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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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14th Jan 2020, 7:07 am | #1 |
Hexode
Join Date: Dec 2017
Location: South East London
Posts: 301
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Bush TV12AM EHT tripler trick?
Hi all,
The overwind has failed on my TV12AM which I’m restoring. It uses the later lopty which I believe to be the same as the TV22s one. What’s the best way to work out the amount of capacitors and EHT diodes I should use to generate the right voltage? Also what’s the best hv diodes to use in this application? Many thanks, Ed |
14th Jan 2020, 3:30 pm | #2 |
Dekatron
Join Date: May 2003
Location: Edinburgh, UK.
Posts: 3,274
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Re: Bush TV12AM EHT tripler trick?
Last edited by peter_scott; 14th Jan 2020 at 3:35 pm. |
15th Jan 2020, 9:31 am | #3 |
Octode
Join Date: May 2009
Location: Stevenage, Herts. UK.
Posts: 1,518
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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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15th Jan 2020, 12:43 pm | #4 |
Pentode
Join Date: Nov 2017
Location: Birmingham, West Midlands, UK.
Posts: 171
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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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15th Jan 2020, 1:30 pm | #5 |
Octode
Join Date: May 2009
Location: Stevenage, Herts. UK.
Posts: 1,518
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Re: Bush TV12AM EHT tripler trick?
But here three diodes are described as a tripler
https://engineeringtutorial.com/volt...-using-diodes/ |
15th Jan 2020, 3:20 pm | #6 |
Pentode
Join Date: Nov 2017
Location: Birmingham, West Midlands, UK.
Posts: 171
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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. |
15th Jan 2020, 3:53 pm | #7 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,394
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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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15th Jan 2020, 3:58 pm | #8 | |
Hexode
Join Date: Mar 2014
Location: Dublin, Ireland
Posts: 398
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Re: Bush TV12AM EHT tripler trick?
Quote:
No contradiction - just dealing with different signals Rgds John |
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15th Jan 2020, 5:01 pm | #9 |
Pentode
Join Date: Nov 2017
Location: Birmingham, West Midlands, UK.
Posts: 171
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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 |
15th Jan 2020, 6:28 pm | #10 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,394
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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. |
15th Jan 2020, 8:44 pm | #11 |
Octode
Join Date: May 2009
Location: Stevenage, Herts. UK.
Posts: 1,518
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Re: Bush TV12AM EHT tripler trick?
Thanks for the explanation!
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17th Jan 2020, 5:10 am | #12 |
Hexode
Join Date: Dec 2017
Location: South East London
Posts: 301
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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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17th Jan 2020, 11:46 am | #13 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,147
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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. |
18th Jan 2020, 12:52 pm | #14 |
Dekatron
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,444
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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. |
20th Jan 2020, 7:12 pm | #15 |
Dekatron
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,444
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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. |
20th Jan 2020, 7:31 pm | #16 |
Dekatron
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,444
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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. |
21st Jan 2020, 9:05 am | #17 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,147
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Re: Bush TV12AM EHT tripler trick?
That looks like a tripler David. These early flyback chassis struggle! John.
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21st Jan 2020, 4:59 pm | #18 |
Dekatron
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,444
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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. |
22nd Jan 2020, 6:05 pm | #19 | |
Heptode
Join Date: Jun 2006
Location: Ware, Hertfordshire, UK.
Posts: 988
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Re: Bush TV12AM EHT tripler trick?
Quote:
BTW, I can't access any diagrams at the moment as I'm sending this from sunny Malaysia. Cheers Andy
__________________
www.youtube.com/user/andyvalve100 Last edited by beery; 22nd Jan 2020 at 6:07 pm. Reason: Grammar |
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22nd Jan 2020, 9:23 pm | #20 |
Dekatron
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,444
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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. |