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Old 14th Jan 2020, 7:07 am   #1
em536716
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Default Bush TV12AM EHT tripler trick?

Hi all,

The overwind has failed on my TV12AM which Iím restoring. It uses the later lopty which I believe to be the same as the TV22s one.

Whatís the best way to work out the amount of capacitors and EHT diodes I should use to generate the right voltage? Also whatís the best hv diodes to use in this application?

Many thanks,
Ed
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Old 14th Jan 2020, 3:30 pm   #2
peter_scott
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Default Re: Bush TV12AM EHT tripler trick?

Hi Ed,

See: https://www.vintage-radio.net/forum/...d.php?t=159716

10nFcaps and 3kV diodes

Peter

Last edited by peter_scott; 14th Jan 2020 at 3:35 pm.
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Old 15th Jan 2020, 9:31 am   #3
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Default Re: Bush TV12AM EHT tripler trick?

The linked post talks of a 'doubler', but the picture then shows three diodes surely that makes it a tripler?
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Old 15th Jan 2020, 12:43 pm   #4
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Default Re: Bush TV12AM EHT tripler trick?

A doubler has 3 diodes and 2 capacitors. A tripler has 5 diodes and 4 capacitors. If you look at the BRC 1500 monochrome circuit in the link below you will see the two variations of eht multiplier used. The doubler providing 15kv for 20 inch tubes and the tripler providing 20kv for 24 inch tubes.https://www.radios-tv.co.uk/wp-conte...4-1500upd1.jpg
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Old 15th Jan 2020, 1:30 pm   #5
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Default Re: Bush TV12AM EHT tripler trick?

But here three diodes are described as a tripler

https://engineeringtutorial.com/volt...-using-diodes/
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Old 15th Jan 2020, 3:20 pm   #6
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Default Re: Bush TV12AM EHT tripler trick?

The only explanation I can give here is that the multiplier in the link above which is used mainly in power supply applications has the advantage of C1 acting as a main reservoir capacitor, hence the need for less multiplier stages. Where as in the TV EHT multiplier the reservoir/smoothing capacitor comes at the end in the form of the CRT inner and outer conductive coatings. Maybe someone with better theory than me can give a better explanation.

Alan.
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Old 15th Jan 2020, 3:53 pm   #7
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Default Re: Bush TV12AM EHT tripler trick?

The classical Cockcroft-Walton multiplier circuits widely described assume a sinusoidal (i.e. symmetrical) drive waveform but in the particular case of TV line output usage, the input waveform is highly assymmetric, i.e. (relatively!) low amplitude, long duration forward scan and high amplitude, short duration flyback. This results in e.g. the typical well-known CTV-usage "tripler" actually being a Cockroft-Walton quintupler, and so on.
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Old 15th Jan 2020, 3:58 pm   #8
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Default Re: Bush TV12AM EHT tripler trick?

Quote:
Originally Posted by wd40addict View Post
But here three diodes are described as a tripler

https://engineeringtutorial.com/volt...-using-diodes/
This deals with sinusoidal signals which is not the case with tv horizontal/EHT circuits which have pulse signals requiring 3 diodes for voltage doubling as has been outlined above

No contradiction - just dealing with different signals

Rgds
John
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Old 15th Jan 2020, 5:01 pm   #9
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Default Re: Bush TV12AM EHT tripler trick?

Thanks to kan turk and turretslug for the explanation. This is something that has mystified me for ages. I cannot remember them going into much detail at technical college years ago. The main emphasis being the use of EHT multipliers with a much smaller overwind on the LOPT was much more reliable than a large overwind and a valve rectifier.

Alan
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Old 15th Jan 2020, 6:28 pm   #10
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Default Re: Bush TV12AM EHT tripler trick?

Oscilloscope EHT PSUs quite often use a power oscillator (valve or transistor) running at maybe a few tens of kHz and feeding diode/capacitor ladder multipliers (particularly for PDA)- in this case, the drive waveform is sinusoidal (give or take....) and the "usual" Cockcroft-Walton multiplying factors apply. It's when potted multipliers for TV use occasionally get tried out in 'scopes (or vice-versa) that folk can get confused at the results!

Yes, the thought of those early CTVs with a full 25kV overwind is quite toe-curling, really.
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Old 15th Jan 2020, 8:44 pm   #11
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Default Re: Bush TV12AM EHT tripler trick?

Thanks for the explanation!
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Old 17th Jan 2020, 5:10 am   #12
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Default Re: Bush TV12AM EHT tripler trick?

Thanks for the reply guys. I’ll do my research! I’ve bought myself a very sick example of the TV12... I’ll hopefully post it up on here soon!
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Old 17th Jan 2020, 11:46 am   #13
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Default Re: Bush TV12AM EHT tripler trick?

The EHT struggles to reach 5.5kv on the TV11/12. You have to keep it close to this figure due to difficulty in obtaining sufficient scan power if you increase it much above 6kv.
Bush issued a modified transformer to give slightly higher EHT and better regulation probably due to dealer/customer queries.
The TV12 series gives a surprisingly good picture considering the TV22 operates at around 8kv. The RF unit is almost a work of art! John.
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Old 18th Jan 2020, 12:52 pm   #14
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Default Re: Bush TV12AM EHT tripler trick?

"The RF unit is almost a work of art! John."

The TV12 "Birmingham" RF deck. A fine example of over engineering.
The line timebase is a follow on from the TV1 and TV2 models. As John says these sets struggle to produce 5KV EHT.
Might be a good idea to connect up a doubler or tripler to the anode of the line output valve and see what happens. An increase of EHT means more scanning power is required.

DFWB.
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Old 20th Jan 2020, 7:12 pm   #15
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Default Re: Bush TV12AM EHT tripler trick?

My "Birmingham" TV12 must have the later line output transformer because the EHT is 6.5KV. Some TV12 sets struggle to produce 5KV.
My rather battered RBM EHT meter was used to measure the final anode voltage.

Here's an idea. When the obsolete Emiscope type 3/4 CRT was replaced by the TA10 tube in HMV 1804 and Marconi VT50 TV sets, EMI service supplied an EHT booster unit to provide an extra 1.5KV to augment the existing 4.5KV EHT supply. The extra 1.5KV was derived from the flyback pulse at the anode of the line output valve. Modifications were required to both timebases to provide the necessary extra scanning power.

A similar device could be made for a TV12.

DFWB.
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Old 20th Jan 2020, 7:31 pm   #16
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Default Re: Bush TV12AM EHT tripler trick?

Hooked up a BRC 950 tripler between the line output valve anode and CRT final anode connector.
The measured EHT is 5.5KV. I might add the CRT is the Mullard MW22-14 without the 'dag coating. EHT might be higher if a capacitor is present.

DFWB.
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Old 21st Jan 2020, 9:05 am   #17
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Default Re: Bush TV12AM EHT tripler trick?

That looks like a tripler David. These early flyback chassis struggle! John.
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Old 21st Jan 2020, 4:59 pm   #18
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Default Re: Bush TV12AM EHT tripler trick?

More info about the TV12M: https://www.thevalvepage.com/tv/bush/tv12am/tv12am.htm

Just to add to the TV12 confusion, wasn't there a version that had the original 1949 timebase deck and the superhet RF unit with EF91 valves?

DFWB.
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Old Yesterday, 6:05 pm   #19
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Default Re: Bush TV12AM EHT tripler trick?

Quote:
Originally Posted by FERNSEH View Post

Just to add to the TV12 confusion, wasn't there a version that had the original 1949 timebase deck and the superhet RF unit with EF91 valves?

DFWB.
Given that the existence of the TV12AM and TV12M would suggest that all the TV12 upper decks were all used up before the EF91 rf deck became available. I doubt any sets left the factory that way. However, the TV22 rf unit might have been available to dealers as a replacement assembly if they were required to tune a TV12 to another channel.

BTW, I can't access any diagrams at the moment as I'm sending this from sunny Malaysia.

Cheers
Andy

Last edited by beery; Yesterday at 6:07 pm. Reason: Grammar
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Old Yesterday, 9:23 pm   #20
FERNSEH
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Default Re: Bush TV12AM EHT tripler trick?

Hi Andy,
It's possible that the owner of a TV12 with the TRF receiver unit had moved to another location which was served by one of the BBC channels the set wasn't tuned to. For good customer relations the Bush service department would then supply or exchange the TRF unit for a five channel one.

DFWB.
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