Magnequest Tech: All choked up on Grid and Anode Chokes

Article 5
All choked up on Grid and Anode Chokes
By VoltSecond - November 14, 2004

NOTE: Many thanks to VoltSecond for the excellent work yet again - ML

1. What does a grid choke actually do, compared to a simple resistor?

A grid choke allows a much lower DC resistance to be placed on the grid of a tube for the same or higher audio frequency impedance. The low DC resistance allows for better bias stability with possibly an easier load on the driver and the choke offers the potential tuning of the low frequency performance of an amp.

2. What is the difference between a common and differential choke?

The iron in a differential choke must support both the DC current and differential AC voltage across it. The differential measurement to look for is the peak volt*seconds. On a linear inductor the peak V*sec = L * Idc + L * Ipk_ac

The iron in a common mode choke (CMC) does not see the differential (dc bias) current or the differential voltage. The iron in a common mode choke sees the common mode (leakage) current and common mode voltage. A CMC allows the choke to have a much higher inductance for a given amount of "bias" current. CMCs are mostly used in noise reduction applications.

When you use a CMC, if you remove it from the circuit, you have to be able to measure an "open" with an ohm meter when you measure across where both windings would attach. A CMC cannot be placed between two points that are DC connected to ground and be expected to operate correctly

There are two main types of CMCs: high leakage and low leakage inductance. High leakage inductance CMCs saturate much easier than low leakage inductance types. High leakage inductance CMC also tend to radiate noise to and pick up noise from adjacent components. Why use a high leakage inductance CMC? The leakage inductance can be used for differential filtering if you are careful and shield the heck out of the CMC. I find it easier to just use a low leakage inductance CMC and use separate iron for the differential inductance.

3. Is there any rules of thumb to judge a single ended transformer
on by the specs the maker gives?

A partial list (Ideally):

** >=8H per kohm of reflected load measured at near rated output power.

** >=8H per kohm of R_plate measured at < 1/10,000 of rated output power.

** -3 dB at rated power at less than 20 Hz (i.e. the saturation limit),

** an upper -3dB point of >35 kHz into rated output impedance when driven by about <1/3 the reflected load impedance. Unless the output loading and drive impedance is specified, you really don't know what you get.,

** a primary side self capacitance < 4500 pF/ reflected impedance (1K reflected Z = 4500 pF, 5K reflected Z = 910 pF). This is to keep the high frequency load line under control. (Note: 4500 pF = 1 Kohm at 35 kHz)

** Low and linear core loss.

** The primary and secondary magnet wire does not come in direct contact unless both primary and secondary are ground referenced. (Magnet wire has a high impulse voltage rating but a poor continuous voltage rating.)

** The coil passes hi-pot and IR. This is for safety.

4. When doing a calculation for a 300b se, would you use to get your inductance, XL= 2 x pi x freq. x L where XL is 3000 ohms say, or XL is 3000 in parallel with the 700 ohms plate res. of the 300b?

Both! XL >8*3000 ohms reflected impedance at 20 Hz at max output voltage to have a good load line and XL>700 ohms at 20 Hz with about 0.1V on the primary for good small signal response.

i.e. for question 3-would smaller core mean more turns for given inductance, which means hi end would suffer

***The high end doesn't have to suffer for more turns on a smaller core. There are many tradeoffs that are made in the design of a transformer or inductor. For example: interleaving and winding techniques can offset having more turns. Remember: 1. Touting just one parameter is faulty and preys on the marketing fad of the week. 2. Every time you improve one parameter, at least one other parameter changes to the worse.

- and even if you do the inductance per 1000 turns, does that tell the whole truth i.e. about core saturation and lf handling? any other 'basics' about the lf side of a transformer? it all seems a bit confusing.

***It is easy to get into trouble with just the basics.

***Most transformer houses are reluctant to specify all their parameters for many reasons: These include: having too many customers don't understand the tradeoffs, giving away enough internal design parameters that someone could undercut the price while not offering the same quality, customers rejecting parts because they are a reasonable percentage off from nominal or they measure the part incorrectly, the raw iron and insulation manufactures not controlling their nominal parameters tight enough to guarantee a number (even when they are the best money can buy (note 1)) and many more reasons.

Note 1: It is common to see 2:1 variations up and down (4:1 total) in some transformer parameters. . .My pet peeve this year is open circuit self resonant frequency (SRF.) On high performance parts, the design with the lower (SRF) can often be the better part! With the same coild, if you trade low nickel laminations (good stuff) for M36 (nasty stuff), the nickel will have a much lower SRF because it has high inductance at the high frequencies.

Play safe and play longer! Don't be an "OUCH!" casualty. Unplug it, discharge it and measure it (twice) before you touch it.

Oh!. . .Remember: Modifying things voids their warrantee.