Single-tube preamp revisited

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

NewYorkDave

Well-known member
Joined
Jun 4, 2004
Messages
4,378
Location
New York (Hudson Valley)
OK, before PRR nails me on the semantics, I'll point out that it's really two tubes contained in a single envelope. But anyway...

Some of you might remember this idea for a simple tube preamp that I posted some time ago:

cheapbastardpreamp.gif


I just wanted to bring this up again since I'm getting ready to actually build one of these and see if the real performance agrees with predicted performance. I wanted to mention, for the benefit of anyone else who's thought about building it, that simulation suggests that running the second triode at a higher plate current will improve performance in driving the fairly difficult load. No big surprise there. I don't trust simulations in general, but the 12AV7 model I've been using has proven to be surprisingly accurate with other circuits I've designed, breadboarded and then simulated after-the-fact.

Anyway, try 470-510 ohms in place of the 1K cathode resistor. This will increase plate current by a milliamp or two, with a corresponding drop in plate resistance. The plate will sit at around half the supply voltage--instead of higher than half, as it did before--which will give a greater maximum symmetrical output voltage swing. The open-loop gain stays about the same, so the feedback factor remains the same.

Or at least this is what the simulator is telling me. We'll see... :wink:
 
Dave,

What was the intended OLG, I forgot?
How much current does it draw?

This also reminds me that I need to get a high voltage power supplies for tube dinking around.

Thanks,
Tamas
 
> before PRR nails me on the semantics

Nail, nail.

I've been thinking on a TRUE "one tube" mike preamp. We just saw the Altec with one stage (but four triodes P-P Parallel). There are good reasons to go P-P but it needs a nasty input transformer.

The best I've come up with, assuming 40dB gain at "Pro" I/O impedances, is not real practical. One 8417 (35W power pentode; cheap hi-gain 6550) will do the job. Overload is Not A Problem. 40 Watts power dissipation and a very-fat output transformer are problems.

> simulation suggests that running the second triode at a higher plate current will improve performance in driving the fairly difficult load.

No shock there. In fact I would consider running it a lot hotter: can it idle 10mA and still swing a little more current? And for 10K load, I'd bring the plate resistor down to 10K and make the decoupler 1/5th of that or 2K. It may become impractical especially in large numbers (that's why almost all classic amps run DC through the iron), but worth a look.

> What was the intended OLG, I forgot?

Who knows, who cares? We want the closed-loop gain. This is almost 1+(100K/2K7) or 38, time the ratios of the transformers: 4:1 output (√(10K/600)) and whatever input. For the popular 1:10 input iron, almost 40dB voltage gain.

> How much current does it draw?

Says right on it. Dave included both voltages and resistances. You could check the cathode resistors and cathode voltages and get V1=0.81mA, V2=4.1mA, no other apparent path for current, 4.9mA. Or you could see that all plate current flows through the 10K which drops 50V: 5mA.
 
[quote author="microx"]Just a thought but would the 0.022mfd interstage coupling cap give some LF roll off[/quote]

Well, of course... -3dB down at 7Hz or so...

Peace,
Al.
 
so if the 10K resistor drops 50V due to the 5ma that the circuit draws, would it be safe to assume that you could use a 350V supply for the same circuit, and use a 20K resistor to drop it 100V due to the 5mA load?

Also, 100V*5mA = 500mW, so I'd want to use a 1W resistor, but a) 20k is uncommon, and b) 1W are generally not as on hand as one would like, is it safe to use 2 10K 1/2W resistors in series as a 1W 20k?
 
>> Just a thought but would the 0.022mfd interstage coupling cap give some LF roll off
> Well, of course... -3dB down at 7Hz or so...


It is inside a feedback loop. Forward gain is over 400. Closed gain is about 40. So there is 10:1 feedback, and a single-pole low-cut inside the loop will be shifted 10 times lower, under 1 Hz. (But we do want the pole to be below 20Hz before feedback so the first stage does not have to work too hard to smack the second stage.)

But there are several more roll-offs in there. A cap on the feedback resistor, a cap to the transformer, the transformer inductance. All these poles can do very unexpected things, like a 20dB bump at 0.8Hz. (The Dynaco tube phono amp has such a resonance.) It is annoying to figure this on slide-rule, and often impossible to prove subsonic stability on the breadboard; this is one thing that SPICE is good for. And Dave knows it. Use his values until you get a sense of how it works.

> 1W are generally not as on hand as one would like

Tube dudes should have a large stock of 1W and 2W resistors. Sure you can lash up a dozen 1/8W resistors if that's what you got, but it's sloppy.
 
Might be interesting to run the 2nd stage at higher Ma like PRR posted. I am thinking 47 and trimax circuits.

Maybe a 2 k input transformer with a lower ratio and more closed loop feedback to drop the gain for high output microphones.
 
My main problem with running the second stage at a higher plate current is that it partly negates the economy of using only one tube. If you're going to bump up the total plate current demand past 10mA, you might as well use two tubes and a cathode follower output stage, which is always going to do better at driving a low-Z load. The thing that makes this preamp cheap 'n cheerful isn't so much that it only uses one tube--12AV7s are cheap--but that the power supply demands are light, 5 to 6mA plus heater current. It's conceivable that one could build several into one box and still power them with a less-than-huge PSU.

If you're always going to be running into "modern" gear with a high-ish (>10K) input impedance, it makes a lot of sense to simply eliminate the output transformer and net the extra 12dB of headroom. The unbalanced connection shouldn't be a problem if you follow the usual precautions. When using a 510-ohm cathode resistor on the second triode, the open-loop output impedance is around 6K, and this is reduced when negative feedback is factored in.

For easily-variable gain, you can bump up the value of feedback coupling cap to 10uF and replace the 100K feedback resistor with a 100K log-taper pot in series with a 10K fixed resistor. This will give you an electronic gain of 12.5dB to 31dB (approx). Add the stepup of your input transformer to that number to figure total gain.

To answer a question raised earlier, open-loop gain is about 50dB and CLG (electronic only, not counting transformer ratios) is 30dB. In preamp circuits using negative voltage feedback, I usually strive to maintain feedback of 20dB or greater.

According to the simulator, maximum undistorted output (<1%THD) under worst-case conditions (plate loaded by 10K load and 10K feedback resistor, with a +14dBU input signal) is +27dBU. But again, simulators are not to be trusted entirely. We shall see.
 
I am thinking of making a "tube-o-lator" out of it with a 10k 1:1 line input transformer. Perhaps reduce the gain a bit. Edcor in, Edcor out, one tube, cheap DIY. Ogh-ogh (caveman runt, Tim Allen style)
 
Well, I actually managed to steal enough time to build up a prototype, and even a little PSU to go with it. I tweaked the circuit a bit here and there. See the "One-Bottle Preamp" thread in the Lab.
 
Back
Top