The Minimalist Superhet Radio project.

[Murphyv310]

A single switchable long wave frequency would be an idea. With the uncertainty of RTE's future on 252 only R4 on 198 is really the last usable station left.

[Geordie McBoyne]

Hi Trevor,
               looking though the circuits of the valve radios in the 1958/59 Radio and Television servicing book we find that most manufactures had adopted the additional tuning capacitor across the local oscillator coil primary for long wave coverage.  However, the full LW coverage of 1000 to 2000 metres can not be quite as good as the separate oscillator coil.  

The oscillator coil was fitted yesterday.  A bracket was found to mount it well away from the rear edge of the chassis.  The original TRF coupling coil was fitted on the rear edge of the chassis but because the new coil has an adjustment screw for the tuning slug I had no other choice but to fit it in the position shown in the attachment.  The oscillator coil is now closer to the waveband switch which can't be a bad thing, keeps leads short.

Geordie McBoyne.

[Geordie McBoyne]

The complete frequency changer stage employing the 7S7 valve has been completed.
According to the rule of Glasnost I must report the performance of the receiver is not very good at all.
Only one strong station can be heard with sufficient volume and quality, a few other stations can be received but they are very weak. The problem is I'm sure with the aerial tuning coils. There is a distinct chance that I have wired the aerial coil incorrectly. It has three windings, long waveband, medium waveband and the aerial coupling winding.
To test the aerial circuits the 7S7 valve is removed. A crystal diode is connected to the hexode control grid valveholder tag. A 470Kohm load resistor wired in, the test meter or scope will be connected across the load resistor. A signal generator will supply 500 to 1500Khz and the various resonance points recorded on the scale. The long wave coverage will also be checked. The set must be disconnected from the mains for these test procedures.

Geordie McBoyne.

[Geordie McBoyne]

The  frequency changer stage employing the 7S7 valve has been completed.
According to the rule of Glasnost I must report the performance of the receiver is not very good at all.
Only one strong station can be heard with sufficient volume and quality, a few other stations can be received but they are very weak.  The problem is I'm sure with the aerial tuning coils.  There is a distinct chance that I have wired the aerial coil incorrectly. It has three windings, long waveband, medium waveband and the aerial coupling winding.
To test the aerial circuits the 7S7 valve is removed.  A crystal diode is connected to the hexode control grid valveholder tag. A 470Kohm load resistor wired in, the test meter or scope will be connected across the load resistor.  A signal generator will supply 500 to 1500Khz and the various resonance points recorded on the scale.  The long wave coverage will also be checked.   The set must be disconnected from the mains for these test procedures.

Geordie McBoyne.

[Murphyv310]

Hi.
I loosely coupled 455khz to the ECH81 on my set and used this as a BFO, results were surprisingly good with the anode bend detector lifted out of its zero operating point. I heard many amateurs on 7 & 14Mhz.
I may add a permanent BFO to the set as a result.

[Murphyv310]

Hi.
I've done a short clip of the results using the signal generator loosely coupled to the FC. The camera battery was just about to expire hence the quick ending
https://youtu.be/aXLy_tql9Js

[Geordie McBoyne]

Finally identified the connections of the aerial input coils. After adjusting the trimmer on the tuning capacitor the frequency coverage is 550 to 1600Khz, that's perfect. The 7S7 valve will be reinserted and the receiver will be tested again as a superhet.
Will report my findings later.

Geordie McBoyne.

[Geordie McBoyne]

Change of plan.   I going to rewire the chassis as a full spec superhet, that's one with a proper IF amplifier stage.  The second IF transformer is fitted under the chassis in the same manner as many small Pilot and Philco radios.
Valve lineup can be either 7S7, 6K7G, 6Q7G and 6V6G/GT.  Alternatively, 7S7, 6B8G double-diode-pentode and 6V6G/GT.
6B8 can be used as a reflex IF and AF amplifier.  Not a nice arrangement though.

Geordie McBoyne.

[Geordie McBoyne]

Decided the reflex IF amplifier is not a nice idea.  Another idea was to employ the 6B8G simply as an IF amplifier and use a high slope output valve such as the 6AG6G or EL33. The minimalist superhet will be redesigned and will become a four valve receiver + rectifier so the valve line up will be 7S7 frequency changer, 6K7G IF amplifier, 6Q7G/GT detector, AVC and audio amplifier, Output 6V6G/GT.  The HT rectifier might be a 6X5GT or metal rectifier.  The chosen circuit will be the series 1 Pilot Little Maestro, you can't get anything more minimalist than that receiver, it has a very low component count. Original 6A8G pentagrid frequency changer replaced by the much better 7S7 triode-heptode.  Like many small Pilot and Philco radios the second IF transformer will be fitted under the chassis.   Rewiring of the chassis has started.

Geordie McBoyne.

[Murphyv310]

Not really a minimalistic superhet any longer with 5 valves if you use one of those useless 6X5 things.
I still think there are ways to improve the Anode Bend Detector. For example use an EF91 or EF80. Perhaps an EF184 would work even better and the addition of some positive feedback on the stage would improve small signal performance. Getting the bias right is important as the valve is turned off and if the bias level is too high then weak stations disappear. it is quite hard to get "reaction" to work well on an anode bend detector as if the valve is biassed off it won't work then when a signal is present it takes off. Ok so there a challenge for a budding home constructor!!!