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Those are still relatively easy to find as you probably know. Two in parallel would limit current to 1 amp. That's enough to begin seeing plate voltage, fan movement and some filament glow.

That radios 8 volt regulator could be 40 years old, isn't made anymore, and is becoming a common failure. I would be hesitant about adding more load to that regulator and as previously mentioned, just install a three-terminal 8 volt regulator. When the MB3756 starts to fail, you notice it first...

The data sheet on L versions also has a derated specification for linear operation. That was a bit surprising. I also took my own advice about using the Mouser website part locator to try and find an inexpensive FET that would work in these applications. What I found was, unless you drop to a...

Not for nothing however, at this frequency how hard do you think it would have been to shift the resonance point one megacycle above 10 meters, rather than one megacycle below 10 meters? I just find this to be a little bit more than coincidental.

Once you get the rectifiers disconnected and the associated bleeder resistors downstream, the 100 watt light bulb test becomes very usable to test the transformer. Now it should be easy to get the filaments to light, through the resistance of the light bulb filament. Although honestly, at this...

If you can find one today even a 100 watt light bulb has an internal resistance of over 140 ohms when lit. It's not unlikely to have 100 watts worth of bleeder resistors in the amplifier alone. It probably won't bring the voltage up high enough to spot things like a leaky filter cap or diode in...

The only thing changing that resistor does, is decrease the current and dissipation on the zener diode. It cannot change the voltage at the electrolytic capacitor however, it can stop that diode from heating the capacitor if they are close together. With the 68 ohm resistor, the zener diode is...

I see a blue Cetron logo on one of those tubes. They are one of the highest quality and most robust 572B tubes ever made. Do not be surprised if they are still making full output. I've got some from the 1970s that are working better than new ones today. In a situation like this the first two...

If you're not showing plate current or output power when driven and only show grid current in proportion to drive power, this is a strong indication that plate voltage is not actually arriving at the tubes. Especially if the grid current is abnormally high, because all drive power is then...

Let's be fair from both points of view. The RM Italy uses 6 cheap metal film resistors that do contain noticeable inductance to perform this process. They are bulky and produce lots of internal heat when operated in carrier modes like AM or FM. Unlike attenuators specifically designed for this...

The more I think about this, the more I think we should get another opinion. In my mind, requiring more than 480 pf across the primary of this input transformer, is a strong indication of a mismatch in the turns ratio or, capacitive loading across that transformers secondary. How many wraps are...

The 105 is its lowest limit, set fully counterclockwise. It is 480 picofarads fully clockwise. That is more padding than I expected to be in this location. Therefore, you will need a larger fixed value in parallel with it. Something between 200 and 300 picofarads should find your best match...

The negative feedback cap can be a simple .1 uf ceramic disk. Generally the cap is connected to the collector and the 100 ohm 2 watt is connected to the base. Capacitors that must hold a precise value in tuned RF cicuits like in parallel with the input, should be dipped silver mica. If you can...

While it looks like you've already achieved near perfection, I'll just point out that you can make small changes by bending the inductor coil. I believe this amplifier uses a tightly wound coil that will only allow you to reduce inductance by spreading the turns apart a little bit. Sometimes you...

Bingo! Nobody uses a real 50 ohm attenuator in a CB linear. They cost $20 to $30 for each 6 or 12 db attenuator and require heatsink mounting. However, switching them in line is seamless without any impedance shift. In the absence of doing that, I recommend the same thing as you, adjust output...

The good news is, this VSWR is actually likely to drop lower with the addition of negative feedback. When your trimmer capacitor is fully clockwise, it is adding the maximum capacitive loading to the input and reducing the input impedance. That is the direction that is producing a lower input...

I wasn't suggesting that there will not be a measurable reduction in input impedance, with the addition of negative feedback. I was simply saying that I have never seen anything close to a reduction from 50 ohms, to 16.5 ohms, which would indicate the 3:1 change, that lowers impedance. My main...

I was under the impression the keying relay only stayed latched, after you were transmitting and unkeyed the microphone. If that is the case, this is very likely related to a lack of negative feedback. Changing the transistor probably only made a difference because it has a lower gain and is not...

While I haven't tried it at 12 volts, years ago I used the Mauser part locator - filter on their website to find decent MOSFET's for RF applications. It might work to find a replacement for the ERF-9530 The trick was using this filter to select package style, dissipation, VDD, and most...

I like this transistor because it's one if the last, still manufactured to OEM Motorola specs, even though it's made by Ma-Com today.