playing with russian ROD valves.

[ruben]

Hello
Just to tell you that I'm still around, doing tests with those valves. I have obtained gains of x20 (Vout/Vin) in RF over 1Mhz, but I don't get gains in higher frequencies (> 20 Mhz). The very simple circuit, Vpp 50V, 1.3V filaments from a NI-MH battery, Anode to positive Vpp with an R of 20K, G3 to ground, G2 to positive with an R of 220K, G1 input with an R to ground of 100K. Some ceramic capacitors between G2 and ground and filaments. I've tried with the 1J24B, 1J29B-V and 1J17B. Measures are made with my dual trace osciloscope (Rigol DS2072 upgraded to 200Mhz), one probe on input (RF generator) and the other one conected to the anode via a small capacitor. All probes in high impedance mode 1/10X. It's posible that my valves are defective?

[Murphyv310]

I doubt the valves are faulty, they are very reliable and I've only had failures due to my own mistakes.
I've had gains up to over 100mhz on the 1J24B and the 1J18B. Even the 1J42A can be useful up to at least 20mhz
Try reducing the G2 feed resistor, make sure your method of construction isn't lossy and screen the valves too even tin foil wrapped around the valves and grounded does for experimental work.
I use a sheet of copper laminate board and use the manhatten method as its the best for RF

[Mike Watterson]

Use either a an HT or a potential divider for g2, never a series resistor. The reason why is in the specs and translated documents. The impedances varies too much with g2 current varying from a milliamp down to microamps. Thus the voltage changes to much.
At 50V HT you can tie g2 to HT. At about 80V HT you can vary the 1j24b from almost no gain to maximum gain by varying g2 from 25V to 55V

Layout is very important. Layout with the valves flat, like the Russians did. You also need to cut the leads, but not too short. Never bend close to body, i.e. at untinned portion. Don't use a push in breadboard except for audio work.
   
Having a screen (tinned steel is fine)  with the shield wire, g3 and f- connected to it is important.

[BusyBee]

I have not tried circuits much at the higher frequencies but that is more down to needs and the test equipment I have. Usually I find there are few issues as long as, mentioned previously, circuit layout is good. I have used stripboard for up to 50MHz but would be in the minority there. Manhattan Style for the frequencies you are talking would be far more reliable as Trevor says. One thing I have not seen mention of (unless I missed it) is to use RF amplifier design with inductance in the anode circuit. This would make a big difference. A previous topic I introduced does have some information and spreadsheet you may find useful (no guarentees but it does seem to work as the EF91 aerial amplifier I produced has been working well every day for a couple of hours or more since then - my most used project). Only low HF but varying the components in the spreadsheet will show you how much an inductance and anode resistor can vary response. From this it is easy to see why lower VHF response won't give good results: https://golbornevintageradio.co.uk/forum...p?tid=7708


Tracy

[ruben]

I have tried the valves "on air" on a circuit strip board, because I don't have a copper board on hand. I know, it's not a good approach with high frecuency, but as a "transistor" man, I was thinking that is enought for a first try.
So I will made the whole circuit again with better layout, shorts legs, and with some screen on valves. I have already noted that are very sensible to magnetic fields...

Thanks

[ruben]

I'm sorry for my absence of more than a month. I've had a very hard month of work (I work as a IT manager / computer technician). In the few moments that I have had free, I have managed to make it work, with a different circuit to the one known in this forum. I still have a lot of work, but I promise that I will put here the scheme and maybe a video.

[Amie]

Hi all.

I've so far only experimented with sub miniature valves that are of standard construction, (6418 for instance), and understand the need for magnetic screening.

Are the rod valves sensitive to mains lighting hum with their construction in high gain situations?

Thanks
Amie

[Murphyv310]

Hi Amie.
Cannot say I've had trouble in that way. I did have some issue when I made an RIAA equaliser and preamp for my Pye Black Box to enable the use of a low output ceramic cartridge. The issue was hum, I tried everything till I screened the valve with some thin steel tube of the correct diameter which effected a cure, it was caused by stray magnetic fields from the gram motor.

[Nick]

Hi Amie.
Cannot say I've had trouble in that way. I did have some issue when I made an RIAA equaliser and preamp for my Pye Black Box to enable the use of a low output ceramic cartridge. The issue was hum, I tried everything till I screened the valve with some thin steel tube of the correct diameter which effected a cure, it was caused by stray magnetic fields from the gram motor.

Picked up a few sheets of mu-metal some time ago - very useful stuff to have around although you have to be careful how you bend it else it can lose some of its effectiveness.

Foil is also available, e.g. https://www.ebay.co.uk/itm/171173026902. The stuff I have is stick-on - I got it to line the inside of very low noise pre-amps.

[Murphyv310]

Thanks for the link Nick.
Will order some today.