EHT generation by an RF oscillator.

[Geordie McBoyne]

Extracts from an article in the December 1960 Practical Television magazine.

The author suggests that this unit can be used as a safe source of EHT for television experiments.
There is no mention the RF EHT generator can also be used to replace the mains transformer derived EHT in pre-war and early post-war sets.
By 1960 it's doubtful that many pre-war TV sets were still in use.
The RF EHT transformer looks to be a simple construction project. The entire assembly will have to be contained in a screening box.

The circuit resembles the EHT generator in the Ekco TS46 TV receiver.

Geordie McBoyne.

[Kalee20]

Interesting!

With the heater winding right at the bottom of the former (less than one diameter away), I'm thinking that the presence of a metal chassis would make a fair bit of difference.

Does the article give an idea of frequency that it runs at?

[Geordie McBoyne]

I believe the oscillator operated at >100Khz in burst mode.

The huge HMV model 1820 TV set employed a separate EHT power supply to provide 12KV for the 21" round faceplate metal cone CRT.
The Emiscope tube soon became obsolete and the service replacement was the Mullard MW53-20.

I'll find the EHT generator circuit and post it up later today.

Geordie McBoyne.

[djrm]

I used to have an oscilloscope made from an old WW2 vintage CRT display unit with VCR97 which had an RF EHT unit, it was inside a screening can with vent holes. The coil was wound on air cored perspex former similar looking to your posted photo. The output voltage in this case would have been around 2.5KV or thereabouts.

[LENINGRAD T2]

Electrically and galvanically independent EHT units can be found in projection televisions.

Adam

[Geordie McBoyne]

The RF EHT generator in the 1952 HMV model 1820. Note the HT voltage doubler with two U107 diodes for the anode of the KT36 oscillator valve. The HT voltage is 380V.
From the service manual. A description of the RF oscillator: "The EHT unit consists of V24 (KT36) working as a "class C" RF oscillator with L46, C78 and TC8 as the "tank circuit" and L43 as the "feedback" winding. Grid current bias is established by R98 and C78.
Coupled to L46 is a low capacitance winding L47 producing 7.5KV feeding a conventional voltage doubler rectifier arrangement."

The metal cone CRT employed in this set had a short in-service life span.

Geordie McBoyne.

[Hartley118]

Electrically and galvanically independent EHT units can be found in projection televisions.

Adam

I recall as a schoolboy experimenting with a commercial RF EHT generator which came my way in a box of bits. It was self-oscillating - at least in the way that I wired it - and fed a bright little MW 6-2 projection CRT with its 25kV EHT. The main EHT generator assembly lived in a aluminium can like an oversize very fat electrolytic capacitor.

I managed to stay alive.

Martin

[Murphyv310]

Electrically and galvanically independent EHT units can be found in projection televisions.

Adam

I recall as a schoolboy experimenting with a commercial RF EHT generator which came my way in a box of bits. It was self-oscillating - at least in the way that I wired it - and fed a bright little MW 6-2 projection CRT with its 25kV EHT. The main EHT generator assembly lived in a aluminium can like an oversize very fat electrolytic capacitor.

I managed to stay alive.

Martin

Hi Martin.
Yes indeed 25kv but very little available current, would certainly give you a jolt.
It's the volts that jolts but it's the mills that kills.

[ppppenguin]

The real danger from a shock like that is that you jump backwards and hit your head on something or take a lump out of your hand as it encounters a sharp bit of metal.

[djrm]

Electrically and galvanically independent EHT units can be found in projection televisions.

Adam

I recall as a schoolboy experimenting with a commercial RF EHT generator which came my way in a box of bits.  It was self-oscillating - at least in the way that I wired it - and fed a bright little MW 6-2 projection CRT with its 25kV EHT.  The main EHT generator assembly lived in a aluminium can like an oversize very fat electrolytic capacitor.

I managed to stay alive.

Martin

I too found a 25KV eht module in my youth, it made a very impressive Jacobs ladder. I think it may have been a module from a projection TV. It was bit scary.