Nuvistors Revisited

[erix]

One of my favorite designers in the whole world is coming out with a new product.

Zachary Vex builds incredibly complex guitar effects used by many many MANY great players. He is now developing a tiny amplifier using a pair of nuvistors.

Dig It!

Says he:

Quote:

one-half watt peak, one-tenth watt clean. it uses miniature pencil tubes, extremely small. the output transformer, caps, and tubes stick out the top of the box and are protected by chrome roll-bars. it runs off 12v dc, from a regulated wall-wart supply or an ordinary car battery works too, so you can use it in the van while touring, or with a tiny 12v lead-acid battery (like the kinds used for home burglar alarms) for several hours while busking in subways. it's got a volume control, brightness switch, thickness switch (full, medium, thin), and a mello switch (mello1, normal, mello2). it has a fan. i think. it seems to need one. that's all for now.

Now, aside from the tone-shaping parts of the circuit - it's that 1/10 watt of clean I'm interested in. This amo is designed to plug into a 16 Ohm speaker cabinet loaded with (4) 92dB/W 12" speakers. It ought to be able to drive a 32 ohm load with ease!

Some more info:

Quote:

the specified life is 100,000 hours, according to one sheet i have on them. however, that's with 150 volts max on the plates, but i'm running 230 volts on one plate, so we'll have to see how that turns out. so far, i haven't blown one... i've run them day and night for weeks. the tubes are soldered in. no sockets would be able to hold them in with the skinny leads they have... they're kinda like resistor leads, not pins. i've experimented with desoldering them, and it's pretty reasonable.

So, there you have it - a small transforer steps up 12V DC to 230V to power a pair of nuvistors in a package small enough to fit in a Hammond 1590B box.

 

[Budgie]

That is really a cool amp. I wonder how avaliable the Nuvistor's are?

 

[runeight]

Certainly a very cool looking amp.

I am having a hard time, however, believing the claims for power output. Since there are no output transformers, the tubes must be in one of the standard output configurations (Cathode Follower, SRPP, White Cathoder Follower, etc.). Unless these tubes can handle huge amounts of current.

If P = I * I * R / 2 then

.1 = I * I * 16 / 2

Which gives me about 111mA peak into the load.

I would be surprised if these tubes can deliver that kind of current in any of the above output stage configurations. I doubt that they can deliver that kind of current at all.

At 1/2 W power output I get

.5 = I * I * 16 / 2

or I = 250mA peak.

This is the current capacity of an EL509 not a nuvistor (at least not any nuvistors that I used to know).

Is the output stage solid state and am I just raving ?

 

[erix]

I have one correction - those aren't nuvistors but 'pencil' tubes or hearing-aid tubes. Somewhat different.

I think you might be right about the output stage - note the placement of the fan. Not on the tubes!! In fact, nowhere near them!

'course there *could* be a little output tranny in there. It's not designed for fidelity.

Knowing zvex I would not be surprised if there were a whole bunch of discretes in the output section. I believe he is using the tubes by overdriving them to get that cranked-tube tone and then it goes to something black with leads....

ok,
erix

 

[kevin gilmore]

Correct you are, they are not nuvistors.

I can guarantee that there are a pair of tab transistors
inside that make ac out of the dc, and the transformer on
the top is for that purpose. Since the power supply is
relatively low frequency it is highly likely that you can
hear the power supply in the output.

Still its a pretty looking piece. And you could probably
drive a car over it without damaging it.

 

[Budgie]

http://www.thevalvepage.com/valvetek...r/nuvistor.htm

 

[zachary vex]

the transformer on top of the nano head is the output transformer.

on the inside there's a switching power supply that generates the 230 volts for the 6021w tubes.

the amp really does generate .5 watts dirty, but it is limited to .1 watt for a clean signal. it is certainly not hi-fi... it's a guitar amp.

there are no semiconductors in the signal path WHATSOEVER.

8^)

there are sound samples on the http://zvexamps.com/ page.

 

[erix]

What are you doing reading an audio board when you should be working...

Glad to see you here!

ok,
erix

 

[Toe]

Quote:

Originally posted by erix What are you doing reading an audio board when you should be working... Glad to see you here! ok, erix

I'm guessing he caught the link in his referer logs...

 

[zachary vex]

Quote:

Originally posted by erix What are you doing reading an audio board when you should be working... Glad to see you here! ok, erix

following up on all of the posts related to my new product IS work! 8^)

sorry i don't make a headphone amp.

 

[erix]

Quote:

sorry i don't make a headphone amp.

You should be! We're not as cheap as musicians.....

I know you must have tons of ciruit snippets littering the floor that were too clean for musical instrument use - turn 'em loose on us!

ok,
erix

 

[runeight]

Quote:

the transformer on top of the nano head is the output transformer. on the inside there's a switching power supply that generates the 230 volts for the 6021w tubes. the amp really does generate .5 watts dirty, but it is limited to .1 watt for a clean signal. it is certainly not hi-fi... it's a guitar amp. there are no semiconductors in the signal path WHATSOEVER.

Right. Very, very clever design. I assumed it was a stereo headphone amp. That's what I get for talking before thinking!! Now where did I put my strat . . . .