Golborne Vintage Radio

Hi.
Revised circuit with small circuit changes.

Thanks for the update Trevor. It's an interesting little circuit. The front end looks like it could be useful for many different circuits.

Tracy

(03-09-2021, 12:34 PM)Amie Wrote: [ -> ]I was very excited once to hear a conversation on top band between what I thought was radio hams, one was talking about being in Paris! then I realised that they were transmitting AM, Naughty... Dawned on me that it was someone local with an old AM house phone. 

Surely you would have only heard one side of the conversation. ISTR one direction was around 1.6MHz, the other around 49MHz. Unless you were close enough to both of them.

Hi Jeffrey.

One side of the conversation was much louder than the other which kind of reinforced my idea they may have been Hams, I found out which neighbour it was afterwards and told them... They were mortified and bought a new phone  

I can only think that they were talking on speaker setting and the quiet side was the distant caller being picked up by the microphone? I recall the conversation being about a visit the caller had just done to Euro Disney... The only thing wrong with that explaination is that the handset tx for the microphone was the higher band as a rule

I can find out though, they gave me the phone! it's about home somewhere, a Binatone thing.

(05-09-2021, 11:06 AM)Amie Wrote: [ -> ]One side of the conversation was much louder than the other which kind of reinforced my idea they may have been Hams, I found out which neighbour it was afterwards and told them... They were mortified and bought a new phone  

I can only think that they were talking on speaker setting and the quiet side was the distant caller being picked up by the microphone? I recall the conversation being about a visit the caller had just done to Euro Disney... The only thing wrong with that explaination is that the handset tx for the microphone was the higher band as a rule

I can find out though, they gave me the phone! it's about home somewhere, a Binatone thing.

Even without using speakerphone, there's always some sidetone. So you might have been hearing that. Some sidetone is inevitable, as a consequence of 4 wire to 2 wire conversion. In fact a little is a good thing. If you don't hear any of your own voice back in the earpiece the line sounds dead and you tend to shout.

Hi.
I've now started to compare the difference with the higher voltage Rod Pentodes. Today I substituted the output valve firstly with a 1J18B then I replaced it with a 1J24B. I had to separate the G2s obviously as the voltages would be wrong for both valves. Using either of the two the optimum value was 47k with a 100n decoupler. There was virtually no detectable difference between the 1J18B & 1J24B. In all honesty the increase of audio level was not that much but the plus is less distortion, no doubt the good output volume from using the 1J42A was due to positive feedback by strapping the G2s.
I'm at present running the 1J24B with 54v HT, dropping by 9v the output loss is marginal. Output valve current is exactly 1ma driving headphones.

I'm full of admiration for the 1j24b, it really packs a punch despite its miserly 11mA filament current.

If there is anything to be wary of, it's the spread in its characteristics. I have two in a circuit, each loaded with 270k anode load, and each with g1 tied to 0V with a grid-leak resistor.

To get the same anode voltage, I need to hit one with 23.6V on g2, and the other 36.4V. So, include trimpots in your build!

Trevor.
Studying your diagram I was a little surprised that at such a low voltage HT there would be enough signal on the "screen" to use as reaction... I was wondering if it was possible to use it for inductively coupled feedback to the filament.

Do you have any ideas on the level of oscillation available at g2 if the reaction is advanced into full feedback?

Thanks.
Amie

Hi Amie.
I've found that Rod Pentodes of any type oscillate very easily. The level of signal at the G2 is actually higher than anticipated. As you can see on my circuit only two turns of litz wire are used for the "tickler" and this is adjusted by varying the HT on this grid. The advantage of a low number of turns is the reaction is easier controlled and smooth, you just want it to oscillate not turn into something unstable and uncontrollable, so many make this elementary mistake of having too many turns on the feedback winding then have to alter the circuit to get a compromise and land up with a stage that virtually has no gain and or poor control over reaction. On this one reaction is so smooth with no adjustment required between 1.5mhz and 750khz then a progressive rise to 530khz but very controllable.
I've made TRFs before using PCF80s but this one I have to say is the best one to date. I'm seriously considering that I'll leave the 1J42A in circuit and not trying the 1J24B, it most likely will need even less FB with perhaps just one turn. I'll see what transpires today or tomorrow. If it works well I'll do my usual YouTube video with the results. Following the openness of Glasnost if it doesn't perform as well then I'll let you know.

"Following the openness of Glasnost if it doesn't perform as well then I'll let you know."



Thanks Trevor! These valves have characteristics different enough from standard types to make them worth investigating.

I had noticed that the 1j24 screen was very sensitive to hum test if not decoupled too. I wonder if they would work in space charge mode at low anode volts...? (With a positive bias on g1).