Powering tupe preamp

[alexidoia]

Hi there,
I am looking to know what power VA need a tube in a mic preamp stage.
I am sorry for this newbies question but I would like to understand how does a tube preamp works.
Can someone help of redirect me into a tutorial about tube preamplification ?
Thanks
Alex

 

[abbey road d enfer]

There was a book "Learn all about tubes in two weeks", but it's out of print ;D
Seriously, a typical preamp triode expect to see ca 150v on its plate, with 1mA of current, and you need a plate resistor of about 100k, so overall, you need 250v x 1mA, that's 1/4Watt.

 

[alexidoia]

Thanks for the reply.
Actually here is what I am trying to figure out. I have a 351 from ampex that I have modified according to this schematic:
http://greg.electrical.com/351-photos/ea351.jpg

It works well and I love it.
What I would like to do is to build a unit with 4 of this in one 2u rack. Thus I need to know what VA I need to get out the transformer and what transformer I need.
Since we are both french maybe I can pm you ?

I would appreciate any help on that project.
Thanks
Alex

 

[PRR]

> what power VA need a tube in a mic preamp stage

That's much too vague. There are battery-powered mike preamps which run on flashlight cells, and monster preamps which take many Watts.

> 351 from ampex that I have modified

Not enough data on schematic.

Some is clear: the heaters need power and you can look-up the requirements in a tube manual. I happen to remember that 12AX7 and 12AU7 need 6.3V at 0.3A or 12.6V at 0.15A. Your schematic shows a rectified DC power supply marked "12V", so you need 12VDC at 0.45A per channel, 12VDC 1.8A for four channels.

The Phantom supply has been hacked-up from an available winding. If you are not going to use the Ampex power transformer, just get a 48V power supply with 10mA per channel.

The plate power demand is not clear. You "could" be running the tubes at 90V and very low current. However 250V-300V at C16 is much more likely. At C26, maybe 350V-450V.

The 12AX7 stages have 100K resistors and can't pull more than 300V/100K= 3mA; in fact they will pull half or less, say 1.5mA each or 6mA for V1 and V4. The 12AU7 stage is trickier to figure; I tell you it is 10mA per side or 20mA for V2. Total 26mA per channel. 104mA for four channels.

Here is the answer: Fender Champ or Tweed DeLuxe replacement power transformer with a 5V3/5V4 rectifier, # 041316. Ignore the 6V winding. This will give at least 350V at 104mA load, and has the 5V winding for the 5V4. It does not have the other voltages needed, but it will be cheaper to use the very-available Champ transformer with generic 12VDC and 48VDC supplies than to order a custom transformer with the "right" windings.

 

[abbey road d enfer]

Thanks for the reply.
Actually here is what I am trying to figure out. I have a 351 from ampex that I have modified according to this schematic:
http://greg.electrical.com/351-photos/ea351.jpg

It works well and I love it.
What I would like to do is to build a unit with 4 of this in one 2u rack. Thus I need to know what VA I need to get out the transformer and what transformer I need.
Since we are both french maybe I can pm you ?

I would appreciate any help on that project.
Thanks
Alex

Sure you can PM me.
I'm a little concerned about the DC heater supply. Instead of a nice clean (?) AC voltage, they are seeing a very distorted spiky rectified voltage, with ca. 1 volt ripple with high frequency components that are too eager to pollute the grids via parasitic capacitance. I'd rather have AC heater voltage with a balancing pot, or cleaner DC. It's a non trivial issue; I've seen many poorly implemented heater rectification inducing bad hum. If you endeavour to build a quad unit, this will be a serious issue. The nice thing with non regulated DC heater is that there is (almost) no energy wasted in extra heat but it takes larger caps to prevent noise. I reckon you need to double the caps and split the 1.5ohms res to make the thing really clean. Also beware that it induces large return currents, making the grounding scheme quite critical.
The other option is to regulate the heater supply, which is done at the cost of energy wasted in the regulator, but then the voltage is much cleaner.

 

[alexidoia]

Thanks PRR,
However if I used a single secondary windings for the 4 channels, if I have a problem of load at some point with one channel will this not create a unbalanced current for the 3 remaining channels. Would it not be better to have insulated circuits?
Alex