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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Dec Sat 02, 2017 12:43 pm 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 9427
Location: Powell River BC Canada
Your radio is of interest. To enhance a good grasp, locate (on line), Radiotron Designers
Handbook, 4th ed, p. 1129 ,Ch. 28, Sect 2-4, Some Characteristics of Reflex Superhet Receivers.

http://www.tubebooks.org/Books/RDH4.pdf

However, an earlier edition of Radiotron's may shed light on as aspect of the problem.
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Amateur Radio Literacy Club. May we help you read better.
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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Dec Sun 03, 2017 10:29 pm 
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Funny Steve, I just picked up your post now, and in fact I have a hardcover of that book and was actually just looking at that section, as well as it's study on various converters including the 6A8. Working through the voltage chart now and finding the plate voltages to be on the low side, by about 20 volts.


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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Dec Tue 05, 2017 10:55 pm 
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Location: Victoria, Australia
The normal variance for voltage is +/- 20%. Having said that I would take a serious look at the filters. There is a risk with modern electrolytic caps of using underrated ones & having them punch through. This being the result of the "B" Voltage surging to double operating voltage, prior to heater tubes conducting. Where I suspect a cap of punching through, or being faulty (new is not a guarantee) I have the facilities to leakage test it.

With a Back Biased set like that, getting the first filter cap wiring wrong, is common. It has voltage greater than the "B" voltage of the tubes on it, many forget that. That capacitor and sometimes the second one does not go to a Negative chassis: It goes to the CT, (most negative point) which is floating above the chassis, normally by the control grid bias of the OP tube. Unless the current through that back bias resistance is correct: It will not work properly.

If the first filter cap is wrong, that can produce hum. If the value of that first cap is lowered, that normally causes a voltage drop.

The other thing that will cause hum & distortion is overload. Overload may be wiring mistake, or a defective component dragging down the volts. I have seen more than one example of a bare metal jacketed cap used as C1 in a back biased set and then it being clamped to the chassis. That destroys the back bias. I have also seen the cap plate to grid on the reflexed tube leak. That will make the tube behave more like a diode & draw more current.

I think this needs walking away from for a while & then you can come back to it with a different approach.

Marc


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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Dec Tue 05, 2017 11:16 pm 
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Thanks for that very helpful info. And you’re right, I am taking a break from this for just a few days as I just found that the power transformer is getting too warm for my liking-as in too hot to touch the laminations. The plate voltages are “only” 30 volts lower than spec, so I’m not sure if there is too much of a drag or not. the 6K6 is sucking the most current but I haven’t gone deeper yet to see if it’s out of the ordinary. Back to it on Sunday!


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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Dec Wed 06, 2017 3:02 am 
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The major issue I have with many schematics is the lack of information in respect of voltages & current. At the opposite end of the spectrum are particularly the German ones, which often list element currents and voltages.

From the latest info, the voltage across the back bias will tell you the current draw, and a pull down of 30V says overload. You can add up the plate & screen currents from the tube data to get a guide as to the "B" current draw if is not listed. Do not assume the problem is B+ alone. You could have a heater cathode short, or a wrong resistor etc.

As said before taking into account voltage dividers, I have before today externally powered "B"; heaters / filaments off to see if it maintains voltage. Chassis is positive relative to the centre tap.

Marc


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 Post subject: Re: Modulation Hum with Distortion - Stumped
PostPosted: Dec Wed 06, 2017 9:37 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 9427
Location: Powell River BC Canada
" I just found that the power transformer is getting too warm for my liking-as in too hot to touch the laminations. "

The Canadian G40 used a 25 and 60 Hz transformer. S-2067 is the part number of the 25 Hz one.
R9 , 390 ohms , should have - 15.5 volts across it.

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VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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