The "Goldberg" -revisited-
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Nachbaur's"Goldberg" (4 tubes)
Evan's Improvements to the original design
New 6v Tube lineup - 12AU7, 6GY6, 12AU7, 6AL5
The real difference/s between Fred Nachbaur's "Goldberg" original and this redesign:
This is the best sounding TX I've built to date. (The new Power TX's are also excellent quality TX's and a bit simpler to build.) Fred, said that his Goldberg is best peaked/tuned with an oscilloscope. I would like to change that comment slightly. After building this TX the oscillator should be tuned by ear! Once you obtain the best volume and crispness, tune the RF output and the antenna using an oscilloscope, but do not alter the oscillator. This will save you some frustration when tuning and shorten the process for you.
Fred said it was the only one he listened to after building his original. With the modifications I've made to linearize the audio even more, I can only believe this new "-revisit-" would have improved the experience.
Check out the video at the bottom.
Click on the image below for a larger schematic.
Diode D1 is installed to protect the 12AU7 triode tube at turn-on, when the cathodes are cold and not conducting. This will help extend the life of the tube. However, do not power up the TX unless the 12AU7 tube is installed, otherwise R1 will see the full force of B- and will not last long! Fred Nacbaur omitted this and you can also, but your triode life might be somewhat reduced. (See FAQ's below, common cathode amp)
TX Power Output 73% more - as mentioned by Fred?
No, I cannot verify this. Of course, Fred was referring to "The Gypsy" his 3 tubes TX.
So if you're looking for more horsepower and you're only using a wet noodle antenna (A wire used as a radio antenna.) you might wish to try the RF output circuit shown below instead of the MU Follower. I recommend the JJ EL84 or a 6BQ5.
Note: To provide power to each tube, the transformer filament winding should be capable of 6v @ 2 amps. So the Hammond 269AX would be the perfect choice.
The print below shows the TX with a modified RF output using an EL84.
The schematic/s above always show the latest updates.
The pictures below may not show every update.
Click on any image below for a larger picture.
The ribbon cable you see goes to the LED's installed in the tube sockets.
All on a single 6" X 8" top!
To answer your FAQ's
Do not use a radial lead power inductor on the plate of the oscillator tube, use a true RF choke. Antique Electronic Supply will sell you what you need. If you use the radial lead choke, which is smaller and looks better, the oscillator may not be playing at it's FULL potential.
Power Supply Design
Within the power supply there are individually decoupled power supply filter sections. The rationale behind this was to provide good decoupling between the three tubes involved, whilst keeping the voltages reasonably high.
Note that this transmitter uses about twice the B+ voltage of any of the previous designs. This is to accommodate the higher requirements of the mu-follower RF amplifier, and to help linearize the performance of the short-tail-pair differential audio amplifier. However, the supply is set up as a split (bipolar) supply, which allows us to ground-reference the output tank coil.
R8,R9,C19,C10 eliminated hum injection and cathode RF leakage.
Modifications to Fred Nachbaur's common-cathode amplifier
.....................The easiest solution is to place an additional plate resistor in the circuit. This resistor should be shunted to ground through a large-valued capacitor to ensure the benefits of the common-cathode amplifier. Many solder slingers do not bother adding the capacitor, introducing the Miller-effect capacitance and losing gain as a result. Laziness, stinginess, trendiness, and ignorance are the usual suspects.
The common-cathode amplifier shown above is nicely fleshed out: the diode protects the triodes at turn-on, when the cathodes are cold and not conducting. Once the they are conducting, the diode falls out of the circuit, as it becomes reverse-biased and no longer conducts. The decoupling capacitor is in place, which will shield the input from a inverted gain at the plate. A feedback loop returns a small portion of the output signal to the inverting input, stabilizing the gain, lowering both distortion and output impedance.
Modifications to Fred Nachbaur's pentagrid oscillator
Applying a little bit of DC bias on the G3 grid of the pentagrid tube (6GY6) can improve the linearity some. This can be done either by biasing the G3 resistor with a source of DC, or using a cathode resistor from ground to the oscillator coil cathode winding (bypass this resistor to ground with a .1uf capacitor).
Modifications to the feedback circuit
Modifying the circuit as shown below changes the feedback pot to act more like a pre-emphasis control. Instead of feeding back all frequencies equally, the feedback network now favors the low frequencies, causing a greater gain reduction at lower frequencies, hence pre-emphasizing the higher frequencies.
1936 - Model 4586 Sears Silvertone radio listening to the,
(with an EL84 RF output)