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PostPosted: 02 Sep 2009, 23:00 
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Since I'm using mostly line level sources of around 2v., I was thinking about building the two-stage input amp. RCA circuit with a gain of 8.

This will be my first tube project, so I'm confused as to why the 12AU7 Tube Mini Preamp only uses about 150v. on the plates, and the 12AU7 tube preamp from the RCA manual uses 300v.?
Can you use 150v. on the RCA circuit, with the same value components listed, and it still work the same?

Another question which may sound pretty dumb:
Why can't you just take the standard 120v. household voltage, rectify it, and use it instead of using a 120v. transformer.

Thank you...


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PostPosted: 03 Sep 2009, 04:00 
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Hi ammel,

just a quick and short one...

The 2 circuits are very different to each other. The first is a "anode follower" and the other one a "cathode follower". They have different specifications on amplification, output impedance and damping factors. I would suggest to read some basic informations about tube circuits. That fills books to get a view on this differences. Maybe someone could describe it with a few words here...
Quote:
Can you use 150v. on the RCA circuit, with the same value components listed, and it still work the same?
No! The values must be calculated for every supply voltage to run the tubes in good conditions. The values affect to the proper operation.
Quote:
Why can't you just take the standard 120v. household voltage, rectify it, and use it instead of using a 120v. transformer.
Some manufacturers used that in the 40's. Not for a long time, because many people was seriously injured from using this devices! :hot: This is a risk for your life. The whole mains in the household then is connected to the device. I don't want imagine what happens, if the device has an short or a failure and the mains fuse doesn't react / blow. :tombstone:

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Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.


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PostPosted: 03 Sep 2009, 12:14 
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ammel68 wrote:
Why can't you just take the standard 120v. household voltage, rectify it, and use it instead of using a 120v. transformer.

:!: While there are projects out there that do this, it is very unsafe. :!:

Here is an example with a lucky end result: http://webpages.charter.net/dawill/tmor ... C5SET.html

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PostPosted: 03 Sep 2009, 15:59 
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Thanks for your replies!
I can now see the safety issues with using household voltage to power a project.

I'm glad to see that the second project is a cathode follower, as it is my understanding they have a lower output impedance which should work well with a higher impedance power amplifier input.

I'm not really looking for much in the way of gain, so I even thought about maybe a tube buffer circuit, but I really haven't found any good schematics for one that is easy-to-build.

Looks as though getting 300v. is going to be a little tricky to get as most transformers I've looked at are around 250-265v.. After rectification, that is going to be way more than 300v.
Could I use resistors to step it down to 300v., or is that not advisable?

I don't have a 500k ohm pot, so will a 100k pot work in place of the 500k shown in the RCA schematic?

Thank you...


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PostPosted: 03 Sep 2009, 16:19 
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You will want more than 300 after rectification. This way you can add more RC stages to your power supply filter circuit. Here is a good program for designing power supplies and you can search the forums and the net on what type you want, i/e RC, LC etc. http://www.duncanamps.com/psud2/index.html

One of the trickiest parts of building amps is taming hum. Good luck.


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PostPosted: 03 Sep 2009, 16:27 
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Thank you Roggom...I'll download that and play around with it.

Do you know if I can sub a 100k pot for the 500k?


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PostPosted: 03 Sep 2009, 21:26 
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The value of the pot should be selected to match your source. You mention 2V, is your source a CDP?

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PostPosted: 03 Sep 2009, 21:47 
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Yes, my sources are usually a cd player or a tuner.


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PostPosted: 03 Sep 2009, 22:00 
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If you look at the CDP documentation it may give you an idea for the load. 100k is a very safe bet and for a CDP 50k is likely good enough, perhaps even 20-25k would do. Basically all this will do is set the sensitivity of the pot (ie. how far you turn it up to get the desired level).

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PostPosted: 04 Sep 2009, 09:33 
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Hi ammel,

Quote:
Looks as though getting 300v. is going to be a little tricky to get as most transformers I've looked at are around 250-265v.. After rectification, that is going to be way more than 300v.
Could I use resistors to step it down to 300v., or is that not advisable?
250-265 is a good idea. Like roggom said: you'll want more than 300 after rect. to set up an C-R-C or C-R-C-R-C filter (if you don't want to use a choke). Each filter stage (pi-filter) steps down the voltage.
Quote:
I don't have a 500k ohm pot, so will a 100k pot work in place of the 500k shown in the RCA schematic?
A 100K is most common with tube amps. Now - there's a little point to think about. The pot on the "RCA" schematic also works as a "grid leak resistor" (500K to 1meg.) If you change the pot to 100K you should ad a 500K resistor after the pot - to ground. The leak resistor sets the input sensibility and the gride leakage drain. (imho - also important).

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Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.


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