Today having a bit of time have begun work on my WLRX. Originally I intended to design a minimal solid state design using modern ICs for everything, but decided to go with vacuum tubes, I have hundreds laying around, all looking at me sadly, wanting my attention.
Going with a rough block diagram of what I need, have bent up an alluminium chassis hopefully big enough to hold enough valves, and around 25 coil cans.. Sorting my valve stock, I can either use miniature 13/15 7 pin types, or the 9 pin european type multis. To decide this, Ill need to have some idea of a schematic and finalise signal flow. Using multi element valves, eg ECF80 triode/pentodes will reduce valve count and real estate needs, but will complicate wiring. With multiple conversion stages it will need the signal doubling back here and there , and without care, it will interfere with itself. For instance it would be folly to use a spare triode in an IF amplifier valve as the BFO oscillator......
The 7 pin valves are probably the way Ill go, but I have not got any 7 pin valve bases as yet. The next issue is heater current, either type take 300mills each at 6.3 volts X 15 = 30 watts or 4.5 amps of heater current, thats one big heavy transformer, and we are gunna need 50 odd mills of HT current too.. Series heater stringing will need 100 volts at 300 mills, still 30 watts but may be possible with a small, light and modern SMPS supply. With the radio covering 100kc to 30 megs, its highly likely this will cause birdies somewhere in the range.... these supplies are filthy with EMI/RFI.
Im wanting to replicate a Racal RA17 without the size and weight. If there is any interest, I will document my progress.
preselector/ RF amp 6BA6
1st mixer 6BE6... if these things even work at 70 mHz???
harmonic generator 6BA6
VFO 6C4 (mHz tune) 37.5 to 67.5 mHz
2nd mixer 6BE6........ if these things will even work at 70 mHz???
37.5 meg IF amp 6BA6
40 meg loop amp 6BA6
3rd mixer ECC81
2nd IF 6BA6
4th mixer/osc ECH81 (kHz tune)
3rd IF 6BA6
3rd IF 6BA6
BFO/product detector ECH81
AM/agc detector/1st audio 6AV6.... may omit thisone and just use a semiconductor diode
Audio output 6AQ5
Magic Eye EM80..... for bling appeal only.....
The front panel is a complicated thing requiring 3 dial scales, one for preselect frequency, another for mHz selection, and the third for kHz tuning. Hoping to use simple dial string arrangement for these. Then add Volume, Mode, some kind of lock indicator and wide/narrow filter selection, AGC fast/slow, and RF gain. If any room left, a front speaker too.