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 Post subject: Using a triode as a diode??
PostPosted: Feb Sat 24, 2018 12:58 am 
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I've never seen this done before. The 76 for the detector/ AVC with the grid and plate connected together. It works, but it seems like a waste. Is this common practice?


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 Post subject: Re: Using a triode as a diode??
PostPosted: Feb Sat 24, 2018 2:12 am 
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Yes it was done. A triode is a diode with a grid added. You can use a triode as diode between grid and cathode. In this case plate was tied to grid.

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 Post subject: Re: Using a triode as a diode??
PostPosted: Feb Sat 24, 2018 2:30 am 
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In the later production Hallicrafters SX-42, using schematic 89D257 : during AM operation the B+ supply to the 7H7 "Limiter and AM Detector" stage is left floating, and the cathode and control grid of this stage function as a diode detector. During FM operation the plate and screen grid circuits are energized and the stage functions as a limiting IF.

-Chuck K7MCG


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 Post subject: Re: Using a triode as a diode??
PostPosted: Feb Sat 24, 2018 2:34 am 
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In the early days sets were judged by how many tubes that they had - as mentioned by others in an earlier post. 76 was not an expensive tube.

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 Post subject: Re: Using a triode as a diode??
PostPosted: Feb Sat 24, 2018 3:24 am 
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Low-end gear uses triodes as diodes. My EMC-500 uses a dual triode. One is actually a power rectifier, the other is the oscillator.
There were several RF oscillators that did this and I think some VTVM's.

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 Post subject: Re: Using a triode as a diode??
PostPosted: Feb Sat 24, 2018 4:18 am 
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I just finished replacing the filter caps in an RCA WO-33A scope (the other caps are mylar).
It uses a 3" CRT, so the anode voltage is only about 650 volts, but they use a 6C4 as the HV rectifier. The tube is rated for a max of 300V on the plate.

The Crosley 1128 uses three 6C5 triodes as diodes - detector, AVC, and muting.

Bob

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 Post subject: Re: Using a triode as a diode??
PostPosted: Feb Sun 25, 2018 3:11 am 
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I can only imagine an advantage where it was in a circuit that used both triodes and diodes, and might allow the use of one type of tube for both purposes, thus saving a mfr the trouble of stocking two types of tubes and a user from having to have more than one spare tube. I guess that situation could exist just as well in some test eqt. as well as some sort of radio circuit.


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 Post subject: Re: Using a triode as a diode??
PostPosted: Feb Sun 25, 2018 3:16 am 
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The famous Western Electric 43 theatre amplifier used two 211s in push-pull, with two more 211s diode connected as rectifiers. That was likely to eliminate stocking two different types at the theatre.

In the tube-count bloating which was rampant in the 1930s and early 1940s, it was easy to diode connect a 6J5, and add another tube or two (or more) to the count. Both Philco and Zenith committed that sin, and there probably were others.

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 Post subject: Re: Using a triode as a diode??
PostPosted: Feb Sun 25, 2018 5:06 am 
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Well, since this example is in a Sentinel 68B I don't think it was to increase the tube count as there are only six, no tuning eye tube on mine. This makes me think it's the stocking/number of tube types is more likely the reason. I find it fascinating.

I have a Kadette 1019, in need of restoration, that is a prime example of padding the count - seven tubes and three ballasts. From the looks of it a rather crummy radio to boot.

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 Post subject: Re: Using a triode as a diode??
PostPosted: Feb Sun 25, 2018 5:46 am 
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In the late 20'a and early '30s, the days of the 27 and 24 tubes, and even a bit later, there just weren't any small signal diode tubes. If one was needed, the only choice was to use a diode connected triode.

Most grid dip meters turn off the B+ when in the absorption mode and use the cathode and grid of the triode as a diode to rectify the incoming RF. When in the dip mode, the B+ is turned on and the triode oscillates. It's an easy way to get two functions and at the same time avoid the many difficulties that switching the RF circuits would cause.

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 Post subject: Re: Using a triode as a diode??
PostPosted: Feb Sun 25, 2018 12:58 pm 
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I have an old Sears console, don't ask me the model, that used a triode, grounded the plate, and used the grid as the diode anode.


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 Post subject: Re: Using a triode as a diode??
PostPosted: Feb Sun 25, 2018 5:41 pm 
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Grounding the plate creates a shield, saves the cost of an external shield. Some of the the later triode / dual diodes had a shield around the diode plates.


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 Post subject: Re: Using a triode as a diode??
PostPosted: Mar Thu 01, 2018 6:42 pm 
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Zenith did this quite a bit.

I know I mentioned this before, but many years ago after I restored a Zenith 765, I noticed it used a type 56 wired as a diode as the detector.

Out of morbid curiosity, I made an adapter to make use of a 3AL5 in place of the 56.

The result was quite an improvement in performance! The radio really pulled in the stations compared to how it performed with the 56!

The reason I thought of this is from a time long ago when I was experimenting with an old Magnavox chassis. I forget what it used as an AM detector, but I tried a 1N34 instead which was putrid. A 6AL5 gave it new life!

Something about those xAL5s are pretty hot!

-Steve

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 Post subject: Re: Using a triode as a diode??
PostPosted: Mar Fri 02, 2018 12:35 am 
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I've got an Atwater Kent 310 chassis in which two of the 56s are connected as diodes, one for the detector and the other for the AVC. I could use one 3AL5 to substitute for both. The two are right next to each other, so I could make an adapter with two 5-pin plugs to one 7-pin miniature socket. Wouldn't that look cool.

Jeff


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