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Old 18th February 2009, 06:34 PM   #1
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Question Building a preamp - somewhat new to tubes

OK, I know you guys get this maybe 100 times per day but, yes, I'm building a tube preamp, mostly to play and learn, not so much because I need one.

I currently have a Dynaco ST-70 that I rebuilt using the VTA driver board and Mullard EL34s. For speakers I have a pair of clone Rogers LS-3/5a's that I made. I also have a pair of Polk RT-15's.

The preamp I was thinking of starting with was the one from diyparadise

First, is this design worth the effort for learning purposes or are there others I should consider. Second, where can I take this design, experimentally speaking, without going overboard ;-)

The power transformer I have for this project is a stancor pm8411 with 375-0-375 b+ and 6.3v @ 4.5A for heater and 5v @3a. Any issues with running the 5687 heater at 6.3v rather than 12.6 or should I maybe use a voltage doubler or separate 24v transformer with rectified/filtered/regulated DC to 12.6v

Thanks everyone!
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Old 18th February 2009, 11:12 PM   #2
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Well, I know less than anyone here but this seems like about as basic as basic gets. It'll invert the phase of your signal if you care about that. It uses an obscure tube if you care about that. It doesn't really preamplify (like a phono preamp) if you care about that.

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Old 19th February 2009, 01:15 AM   #3
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Actually, that 5687 based design is quite reasonable. However, it does amplify the signal it's fed. If, as I suspect, your signal source is a CDP, you rate to have excessive gain and a "hair trigger" volume control.

Let's find out what your gain structure requirements are. Cobble a passive control center up from a pair of 10 KOhm log. taper potentiometers. Mouser and "Rat Shack" have suitable inexpensive controls. If you get satisfactory control of the listening level, employing the passive set up, your project should (IMO) be a buffered volume control, which is cathode followers, not common cathode gain blocks.
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Old 19th February 2009, 03:21 AM   #4
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Default Re: Building a preamp - somewhat new to tubes

I've been a diy tube preamp addict for a couple years, and I'm still tweaking and redesigning my own first one, as my knowledge and tastes evolve.

The circuit you cited is a good place to start, and there are many things you could do to tweak it (such as remove the cathode bypass capacitor, which will lower the gain a little along with distortion, at the possible expense of output capacitance). The only issues I would have with it personally as a stand-alone preamp are:
1. (as mentioned) It inverts the polarity, and might give you too much gain
2. The pinout of the 5687 is such that swapping in another tube like the 6922/6DJ8/6N23P/6N6P/6N1P or 12AX7/12AT7/12AU7/etc. would be a major undertaking, as opposed to substituting a couple resistors and/or minor heater wiring (easy with a switch if you're into overengineering things like me and want a lot of versatility when experimenting).
3. Unless you have other plans for the 2nd triode, I am always against two channels sharing the same tube; granted, the internal shield will help with inter-channel isolation.

As for running the dual-triode 5687 (or a 12A*7 for that matter) at 12.6 vs. 6.3, it makes no difference; it's just a matter of whether you wire the two heaters in parallel or series.

I personally would go with a more ubiquitous tube, which will leave yourself more room to play with alternatives. If you want something else slightly off the beaten path, I've had good luck playing with Russian tubes like the 6N1P and 6N6P which are cheap and easy to find.

If you want to utilize both triodes for one channel, you could either parallel the circuit in question, or look at building a cathode-coupled amplifier (aka common-cathode) for a non-inverting stage with gain and the option of feedback if you want; or a White cathode follower if you don't need gain and just need a good buffer. Similarly, a regular (or parallel) cathode follower would work, and would be the easiest to implement by far.

Good luck!
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