The Minimalist Superhet Radio project.

[Murphyv310]

Hi.
I was chatting to Geordie on the dog and bone this morning about my little set. I noticed after I built the set it was a little unstable on long wave, this was due to the wrong valve of RF filter cap prior to the volume control. It actually improved reception no end on long wave only oddly enough, it tightened up the selectivity and I was able to receive Poland on 225Khz, it is the only set I have heard this station. Reverting to the correct filter cap and it goes.

Well this spurred me on today to add some reaction to the EF91 Anode bend detector stage with remarkable results. I added another IF TX to its anode and fed the output back to the first IF via an RC network and a pot. It is quite touchy but brings up the weak short wave signals immensely, I also was able to resolve some SSB on 7mhz.
I'm going to try and tame it a bit and if it works well enough post up here the circuit changes.

[Geordie McBoyne]

Today, the beat up SenTerCel metal HT rectifier has been removed. The old rectifier will be substituted with a 6X5GT valve. Although designed as a full wave rectifier the 6X5GT was often used as a half wave rectifier, think of the KB "toaster" FB10.  A 1 1/8" Q-max cutter was used to make the hole for the octal valve socket. Because of the introduction of the new valve holder many of the under chassis components will be repositioned. One not particularly nice feature of the circuit design is the position in the circuit of the single pole on-off switch, it is only the in neutral side of the mains which is switched, nasty! Switch the set off and the chassis becomes live. However, a replacement double on-off switch was fitted some time in the distant past so the now the mains will be fully switched and later on a mains fuse will incorporated in the mains supply circuit.                               

I'm sure the existing mains transformer will cope with the additional 0.6amp heater current demanded by the 6X5GT. Total heater current including the 6X5 is 1.7amps. The dial lamp is supplied from a separate winding on the transformer. 
The large 6V6G output valve has been substituted by the much more compact 6V6GT. The 6V6 has a modest slope of 4.1mA/V, in the development mule the output valve is an UL84 has the much higher slope of 10.5mA/V.   

Geordie McBoyne.

[Murphyv310]

Hi David.
Don't let anyone know I've one of these, you ken what those Radio types are like..... a load of Kleptomaniacs, it's yours if you want it, fully tested and working.   

   

[Geordie McBoyne]

Hi Trevor, thank's for the offer of the SenTerCel rectifier, but now the 6X5GT HT rectifier valve is well and truly established in the radio. The radio is working at last, but it wasn't an easy fix. As a TRF this set has absolutely hopeless selectivity, the powerful Virgin Radio transmitter swamps out the adjacent stations Absolute Radio and Talksport. This is the candidate for conversion to become a minimalist superhet.
The W61 RF amplifier, the 6SH7 anode bend detector and the screen grid of the 6V6 output valve are supplied from only 120 volts HT. The anode of the 6V6 gets 240volts. The detector valve is surely on a starvation diet. the anode volts are 93, the screen grid 13volts and the cathode 1.5V. Anode current is about 60microamperes.

Geordie McBoyne.

[Geordie McBoyne]

I have decided the best frequency changer valve for this project is the 7S7, a triode heptode. This valve has a suppressor grid which means it shouldn't have RF tetrode characteristics, has a very good rA figure of 1.25Mohms. gc not bad either @ 0.525mA/V

The 7S7: http://www.radiomuseum.org/tubes/tube_7s7.html

Geordie McBoyne.

[Nowhere-Man]

I learn that the noise performance of the ECH81 is much better than any of it's predecessors.

There is the Telefunken version too as well as the mullard. Both are covered on the Valve Museum with the Telefunken getting more detailed cover.

[Geordie McBoyne]

So much so that contributor Synchrodyne mentioned in an earlier post that because the noise performance of the ECH81 is so good the requirement of an RF amplifier can be obviated.    The ECH81 has the best conversion conductance of all the known frequency changer valves, better than 0.7mA/V.
From the Valve Museum:    
http://www.r-type.org/exhib/aaa0036.htm

Geordie McBoyne.

[Geordie McBoyne]

Fitted the IF transformer today.  The next stage will be to find a local oscillator coil for Medium and Long waves.  Might retain the existing aerial coil if the tracking can be maintained.

Geordie McBoyne.

[Geordie McBoyne]

I've a choice of two medium wave local oscillator coils. The one close to the metal can is a genuine Denco Maxi-Q component. Range 2 is the MW band and the oscillator coil is coded red.  Blue is the aerial coil and yellow is the RF interstage coil.
The coil on the right is I think not a genuine Denco part, however, it is red and has the number 2 on it so it might be OK.  
For the long waveband I'll try that cheapo trick of switching in a 300 to 390pf capacitor across the oscillator tuning winding.  Check out the circuit of the Pam TR26 to see how it's done.

Geordie McBoyne.

[Geordie McBoyne]

Circuit diagram of the front end of the Pam TR26.

Geordie McBoyne.