Im just surprised to understand that the 12AX7 B+ consumes just about few milliamps but why do we need 75ma as stated here can anybody let me know how many ma of current rating only to power one channel of 12AX7 B+
To power one 12AX7/ECC83 (i.e. two triodes) you need almost 2VA for AC heaters and about 6VA for a cap input PSU for HT/B+. Some degree of overspecifying does no harm.
Where does it say that you need 75mA? All it says is that the specified transformer could deliver 75mA. You won't find transformers much below 50mA anyway.
The limiting parameter becomes wire thickness.
Even hair thin wire can provide 50mA or more, and thinner one becomes a b*tch to wind, it breaks so easily.
So even if you need 1mA, wire will be overspecified simply to design a reasonable transformer.
To boot, the smaller the core, the more (thousands) turns you need, complicating manufacturing even more.
Even hair thin wire can provide 50mA or more, and thinner one becomes a b*tch to wind, it breaks so easily.
So even if you need 1mA, wire will be overspecified simply to design a reasonable transformer.
To boot, the smaller the core, the more (thousands) turns you need, complicating manufacturing even more.
This schematic suggest a transformer ( which might be larger then needed).
A smaller one could be pa211 ( the transformer used in PAS3, available from
PA211 Power Transformer Dyna PAS for Dynaco Replacement MADE IN USA )
This transformer feeds 4 ecc83 + a pilot light ( just barely )
I've seen that "4S" pre schematic before. That schematic is design to be used with all of the 12**7 tubes interchangeably I believe. Some of them will draw more than the 12AX7's, but all should only be a couple mA. You certainly don't need 75mA.
This little guy is pretty decently made at a good price (have one in my parts collection right now):
Hammond Manufacturing - 6K88VG - LEADS 6.3V @ 2.0A 500VCT @40DCMA 50/60HZ PRI:110-120V TRANSFORMER - Allied Electronics
I've used these in tube amps before, good price, but overkill on the specs for your use:
AS-05T200 - 50VA 200V Transformer - AnTek Products Corp
If you use something other than the model spec'd, you may have to drop some volts in order to reach the 250V DC in the schematic. Duncan PSU designer software (google it) is free and is your best friend for this.
Also check Triad magnetics for some good inexpensive choke options (you can get a lot more than 2H for Hammond money).
This little guy is pretty decently made at a good price (have one in my parts collection right now):
Hammond Manufacturing - 6K88VG - LEADS 6.3V @ 2.0A 500VCT @40DCMA 50/60HZ PRI:110-120V TRANSFORMER - Allied Electronics
I've used these in tube amps before, good price, but overkill on the specs for your use:
AS-05T200 - 50VA 200V Transformer - AnTek Products Corp
If you use something other than the model spec'd, you may have to drop some volts in order to reach the 250V DC in the schematic. Duncan PSU designer software (google it) is free and is your best friend for this.
Also check Triad magnetics for some good inexpensive choke options (you can get a lot more than 2H for Hammond money).
so
heater 150ma if used in series
B+ 5ma each so for each tube 10ma
so in practical case can we double the rating for the trafo to be on the safer side?
heater 150ma if used in series
B+ 5ma each so for each tube 10ma
so in practical case can we double the rating for the trafo to be on the safer side?
"150mA in series" would imply 12.6V heater supplies. Don't forget you need 6.3V @ 1A of heater for the rectifier. It will probably be easiest and cheapest to go with something over-spec'd as they're more common (see Allied and Antek links in previous post) as well as comments by others.
If you search the Edcor catalog, you want a center tap on the HV and 6.3V heaters (.3A draw for a 12**7 plus 1A for the 6CA4, look for at least 1.5A, better 2A+). Probably anything between 350V and 400V with a center tap is going to work with this design (find a low DCR choke if on the low end of B+ voltage, add an extra filtering stage if too high).
Edcor PTX:
https://edcorusa.com/xpwr024
https://edcorusa.com/xpwr105
Chokes:
Low Frequency (Inductor) Chokes On Triad Magnetics
Sticking to 6.3V heaters also means you can play with other 9-pin tubes with different pin-outs down the road if you change some pin connections (6DJ8, 6CG7, etc). Note that this will probably mean re-optimizing the bias, but your chassis work and power transformer/choke/sockets can be reused. You need a 6.3V heater for the 6CA4 anyways.
If you want to redesign the power supply or go solid state rectified, some of this goes out the window (wouldn't need a CT with a SS full wave rectifier, could rectify and regulate the heaters, etc). Train of thought above is in the interest of changing as little as possible with the schematic as is (but saving some money on the transformers).
If you search the Edcor catalog, you want a center tap on the HV and 6.3V heaters (.3A draw for a 12**7 plus 1A for the 6CA4, look for at least 1.5A, better 2A+). Probably anything between 350V and 400V with a center tap is going to work with this design (find a low DCR choke if on the low end of B+ voltage, add an extra filtering stage if too high).
Edcor PTX:
https://edcorusa.com/xpwr024
https://edcorusa.com/xpwr105
Chokes:
Low Frequency (Inductor) Chokes On Triad Magnetics
Sticking to 6.3V heaters also means you can play with other 9-pin tubes with different pin-outs down the road if you change some pin connections (6DJ8, 6CG7, etc). Note that this will probably mean re-optimizing the bias, but your chassis work and power transformer/choke/sockets can be reused. You need a 6.3V heater for the 6CA4 anyways.
If you want to redesign the power supply or go solid state rectified, some of this goes out the window (wouldn't need a CT with a SS full wave rectifier, could rectify and regulate the heaters, etc). Train of thought above is in the interest of changing as little as possible with the schematic as is (but saving some money on the transformers).
If you take a closer look at the scematic, you will see 250v @ 1.8mA on the right side.
What do you want to use this circuit for?
What do you want to use this circuit for?
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Hifi preamp both for stereo and home theater use.
The reason why I was asking for 2A for heater is if we use 6 of them we need about 900ma of current for series heater.
I can get custom transformer done so no issues with standard offshelf.
Just needed the advice.
Here are some things I would consider:
1. The output will have incredibly low power, so its not going to be able to drive the amplified signal through any reasonable length of cable. It will definitely sound pretty bad. If you decide to stick with the 12ax7 you will definitely need an output buffer stage.
2. 12ax7 has an amplification factor of 100. I seriously doubt that you need so much amplification in a pre-amp. I would ditch it for a medium-mu or low-mu triode. Do you like the look of WWII tubes? How about a 6SN7 instead? Prefer the 9-pin noval? how about 6dj8?
3. The load resistor on the 12ax7 is only 100k which is too small a load relative to the plate resistance. This means your signal amplification will not be linear... this is easy to solve by simply chosing a different type of tube (i.e. one with a lower amplification factor). Believe me, you want a nice clean linear singnal amplification.
4. That naked cathode resistor won't work so well at maintaining bias. The standard solution is to by-pass the cathode with a capacitor, but this too has pitfalls. I won't go into too much detail here, but simple solutions exist that work extremely well here as soon as you chose to use a different, medium-mu or low-mu tube instead.
I could go on quite a bit, and I am sure that others will . Suffice it to say that far better pre-amp schematics exist than this one.
1. The output will have incredibly low power, so its not going to be able to drive the amplified signal through any reasonable length of cable. It will definitely sound pretty bad. If you decide to stick with the 12ax7 you will definitely need an output buffer stage.
2. 12ax7 has an amplification factor of 100. I seriously doubt that you need so much amplification in a pre-amp. I would ditch it for a medium-mu or low-mu triode. Do you like the look of WWII tubes? How about a 6SN7 instead? Prefer the 9-pin noval? how about 6dj8?
3. The load resistor on the 12ax7 is only 100k which is too small a load relative to the plate resistance. This means your signal amplification will not be linear... this is easy to solve by simply chosing a different type of tube (i.e. one with a lower amplification factor). Believe me, you want a nice clean linear singnal amplification.
4. That naked cathode resistor won't work so well at maintaining bias. The standard solution is to by-pass the cathode with a capacitor, but this too has pitfalls. I won't go into too much detail here, but simple solutions exist that work extremely well here as soon as you chose to use a different, medium-mu or low-mu tube instead.
I could go on quite a bit, and I am sure that others will . Suffice it to say that far better pre-amp schematics exist than this one.
That circuit isn't a hi-fi preamp. It looks more like a guitar preamp. Too much gain. Too high output impedance. Possibly too much distortion.rhythmsandy said:
A cathode resistor (bypassed or not) is an excellent way of maintaining bias for a small signal stage.soulmerchant said:
Oh, I didn't realize that the original poster was considering using it for microphone or phono signal input. If that is the case then I do agree (even though I would still not use this design).
I would avoid using this as a pre-amp with line-level signal input (or higher). There are just so many better ways to build a pre-amp. The 12ax7 is an excellent triode, but not when used this way.
As a pre-amp this schematic is not a winner.
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A 'universal preamplifiier' will be equally bad at doing all possible applications. There is a reason why people normally design different circuits for mike preamps, guitar preamps and line stages.
The only reason why Im considering the universal preamp is because of its single ended operation.
I see alot of people have built it and are happy with it.
May i know the reasons why the single ended preamp wouldnt be good? I agree the distortion is in the order of 1 to 1.5% range for certain tubes but i usually like the signature of the single ended design.
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