Fred
I have printed out a copy of the circuit of your radio from your PDF file and have been using it as a guide as I traced out the circuit of my radio.
It is similar, mine has all cathodes grounded, with the CT of the secondary of power transformer going to ground via 2 470 ohm 1 w resistors in parallel. The negative side of the HT filtering goes to this point.
Mine uses the diode section of the 6N8 for demodulation and not the diodes in the 6AV6.
Still rechecking my circuit diagram for accuracy with the actual radio.

Mine uses the diode section of the 6N8 for demodulation and not the diodes in the 6AV6.

If the 6AV6 is only used as a triode and nothing more, and if you were into modifications, you could could replace it with a 6AU6 wired up as a pentode. The radio would be a really super hot performer.

I have the transistorized version

and its dial:


It's only MW AM though. Notice the "missing" 5th knob. IIRC, it has a 3 gang tuning cap. And a ferrite rod antenna.

It's happy with American 240V 60Hz.

Hi Robbert, I love hotting things up too!

I think the designer used a 6AV6 to keep the overall gain of the audio section down.
The front end being RF/Mixer/IF will have oodles of gain and selectivity and the detector output will be like mine at the Volts level with a short aerial!
Thus to get that volt level to tens of volts to drive the 6AQ5 to clipping, only needs a low gain triode, almost only needs a stage there so you can have a volume control!
I note mine has damping resistors across the IF coils, I suspect a bit of damping there to flatten off and reduce the IF response again indicating an abundance of gain in that stage.
The "Farm set" here has a very good signal layout for stability it looks like a bit of thought went into it.
Probably influenced by designing TV IF strips.
Souping up the audio would probably just make the thing unstable for no extra gain performance gain.
I know what happened when I souped up the audio section in my "Little General" a previously stable front end went barmy!

I guess you could call these sets the "cutting edge" of design at the time using the influence of TV design.
Fred.

The 6AV6 is a high-gain triode with a gain of 100, like the old 6B6G which preceded it. I guess if you wanted lower gain, which might give smoother volume control action, you try a 6AT6 which is plugin-compatible. Of course there's probably another dozen different valve types that can be plugged in safely. One could even try out the various types to find which seems best.

There seems to be a lot of same valves hidden in others. If one refers to the valve data it can be very interesting. I am of the opinion that 6M5 appears in other valves and 6CK6, is a 6M5 with the suppressor grid connected to pin 6 instead of internally to the cathode as in 6M5.

In a pinch I have tied pin 6 to cathode & used one. The pentode in 6GW8 also looks suspiciously like a 6M5 spec. (why reinvent the wheel)

In that under chassis shot I realised I can see two strategically placed tin plate shields, one between two coils and one about the IF stage.
That indicates a bit of extra care taken at getting the best stability.

I think Marcc mentioned once in a post about leakage of signals between sections in a radio.
I can vouch for that having made unstable circuits as some plate leads act as little transmitting antennas.
That can spray, RF, Oscillator and IF all over the place! .

Then there is always a grid lead somewhere that picks that up and you can have distortion with audio circuits overloaded by RF or just plain instability with RF to RF feedback. Ditto if you use an "unshielded" valve in place of a shielded (internal) type.
The more I look at the Calstans the more I see experience in laying out a stable high gain (for the time) valve line up.

Its interesting to scope a valve plate lead in a radio and look at what is actually there. An audio valve may have RF and IF components in it as well as the audio signal or an IF with AF !

Pity about the slap dash wiring and component placing, although on reflection that may be part of the stability layout and I guess they were made for a price. I have tag strips holding most parts in my designs, i'll bet that was a luxury in the 1960's radio factory!
Fred.

I have indeed mentioned some of the horror stories that invariably arise when you have seen a lot of radios. It is true that many were made for a price in a competitive market. It is no different now, lots of people buy on price, often without a lot of choice.

It is sometimes amazing what an Oscilloscope sees, that you had no idea was there. It is but one reason why I use one as the meter when calibrating. In a hacked Autodyne set (Screen Grid Circa 1932) where I had lost signal, the oscilloscope (being used as a signal tracer) was showing a secondary oscillation at the mixer?

That was the common error related to lead dress. The second detector first AF is the worst area for that, however, in this one it was related to running the mixer screen wire a long distance parallel to the plate wire then crossing an oscillator wire across it. To help make it happen, one wire was not close to the chassis.

Wires floating through the air are antenna's or radiators, & the signal can induct. Never unusual to find some RF in the OP tube. Sometimes a little regeneration is fine, but it can destabilise. If there is oscillation in the Det / AF, or before that, that can apply enough AGC voltage to really desensitise the set (which is its job). In one set running unshielded RF wires through the audio was crazy.

Unshielded volume & tone wires travelling long distance are a constant issue. & some modern caps, especially disc can be inductive.

The miniature valves tend to be internally shielded, but some end up needing more. 6AV6 can be really unpredictable.

Being tidy & thinking about the layout & careful shielding can result in a really good set. I have seen shielding that is a mere decoration.

All part of the entertainment when trying to fix them & some were never designed for fixing.

Marc

Have finally traced out the circuit of the radio. It is different to the circuit of your radio Fred.
have sent copy of pdf of circuit for Brad to upload.
Would be interested get and idea of the working of the radio as I am not real good with my cct theory.
The coils mentioned as can be seen in the under chassis photo, I have designated from top going cw,
L2 SWA L1 BCA and the coil AC1 can be seen as the coil mounted above the chassis.
Will add more photos later.

Calstan Model 607 Circuit Diagram

Document uploaded to Post 24.

Hi Norm, I like hand drawn circuits, always a bit more personal that cad drafting!

I sat down with my coloured crayons and hi-lited the various buss rails, red for ht, orange for screen grid, green for signal path and so on and considered the design.

1/ I think you have nearly nailed it, just a bit of a thing around the 6BE6, where is pin 7?. I think there is a mix up with 1 and 2.

2/ The use of the RF stage is interesting but it makes sense. The have the RF stage active on BC but bypass it on SW. That may sound weird but might be so. You can double check this by looking at the coils and make sure the coils you have labeled SW have the lesser turns.

3/ The coil you have labelled BCA should be "BCRF" and have many turns not fewer turns as it works on BC only.

As Marc points out this is a farm set and probably meant to work on a long antenna, with the greatest variable gain and selectivity on the BC band to sort out all the competing state stations. On SW they have dumped the RF stage and have SWA driving the 6BE6 convertor directly, but with no grid bias so max gain on lower signal strength, the AGC only "works" on the 6N8 if any is generated on the SW signals. On BC the 6BA6, 6BE6 and 6N8 have AGC applied to stop overload on varying local or long distance signals.

The rest of it is pretty normal, with switching for a PU entry and a lot of care about the feedback and tone loop to help with SW listening.

How does that sound?
\Fred.

Thanks Brad for uploading circuit diagram.
Hi Fred
Thanks for your knowledgeable input.
Pin 7 on the 6BE6 is the screen connection that goes to TC3, I have incorrectly labeled it pin 2 as pin 2 is the cathode connection.
The coil you designated as BCRF has a large number of turns, L1 has more turns than L2.
As can be seen on the under chassis photo the white wire going to SWA coil is the antenna wire and comes out the back of the chassis and is about 2 foot long. Just using this as an antenna I was picking up quite a few SW stations, it seems to be a lot more sensitive on BC.

Nice job tracing the circuit.
I particularly like the neg feedback network around the audio stages, and using the tapping on the volume control giving loudness boost at the lower volume levels, also convenient power switch on the vol control. No twisted power wires here hope there's no radiated hum.

One minor point, looks like there is no grid leak resistor to ground pin1 6BE6 osc grid...

Hi Wirelessfan
Thank you for your reply.
There is no grid leak resistor on pin 1 of 6BE6.
The under chassis photo is how I received it.
I have rewired the mains and included mains fuse, will have photos latter.

Not a great fan of fuses mains side. I have noted before a Thorn that arrived here with a fused transformer. As the mains side did not overload the only damage the fuse suffered was to its holder: Waste of space.

That set is back biased. One of the tricks was to sail the back bias resistor with the wind. You had it at the lowest wattage you could get away with. OR You did similar to the plate windings of the rectifier. If there was a short, or overload, the resistor fused not the transformer.