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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 2nd May 2020, 1:27 pm   #1
Mike Fulton
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Default 41MP replacement with MKT4

Hi Chaps,
I have a Lotus 3 valve AC mains set which I'm restoring, the one reviewed in 6th Jan 1932 WW.
I don't know the original valve line up but it uses an indirectly heated 4v output triode (which is missing) but I assume, judging by the circuit, it was probably a 41MP or similar. I don't have the output transformer but the set has a 330 ohm bias resistor fitted so that seems about right for this valve.
Unfortunately, prices for the 41MP are out of my league but I do have an MKT4 on the correct B5 base which measures as new - has anyone used this as a replacement for the 41MP before?
Obviously, I'll have to supply a screen feed for it - the output transformer I've managed to source is 7000 ohms impedance so this should be suitable for the MKT4 I believe.
Any input would be gratefully received,
Thanks
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Old 3rd May 2020, 3:54 pm   #2
boxdoctor
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Default Re: 41MP replacement with MKT4

There seems to be no difficulties wth this substitution.The heater currents are the same, so no extra load on the mains TX from that point of view.
You could triode-connect the MKT4, this would enable you to have an inconspicuous wire from the side terminal to the anode pin of the valve at the base, (being careful to avoid any short to the chassis etc). Output transformer is for a triode, so no problems there, I believe.
The cathode bias resistor may need to be raised to possibly 470 ohm, but this is unlikely in my opinion. Check the anode current. Tony
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Old 12th May 2020, 8:27 am   #3
Mike Fulton
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Default Re: 41MP replacement with MKT4

Thanks for the input Tony!
I fitted the valve yesterday and connected the screen straight to HT as that was the arrangement on the following model with the metal rectifier, which appears to use a beam tetrode in the output. The stage works very well and voltages are exactly where they should be first time!
Unfortunately, all I get from the set is squeaks & whistles but the radio has a VP4 fitted as the RF amplifier which is clearly incorrect as it's variable mu pentode. I have a few MS4B in the loft so when I'm feeling adventurous I'll dig out the ladder and root them out,
Thanks again,
Mike
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Old 12th May 2020, 12:25 pm   #4
boxdoctor
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Default Re: 41MP replacement with MKT4

Pleased to see you got it to go Ok, Mike.
The reason I was suggesting triode connection was the probability of a better match to the speaker, apart from making it simpler to do.
Anyway, your arrangement appears to have done the trick admirably.
Your "squeaks and whistles" appear to be, as you say, instability in the R.F. stage (I am not familiar with this model, though), to which a replacement R.F. amplifier may well be the answer. I would suggest that you check the metallising on the replacement valve is soundly connected to the appropriate pin on the base, as they are prone to becoming a poor connection over the years. Good luck ! Tony.
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