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Old 12th Sep 2017, 8:04 pm   #1
beery
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Default Cossor 54 4TP substitution

Hi all,

I've been having a go at making a substitute for the 4TP triode pentode valve that is used in the frame timebase of the Cossor 54. The orignal valve is made from unobtainium.
Luckily my set does actually have a good 4TP in it, so I have something to compare with.

David Boynes has looked into this before and found that an ECL80 is a good replacement. The problem is how to get 6.3 volts for the heater. Also, if the valve is not warm before the EHT rectifier comes on it would cause a flashover...

I decided to wind a transformer that would fit in a 7 pin valve base...

Now the ECL80 requires 6.3V at 0.3A which would mean a 1.89VA transformer which would be too big to fit in the base. However we don't need isoloation, we simply need to step up the 4V heater supply to 6.3V, so an auto transformer will do.
Oh the joy of the autotransformer... The transformer only needs a VA rating based on the voltage difference required. 6.3V - 4V = 2.3V. 2.3V x 0.3A = 0.69VA, which makes for a small enough transformer.
Note that 6.3A at 0.3A equates to 1.89VA. 1.89/4V = 0.4725A at the 4V input to the transformer. Of this 0.3A will pass into the secondary and the remainder 0f 0.1725A forms the primary current. Of course the transformer is not 100% efficient, so the primary current is in the order of 0.2A.

The laminations and the bobbin were obtained from a scrap GPO 746 dial telephone. I experimented with different ratios to work out the primay turns per volt which worked out to be about 60 (I can't remember the exact figure).
That means 4 x 60 = 240 turns for the primary. The secondary is 2.3 x 60 x 1.1 loss adjustment = 152 turns. In the end I had add more seconday turns because such small transformers are not very efficient.

The transformer then had a pancake style B9A socket attached to it and then some flying leads to enable it to be mounted into a B7 base.

The result was, it worked!!

I'm now planning to hide it in an old valve envolope...

The first picture shows the 4TP in situ.
The second shows the ecl80 sitting on the auto transformer.
The third shows the completed valve and adaptor.
The fourth picture shows the valve in situ.
The fifth picture shows the result. Note that I can't easily adjust the height as in this set it is done via resistor substitution. The poor resolution is due to a gassy CRT and the fact that the MVS-PENB sound IF valve was having a fit.

Cheers
Andy
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Old 12th Sep 2017, 8:23 pm   #2
FERNSEH
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Default Re: Cossor 54 4TP substitution

Hi Andy,
the ECL80 is well up to the task as serving as a substitute for the now impossible to find Cossor 4TP. According to the information in the Mullard Maintenance Manual the peak anode volts for the pentode section is 1.2KV Triode Va max is 200V. Last year I serviced a 1952 17" Dynatron TV. In that set the frame output valve is an ECL80. The anode of the pentode section is supplied from the boost HT supply which is almost 300volts.

DFWB.
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Old 12th Sep 2017, 9:35 pm   #3
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Default Re: Cossor 54 4TP substitution

Hi Andy, a nice solution using an autotransformer. I've made a few of these for people who needed to use CRT's with different heater voltages.

It will also work on AC/DC sets (but only on AC) provided the VA requirement for the 2 valves are approx. the same.

Turns a volt looks a bit high for that lam set, but the quick and dirty way to check is to plot a volt/ amp curve for say 100 turns wound on the former and look for the mid point of the "knee". That will give the t/v figure.
Depending on the wire used you may need to add compensating turns to the output winding.

Ed
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Old 12th Sep 2017, 9:58 pm   #4
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Default Re: Cossor 54 4TP substitution

I too use the miniature valve inside a large envelope, stuff on!
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Old 13th Sep 2017, 4:19 am   #5
Argus25
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Default Re: Cossor 54 4TP substitution

Andy, That is a very good idea, well done. And it has reminded me of a project.

I have been thinking of something similar for a replacement for an electrostatic TV CRT. In the future, unless new ones can be made, they will all fail. I was looking at some modern compact 5 inch monochrome electromagnetic deflected CRT's that are much shorter than electrostatic tubes, that are still plentiful in a range of sizes and realized they could, with added electronics, be fitted to a housing the same outer shape as the electrostatic tube and be made from the outside at least to replicate the behavior of the electrostatic CRT, pin for pin. The heater connections for the most part could be used to power the internal electronics. I was going to have a go at replicating a tube like a 5AP4 or 5BP4 initially, its on the to do list!
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Old 14th Sep 2017, 2:49 pm   #6
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Default Re: Cossor 54 4TP substitution

Hi all,

I've attached the circuit of the frame timebase to explain the flashover risk that I mentioned earlier.
The line which connects the to the top ends of R30, R32, R37, R38 etc. is the EHT rail of around 3KV. So if the pentode of the frame valve is not starting to conduct when the EHT rail comes up, it will have 3KV across it!

I found some details for the 4TP in 'The Practical Radio Reference Book' from Odhams Press Ltd. It is listed as a frequency changer.
Fil 4V 1.4A
Typical operating conditions (Pentode only)
Va 200V
Vs 200V
Bias -5.0V
Conv. Conductance Mhos 4500
I'm not sure what Mhos is, I was expecting a figure in Siemens.

It is quite strange to think that Cossor were ahead of the game in some ways before the war, with special dual TV valves like the 4TP and even being the first to come up with the ion trap CRT. After the war they seemed to simply rebrand Mullard stuff.
Perhaps I am using the wrong valve then, it should be a 6AB8!!

Argus25. Substituting an ES CRT with a magnetic one is very ambitious. The Cossor 54 has and ES CRT and it is a real problem. Mine seems to have some unwanted gas in it and it seems to be a problem with some ES CRTs due to the amount of metal surface area inside them (that can trap gas molecules). Even when the last CRT rebuilders were still going, I never heard of one having 2" neck stems to re-mount ES guns...

Cheers
Andy
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Old 14th Sep 2017, 3:11 pm   #7
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Default Re: Cossor 54 4TP substitution

Quote:
Originally Posted by beery View Post
I never heard of one having 2" neck stems to re-mount ES guns...
Andy,

Yes, even if a tube re-builder was to be re-born in our modern world, it is highly unlikely an ES tube would get re-built. This is why I was contemplating other solutions.

On your initial 3kV problem, I came up with a very simple way of delaying the EHT in electrostatic sets until the heaters had warmed up. In the Andrea KTE-5 for example, the EHT is derived from a separate transformer & 2X2, it came up much faster than the scan stages, as did the CRT heater. So the beam could burn the crt phosphor in that interval before the HT and scan got established.

One day I notice that a typical audio output tube, like a 6v6 (and many others) have about a 270R to 330R cathode resistor and it drops roughly around 12V for the bias. Suddenly the penny dropped and I realized the cathode resistor could simply be replaced with a 12V, 270R to 300R relay coil from a common garden 12V relay.

So if the relay coil is used or incorporated into the audio output tube's cathode, the audio stage functions normally...but, it takes time for the tube to warm up and get enough current to close the relay and the HT has to be established to do that and by then all other stages are running. A perfect delay, the relay contacts were simply put in series with the mains supply to the EHT transformer primary, to delay the EHT. So its a quick & easy way to create a delay after turning a set on, if you need to, so as to benefit some part of a circuit in the set.
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Old 14th Sep 2017, 4:58 pm   #8
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Default Re: Cossor 54 4TP substitution

Andy- Excellent and ingenious plug-in conversion there, probably an ever-spreading problem as 4V valves become scarcer. I've noticed that 6/6.3V 20VA transformers seem to be remarkably inexpensive from some sources, so adding one of these to sets that have the space could also offer new zest to pre-war technology.

Quote:
Originally Posted by beery View Post

I found some details for the 4TP in 'The Practical Radio Reference Book' from Odhams Press Ltd. It is listed as a frequency changer.
Fil 4V 1.4A
Typical operating conditions (Pentode only)
Va 200V
Vs 200V
Bias -5.0V
Conv. Conductance Mhos 4500
I'm not sure what Mhos is, I was expecting a figure in Siemens.

Cheers
Andy
I sincerely apologise if a bit of sarcasm flew right over my head (not unknown) but Mho is an Ohm backwards, or unit of conductance in olde-tech speak. 4500 micro-Mhos seems more credible for a 'thirties pentode, though! All the same, conversion conductance is (once the maths has been waded through) typically a quarter or less of the mutual conductance for a given valve, so around 1mA/V seems more likely. Always possible I'm barking up a mis-construed tree,

Colin

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Old 14th Sep 2017, 10:37 pm   #9
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Default Re: Cossor 54 4TP substitution

Mhos and Siemens are the same thing. The unit (Amperes per volt) was given a new name, most probably on account of it seeming unfair to have two units effectively named after the same person, when there were other people worthy of having a measuring unit named after them.
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Old 8th Nov 2018, 10:30 pm   #10
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Default Cossor 54 1938.

Andy has kindly constructed and supplied me another 4TP substitute module which again employs the ECL80 triode-pentode.

The original Cossor 4TP can now be considered impossible to find.

The attachments show the results. The Cossor 3244 CRT in my set is very poor condition and the workshop lights must be switched off to view any images on the screen.
The oscilloscope trace was taken from the anode of the triode section of the ECL80. It is quite safe to connect the oscilloscope X10 probe to this point but in order to take any scope traces from the anode of the pentode section a X100 probe must be employed to do this. Very high voltages exist on the anode of the pentode section.

Adjustment of height and width is achieved by selecting various values of anode load resistors for the pentode section of the 4TP frame oscillator and the MS-PEN line oscillator valves.

DFWB.
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Old 9th Nov 2018, 1:07 am   #11
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Default Re: Cossor 54 1938.

It's amazing to see that the CRT works at all at 80 years old!

Regards,
Paul
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Old 9th Nov 2018, 10:18 am   #12
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Default Re: Cossor 54 1938.

I bet that modification was not included in the Mullard Application laboratories notes on the ECL80..

Great little conversion. That 4TP sounds familiar David but it would take me a week to discover if I have one.

All electrostatic tubes suffer from very short life. Massive gun assembly filled with air molecules. Have you tried increasing the heater volts by 10%? This can have a dramatic effect on picture quality. John.
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