"Impossible" circuit which seems to amplify anyway.

[sixSixSeven]

So I was going to build (or try to build) a capacitor coupled tube preamp out of the normal pentode in an 11BQ11 Compactron from an old TV. Long story short, I had nothing to make a B+ supply out of.

Well... I got a 32/16V printer supply and a 12V switching supply of unknown origin and powered the B+ with the 32V part, and it works and actually sounds pretty good! The filament is powered one diode drop off of the 12VDC, and I measured it to be a perfect 11.2V. The circuit is working perfectly... with 32V on B+!

Is there a reason this works? I always thought you needed upwards of 100V to the plate. The tube's running in triode mode. It's capacitor coupled between my PC's sound card and my stereo, and the amplitue of the output is noticably larger.

I'll give you guys a schematic if you wish.

[rsumperl]

That would be great if you could post a schematic.

Ray

[bichela]

Originally Posted by rsumperl

That would be great if you could post a schematic.

Ray

Yes please post one! I was thinking of doing some simple amp (using batteries) to mess with tubes without electrocution risk. a 6v lantern cell for the filimants 3 9v transistor batteries for the b+ and one 9v for the C (grid). mostly to expirement with.A nice low voltage amp would be a good basic start. with those 1w computer speakers that go on a motherboard. with a 1kohm center tapped transistor output transformer. Something kiztch but useful for testing work.

[Miles Prower]

Originally Posted by sixSixSeven

Is there a reason this works? I always thought you needed upwards of 100V to the plate. The tube's running in triode mode. It's capacitor coupled between my PC's sound card and my stereo, and the amplitue of the output is noticably larger.

I'll give you guys a schematic if you wish.

So?



From: Steve's Tube Pages. There is no reason why it shouldn't work. Just because it isn't typical, and it's more usual to see higher voltages, and it's not in the spec sheets, doesn't mean it can't work.

For uber-high input impedances, there are the "electroscope tubes" which operate on voltages as low as 4.0Vdc. The low voltages being used to minimize ionization, and drive the grid input impedance to the point where the limiting factor is the glass between the grid and cathode connections.

[bichela]

Originally Posted by Miles Prower

So?



From: Steve's Tube Pages. There is no reason why it shouldn't work. Just because it isn't typical, and it's more usual to see higher voltages, and it's not in the spec sheets, doesn't mean it can't work.

For uber-high input impedances, there are the "electroscope tubes" which operate on voltages as low as 4.0Vdc. The low voltages being used to minimize ionization, and drive the grid input impedance to the point where the limiting factor is the glass between the grid and cathode connections.

For my first teeny amp.... some of these may be fine. got any good tube numbers?

[sixSixSeven]

It's now running at 12V.

[Miles Prower]

Originally Posted by bichela

For my first teeny amp.... some of these may be fine. got any good tube numbers?

I have a couple, but what I don't have is any information about availability.

Electroscope Tubes

-----------------------------------|----- FP-45 ----|----- D-96475
Filament: ----------------------| 2.5V@90mA | 1.0V@0.27A
Space charge grid bias ----| 4.0V ----------| 4.0V
Vpk ------------------------------| 6.0V ----------| 4.0V
Grid Bias -----------------------| -4V ------------| -3.0V
Ri---------------------------------| 10E16R ------| 10E16R
Igg-------------------------------| 10E-10A ------| 10E-10A
Ip---------------------------------| 60uA ----------| 85uA
rp --------------------------------| 45K ------------| 25K
Gm-------------------------------| 20uA/V -------| 40uA/V

Both types include a "space charge" grid that acts as a shield to prevent any positive ions coming from the filaments (DH types) from causing grid current that decreases the Zi. The u-Factors are quite low, but gain isn't what they were designed for: maximizing Zi was the whole point in order to make highly accurate and sensitive DC amps for instrumentation.

These are highly specialized types, and getting them these days may prove to be a problem.

[bichela]

Originally Posted by sixSixSeven

It's now running at 12V.

That picture is very teeny. Can you blow it up to about 640x480 native resolution so I can actually read the values of the parts?

[bichela]

Originally Posted by Miles Prower

I have a couple, but what I don't have is any information about availability.

Electroscope Tubes

-----------------------------------|----- FP-45 ----|----- D-96475
Filament: ----------------------| 2.5V@90mA | 1.0V@0.27A
Space charge grid bias ----| 4.0V ----------| 4.0V
Vpk ------------------------------| 6.0V ----------| 4.0V
Grid Bias -----------------------| -4V ------------| -3.0V
Ri---------------------------------| 10E16R ------| 10E16R
Igg-------------------------------| 10E-10A ------| 10E-10A
Ip---------------------------------| 60uA ----------| 85uA
rp --------------------------------| 45K ------------| 25K
Gm-------------------------------| 20uA/V -------| 40uA/V

Both types include a "space charge" grid that acts as a shield to prevent any positive ions coming from the filaments (DH types) from causing grid current that decreases the Zi. The u-Factors are quite low, but gain isn't what they were designed for: maximizing Zi was the whole point in order to make highly accurate and sensitive DC amps for instrumentation.

These are highly specialized types, and getting them these days may prove to be a problem.

Who is the manufacturer? Electroscope? Never heard of that company. I do know RCA made some space charge tubes that were used in many later model car radios.

While the 12ax7, one tube I do have here, isn't designed for low voltage plate operation. An Austrailian on Youtube demonstrated a 12AX7 being used in a radio tuner he built. the amp (so we could hear it) was a transistor amp.

Given the output of such a tube, you could get maybe 1/2 or 1/4 Watt of output. With some of the modern pcboard speakers that would work great as they don't need much wattage to get good loudness.

[kevinkr]

Originally Posted by bichela

Who is the manufacturer? Electroscope? Never heard of that company. I do know RCA made some space charge tubes that were used in many later model car radios.
<snip>

Not sure that there ever was a company called Electroscope, but an Electroscope is a device designed to measure static charges. The tubes listed don't come up in any familiar database. In any event these would not be the type of tube I would use in an audio application, their relatively high rp combined with extremely low transconductance would probably result in a lot of unnecessary noise, but I SWAG..

You might want to take a look at the 6GM8/ECC86 as this tube was designed expressly for low voltage operation. The 6DJ8/6922/7308/ECC88/E88CC also apparently exhibit pretty good performance on low plate voltages. Take a look at the 6088 and 6418 direct heated sub mini pentodes as well - although their linearity isn't spectacular they can be useful in audio applications.. See here for a hint of interesting things you can do with these tubes: http://greygum.net/sbench/sbench/6088pre1.html