tube stage: low ouput z / no gain / capable of driving 600 z+ loads

[ChrioN]

Is there such a thing as a high current output, low z output, tube buffer stage? No solid states, just tubes.

Basicly, what I need is a preamp with no gain, with low output impedance, and capable of driving 600ohm impedances.

If there is, please point me in that direction  ;D

 

[NewYorkDave]

A White Cathode follower, using suitable tubes, run at a fairly rich current. That's one way. If you don't mind using a stepdown transformer, there are other ways.

Also see this recent thread:

http://www.groupdiy.com/index.php?topic=31534.0

 

[ChrioN]

Thanks

Actually, what I meant by low z output was more like less than 200

 

[NewYorkDave]

You can achieve output Z of 200 ohms or less using a WCF. A few years back, I whipped up a WCF on my bench, using a 12BH7, that could drive 600 ohms to about +24dBM before crapping out.

I won't even ask WHY you want to do this.

 

[mcs]

You could also use a 6AS7/6080 cathode follower. Low impedance driver and space heater in one

Best regards,

Mikkel C. Simonsen

 

[gyraf]

If you don't need high output levels, you can passively reduce output impedance by a simple passive divider, sacrificing level for impedance...

I think we need more info about the design criteria?

Jakob E.

 

[ChrioN]

Jakob, what exactly is a passive divider? A voltage divider with the shunt to ground as output impedance? Or just a resistor in series with the output?

 

[Wagtbtoobz]

Hej!  The passive summing busses of the old 60' ALCATEL tubed mixing consoles of the ORTF were driven by trioded 6AU6 in CF mode. Cathode was grounded by a 30Hys inductor (10mA idle) , output was given with a 2,2uf cap , and then 9k1 serial output res tied to a 200ohms bus . 12 amps were tied together . The 6AU6 had 50dB gain before , the summing amp had 40 to 50 dB again .  Pierre

 

[gyraf]

Jakob, what exactly is a passive divider? A voltage divider with the shunt to ground as output impedance? Or just a resistor in series with the output?

yes, the voltage divider..

 

[PRR]

> Is there such a thing as a high current output, low z output, tube buffer stage?

Yes. Give more criteria. Otherwise.... a Macintosh MC-xx Big Tube Amp will give >10V >1A into ~~10r with <1r output impedance. I have used my Fisher stereo-24W as a line driver. And it keeps coffee hot!

Some would say the big Macs don't sound like "tube amps". Over-develop one aspect, change another.

> what exactly is a passive divider?

My Dyna tube preamp has Zout and load rated at 50K. If I put 49,999 ohms and 1 ohm in series, and tap across the 1 ohm, Zout is now 1 ohm. However if it could do 5V in 50K, now we get 0.1 milliVolt un-loaded or 0.05mV in 1 ohm load.

The voltage-divider will NOT increase current, only lose some.

The transformer can swap current for voltage. Tube runs at 250V 50mA, we wind a transformer 5,000 turns and 10 turns, we get 0.5 Volts and 25 Amps. Since we rarely need 25A of audio, typical windings are 5000:1000 or thereabouts.

Run the 16 ohm output of your tube speaker-amp into the 120V side of a Weller Soldering Gun. The secondary impedance will be about a milli-Ohm, altho the output level may not exceed 10mV.

There is a Russian MIG-radar tube which can pass almost 1 Ampere. Parallel these until you reach your Iout or Zout spec, or your heater-transformer falls through the floor.

Tubes are medium-high impedance things. You should never just ask for "low Z", you must think what Z you really need, and how much you can pay to get there.

As the old 50K Dyna shows, if all the gear is in arm's-reach, nutty-low-Z is NOT needed.

> no gain, with low output impedance, and capable of driving 600ohm impedances.

Driving "how hard"?? 1V peak? 100V peak?

6DJ8 in WCF with 200V-300V supply can put over 5V in 600r. Zout will be near 50 ohms.

12FX5/60FX5 with 200V supply flowing 40mA as simple cathode follower can put over 10V in 600r and Zout is near 80 ohms. And 12FX5 is a heap cheaper than 6DJ8.

100V in 600r would be hard work for a 2A3. And Zout would be "only" 800r before NFB.

Is 50r "low"? Compared to what? If you drive a single 3 or 30 feet cable, 50r is "zero". If you drive two loads, and one may get stepped-on and turned to a 1-ohm short, 50r is not low enough to prevent loss of signal to the other load; but if each is isolated with say 470r, then a short on one is only a 1dB error on the surviving load, the gig ain't ruined.

Why are you driving 600 ohms? Didn't that go out of style 50 years ago?

 

[CJ]

"what I need is a preamp with no gain..."

now that is a classic.
like a horse with no name, only different.

You could build the Tube Tech op amp.
I do not know the output Z.