12B4-A Linestage

[sorenj07]

I'm not happy with my 6SN7 linestage and would like to use 12B4's instead - I have too much gain! I simmed like .3% THD in Tubecad with a grounded grid setup as follows:

100K input pot, 1M grid stopper, 750R cathode resistor, 5.6K 3W plate load, 1uF 400V/470K RC output. B+ 300V, 15mA per side.

I would like to use a regulated voltage supply. I found a simple-looking one here:

http://members.nuvox.net/~zt.robgrow...wersupply.html

I'd be using a 269EX for around 400-0, a bridge of 1N4007's, choke input to a 193C (20H 100mA 181R) and 550uF can cap. That gives me 360V into 30mA. Could i feed that into the regulated voltage supply PSU right at the 10K 5W resistor, and use the schematic from there? I can also put a 1K 3W after the choke to have 330V into 30mA.

http://members.nuvox.net/~zt.robgrow...upply/rps.html

Thanks for the help! This is my first foray into regulated supplies.

[ashok]

Why do you have a 1Meg grid stopper ? The usual values are MUCH lower I believe. Typically less than 10 K and often 1K or thereabout. A very large resistor in the grid should reduce hf performance due to the feedback through the miller capacitance .

My sim shows -3db at about 7.6Khz !! It should sound really dull !

You could also raise the plate resistor to 10 K ohms .

If you drop the grid stopper to 1K or IF you want , to 10 K when the 20Khz response comes up very well. Gain will be in the region of about 6 . Is that too much for you ?

You can drop it further by using a grid stopper of 47K ohms and connect the grid to the load ( ie after the output cap ) with a 220K resistor. You will get a gain of 2.4 ! Increasing the value of the 220K resistor will get you more gain.
Since the miller capacitance is still active but swamped by the feedback resistor , the HF doesn't suffer as much . However I suspect the HF distortion levels might have to be checked . We can't get anything for free !
The input impedance will drop and be less than 100K ohms. You could raise the input resistor to 1 Meg when you should get a Zin of about 70 Kohms.

You should get a good sounding line stage .

[sorenj07]

waaait, I think I misused the term "grid stopper" - I meant the resistor that goes from the grid of the tube to ground. I don't have any resistors in series from the 100K pot to the grid. TubeCAD doesn't show one either, and I worked with it to have very nice distortion and frequency response filters. I have no clue if those will work in real life, but hey...

I'm interested in using shunt regulation instead of series transistor regulation, is there a simple way to do this? I really don't know much about the theory... time to dust off my copy of Valve Amplifiers, I guess

Thanks for the help though, I'm still really feeling my way around here

[Peter M.]

Lots of good info in this thread :
Yet another 12B4 line stage, or is the 12B4 better than the Grounded Grid.....

[sorenj07]

Yeah I guess I don't have to reinvent the wheel. I'd glanced at that thread but I guess it warrants a closer look. In the meantime I figure I'll do some mods to try and reduce gain in my linestage, and fix some hum issues. I have PT and choke 1/2" away from each other, with iron going the same direction :O

[Sheldon]

Here's a shunt regulated power supply that I recently made (the zener symbols are actually OB2 gas reg tubes). I used a voltage regulator in front of a current regulator, in front of the reg. tubes. Overkill, but that way both voltage and current are limited when the amp is warming up. You could easily sub in your voltage regulator, and you could use an appropriatly sized resistor instead of the IXCP current regulator (though it is very simple to use, only a pot or resistor required). You would have a very quiet supply.

http://www.diyaudio.com/forums/showt...31#post1065231

Sheldon

[sorenj07]

Ummmmm. wow. 0_o. I don't have a clue how that thing works, but are there simple ways of getting an output of 300V 30mA? sorry for the hassle but honestly it's over my head.

Also, what program is that in?

[Sheldon]

Originally posted by sorenj07
Ummmmm. wow. 0_o. I don't have a clue how that thing works, but are there simple ways of getting an output of 300V 30mA? sorry for the hassle but honestly it's over my head.

Also, what program is that in?

Lots of ways to get 300V at 30mA. As I said, that power supply is overkill. I wanted to try several different things, and I was looking to protect against a variety of fault conditions. You can go much simpler and still have fine results. That one is for a phono amp and I wanted a very quiet supply.

If you want a shunt regulated supply, look at the 12B4 thread referenced above for some examples. Or (I'm by no means an expert) the supply you referenced looks fine for starters. If you want shunt regulation, just delete the large cap after the regulator and add a resistor in series to limit the current then a string of gas regulator tubes to ground to give you the output voltage you want. Plug in the values in your schematic and the folks here will help you find the right combination.

Also, if you want to lift the filament supply (recommended), a simple voltage divider from the B+ with a cap to ground will do fine. Again, what I show is a fancy device from Morgan Jones's book and maybe overkill for a regular preamp.l

The program I used to draw the schematic is called Microcap, and they have a free version.

Sheldon

[colin.hepburn2]

HI
How about this one
http://www.tubeaudiokits.com/LIBRARY/Marantz-7.htm
more here @
http://www.tubeaudiokits.com/Modules/modules-init.htm

[sorenj07]

The power supply at the bottom of the first link looks interesting. Unfortunately the Zener diodes aren't labeled, and my transformer is 190-0-190, which translates to 206-0-206 or 412-0 on my 125V mains. It's interesting to see more use of the TIP-50.. hopefully this is an indication of its reliability or ease-of-use.