Ohmite offers bolt down fractional value resistors in metal casings of 10 and 25 Watt values.

For a single 3-500Z dropping 0.5 Volt at 15.5 Amp requires an 0.17 Ohm resistor and will dissipate 7.25 Watts of power.? Bolt the down under the chassis near the base of the tubes.? This will place the resistors in the cooling air blast for the tubes.? Use one resistor for each tube.

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I have two Dale 0.02 Ω 25 Watt sitting here in front of me that I ordered from Mouser.? I plan to use them in two amplifiers that have a single 3CX1500A7 tube in them.? I too have high mains voltages that I use a Fluke meter to read.

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73,

Dave, w6de

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Not to belabor the point but it is wise to monitor you incoming voltage and note that at times it will change. I have quite a bit of classic equipment that was designed to run at 115~117VAC. I recently purchased a Heathkit IM-103 and calibrated it with my Fluke 85 III. What’s interesting to note is the incoming leg(s) is 113~117VAC during the weekdays and jumps up to 122VAC during the weekend.

I’ve had incoming once at 135VAC. When lights started ‘burning out’ in succession, I checked the line and contacted the power company, (PPL). They confirmed what I was seeing and rectified the problem that evening. ± 5% is their usual tolerance from what the lineman told me when he did the repair. It was actually a capacitor bank that fed our line so I wasn’t the only one with the problem.

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Mod-U-Lator,

Mike(y)/W3SLK

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Additional comments and another choice for your TL922 3-500Z filaments.

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I just downloaded and examined the TL-922 amplifier service manual.? I traced the input power circuit and the TL922 doesn’t have any surge protection on the input circuit.? Instead of using fixed value resistors in your filament circuit you might choose to use Power NTC Thermistors (NTC = Negative Temperature Coefficient).? These start with higher resistance and when hot reduce to lower resistance.

E.g., for a single 3-500Z two 2 Ω NTC resistors in parallel would offer 1 Ω at power on and drop to 0.018 Ω after 130 seconds.? 10 Watts dissipation with two in parallel with a Bournes BN-LG25Y2R0Mx. A parallel pair of these for a single 3-500Z would meet your needs for a 0.5 Volt drop and offer inrush protection—but require approximately two minutes to reach operating temperature.

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I just measured my 240 mains voltage and the line voltage is now at 240 Volts.? I then recalled that when I called my Electric utility (PG&E) and they came and measured my mains voltage it was 250 Volts.? The service man said they increase the mains voltage in the summer time to better handle the Air Conditioning load.? Not good,? I may have to rethink the values of the resistors I put into my 3CX1500A7 amplifiers.? Don’t target the exact 5 Volts but straddle the +/- tolerance limits. So, the filament voltage to ensure the summer doesn’t go to high and the winter voltage doesn’t fall to low.? I have some notes about the summer line and filament voltages and I’ll have to now take some notes about the winter mains and filament voltages.? I’m not going to assume it is a linear drop across the amplifier circuitry for the filament voltage.? One of the amps is an Alpha 9500 with so called automatic line voltage selection [whose upper limit seems to be 240 volts] and the other amp is a Henry 3002A 2 Meter amplifier.

73,

Dave, w6de

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The meter one uses for AC mains or any AC measurements should be a TRUE RMS reading meter to be accurate.?

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Many of the higher quality Fluke ?meters (read higher priced ones) are True RMS reading AC meters but not all Flukes are.

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There are other brands that are as well.? If they are the spec will say so.

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73,

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Walter

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Before retiring and while still active in the field, precision

D.M.M.’s were important, not in themselves, per se, but as

a reference point in any possible report ensuing from having

to have employed a meter at all.

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Being able to say ” “a Fluke 89-4” multimeter confirmed the

proper voltage,”” would immediately be acceptable as an

observation deemed correct.

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So there in and there out different Fluke meters were supplied

to people doing these reports, and they were deemed calibrated

when received, but no Proof of Performance quality checks were

ever specified to even see if those were correct…

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As the years wore by the collection of those ubiquitous yellow

rubber handhelds accumulated. I remember the last time I

sent in the ’89 to the factory, only to be told:

1. We don’t have any more parts to fit the meter, and,
2. We can’t do any minor repairs, and,
3. We can’t do any more calibrations.

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Well, the battery holders were turning to cheese and in order to

get the meter to come up it was necessary to jury-rig a connection

between the AA cell compartment and the meter, proper.

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Then, the AA battery holder hardware turned to cheese on the ‘289,

and Lo! and Behold!? “ Fugeddaboudit.” There are no parts to fix it.

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There are other meters, newer models, and they seem to be working,

and when setting up a bench to observe a “standard,” the readings

are all over the place. Fluke. Thousands of dollars to buy and then,

when the rubber needs to meet the road, they can no longer be trusted.

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A case-in-point:? Recently an experiment regarding the various measurements

of signals and amplitudes came up. Both the ’89 and the ‘289 can be

configured for dBm, and even the reference load values adjusted for

a wide range of activity, like 50 or 300 or 600 ohms.

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Even with a new set of AA cells, and a proper warmup, using the meters

to show what the new bench supply says is “something” in the 50 ohm

dBm Land, the readings are different. Not too, too different, but still not

right on the money as Fluke would have been relied upon to produce.

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So yes, there are a new set of DMM’s coming out that are Orders of

Magnitude cheaper than new Flukes, and best, is that many of them

come with certificates of calibration right in the box.

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Forums like the eev blog compare quality and consistency to test

gear day after day. They are the first to raise a ruckus when something

is squirrely.

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Hal Mandel

W4HBM