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Old 17th Sep 2019, 10:05 pm   #1
Thyratron Valve
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Default Regentone A133 HT smoothing capacitors, V4 cathode resistor

Good evening I am working on a A133 and a little puzzled that the two caps 23 and 24 in the set are both 50uf, but the Trader sheet shows these as 32uf, the can looks original also another clue is most of the solder joints have a light red paint mark on them which was also on these, Have tested these out of circuit and are not very good so will be changing them anyhow along with all the other waxy ones.

The other question I was thinking of taken the load off V4 by changing the cathode resistor R14 from 180 to 220 ohms
I would be interested in any of your comments

Kind Regards
John
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Old 17th Sep 2019, 11:41 pm   #2
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Default Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor

I don't know how long this set was in production for, but I wouldn't be surprised if the later ones got the 50 + 50uF (double unit?) version, probably by then the 50uF types were the same size and price as the 32uF version and the latter may even have been superceded. Electrolytics have a very wide tolerance anyway- particularly back then- often -20+50% or even -20+80% or the like, so it's not critical. Quite honestly, I'd go for "British compromise" here and make the first (reservoir) section replacement a 33uF (being kind to the rectifier) and the second (smoother) a 47uF (good hum reduction)! I'm assuming that you would be using modern separate types here, which will almost certainly be compact enough to tuck away tidily underneath.

The cathode resistor change isn't a bad idea at all- it's not often that the volume needs to be anything like maximum even in a modest radio and output valves are a diminishing resource, so wise to give it an easier time. You might even find that 270 ohms is an acceptable compromise,

Colin
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Old 18th Sep 2019, 10:51 am   #3
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Default Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor

Colin thanks for your reply that's is very helpful, I will changing them as suggested actually already have some 33uf for this particular job so will properly stick to them. I will try the 270 also I am certain there will be plenty of volume left.
I noticed on Trader Sheet there is a PU socket fitted but no switch, is this usual?

John
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Old 18th Sep 2019, 1:51 pm   #4
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Default Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor

I gather that where there was a PU connection but no switch, the user was simply expected to tune to a quiet spot on the dial to make use of it! I'm not sure how common this practice was but even the GEC BRT400, a large, expensive and sensitive communications receiver adopted this way of doing things, so I guess it would have worked most of the time. Seems odd when the saving was a simple bit of switching, though.
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Old 18th Sep 2019, 5:35 pm   #5
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Default Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor

I have two of these, still unrestored. I'll have a look and see what smoothing caps are in them. Geoff
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Old 18th Sep 2019, 9:31 pm   #6
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Default Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor

Both of my sets have 32/32uf cans, so someone has probably replaced yours with something they had handy.
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Old 18th Sep 2019, 10:03 pm   #7
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Default Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor

Here's a picture I happened to have handy.
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Old 18th Sep 2019, 11:11 pm   #8
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Default Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor

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Originally Posted by turretslug View Post
I gather that where there was a PU connection but no switch, the user was simply expected to tune to a quiet spot on the dial to make use of it! I'm not sure how common this practice was but even the GEC BRT400, a large, expensive and sensitive communications receiver adopted this way of doing things, so I guess it would have worked most of the time. Seems odd when the saving was a simple bit of switching, though.
Seems a strange basically cost cutting idea, I presume the owners instruction book/manual would inform how to use the PU socket.
John
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Old 18th Sep 2019, 11:23 pm   #9
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Default Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor

Geoff Thanks for the info, they do look original caps as I said, all the solder joints were marked with a red opaque paint as like form of warranty seal, the three connections on the caps also had this 'seal' on them, that's what made me think they were as fitted original

John
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Old 19th Sep 2019, 8:46 pm   #10
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Default Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor

Hi John, the key thing is not to exceed the max spec reservoir capacitance for the rectifier. EZ40 has a max spec cap of 50uF. However it also recommends surge limiting resistors in the anode circuits ranging from 125R for an HT of 250v to 300R for an HT of 350v. It would be interesting to know if your set has any anode series resistors fitted. Alternatively you might find a surge limiting resistor between cathode and the reservoir cap. Disadvantage of that location is it takes full HT current so needs to be higher wattage than those in the anode circuits. The higher the value of capacitor used, the more important is the need for surge limiters. Hope I'm not teaching granny to suck eggs
Cheers, Jerry
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Old 19th Sep 2019, 9:16 pm   #11
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Default Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor

The series resistance will be the resistance of the transformers secondary plus a proportion of its primary resistance, I make it approx. 388 ohms for each anode for the Regentone A133.

Lawrence.

Last edited by ms660; 19th Sep 2019 at 9:45 pm.
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Old 19th Sep 2019, 9:17 pm   #12
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Default Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor

Hi John,

It's possible that the replacement was done by a professional repair shop, and marking the connections was just something they did. Mine don't have red marks. The shop may also have "upgraded" to 50uf as they may have believed it "increased performance". I myself am usually dubious to change off of manufacturer's specs, as the circuit was designed by engineers. Jerry made some great points as well.

Cheers Geoff
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Old 19th Sep 2019, 10:48 pm   #13
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Default Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor

I have just acquired one of these sets. I was collecting a vintage telly (Philips 17TG106U) and the seller had some wirelesses he wanted rid of.

The A133 was in a very poor state but I felt sorry for it! Once on the bench it was found to be complete and all valves present and correct. The usual wax caps were replaced and the HT smoothing capacitor was removed. An attempt at slow, current limited, re-forming was made. This failed and I decided not to persist but replace it and put in modern caps. The original was put back, but out of circuit.

The Radio and Television Servicing Manuals circuit shows the HT capacitor to be 32+32uF and the one in my set is indeed 32uF. I'd be a bit worried for the rectifier with 50uF.

Actually, I didn't have 32uF so fitted a pair of 22uF 450V caps. There is no trace of hum from the set.

Despite the usual wax caps being replaced, the set didn't work first time. There was a nice crackle/his from the set when I attached an aerial, but no stations were found on any band.

That is of course "classical" for a functioning IF but a local oscillator that doesn't. With a sig-gen on my bench, I thought it worthwhile to switch it on and set it to 1KHz modulated 470KHz. Bringing the output probe near the aerial brought in a loud 1KHz tone.

As it was dead on all wavebands that lead me to suspect C9 (65pF) in the local oscillator circuit. Changing that produced a nicely working set. In the first pass of capacitor replacements, I don't usually replace low value caps.

Since I'd got the sig-gen on, I thought I might as well sweep the IF and peak it up. The sig-gen is connected to my 'scope for use as a wobbulator.

It was slightly off 470KHz and the peak was a bit asymetric, suggesting the two transformers were at slightly different frequencies. Each transformer has two cores to be adjusted. Its an iterative process and, if you are used to it, fairly easy. Its possible to peak each transformer separately, but with only two transformers, I just do it in one. Different story (elsewhere here) with my Eddystone 770, which had faulty IF transfomers. Each stage had to be fault found and peaked, then peaked end-to end.

Just finished stripping the varnish off the veneer. That will be varnished tomorrow...
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Old 19th Sep 2019, 11:07 pm   #14
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Default Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor

Quote:
Originally Posted by cathoderay57 View Post
Hi John, the key thing is not to exceed the max spec reservoir capacitance for the rectifier. EZ40 has a max spec cap of 50uF. However it also recommends surge limiting resistors in the anode circuits ranging from 125R for an HT of 250v to 300R for an HT of 350v. It would be interesting to know if your set has any anode series resistors fitted. Alternatively you might find a surge limiting resistor between cathode and the reservoir cap. Disadvantage of that location is it takes full HT current so needs to be higher wattage than those in the anode circuits. The higher the value of capacitor used, the more important is the need for surge limiters. Hope I'm not teaching granny to suck eggs
Cheers, Jerry
Thanks Jerry,
I will be limiting the capacitors to the Trader service sheet 32+32uf, there is no anode resistors in the set I have, also no surge resistor between cathode and the first cap, but a 1k 4 watt in series between 1st (C24) and 2nd (C23) caps.

John
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Old 20th Sep 2019, 8:49 am   #15
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Default Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor

Hi John, Lawrence makes a fair point about the transformer HT winding DC resistance providing effective surge limitation. If you are going back to 32uF caps as well then the additional resistors would probably be overkill. Cheers, Jerry
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Old 20th Sep 2019, 9:47 am   #16
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Default Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor

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Originally Posted by cathoderay57 View Post
Hi John, Lawrence makes a fair point about the transformer HT winding DC resistance providing effective surge limitation. If you are going back to 32uF caps as well then the additional resistors would probably be overkill. Cheers, Jerry
Thanks Jerry and Lawrence, I am learning fast, I think that if I can give those two valves V4 El41 and V5 EZ80 a easier life without drastically altering from the original circuit will be beneficial.
John.
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Old 20th Sep 2019, 11:09 am   #17
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Default Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor

The presence of an EZ80 may indicate a late production version- the Rimlock valve series had a relatively short reign before Noval types steadily displaced them. Radiomuseum lists an EZ40 for this model but, whilst it's a very useful and wide-ranging site, many of its model descriptions are single-example snapshots and it's not unusual to find some differences in the set that's in front of you, so to speak. The EZ80 spec. suggests that it would be more amenable to fitment of a larger reservoir capacitor, too, which could explain the apparently original 50 + 50uF component.
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Old 20th Sep 2019, 8:53 pm   #18
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Default Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor

Apropos V4 cathode resistor, I like the idea of increasing it a bit. In my set, I have noticed the EL41 is very hot. I know its biased to class A and will get hot but thought I'd check the current through R14. The easy way, was to measure the cathode voltage, and check the resistor value. R14 measured 200 ohms and the cathode was at 10V, so according to ohm's law that means 50mA.

The data-sheet for EL41 says the max anode current is 36mA and a screen grid max of 5.2mA which gives 41.2. The measured current is significantly higher than I'd expect. I should say that the coupling capacitor (C15 0.01uF) has been replaced and disconnecting it completely makes no difference. The HT was also checked and measures 250V.

The control grid reads 1V to chassis which I wouldn't expect. There is still a fair negative voltage on the grid relative to the cathode (because of the current through R14). I have checked R12 (680K) that connects the grid to chassis. It is slightly high but not enough to be the cause. I'm wondering about the valve and have a spare on order.
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Old 20th Sep 2019, 9:10 pm   #19
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Default Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor

The EL41 (and its designed-for-series-heater-chain sidekick the UL41) is renowned for internal leakage that can put a positive bias on the control-grid, and give the excess-current effect you've noted.

Various people (myself included) have over the years tried a range of different ways of 'zapping' the conductive film within the valve that causes this leakage, and report different levels of success.
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Old 20th Sep 2019, 9:15 pm   #20
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Default Re: Regentone A133 HT smoothing capacitors, V4 cathode resistor

Sounds like the EL41 is getting tired. Another possibility (thinking UL41 DAC90A problems here)- have any of the unused pins on the valveholder (3,4,6) been used as tie points for wiring? If so, there may be some mileage in "floating" any such connections.


You can try reducing the grid resistor from 680k to ~330k to see if this helps a bit and just increase the cathode resistor to get the total current down to around 35mA. This will allow the valve to soldier on a while longer- probably for ever if the set is not used much!
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