12AU7 Preamp build

[revintage]

Measure with minus of your meter connected to ground. Ua could also be 150V, depends on B+.

 

[nkg]

And just a reminder to Alex everybody on this site at some stage had a "knowledge level below the minimum level " and that is why this site excels as the older more experienced can assist in keeping beginners safe , to do it on your own is far more dangerous . Keep up the good work Matt
Nigel

 

[mattthegamer463]

What you show in your picture is almost what I have (150V instead of 160-180) so I'm afraid I'm still not quite sure what I'm looking for. Ill measure my cathode voltage in relation to ground when I get home.

 

[revintage]

Hey Matt,
Dont mind measuring anymore, your working point is OK ! 150 or 180, it is just a matter of tube specification spread. No need to measure cathode voltage.

Listen instead! Does it sound OK, is gain OK?

 

[mattthegamer463]

It works!!

In a word, I'm amazed. Its such a tangle right now... but I'm quite happy that it does work and sounds pretty decent, as far as I can tell. Its hard to say with only one channel and some very improvised connections to hook it up at this stage, and my ears are pretty far from audiophile.

 

[mattthegamer463]

So I was busy for about 3 weeks and didn't have any time to work on this project, but then I just sat down and completed the whole thing in one night. Here are the pictures.



















I want to thank Revintage for the incredible amount of help he gave me. Without you, this project wouldn't even be started. Thank you so much.

Thanks to everyone else who gave me useful tips on what to do.

 

[Alex Kitic]

And just a reminder to Alex everybody on this site at some stage had a "knowledge level below the minimum level " and that is why this site excels as the older more experienced can assist in keeping beginners safe , to do it on your own is far more dangerous . Keep up the good work Matt Nigel

It is very nice that some of you have enough time and patience to do "electronics kindergarten" (no pun intended, indeed). I am just too old and too busy to do that -- and as far as kindergarten goes, my children actually go to kindergarten so I have to preserve all my patience for them.

I am also to blame for being self taught, and for having done my studies and my intiations without help from others: basically, I never needed it.

Last but not least, CONGRATULATIONS, MATT! No piece of equipment sounds better than the one you build by yourself! With this gesture you have joined the DIYers as opposed to the consumerists.

 

[mattthegamer463]

I want to do some oscilloscope testing, using my PC as a signal generator, just to see what kind of waves this thing makes. Since its a preamp, do I need to be concerned about dummy loads or can I just hook the outputs to my oscope leads?

 

[kevinkr]

I want to do some oscilloscope testing, using my PC as a signal generator, just to see what kind of waves this thing makes. Since its a preamp, do I need to be concerned about dummy loads or can I just hook the outputs to my oscope leads?

No dummy loads required, just connect it to your scope. (If you want to be particularly accurate you might want to load it with the same resistance value as is present at the input of your power amplifier.) Very nice job btw.

 

[mattthegamer463]

Thanks.

One thing, whenever I switch the high voltage switch on, thus putting HV into the amplifier circuit, theres quite a pop from my speakers. This happens when turning it on and off, and regardless of the volume level on my main amp. What gives?

 

[kevinkr]

You should turn on the power amplifier after turning on the high voltage in your pre-amplifier, better still you should remove that switch for the circuit. (Replace it with a dpdt and use it to mute the outputs.)

Turning on the High V generates one heck of a transient at the output of your pre-amplifier, going from 0 to a 100V or more almost instantly and the coupling cap has to charge as well. Not recommended..

 

[mattthegamer463]

You should turn on the power amplifier after turning on the high voltage in your pre-amplifier, better still you should remove that switch for the circuit. (Replace it with a dpdt and use it to mute the outputs.)

Turning on the High V generates one heck of a transient at the output of your pre-amplifier, going from 0 to a 100V or more almost instantly and the coupling cap has to charge as well. Not recommended..

Oh. I thought that it was bad to have the HV on the tube before the filaments had properly heated up... could I get a second opinion?

 

[kevinkr]

Largely a myth, and in particular cathode stripping is not a problem with the 12AU7A.

In 25yrs of building and selling tube hifi I have never seen a single instance of cathode stripping with any tube I have used in my designs, and I don't know anyone amongst my extensive circle of friends who have run into this problem.

It is a concern with thoriated tungsten filaments in some dhts operated at very high voltages.