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Old 15th Nov 2019, 11:52 pm   #1
lloydwells
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Default TRF sound problem a list of suspects please

I have a large console TRF possibly bodged together out of bits the output valve is a PX4 the set is about 1930 ish and other than some power supply components pretty much unrestored the set has no tone control of course.

When it's in use it will tune plenty of stations but the output completely lacks treble and is slightly distorted bit mainly it's just bassy I have tried three PX4s in it no change and substituted all the other valves no change either I know this is a complete pain with no schematic but where the heck should I look for something that will effect the tone. The set runs the px4 and two AC/HLs as well as the detector ( can't remember what that is )
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Old 16th Nov 2019, 10:02 am   #2
Herald1360
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Default Re: TRF sound problem a list of suspects please

Look for any capacitors across parts of the signal path- anode loads, output tx primary, grid to ground etc. Stab a guess at circuit impedance at that point and see what frequency the cap in parallel looks the same (1/2piRC).

Could just maybe be output tx limitations......

Replace the PX4 with a pentode? Flog it and buy a better set?
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Old 16th Nov 2019, 1:03 pm   #3
ms660
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Default Re: TRF sound problem a list of suspects please

To much regen can cause distortion and no treble, eg, muchas reduced bandwidth of the tuned circuit(s)

Lawrence.
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Old 17th Nov 2019, 12:02 pm   #4
boxdoctor
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Default Re: TRF sound problem a list of suspects please

If it was somebody's own design and layout, a design or construction flaw may have resulted in audio instability which may have been "cured" by a treble-cut capacitor being fitted somewhere, most likely across the speaker transformer primary winding, or the input circuit of the output valve. Tony.
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Old 18th Nov 2019, 6:11 pm   #5
Neil Purling
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Default Re: TRF sound problem a list of suspects please

If the set was home-brew it could plausibly have been made post-war from a stash of old parts that progress had rendered obsolete. In the absence of an image of the chassis I cannot do much but guess.
Have you tried sending the signal from the detector to another amplifier & see if that has the same audio issue?
My experience of TRF's has been with post-war 6.3V pentodes regenerating by varying the screen-grid voltage. One of those loses the bass response at the point of oscillation.
Have you looked for capacitors between the PX4 anode and chassis? I have used 0.01uf @ 750V to roll-off the treble. Maybe the original constructor has undesired instability & was struggling with RF getting through to the o/p stage. Many designs had a choke in the HT feed to the detector's anode to keep RF off the HT line, or reduce it.
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Old 18th Nov 2019, 6:25 pm   #6
G6Tanuki
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Default Re: TRF sound problem a list of suspects please

Try - as a test - feeding the output to a different speaker.

In the past I've seen plenty of old speakers with rust between the pole-pieces and the moving-coil, the moving-coil's former being only loosely-attached to the cone, the cardboard of the cone having age-degraded to something with the structural-rigidity of toilet-paper, the locating 'spider' in the centre of the cone breaking and letting the moving-coil foul on the centre-part of the magnet....
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Old 18th Nov 2019, 11:53 pm   #7
PJL
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Default Re: TRF sound problem a list of suspects please

We would need a circuit diagram with component values. If it is a homemade it may simply be down to design/build errors. You could start by posting pictures.
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