Lafayette HE-40 "BFO" Curiosity

[AnalogueMan]

I've just finished restoring a Lafayette HE-40, a pretty basic receiver which I believe is equivalent to a Hallicrafters S120 and also appears in other disguises.
The set is working well, but I'm curious about one detail. It doesn't have a BFO as such, according to the circuit diagram the knob labelled "BFO" is actually a pot connected to the suppressor grid of the first IF valve. This pot also has an integral switch that turns the "BFO" off.
When you turn the knob past the initial click to the On position, the first thing you notice is an immediate increase in gain. Turning the knob up more further increases the gain until the stage starts to self-oscillate, thus providing the "BFO" signal, but it does so while still managing to continue working as an IF amplifier.
I've attached a screenshot of the relevant area of the circuit disgram but it's not very clear, particularly in respect to which switch is the BFO on/off one that is ganged with the rotary control.
Can anyone explain what is going on here and how this works?

[Radio Wrangler]

In a pentode, both g2 and g3 are usually at RF ground through decoupling. They have different DC voltages in order to bias the valve into a condition where it has useful gain. These two decoupled grids act as screens to protect g1 from the electric field of the anode. Being grids, they have gaps and so each gives only partial screening, but with both acting, the screening effect is greater. This results in a lower effective feedback capacitance from anode to g1 than if g2 and g3 weren't there.

If triodes are used as RF amplifiers, it is common to apply 'neutralising' circuitry where a phase-inverted version of the anode signal is capacitively coupled via a small capacitor to the grid. This when analysed looks like a balanced bridge circuit, nulling the current into the from the a-k capacitance. This allows the stage gain to be higher, but it also fixes an issue of instability. The feedback can create negative resistance effects, and that, if great enough can promote oscillation of either grid or anode circuits.

The screening of g2 and g3 in a pentode reduce the unwanted feedback from a to g1 to the point where you can get the best stage gain AND have reliable stability without the faff of having neutralisation and having to adjust it.

Sticking a resistor in the g3 to ground connection spoils its screening effect and in this case g2 alone isn't enough to ensure stability, and the thing bursts into oscillation. Oscillation is a sure indication of positive feedback and positive feedback increases gain. As the g3 resistor is increased, just before the onset of oscillation, the gain increases dramatically. A running oscillator is also very sensitive to any signals close to its operating frequency.

That valve is being used as a 1-valve super-regen receiver in its own right, set to the IF frequency of the receiver it is embedded in.

The circuit was used here because it saved a few shillings. Most receivers avoid this technique because it is so easily pulled by whatever signals are around. It becomes a right pain to use in congested bands.

David

[AnalogueMan]

Excellent explanation, thank you.
You're right, it's a terrible BFO in a receiver that's not very good anyway, you can receive CW okay as long as you're happy to listen to several stations at once but SSB is a dead loss.
I'm thinking they must have done a fair bit of experimentation to get the behaviour of this stage just right - if 'right' is the appropriate word. But as you say, it saved them a fair bit of money while still allowing them to claim they were selling you a receiver with a BFO.

[frsimen]

The Hallicrafters S-120 circuit is rather clearer. It shows that when the BFO is switched on, a 470 ohm resistor is connected across the AGC line. That will reduce the AGC voltage significantly and accounts for the observed increase in gain.

Paula

[AnalogueMan]

Thank you, I'll track down the S-120 circuit and have a look.

[M0AFJ, Tim]

The HE30 I had was also dreadful, the BFO drifted like mad, had to have a trimming tool inserted so it could be adjusted during overs!.
I did improve it by building a separate solid state BFO and adding voltage stabilisation in the receiver. Was not sorry to see it go!

[Radio Wrangler]

It was a receiver you could afford, if you saved up a bit as a kid back in the day and were seduced by the artist's impressions in the adverts of G W Smith & Co in PW and SWM. But it wasn't a receiver you'd want to use if you'd ever tried an AR88 or HRO.

It was a thing of its time and of its market segment. An interesting historical artefact but not one you'd want to use today. Getting one up and running is an interesting lesson in what we were prepared to put up with as kids. We were exploring and anything was better than nothing. An interesting illustration of what was actually behind those artist's impressions in PW and SWM ads..... Maybe it's why I've never since trusted artists?

David

[AnalogueMan]

Yes I've got several other receivers including an Eddystone and an AR88, The Lafayette was given to me FOC in a fairly sorry state so I took it on and sorted it. As part of a collection it forms an interesting contrast to the other, better sets, and is a great reminder of that first cheap set that got many of us hooked on wireless

[ms660]

Here's a schematic for the Knight Star Roamer, the IF amplifier/BFO arrangement is almost the same:

https://bama.edebris.com/download/kn...rmr/strrmr.pdf

Lawrence.

[Oldmadham]

Originally Posted by Radio Wrangler

In a pentode, both g2 and g3 are usually at RF ground through decoupling. They have different DC voltages in order to bias the valve into a condition where it has useful gain. These two decoupled grids act as screens to protect g1 from the electric field of the anode. Being grids, they have gaps and so each gives only partial screening, but with both acting, the screening effect is greater. This results in a lower effective feedback capacitance from anode to g1 than if g2 and g3 weren't there.

If triodes are used as RF amplifiers, it is common to apply 'neutralising' circuitry where a phase-inverted version of the anode signal is capacitively coupled via a small capacitor to the grid. This when analysed looks like a balanced bridge circuit, nulling the current into the from the a-k capacitance. This allows the stage gain to be higher, but it also fixes an issue of instability. The feedback can create negative resistance effects, and that, if great enough can promote oscillation of either grid or anode circuits.

The screening of g2 and g3 in a pentode reduce the unwanted feedback from a to g1 to the point where you can get the best stage gain AND have reliable stability without the faff of having neutralisation and having to adjust it.

Sticking a resistor in the g3 to ground connection spoils its screening effect and in this case g2 alone isn't enough to ensure stability, and the thing bursts into oscillation. Oscillation is a sure indication of positive feedback and positive feedback increases gain. As the g3 resistor is increased, just before the onset of oscillation, the gain increases dramatically. A running oscillator is also very sensitive to any signals close to its operating frequency.

That valve is being used as a 1-valve super-regen receiver in its own right, set to the IF frequency of the receiver it is embedded in.

The circuit was used here because it saved a few shillings. Most receivers avoid this technique because it is so easily pulled by whatever signals are around. It becomes a right pain to use in congested bands.

David

Not "superregenerative" as it doesn't have a "quench" oscillator---it's just a plain regenerative stage. In theory, it should sharpen the frequency response of the stage a bit, too.

[ChibaJoe]

On the similar Knight Star Roamer, the BFO switch disconnects the cathode bypass cap as regeneration is advanced. Presumably, full stage gain is desired when there is no regeneration. However, control of regeneration, at or near the onset of oscillation, is smoother, more positive without the cap. Is the HE-40 wired the same way? It's not clear from the schematic.

I think it's better to approach the operation of these receivers as a regenerative with a superhet tuner, rather than as a superhet with a regenerative IF stage. Regen users know strong signals reduce gain, increase bandwidth. Very strong signals can completely shut down oscillation: CW becomes thumps. Sometimes advancing the regeneration control well past the onset point will help. The better way is to attenuate the signal between the antenna and the radio. A 1K pot works adequately for this purpose. It's good practice to set the attenuator so that band noise just slightly exceeds 12BE6 generated noise as the basic setting, increasing attenuation for very strong signals as needed. Again, regen users know that it's best to keep incoming signal level low, and the audio gain high. This approach also works for Star Roamers, S-120, S-38, NC-60, et al. They can sound surprisingly good.

There is an excellent paper on how signal level affects regenerative gain and bandwidth on the Radiomuseum site: Jochum Bauer, Regenerative circuits with real-time feedback limiting.

The US patent holder describes this circuit as a tuned grid, tuned plate oscillator when the suppressor is disconnected from ground.

https://patents.google.com/patent/US3107333A/en

The patent was applied for in 1960, granted in 1963. Let me point out that this circuit was already known and used in Japan by 1950.

Rob

[Radio Wrangler]

Originally Posted by Oldmadham

Not "superregenerative" as it doesn't have a "quench" oscillator---it's just a plain regenerative stage. In theory, it should sharpen the frequency response of the stage a bit, too.

It's not simply regenerative, either, because it is oscillating sustainably and a regen is on the side of stability.

It amounts to a super regen without a quench system, it is just left to oscillate. This is how a super-regen receiver can be set up to demodulate CW or SSB signals.

It is neither a regen nor a super-regen in the normal use of those terms, but I chose the phrase super-regen because it is most easily understood coming from that direction. Describing it as a regen misses out the oscillation action. What it does is what a super regen does in the time betwen quenches.

It operates as a combined BFO and product detector.

It's still an 'orrible bit of cost-cutting when all's said and done.

David

[G6Tanuki]

IF regeneration [below the point of oscillation] is basically the same as a Q-multiplier, which was a popular way of getting extra selectivity in cheap radios where you couldn't afford a crystal-filter.

But as noted upthread, pushing the IF stage into oscillation so it also acts as a BFO comes with a slew of additional horrors.

Pat Hawkers ART book shows an alternative approach using a forward-biased diode to adjust the degree to which the suppressor-grid is grounded.

See attachment.

If you want to do a "BFO on the cheap" you can always follow the approach of the WWII-era Collins TCS military radios which used the triode of the first-audio stage to also oscillate at IF. Sounds wierd but it worked surprisingly well [I've got a Stewart-Warner built TCS receiver here awaiting rework].

[AnalogueMan]

I've collected a few old articles describing improvements to this and similar sets including building and fitting a real BFO. I'm not going to bother, however, with this set - it may be nasty but it's "fine" as it is. If anyone's interested I'll dig them out and post them