Technically, it's called "modulation acceptance". To build a better AM detector in our 5 tube radios. Virtually all American 5 tube radios used a tube with one triode and a pair of diodes, like the 6AV6 or 6SQ7. Most radios only used one of those diodes and tied the other to ground. But this tube was designed for circuits that would use one of the diodes as the audio detector, and the other a source of AVC voltage. In radios that only used one of the diodes for both these uses would run into a problem, in that the AVC filter cap would tend to back bias the diode, making detection of weak signals distorted. But to avoid this we meed a few more resistors and a cap, which in a mass market consumer product was still too expensive for such a thin profit margin product. But we in this forum can afford them, so. What we do to modify an existing set that has the dual diode detector tube, first disconnect the AVC resistor (typically 2 megs or so) from the audio signal (usually connected to the node at the top of the volume control). That now free end of that disconnected resistor will now connect to the 2nd diode (once you unground it). Connect a 1 meg resistor from this diode to ground. And connect a 100pF cap from the 2nd diode to the first diode, to pick up the IFsignal to develop AVC from

This diagram shows a slightly different connection for the cap, but this cap connecting to the other diode is likely easier.

Be sure to check that strong stations are received undistorted, to be sure you didn't kill the AVC function by mistake.

Another enhancement is to reduce the heater voltage on the 6AV6 from 6.3V to a little more than 4V. A 6 ohm resistor in series with the heater from the 6.3V supply wil get you this.

This reduced heater voltage actually helps the triode section, makes it more likear. This won't hurt the cathode, as the currents through the diodes and triode are less than an milliamp.

Some radios used the 2nd diode's tube pin as a tie point, and this would also allow some negative potential (from stray electrons) to develop. This doesn't really help us here, so reclaim the diode for the above mod.

Wa2ise,

These are the type of articles on this Forum that I like ... they challenge my knowledge of how these old radios function.

I have got a grasp of the principles (but not the detail) you have outlined and will put a bit more thought into them soon.

I am travelling at the moment to a warmer part of Aussie ... but have got a few circuit diagrams with me to contemplate when I find a suitable beach.

However, a couple of questions ...
*** what is meant by 'contact potential voltage'????
*** re reduction in hum ...... I assume that are we talking about an a/c indirect heater (?) ....... is the hum caused by the heater cycling, hence the number of electrons boiled out of the cathode cycling ????
*** can you elaborate on "This reduced heater voltage actually helps the triode section" ?????

Cheers,
Ian

There is always the risk with AC heaters that they will modulate the cathode and that introduces hum at often half of the frequency of the AC (Rectification).

In some special applications, to avoid this the heaters are actually run on DC.

I would note that the use of the two plates of duo diode Triode or Pentode are normally used separately for delayed AGC. Delayed AGC is called this as it, is set such that it only starts to impinge on the valve /s that it is applied to, when a certain voltage is reached, often via a voltage divider.

One should note that in most cases in a receiver like the above, the current draw of the grid is insignificant.

Marc

"However, a couple of questions ...
*** what is meant by 'contact potential voltage'????

Thanks. Contact potential is what you get when, in a high impedance circuit, on a grid or diode plate, from the electrons boiled off the cathode. It's used to bias the grid on a 6AV6 triode, and with the grid resistor being around 10megs, will produce about -1V. Likewise on the detector plates, though to a lesser extent. Reduced hester voltage will make less electrons, and lower voltage. But we still get enough for the triode to still function.
*** re reduction in hum ...... I assume that are we talking about an a/c indirect heater (?) ....... is the hum caused by the heater cycling, hence the number of electrons boiled out of the cathode cycling ????

No, just a bit less leakage from the heater to the cathode.

*** can you elaborate on "This reduced heater voltage actually helps the triode section" ?????"

Looking at this diagram

are curves of the triode. Divide by 2 to get heater values for a 6AV6. The curves get a little straighter on reduced heater voltage, but at the loss of max cathode current capability. But the 6AV6 tridoe current os low, so it's not a problem. Some hi-fi preamps use this trick.


"to a warmer part of Aussie" I've heard that it gets cold there in the Summer, and hot in the Winter. Or at least during American Summers and Winters

The thing not to be overlooked is that the valve is high impedance & voltage controlled. The voltage swing for the next valve is often provided by having a high resistance cathode resistor.

Australia is big, it is actually divided by the "tropic of capricorn".

To North of it tends to be tropical. The centre Arid & desert .

You do not go holidaying at Christmas (Summer) in the northern regions as that is when the tropical "wet" arrrives & travelling by road can become impossible due to flooding.

Marc

Here in my home state of New South Wales you can be at the beach, skiing the slopes, hiking through tropical rainforest or trying to stop your radiator cap popping in the desert or having a picnic on the lush green slopes and plains, all within about five hours' drive, maybe six.

I spent two nights camping in the gold town of Sofala on Sunday and Monday and realised there and then the importance of having a decent radio.

I had the misfortune of missing the call of a footy match between the mighty Canterbury-Bankstown Bulldogs and the Cronulla-Sutherland Sharks. The Doggies won by 20 points but I couldn't hear it - no valve radio in camp.

The car radio picked up 2GZ (freq. 105.1MHz) from Orange but just barely and the score couldn't be heard. My digital radio can also pick up the FM band but it simply couldn't pull in the signal.

If I had a valve job in camp with a modest aerial wire I'd be able to hear the broadcast on just about any AM station within 1,500km.

As for the temperature, -2°C at night and upto 20°C during the day with clear skies. Just the way it should be in late autumn.

I understand how a triode amplifier works and individual diodes ..... but when built into a Double diode Triode, I am confused.

For example, using the circuit diagram posted above by Wa2ise ..... an audio modulated IF signal appears on pin6 the top diode ..... this diode will conduct when the IF signal is positive, and will not conduct when the IF signal is negative. This is the 'rectification' part of the process.

Question ....
when the signal is positive and the diode conducts, does it 'absorb' all the positive signal, leaving only the negative signal to be amplified by the triode??
OR,
are these double diode triodes constructed in such a way that the triode only conducts when the IF signal is positive and the diode is conducting, with the triode amplifying the positive portion of the signal??

I am deliberately ignoring the IF filtering and AVC at the moment for simplicity.

Cheers,

The diodes are independant of the triode and pentode when it is used (eg 6B8).

The diodes behave much the same, as they do in a crystal set.

Marc

In the circuit diagram at the top ..... why is the AGC diode (pin 5) feed from the primary, versus secondary, of the IF????

Re my last Post for this Thread ...... for those who maybe interested ......

Did some further reading and found out that this configuration (feeding the AGC diode from the IF Primary rather than the Secondary) is to reduce the screech caused by sideband frequencies when tuning into a station. The wider frequency resonse curve of the Primary activates the AGC earlier. When tuning through the station to the other side, the AGC usually retains some residual affect and sideband screech is not normally heard.

Don't know that I entirely agree with that. there are about five different ways to wire up the detectors.

Quite often when the two diodes are seperated, one is doing the signal detection and the other, when fed from the Plate of the valve prior to the detector, is providing delayed AGC.

The voltage developed on the AGC has to reach a certain voltage, before it starts to impinge on the control grid /s of the valve, or valves it is applied to; often via a voltage divider.

Marc