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Making a Four Vacuum Tube Radio from scratch - need help with distortion

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Daniel Bingamon:
I'm making this radio from scratch, I'm close to completion but I have a little distortion problem.
My design I call the AS4 - for American Safe Four since it has Transformers for the Heaters and the B+ as well as an Audio transformer.
It has four vacuum tubes, instead of a tube rectifier I'm using a full wave diode bridge.
The B+ transformers is a 1:1 ratio of American mains, loaded it has about 171VDC across the first filter capacitor set.
My outdoor antenna is two slinkies (yes, the metal toy) stretch out in parallel on top my workshop.
There is a 50,000W AM radio station, WLW less than 5 miles away by air - I'm a little concerned about that getting into the set.
The chassis is an old small computer that had two drive bays.  The speaker is on one bay and the variable capacitor is on the other.

A photo of the unit is attached.

And here is a video of some testing with a station playing in the background.

My questions:
1. What could be causing the negative going spikes that are stimulated by increased amplitude?
2. Is a 1M or 500K a better choice for a volume control pot?

SuzyC:
Your problem might be the output screen grid resistor 100k ..should be around 2.2K and with a bypass cap. (20uF)

Should also have .01-.1uf bypass caps near each plate supply to I.F. transformers and maybe a decoupling resistor(1k) between mixer plate supply and IF stages power supply.

C8 is also a rather large capacitance for its intended purpose. L1 should have a tap around 20uH to couple to the slinky ant and this would greatly increase your tuning selectivity. This is needed when nearby to Hi Power AM station.
500K is fine for volume control.

Where's the picture?

Circlotron:

--- Quote from: SuzyC on May 05, 2021, 03:13:49 am ---Your problem might be the output screen grid resistor 100k ..should be around 2.2K and with a bypass cap. (20uF)

--- End quote ---
Given the lowish 171 VDC HT supply I'd say connect the screen directly to HT without any resistor. Quite normal practice. 100K is way way high for a power output stage.

Daniel Bingamon:
The screen measures about 101VDC.  Is that too low?

T3sl4co1l:
That's probably about right at DC, but without a bypass cap it'll completely tank as signal current rises.  Even with a bypass cap, it'll heavily compress the audio (including distortion from the reduced maximum power output under that condition).

You might couple the detector audio output to another amplifier to see if it's linear at that point, in which case that implicates the output.

Speaking of maximum power and matching, what is that OPT anyway?  This?  https://www.parts-express.com/70V-10W-Line-Matching-Transformer-300-040 The wire colors don't seem to match up at all, I'm not too sure.  These also aren't made for DC, it's not obvious how much they can really handle before distortion sets in.

Tim

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