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Maco/ Brute 75
- Thread starter 238
- Start date
Well i have changed all the caps except the ssb cap. Changed the coax inside to 316 grounded on both ends. Moved the plate control to the front of the case. I still have a little hum in the amp, when i transmit, my voice covers it up when i talk. I have noticed on my Super 75 the coil between the tune & plate control has been tapped back so you only adjust a cross 4 coil. The Brute 75s adjust a cross all 12 coils. If i turn the tune all the way to the right the hum gets lower. But i run out of adjustment. 238
The Plate Tune control should peak with the plates between the two extremes. It has no end stop, so if it gets to the end of its range, this will appear to be a peak on the meter.
You can look at the plates to see if they are all the way apart (minimum capacitance) or if they're fully meshed (max capacitance setting).
If the control shows what looks like a peak at either of these extremes of rotation, it's not really a peak. It's the control running out of range before it can reach a resonant peak.
There's a way to tell this from the outside without looking at the plates. If the Plate Tune control has ** two ** peaks in one whole 360 turn, this tells you that it really is reaching a resonant peak.
And if you see just one peak on the meter in one full turn of the knob, it's time to see if the plates are fully apart or fully meshed together.
That will determine how the coil that it's connected to needs to be adjusted.
That control is really just the "fine" Plate Tune control. The coil is the "coarse" adjustment that brings the fine control into range.
73
You can look at the plates to see if they are all the way apart (minimum capacitance) or if they're fully meshed (max capacitance setting).
If the control shows what looks like a peak at either of these extremes of rotation, it's not really a peak. It's the control running out of range before it can reach a resonant peak.
There's a way to tell this from the outside without looking at the plates. If the Plate Tune control has ** two ** peaks in one whole 360 turn, this tells you that it really is reaching a resonant peak.
And if you see just one peak on the meter in one full turn of the knob, it's time to see if the plates are fully apart or fully meshed together.
That will determine how the coil that it's connected to needs to be adjusted.
That control is really just the "fine" Plate Tune control. The coil is the "coarse" adjustment that brings the fine control into range.
73
I have noticed on my Super 75 the coil between the tune & plate control has been tapped back so you only adjust a cross 4 coil. The Brute 75s adjust a cross all 12 coils. If i turn the tune all the way to the right the hum gets lower as the power gets lower. But i run out of adjustment. The plate meshes after i spread the coil a little. The main problem i still have is a hum all the caps have been changed. I reckon that just leaves the transformer. How would i check that. An as always i thank you for the time you take to help us. 238
Two kinds of "hum".
Real, live sinewave 60-Hz alternating current hum. The sound you get if you unplug the cord from an electric guitar only half way.
And 120-Hz "ripple' from one or more of the DC power supplies that power the tube.
A 'scope serves to narrow this down pretty quickly. Lets you see if it's 60 or 120 Hz. Aso lets you see if it's the sinewave "snake" shape waveform, or a sawtooth wave you get from a DC rectifier circuit with a bad filter cap. Or a filter cap that's too small.
My intuitive guess is that the negative-polarity bias voltage has some rectifier ripple on it. The filter cap for this voltage has the positive end grounded.
What size is the cap in your Brute that filters the bias voltage?
73
Real, live sinewave 60-Hz alternating current hum. The sound you get if you unplug the cord from an electric guitar only half way.
And 120-Hz "ripple' from one or more of the DC power supplies that power the tube.
A 'scope serves to narrow this down pretty quickly. Lets you see if it's 60 or 120 Hz. Aso lets you see if it's the sinewave "snake" shape waveform, or a sawtooth wave you get from a DC rectifier circuit with a bad filter cap. Or a filter cap that's too small.
My intuitive guess is that the negative-polarity bias voltage has some rectifier ripple on it. The filter cap for this voltage has the positive end grounded.
What size is the cap in your Brute that filters the bias voltage?
73
22uf is probably way too small. A look at the bias voltage with a 'scope would tell you how much ripple there is on the bias voltage. I would use a 100uf cap for the bias filter.
That's my first guess as to the source of the hum. Again, if you could see the amplifier's output on a 'scope I suspect you would see that sawtooth shape of power-supply ripple modulating the carrier.
Found the folder with the 1979 diagrams in it.
The ECG5221 zener diode is a 105-Volt 10-Watt part.
Here's what the RF schemo looked like before it got marked up.
It's worth noting that the tube's control grid and screen grid are reversed in this diagram. At least they left off the tube socket's pin numbers.
You may or may not need the zener diode.
The exact negative grid voltage needed will change from one tube to the next.
73
That's my first guess as to the source of the hum. Again, if you could see the amplifier's output on a 'scope I suspect you would see that sawtooth shape of power-supply ripple modulating the carrier.
Found the folder with the 1979 diagrams in it.
The ECG5221 zener diode is a 105-Volt 10-Watt part.
Here's what the RF schemo looked like before it got marked up.
It's worth noting that the tube's control grid and screen grid are reversed in this diagram. At least they left off the tube socket's pin numbers.
You may or may not need the zener diode.
The exact negative grid voltage needed will change from one tube to the next.
73
Not usually a transformer problem. A transformer generally either works or doesn't. Don't see a lot of "in between dead and alive" problems.
Hum on your transmit signal tends to originate from ground faults, bad rectifiers, bad filter capacitors, or a filter cap that's just too small.
Leakage current, or a short between the tube's heater and the cathode can do this, as well. Much more common to see in a receiver tube than in a linear.
RF feedback can create odd noises on the amplifier's output even when all the filter caps are functioning perfectly. If turning the Plate Tune knob causes the 'hum' to change that points to RF feedback trouble.
73
Hum on your transmit signal tends to originate from ground faults, bad rectifiers, bad filter capacitors, or a filter cap that's just too small.
Leakage current, or a short between the tube's heater and the cathode can do this, as well. Much more common to see in a receiver tube than in a linear.
RF feedback can create odd noises on the amplifier's output even when all the filter caps are functioning perfectly. If turning the Plate Tune knob causes the 'hum' to change that points to RF feedback trouble.
73
238,
When you are tuning the amp, are you tuning with a modulated signal? Or are you tuning for max dead key watts?
73
David
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