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Old 26th December 2004, 03:41 AM   #1
andy2 is offline andy2  United States
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Default What are terminal 1 & 4 of 300B tube

I usually see terminal 1 & 4 connected to a pot with the center tap to a resistor at the cathode.

How do you know what voltage to apply to terminal 1 & 4? Should the voltage be an AC or DC source?

How do you know how to adjust the pot? Should the pot be at the center point?
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Old 26th December 2004, 03:55 AM   #2
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Sounds like the filament. You need 5VAC at some amps, although some argue DC is better/worse (with absurd logic of course, but we're talking the 300B here -- these things trancend logic, as jewelry to a woman).

The pot nulls the AC voltage so it isn't amplified (since a voltage at the cathode looks the same as a voltage at the grid). As such, you want to adjust the pot (by meter or by ear) to yield the lowest hum in the output. A pot is unnecessary with DC heating.

The resistor to ground sets bias by the current through it, which since it also passes through the tube, is a very stable source of this bias. It provides a positive bias, but since the grid is grounded (through an interstage transformer, grid choke or grid leak resistor), appears negative to the tube and thus sets it in a comfortable position.

Tim
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Old 26th December 2004, 03:56 AM   #3
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I would like to add that what are the effects of biasing terminal 1 & 4?

How does it affect the operating condition of the tubes?

Does bias affect the standing current of the tube?
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Old 26th December 2004, 06:39 AM   #4
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Bias IS the standing voltage or current, on or through a tube.

Tim
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Old 26th December 2004, 10:07 AM   #5
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Pins 1 and 4 are indeed the filament of the 300B (As Tim has said). It needs 5VDC at 1.2A or 5V RMS at 1.2A RMS to operate correctly. The filament must be heated so it can create a cloud of electrons around it through thermionic emission. These electrons are attracted to the anode (which is positive with respect to the filament). The grid is negative with respect to the filament (usually), so it does not attract electrons to it. The grid voltage thus can exercise control over the current through the valve (more negative -> less current, less negative -> more current). That was a very simple explanation of the basics of how thermionic valves operate, from the 300B to the little 5702.

Quote:
Originally posted by andy2
I would like to add that what are the effects of biasing terminal 1 & 4?

How does it affect the operating condition of the tubes?

Does bias affect the standing current of the tube?
What Tim means, is that when you connect a resistance between the centre-tap of the filament transformer, or the hum pot wiper, and ground, a voltage must develop across that resistance when current flows through the resistor: Ohm's law, V=IR. Thus, the filament (acting as the cathode) will be positive with respect to ground. Yet, the grid is at ground potential (since it is grounded through the grid resistor, or choke, or interstage transformer, or whatever). Therefore, the grid is negative with respect to the filament. The size of this resistance between the filament and ground determines how negative the grid becomes with respect to the cathode, and thus determines the standing current through the valve.

I'm not sure if this answers your question a little more clearly (actually I'm not entirely sure what the question was), but I hope this helps. Perhaps something like the "Grounded Cathode Amplifier" article at the TubeCad Journal may be more helpful.

It's much easier to understand something if you've got something in front of you, like a schematic of a 300B amp or something. If you're still confused, post something like that, and I'll try (or anyone else for that matter) to explain what the components do, and how they affect the operation of the valve.
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Old 26th December 2004, 03:39 PM   #6
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Quote:
It's much easier to understand something if you've got something in front of you, like a schematic of a 300B amp or something. If you're still confused, post something like that, and I'll try (or anyone else for that matter) to explain what the components do, and how they affect the operation of the valve.
OK-- if this is considered hijacking the thread, then please say so, but I think this insane offer to describe the workings of a tube circuit is an opportunity I cannot afford to waste.

I've attached the schematic for the JE Labs 300B amplifier from Angela, which I think is a good example 300B circuit. Its reasonably simple (reasonably) and probably offers a look at a basic amplifier topology for a variety of SET designs.

Hopefully I got all the lingo right--- we're talkin' noobie here.

Let's look at the main circuit (i.e., not the power supply, right now). It seems (based on my VERY limited understanding of circuits) that the 100K pot is to vary the amount of signal passed to the grid of the driver tube (6SN7). The first half of the triode is cathode-grounded though a 470R resistor.

I imagine that Ohm's Law is used to determine the resistor value to properly reference the cathode to ground, but could someone explain this in simple terms (remember, I'm a stupid newbie)? Why couldn't the cathode by tied to ground using any old resistor (I think I sort of know why, but I want to actually, really know why), or simply be grounded without any resistor at all?

This is likely to be the beginning of quite a few questions, so please stop me, andy2, if you think I'm hijacking this. I just thought that since your initial questions were similar to my own that we could both benefit from the explanation of this circuit.

Also, feel free to take us through the mind of the electrons as they pass though the various components here. I really have a very limited knowledge, so some kind of puppet show-style explanation would NOT be considered patronizing.

Hey-- how about a stream-of-conciousness, one-act play? Take me to the SOUL of the electron. I want to feel its reaction to resistance... its malaise over capacitance...

OK, too much Kofi. You're raising too many eyebrows.

Seriously, any help explaining this (or any) tube circuit would be much appreciated!

Thanks,
Kofi
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Old 26th December 2004, 08:19 PM   #7
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In the attached schematic, where would you connect the transformer terminals: 6.3VAC 6SN7.
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Old 27th December 2004, 02:42 AM   #8
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Those would go to pins 7 and 8 as I recall, heater for the 6SN7. Don't trust me, I haven't wired a 6SN7 in a while...

Tim
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Old 27th December 2004, 03:36 AM   #9
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OK.

I see that those terminals are connected to pin 7 and 8 of the 6SN7 pair.

Why don't you need the same pot used in the 300B for the 6SN7 for the filament heater?

Why is it that you can connect them directly to 7 and 8?
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Old 27th December 2004, 06:12 AM   #10
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Quote:
Originally posted by andy2
OK.

I see that those terminals are connected to pin 7 and 8 of the 6SN7 pair.

Why don't you need the same pot used in the 300B for the 6SN7 for the filament heater?

Why is it that you can connect them directly to 7 and 8?
You've missed something that the 300B is famous for (and part of the reason why I think it's a bit of a pain...). It doesn't have a real cathode, i.e. it is directly heated (you have to have heard DHT: 'directly heated triode' before in marketing material somewhere?)

In the 6SN7, the part of the valve which emits the electrons is the cathode, a metal sleeve coated with alkali metal oxides, which is heated by a thin tungsten wire inside it, not surprisingly called the heater. There is insulation between the heater and the cathode, so they are electrically separate (not completetly true, the insulation isn't perfect). Thus, you can apply a voltage to the heater of the 6SN7, heating the cathode and allowing it to emit electons, and use a seperate components between cathode to ground for biasing. (One of the pins is connected to the metal sleeve of the cathode).

In the case of the 300B, this valve has a FILAMENT, like a light bulb. There is no separeate heater and cathode. The filament is like the heater (a thin tungsten wire), except the oxide coating in this case is applied directly to the tungsten wire instead of to a metal sleeve surrounding it. All the hum pots and what-not are circuitry to create a "virtual cathode", i.e. to emulate the function of the cathode in an indirectly heated valve.

I have a few pictures which might explain the difference between directly heated valves and indirectly heated valves better (from an old RCA valve manual from the late 1920s or early 1930s). When I find it, I'll post it.
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