Re: {Collins} 75A-4 Issues - Hum V13 DC filament

Note that AC voltages are usually stated as RMS. The RMS value of an AC voltage is its DC equivalent. You are probably not hurting anything by running the filaments at 5.5 volts DC but its the same as if you were running them at 5.5 VAC.

If increasing the value of the coupling caps increases the hum it means the hum is coming from an earlier stage. Feedback may reduce distortion but must be applied with some care. The amplifier must have reasonable phase linearity over the frequency range the feedback works or you will have problems. As the phase changes the effect of the feedback is reduced until you reach a point where the distortion will increase. At some point the amplifier becomes unstable. Sometimes this is at an ultrasonic frequency or the oscillation occurs only on peaks or some other unpleasantness. One of the main problems with the simple amplifiers used in many communication receivers is that it puts DC on the output transformer primary. The DC tends to saturate the transformer core which limits its low frequency response and also tends to produce distortion. Push-pull amps also have DC running through the primary but the two halves are opposing so the effect of the magnetization is canceled. It seems to me that the designers of these receivers just assumed that the audio quality needed was not very great and never listened to them. In fact, the harmonic and especially intermodulation distortion of these rather crude amplifiers causes the effects of noise and interference to be exacerbated. Even CW is easier to read through a low distortion amplifier. I don't know of any simple way to fix this. Going to a shunt-fed transformer results in low efficiency of the supply plus has some other problems and converting to push-pull is probably not practical in most receivers. Using an external amplifier fed by the detector is perhaps the best solution.

Richard Knoppow
Los Angeles

----- Original Message ----- From: "Bill Cotter" <n4lg@xxxxxx>
To: <collins@xxxxxxxxxxxxx>
Sent: Sunday, December 09, 2012 8:34 AM
Subject: {Collins} 75A-4 Issues - Hum V13 DC filament

WARNING: Those that abhor modifications to BA gear - quick, leave now!


I have struggled to eliminate 60/120Hz hum from
my 75A-4 (SN#2887 W3HM restored), which was
exasperated after I recapped the AF stages with
0.1uF caps in the audio chain to improve the low
end fidelity, and the negative feedback mods to
lessen audio distortion. I have moved wires
carrying AC around the chassis, shielded and
relocated the AC switch leads, all to no avail.

The problem turned out to be AC filament pickup
in the first audio stage (12AX7 v13). No amount
of swapping tubes cured it. As one of the members
noted, the AF chain has tremendous gain. I
finally converted the filament supply for that
tube to DC, and all my struggles went away.

Email me for a schematic of my mod. Note: I run
the filament closer to 5.5VDC to compensate for
the RMS power when on 6.3VAC. The mod is done on
terminal strips stood up on hex spacers screwed
into the IF screws, making it easily reversible.

73 Bill N4LG

At 03:38 PM 12/5/2012, you wrote:

For those of you with 75A-4's ... do this little "test;"

Turn RF Gain full CCW and turn up AF Gain to listening level. Switch back and forth between AM and CW-SSB. Do you note a significant increase in hum in the CW-SSB position (and nearly none in AM)? I have done with with five 75A-4's and they all hum in the CW-SSB position.

I realize we seldom use the receiver with RF gain full off (hi) but it's an interesting test. Why the hum? Where is it coming from?

73, Bob


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