What Is This Strange Valve Circuit?

The valve circuit below appears to be a type of four stage amplifier.

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Can anyone tell me the approximate gain and bandwidth? The 60" antenna would tend to indicate 197MHz full wave, but is this (or quarter wave) a valid assumption considering the components used?

Does the circuit do anything other than amplify, e.g. oscillate due to high gain, modulate? I ask because it is built into a transmitter.

Thanks for any insights.

Robert Miller

Reply to
Robert Miller
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It is strange: no B+ HT smoothing filter capacitor and the cathode current meter polarity looks wrong?

piglet

Reply to
piglet

It really needs to be drawn clearly to make sense, too many details are too ambiguous.

1.5v cell biasing a mains circuit is a feature of pre-war circuits. The fun ny layout is typical of 1920s or earlier, as is the lack of decoupling. Pen tagrid tubes appeared in 1933 so the engineer was using dated methods with new tubes. Lack of smoothing would have made it distort a lot - it must hav e been for voice since telegraph circuitry could have been far simpler. Unl ess perhaps it broadcasted a whole stack of telegraph signals at once. And why 4 pentagrids?

One thing's for sure: it's odd.

NT

Reply to
tabbypurr

My guess is that it is for around 50MHz since 60" would be 1/4 wavelength.

Reply to
John S

Small signal gain for a common cathode pentode amplifier operating in the "saturation" region is simply the transconductance at the operating point times the plate resistor, which for the 6AC7 with a 50k load resistor is probably around 400 times - which doesn't seem like an unreasonable amount of gain for a single pentode stage.

It looks like it's using a combination of battery bias and self-bias to set the operating point, but it's a very sloppy schematic and it's had to tell WTF is connected to what. or what the supply voltage is, and why there doesn't appear to be any filter cap after the rectifier tube. I don't think the output antenna can possibly be very well matched to the plate of the final 6AC7 with just RC coupling like that.

Reply to
bitrex

Too illegible for me at least I'm sorry to say. Pity, as it would be a simple matter to analyse it - if only one could read it!

Reply to
Cursitor Doom

On Friday, July 28, 2017 at 8:57:34 AM UTC-7, bitrex wrote: ...

Not at the 50MHz and above frequencies that have been mentioned in this thread. The dominant anode load will the capacitance of itself and the grid of the next stage.

Wth the 50K anode resistor the current cannot be more than a couple of milliamps - that will bring the gm down to only a few mA/V as well.

At high frequencies I would be surprised if the circuit has any gain at all.

I found a reference to the 6AC7 that it was introduced in 1939 so that sets the earliest date of the design.

kevin

Reply to
kevin93

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