Quote:
Originally Posted by old_coot88
Dude, just to reiterate, check the heater wiring to verify that the 1st audio tube's heater is at the bottom (ground) end of the string. If it's not, it could be introducing hum downstream of the vol. control
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I will add the common mode fixed AC hum and thd+n high distortion is present at VOL control center tap and other end presumably from the 12AT7 First Audio AVC dual-triode > Diode plates that are parallel wired in this radio.
Including the 2 Vol.pot caps and 50C5 final beam power tube correct uF cntrl.grid cap ,
I have 4 NOS not spendy tubes shipped and ~10 discrete circuit caps with the uF # @630v values from Hayseed Ham-fest ready made made cap kit parts listing for this radio but USA name brand caps not whatever they sell there and noting new Lytics are installed in this radio anyway so pricing was more favorable this way and total $2.50 USPS .
Instead of getting stuck up for small order at Amazon, E bay, Mouser or Digikey
I found them all cheap at Tube Depot store with the over priced tubes I wont buy there :LOL
If all that does not fix it I will jimmy rig a sig. tracer and diode detector with a Ti PDIP- 8 op amp and some buffer caps and a battery & whatever else it may I need all I probably have sine its nit tube radio stuff & I can use the Senns. HD280 phones and for the amp/tracer a capacitor /diode detector series probe for the radio sig path before AF conversion ofc with a cap uF verified that passes the AC hum and see if I can find the origination of the AC hum if it is not the bloody tube heater wire order you kindly informed me of ,they could have skewed that too , noting the 50C5 outpt. cntrl. grid cap is way wrong uF from factory ,will keep advised,
OTOH i could use my Schitt Audio Magni headphone amp it is very quiet and runs on DC wall wart .what do you think about that + diode and cap probe instead of op amp and maybe a cap on audio in to HP amp ground or no ?
full stop.
regards