I'm wondering if someone could point me to some schematics. The radio in question has 24A, 47, 80, but my interest lies in improving its performance, getting the voltages around the 24A correct, etc.

The radio actually works, and can get a roomful of sound from 2FC, but it drops off rapidly as you tune upwards. 2BL makes moderate sound, the rest are pretty much inaudible.

The radio's main resistor bank is a paper-covered dropper with taps at various points. Unfortunately the taps started to spark and the paper started burning, so I had to disconnect it and use normal resistors instead. There's no markings so I had to guess at values. With some schematics I may be able to use better values.

Is anyone able to help? Schematics for any regenerative radio that uses a 24A are what I want. It would help greatly if the voltages were marked.

Thanks,
Rob

When things get hot & sparky, that tends to indicate a major fault. 24A is a Tetrode and I have a set on the bench with two. It is however an autodyne with big problems.

What it may be better to do, as I quite often have to do, is draw out the circuit, that should not be a major hassle with only three valves. The plot for the rectifier will be straight forward.

I normally draw the valves & transformers with an AutoCad & then play join the dots. I have a socket drawn for 24 as I have done that with this set as I have had to extract 7 caps from one can full of pitch.

The wiring will be interesting as like the one on the bench 24A is a heater valve & 47 a Pentode with filaments.This set (1932) has both on the same winding, with a "Humdinger" to ground.

What will be of interest is the cathode of the 24A If it has a resistor that may give an indication of 180 or 250V Plate (Screen 90V both cases) grid -3V. If it is used as a plate detector, rather than a grid leak, it may have a cathode resistor of several K and that's where voltages become critical. The screen cuts to 20-45V, bias -5V & the aim is to have the tube draw 0.1mA no signal. This type of detector is notorious for distortion, albeit a pentode is better.

The long wire wound adjustable resistors have a habit of failing. Where that happens & all of the wire is the same, ohms per unit length can get you out of trouble.

I'd still like some schematics though.

Just to make it clear: unlike most radios where things are kept as original as possible, this one is one of my experimenters. That means that I use the same valves and mostly original hardware, but the capacitors, resistors etc can have new configurations, the object being to get the best possible performance, hopefully much better than the original setup.

That is why I need schematics of other setups that work, in order to get some ideas.

My general thought is that, all radios will, by default, have some regeneration and by laying out the wiring in specific patterns, a designer either encourages it, or discourages it and one false move in repairing / building a set can see a secondary oscillation (destabilisation) occur and that can kill the performance of the set.

I have frequently had to make adjustments to compensate for stupid manufacturing oversights and differences in component construction. On that, the capacitor bank in the current fixer, is in a tin can: Fine. But all of those caps will be outside foil to reduce the interaction (coupling) between them. So in rebuilding the capacitor bank, I have to allow for modern caps that are not like that. The one way to have any hope of achieving that, is to "zig zag" an earthed brass strip through them & between them.

So the combination of valves & resistors are not the only factors to be considered. Even with what is essentially a TRF the "Q" of the coils matters & in coil sets supplied to manufacturers the coils and the tuning gangs were a matched grouping & you cannot just use any salvaged coil set with any salvaged gang. The frequency & tracking could be all over the shop, if they are not compatible.

That could be the issue with the front end. The difference between a crystal set & some sets front end aerial section was not overly different.

Now the other question is, are you trying to make a Superheterodyne, or TRF? The detector (2nd in superhet.) Is also significant. We have the Infinite impedance detector & Anode bend which is used in the fixer using 24-S.

I would suggest having a look at National USA "Auto Box" (Nostaligiaair it's in "Rider") it is entirely 24"s

The radio is a regenerative set, not TRF, not superhet.

I'm not changing the aerial coil, the regen coil, the tuning capacitor or the regen adjustment capacitor. So it will still have the same basic functions as before.

The aerial coil is wound on a big black plastic former mounted on the chassis, and enclosed in a big metal can. I wondered about the can, as radios from the 50s have their aerial coil in the open. So, I removed the can, and was rewarded by a large increase in volume. Stupid design fault right there.

The regen feedback is healthy. The weaker the station, the more likely that oscillation will occur. The oscillation is accompanied by a strange zizzing sound... maybe that is normal for a regen radio? I don't have any others to compare it with. It's a problem for faraway stations, as it will suddenly start oscillating when the station fades, and won't stop unless you manually ride the control.

The big problem with the radio, as noted in the original post, is the dramatic drop-off in sensitivity once you tune away from 2FC. This is what needs to be fixed.

Regenerative is a form of TRF as you have no oscillator. White noise is always a problem, & more hiss. if its a pulsing type Zizzing that is liable to be squegging. Beware the CFL & switch mode PSU.

It would be interesting to know if the band spread & resonance of your coil set is actually part of the issue and it is in fact not tracking the full BC band? Oscillation can be a product of poor decoupling allowing feedback, or over coupling & too much regeneration.

By removing the can you may also leave the coil susceptible to radiation. 6D6 / 6U7 un-shielded are one fantastic valve for picking that up & using it to destabilise the entire stage. Most of those old radio's worked with cans on coils, if they didn't, they would have never left the factory with them on. The Grigsby-Grunow on the bench has both sets of aerial coils & the oscillator one in cans.

I've done some research on the internet, and while I couldn't find anything specific to the 24A, there's lots of information for generic circuits. I've written down various diagrams and voltages that I can try out.