• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Plate choke on a line stage?

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#121
Dear Poobah,

thank you so much for the help you're giving to me.

The only FETs I've got are:
2SD1426
2SK2545
2SK2666
SSP35N03 (yes, it's a mosfet, very obsolete)
IRF840 - plenty
BF256B - small, unusable for that, but plenty

But I've got many bipolars, like 3055s, BD***, TIP110, also 2SC4977 (that hi-volt one), also many BDX53C etc etc...

I've also just found big TO3 Germanium power transistors :bigeyes: like 2N251A...
 
#124
#125
Coming back to the phono preamp, take a look here:

http://www.videohifi.com/forum/topic.asp?ARCHIVE=&TOPIC_ID=27017

Here there is a Spice simulation of that RIAA curve, showing that's inaccurate.

An externally hosted image should be here but it was not working when we last tested it.

My RIAA

An externally hosted image should be here but it was not working when we last tested it.

Correct RIAA

It has been pointed out that the 33k resistor (R6 in this schematic) should be 3.3k, a typo.
An externally hosted image should be here but it was not working when we last tested it.


What do you think about it?
 
#129
Here I am, after a rapid trip to National's website.

I've read the application note, and I've found those 2 schematics:
An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


How to adapt these to the use of a Mosfet? And note that I haven't got a POWER trimmer (5W??? mmm my electronic shop surely doesn't have it), I do have precision multiturn pots.

edit: I only have LM317T. Is there any problem to use it? It would have the full Vout voltage on the tab... I'm planning to isolate it from the heatsink with a silicon pad.
 
#130
You can use the MOSFET is place of the Darlington, being sure to place a gate-stopper resistor (say, 220-470 ohms) in series with the gate lead, as close to the FET as possible. The 6.2V diode will be a bit marginal because of the higher Vgs of a FET; you can use something more like 8-12V there. I'll sketch something for you, if you like.

One other thing that Maida left off- put a reverse-biased diode from input to output of the 317 to protect it at turn-off.

For a fixed output voltage, you don't need a trimmer.
 
#133
An externally hosted image should be here but it was not working when we last tested it.


Here's the supply I had in mind. Questions:
- Is the raw supply ok or I need more caps / more decoupling?
- the 1N4742 is a 12V zener, I choosed it just because it was 12V. In reality, I should have in my parts bin some 6.2V zeners, I will series two of them.
- The IRF840 does need a fairly large heatsink. I have some...
- My simulation doesn't start (Multisim is a bad program or I can't use it properly), I wanted to calculate the proper value for the trimmer, to get 200/250 volts out. I would go by temptatives, I guess...
- Do I need that snubber net on the output?
- Do I made some other errors?

Thanks to all!!!
 
#136
You want to make sure that the ripple is low enough that you don't have regulator dropout on the ripple troughs. Past that, the reg will do a pretty darn good job of ripple attenuation.

In your case, the current draw is pretty low, something like 20mA. So the ripple will be quite low with those big filter caps, no worries. If the input to output voltage is 20V or higher, you've got more than enough margin.
 
#138
I think you should wire the floating pin on the pot back to the tapper. But I'm not sure. This is usually how a variable resistor is done with a pot.

Also if sim doesn't work there can be something wrong with the schematic and the real circuit may fail. Try to get a good sim before powering up the real thing. That's what I would do anyway.
 
#139
Dear friends,

the sim didn't work because of the aforementioned error in the schematic. Now it simulates, but I have some problems with it: here it is
 

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    psu simulation.gif
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#140
(sorry for the psychedelic colours)

well, you can see the dissipation of the mosfet and the output voltage. But if I connect a 33k resistor in place of the trimmer (more or less the same of the setting of the pot), why do I get a slowly ramping voltage that ends up at much more than the previous output voltage? I stopped the simulation when it was about 280V... 😕
 
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