1Ж42А. As a mixer/Frequency changer????

[BusyBee]

Some while ago I purchased a cheap hobby crystal radio manufactured in the far east just for a relaxing change and finding (as many have) that the signal diode provided was actually marked ST4148 which is probably a 1N4148 in another guise (and later tested as silicon) I can say from that experience that a point contact germanium will be far superior for small signal. I have never had a 1N60 so cannot provide experience. I do have a 1N5819 but have not tried. I think, at least for those I have used, PIN diodes are generally for frequencies over 10MHz although I may be confusing the application as I have only used them for attenuators. The D311 (Russian) I replaced it with worked very much better with several stations that could be sensibly heard rather than a vague indecipherable murmer at one point in tuning which may have been anything.

Tracy

[Mike Watterson]

No, the 1N5819 is only a power diode. I confused it with the specialist Schottky diodes. It's no use for RF.

PIN diodes are only for switches and attenuators, no use at all as detector. They are either off (reverse biased) or variably on depending on the DC current, They are very very slow. So a small DC voltage will hold off a larger RF voltage, so they do have a per model minimum frequency. Also only a small forward DC voltage holds it on, a larger RF signal won't rectify. The 1N4007 has such wide junction and slow speed that it can be used 50 kHz to 30 MHz as a sort of poor PIN diode. It also works as a varicap! So it's not much use as an attenuator or switch at higher HF frequencies or VHF as the capacitance is too high. It seems it must be differently made to the 1N4001 to 1N4006. It also has a higher forward voltage drop than a 1N4001.

[Murphyv310]

Hi.
I dug out my Super Regen rod pentode FM radio last evening.
I was always dubious of the quench coil and after some rewinding then trying another coil that was pretty close to the desired value I tried it with great success.
Although the set was quite sensitive it was not very loud, on replacing the quench coil it is much louder. It uses the 1j29b, with 100v HT derived from a tiny little transistor inverter that came from a plasma battery operated rope light. It is now regulated to 100v, and works fine with the 1j29b, again I'm using HiZ phones. I can confirm the 1j42a wont work in the circuit and of course with 6 to 9 volts HT.
The radio is very sensitive now and the audio quality quite good even though its slope detected.

[Murphyv310]

Hi.
Well we are now running with an 1N60P. Initially I wasn't impressed as the distortion was very bad but adding a 5.6 Meg between anode and the filament supply of just over a volt totally cures the distortion and the whole set is much more lively. I'll redraw the circuit with the mods. I am getting over 2 volts P-P audio across the headphone and 200mv P-P on Spirit Radio, which is still perfectly loud enough for normal listening.

[Mike Watterson]

Great.
I've been doing a belated "spring clean" of my workshop. I've a few projects to finish and then I'll take a look again at the 1j24b, 1j29b, 1j42a and compare the DM70 with the Russian battery Magic Eye and maybe the DM160 with the Russian VFD indicator which only uses a 0.6V filament. I did try two of the 0.6V Vf Russian indicators in series with one as a phase splitter and one as an AGC amp. The idea also was to mount them on a perspex the tuning cursor to illuminate it. The set uses 2 x 1j24b as push pull using a very miniature mains transformer; the push pull means no risk of Saturation and it's more audio power than DL94 with 1/4 of the filament current.

I've not done the AM and VHF RF and mixer stages yet. For first iteration I'm using the rectangular small Philips AM/FM IFTs as used in the 1950s Philips mains and battery AM/FM sets. Later I'll try home brew air core IFTs using capacitor tuning to see if the Q is better.

AM detector is 1N60 and the FM diodes are 1N4148. AGC via g2, hence the diode detector for AM is Positive and the indicator amplifies a little, inverts and level shifts to the +20 to +55 AGC range needed for the 1j24b g2. The +55V being the indicator off. The negative bias for that and the 1J24b are from a battery for testing, but will be from a low pass filter connected to the 1j24a g1 local oscillator. Valve oscillators generate a more negative voltage on the grid than on the cathode/filament. The Philips Colette uses this (even on Gram and VHF) from the DK96 g1 as output valve bias as it's push pull.

[Murphyv310]

Hi.
Well a set back yesterday. I was tidying up the board and silly me didn't remove the PP3. So you've guessed it...... 9V across the filaments. It blew the audio valve but appeared to have not blown the filaments of the RF amp. So I replaced the AF output, oddly the performance is dreadful, as weak as water with odd effects in the audio stage.
Firstly it works better without the 1N60P, as a leaky grid detector but still very quiet, the reaction doesn't now behave as it did either. Also there is more AF gain with the filament on the AF amp running at 0.7v.
Im now thinking that the RF amp filament has been damaged effecting the focus of the electrons onto the anode which could be giving me low output from that stage coupled with a possible faulty new AF amp valve?

[Mike Watterson]

Dunno.
I did look at the single diode detector to 0V and found that it doesn't work properly without a large series resistor between the diode and an RF decoupling cap. Because otherwise with no RF decoulper there is RF on the grid, and with one the previous stage is loaded badly. If you look at actual valve radio circuits you'll see a resistor between the diode anode and the volume control, which has the RF decoupling on it. Similarly an AGC line has a series resistor.

So I then tried the peak detector. The first diode can be like on a valve radio, cathode to 0V. Then the next diode, D2, has the cathode to D1's anode. The grid leak and RF decoupling are then the load on the Anode of D2. A peak detector is usually drawn with the diodes the other way round. If the g1a and g1b are separate but connected by a 10nF cap, and g1b has 2.2M to 10 M Grid leak, and diodes fed DC to g1a with a same value grid leak, then at low signals the g1a and g1b have same voltage, but as signal gets larger the g1a is more negative and the gain drops. I've not tried it yet. Maybe tomorrow.

However EITHER you use one diode and need a series resistor before the RF decoupling, OR you use two diodes wired as a peak detector. Analysis will show that the load resistance is the AC load, but that because there are two diodes, the RF decoupler across the load resistor (the grid leak) is actually invisible to RF. The series capacitor from the previous stage only needs to be large enough for the equivalent RF AC load of the load resistor. The capacitor on the load resistor only needs to be small enough to not too much attenuate the audio given the Grid Leak value. At an IF/RF of 485 kHz, 3M3 Ohms load with 10M scope probe (10:1 into a 1M || 30pF scope) an RF decoupler of 680 pF was enough. 100% RF without it.

No extra series resistor is needed with the peak detector, so in practice the audio level is about 2.3x to 2.5x a single diode with the series resistor to feed the RF decoupling, depending on the source impedance and the final load resistor (the grid leak).

The capacitor to feed the peak detector was 220pF from the source coil.
An IFT was simulated by having 2 x 100 uH "resistor body style" inductors on either side of the breadboard divide that's for down the middle of DIL sockets. These were tuned with 1.22nF. An old Advance signal gen fed one side via 1K, though even at direct (50? 75? what?) connection the peak was still fairly sharp.

I can't see how the RF valve wouldn't have been damaged.

[Mike Watterson]

Oh, and my sympathy on the battery accident.
At least they aren't totally unobtainable, or madly expensive like an 1L6. I've accidentally zapped filaments and sometimes transistors with power misapplied.

I'm tempted to 10mA current limiters and diode clamps if experimenting with something irreplaceable.

[BusyBee]

Hi.
Well a set back yesterday. I was tidying up the board and silly me didn't remove the PP3. So you've guessed it...... 9V across the filaments. It blew the audio valve but appeared to have not blown the filaments of the RF amp. So I replaced the AF output, oddly the performance is dreadful, as weak as water with odd effects in the audio stage.
Firstly it works better without the 1N60P, as a leaky grid detector but still very quiet, the reaction doesn't now behave as it did either. Also there is more AF gain with the filament on the AF amp running at 0.7v.
Im now thinking that the RF amp filament has been damaged effecting the focus of the electrons onto the anode which could be giving me low output from that stage coupled with a possible faulty new AF amp valve?

Bad luck

I was only thinking yesterday about a 1ж37б I have which I need to test again. I think I zapped it in a similar manner a while back. It appeared that the emission had dropped to very low but the valve still worked. Because it worked I didn't bin it, just intended to investigate further. I don't know whether it is just the confusion of wire contacts but I have never really heard of it with traditional valves.

I presume their filaments are coated similar to what the cathode of an indirectly heated valve would be to improve emission?

Tracy

[Mike Watterson]

I presume their filaments are coated similar to what the cathode of an indirectly heated valve would be to improve emission?

All Battery valves since about 1925 used coated filaments. Uncoated filamentary cathodes are what are called Bright Emitters. The insulated cathode (indirect) was much later and enabled RF, IF, Oscillators and preamps on an AC supply. I think the formulation or process to coat filaments used by Mullard / Philips changed in about 1928.

It's plausible that low emission after excessive filament current is caused by evaporation of some of the surface. It would be interesting to measure the hot and cold resistances of an undamaged filament and suspect but still continuity filaments. Note that the "hot" resistance is nearly double the cold resistance for practically all valves. The temperature coefficient of tungsten. Which is why a filament lamp gives more voltage regulation than a resistive dropper, but may need a thermistor. TVs need a thermistor. A barretter is a primitive regulator using iron in a hydrogen atmosphere to give even steeper regulation for a series current chain.

The coating is a mixture of Barium Oxide and other "rare earth" oxides. Some specialist valves even have a slightly radioactive coating.