieve this?
:
:Milton
:
::Do try to modify the circuit....it will perform well without the battery. The only problem with a zener and a dropping resistor is that there will be 22 volts less flowing through the tube in order to build up the 22 volt differential between the cathode and the grid.
::
::T.

Milton, when you put the cathodes 22 volts positive with respect to the chassis--the thing that the grids are tied to through the transformer, you effectively make the grids 22 volts negative with respect to the cathodes. Look at it any way you want. The circuit makes the cathodes 22 volts positive with respect to the grids, and makes the grids 22 volts negative with respect to the cathodes.

Also, regarding Radiodoc's suggestion for a separate transformer supply, I don't think that this is necessary....22 volts less to the cathode isn't likely to make any difference in audio reproduction.

Thomas

:Milton, when you put the cathodes 22 volts positive with respect to the chassis--the thing that the grids are tied to through the transformer, you effectively make the grids 22 volts negative with respect to the cathodes. Look at it any way you want. The circuit makes the cathodes 22 volts positive with respect to the grids, and makes the grids 22 volts negative with respect to the cathodes.
:
:Also, regarding Radiodoc's suggestion for a separate transformer supply, I don't think that this is necessary....22 volts less to the cathode isn't likely to make any difference in audio reproduction.
:
:Thomas

Thomas,

I agree with you that my suggestion was pitiful after looking at some pictures of an AF68. It is very compact inside and uses a separate power supply. Inside and out it is one beautiful piece of equipment. With -22.5 volts of bias, the 6L6's are probably being operated AB1.

Radiodoc

Not a pittiful suggestion. Quite a good one, I think. I just don't think that it will be necessary. If a resistor-zener doesn't work, and he doesn't want to go back to a battery, though, the power supply will be an excellent one. I think that the resistor-zener should work fine, though.

Thomas

:Not a pittiful suggestion. Quite a good one, I think. I just don't think that it will be necessary. If a resistor-zener doesn't work, and he doesn't want to go back to a battery, though, the power supply will be an excellent one. I think that the resistor-zener should work fine, though.
:
:Thomas

Yo Thomas,

If you would like to take a look at an Elmac AF68, may want to take a look http://www.miami.muohio.edu/president/personal/w8zr/vintage/transmitters/af68.htm

Radiodoc

It truely is compact. Interesting transmitter!

T.