Confused with tube radio

[RayRay1132]

I acquired a fada neutrodyne 175 a battery powered tube set at a flea market and noticed that it has three tuning dials. Got a few questions. How do you tune this set? Whats the easiest way to power this thing? (I know it needs all three voltages a,b,c.) Never worked on battery powered tube radios before so any advice would be appreciated.
Thanks in advance -Ray

 

[Nigel Goodwin]

It's a TRF set (not a superhet), hence the multiple tuning capacitors - one for each RF stage.

Why three batteries?, usually there's just heater and HT.

 

[danadak]

TRF Tuning -

UK Vintage Radio Repair and Restoration - TRF Radios

Regards, Dana.

 

[RayRay1132]

Why three batteries?, usually there's just heater and HT.

It needs three separate voltages for grid, heater and the b+

 

[Nigel Goodwin]

Right, this is probably the schematic (or similar):

13

And (rather confusing) battery connections here:

21

Looks like one too many batteries there, giving 135V on the 90V supply.

The extra battery doesn't seem to be for grid bias (if the diagram is correct), but a separate 22V HT supply for the headphone output/detector stage.

It all seems a bit bizarre?, but it's from back in the days before they found the best ways of making radios.

 

[RayRay1132]

Right, this is probably the schematic (or similar):

13

And (rather confusing) battery connections here:

21

Looks like one too many batteries there, giving 135V on the 90V supply.

The extra battery doesn't seem to be for grid bias (if the diagram is correct), but a separate 22V HT supply for the headphone output/detector stage.

It all seems a bit bizarre?, but it's from back in the days before they found the best ways of making radios.

Thanks. Mist of that schematic looks pretty similar to my radio but there is also definitely a c+/c- voltage terminal. (Completely separate from the 22 volt terminal)

 

[Dick Cappels]

It's a TRF set (not a superhet), hence the multiple tuning capacitors - one for each RF stage.

Why three batteries?, usually there's just heater and HT.

Some tube circuits require a negative bias source in addition to voltages for the heater and plate.

 

[unclejed613]

It's a TRF set (not a superhet), hence the multiple tuning capacitors - one for each RF stage.

Why three batteries?, usually there's just heater and HT.

"C" battery is for grid bias... "A" for filaments, "B" for plates. those older sets used something like a 3 or 6 volt battery for grid bias, 45 or 90 volts for the plates, and you need to check the tube numbers against a tube chart to get the correct voltage for the filaments. the reason for the grid bias is that in the early tubes, there weren't separate heaters and cathodes like in more modern tubes, so the tubes can't be self biased using cathode resistors, as the filaments ARE the cathodes, and are all wired directly in parallel, with one side of the filament grounded. so a grid bias supply is required to bias the grids properly. the correct term is "dirrectly heated cathode" vs "indirectly heated cathode".

 

[Nigel Goodwin]

Rather confusingly the only circuits I could find show directly heated valves, but with no separate grid bias - but an even more confusing array of batteries, which don't even match the voltages on the circuit? (three 45V in series to give 90V?).

Grid bias is still common on more modern valves, simply to increase the power of the output stages in many guitar and PA amps, avoiding the losses across a cathode resistor.

 

[Dick Cappels]

90 volts was a popular plate voltage when I was a kid. 90 volt batteries were available all over the place but since the 1960's when I moved over to transistors where would you use a 90 volt battery?

 

[Nigel Goodwin]

90 volts was a popular plate voltage when I was a kid. 90 volt batteries were available all over the place but since the 1960's when I moved over to transistors where would you use a 90 volt battery?

Popular over here as well, with portable radios using D series valves specially designed for portable use, with 1.4V heaters.

You can actually buy, or make your own, 90V inverters for running old radios.

 

[danadak]

And radios built with low plate V tubes -

https://www.junkbox.com/electronics/lowvoltagetubes.shtml (Space Charge tubes)

Low Plate-Potential Tubes, January 1957 Radio & Television News - RF Cafe

Having been invented only a couple years earlier, transistors were being designed into the power supplies, but low-voltage tubes

www.rfcafe.com

Regards, Dana.

 

[Nigel Goodwin]

And radios built with low plate V tubes -

https://www.junkbox.com/electronics/lowvoltagetubes.shtml (Space Charge tubes)

Low Plate-Potential Tubes, January 1957 Radio & Television News - RF Cafe

Having been invented only a couple years earlier, transistors were being designed into the power supplies, but low-voltage tubes

www.rfcafe.com

Regards, Dana.

Pretty cool, but falls down on no output valves

If you had to use a vibrator for the output stage of your car radio, you may as well use it for the rest as well.

There's quite a few audio preamps etc. these days using high voltage valves with low voltages, just for the fad of a 'valve amplifier'.

 

[danadak]

Using space charge power tetrodes at 12V, no vibrator needed, workable. Not boom box
power but still enough to build a radio and get a few mW.

 

[Nigel Goodwin]

Using space charge power tetrodes at 12V, no vibrator needed, workable. Not boom box
power but still enough to build a radio and get a few mW.

Not much use in the noise of a car - which is what they were designed for

 

[danadak]

Cars not the only place for radios.....

Space charge tubes only designed for cars, no other applications requiring low V
amplification and control pre semiconductor era ? Huh.....

 

[unclejed613]

Thanks. Mist of that schematic looks pretty similar to my radio but there is also definitely a c+/c- voltage terminal. (Completely separate from the 22 volt terminal)

maybe an improvement on the radio Nigel linked, most of the grids in that schematic look like they are operating at zero bias.

 

[Dick Cappels]

Hey, with only 12 volts of plate voltage, how much grid bias can you have and still get any plate current?

 

[danadak]

Here is the 12K5 Tetrode at various bias -

http://www.r-type.org/pdfs/12k5.pdf

Regards, Dana.

 

[debe]

Here in Australia there was car radios in the early 60s that had 12V valves in the front end & Germanium transistors in the audio stage. They seemed to work well.