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Old 8th Jan 2012, 9:25 pm   #1
2E1CIH Mike
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Default Marconi CR300 PSU (and other things).

Hi,

Although I'm nowhere near ready, I've been thinking ahead re building a PSU for my Marconi CR300 restoration project. (It was bought without the dedicated Marconi one).

I must firstly "unconvert" it from its modified, 6.3V (valve heaters) state, back to 24V series/parallel configuration.

I have seen a 24V transformer advertised, but then became unsure when I stumbled on a website (for guitar valve amplifiers) which talks about hum problems originating from heater supplies. (It's a little beyond my knowledge level). Please see below:

The following tricks can be used to reduce heater hum:

Transformer centre tap: The traditional way to reduce hum is to use a heater supply with a centre tap and connect it to ground. In this way the valve heater will be at a positive voltage along half its length, and at an equal but opposite voltage along the other half, at any one time. The average stray current between filament and cathode is therefore reduced by a little more than half, and the frequency of the ripple voltage produced in the cathode is also doubled, so can be shunted to ground more easily by the cathode bypass capacitor. This method is usually enough to bring hum to a satisfactorily low level. Additionally, in a perfect world the out-of-phase radiated fields will cancel, and no hum will be induced in the cathode by that method either. The only problem with this system is that it is impossible for the centre tap on the transformer to be precisely centred to AC and DC, so perfect cancellation will not occur and some low level hum may be heard.

Artificial centre tap: A better way that can also be used with non centre-tapped heater supplies is to create an artificial centre tap with resistors. The resistors should have a low resistance so the maximum heater-to-cathode resistance of the valves is not exceeded, and so the reference to ground is as close to zero as possible. Values of 100R (1/2W min) and 220R (1/4W min) are usual. They will of course cause a small amount of extra current draw from the transformer (32mA when using 100R resistors at 6.3V) so bare this in mind.
The advantage of this system is that close tolerance or matched resistors can be used to create a perfectly centred ground reference, and the extra resistance to ground will reduce the chance of an arc occurring within the power transformer in the event of a speaker being unplugged.
Another traditional method uses a potentiometer with the wiper grounded- a so-called "hum dinger". This allows minimum hum to be dialled in precisely by creating a Wheatstone bridge with the heater-to-cathode capacitance within each valve.

I tried changing the colour of the copied font above, but it wouldn't....so it's me speaking again.

Could anyone advise me (simply please) if I need to be concerned about this, or could I use a 240V/24V transformer (5A....so ample) to supply the heater circuit?

Also, sadly, I pulled a very awkwardly positioned valve envelope out of its plastic base when trying to remove it. Is it feasible to repair? All the wires seem OK. I had thought of cutting a piece of dowel and positioning it below the chassis in such a way as to be able to push the spigot by pressing down on the chassis, but didn't do this......Well, I did originally see this restoration as a learning process.....Here's a lesson learned!

Many thanks......Mike
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Old 8th Jan 2012, 9:38 pm   #2
Sean Williams
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Default Re: Marconi CR300 PSU (and other things).

Mike,

If the CR300 was designed for 24v heater use, then going back to that will make sense. Ignore most of the HI-FI/PA stuff regarding heater supplies - the radio will not be fussy, and was designed to run of some frankly disgusting mains power - I very much doubt you will suffer greatly with hum.

As for the decapitated valve base, chalk it up to experience, and replace the valve completely - you have enough of a challenge without trying to repair a sick valve base.
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Old 8th Jan 2012, 9:52 pm   #3
g4aaw pete
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Default Re: Marconi CR300 PSU (and other things).

Hello Mike

The article you’ve drawn attention to is more applicable to audio amps. In particular, the very sensitive early or ‘first’ stages.
In radio receivers, hum picked up from the heater supply tends to be far less of an issue.

Personally, I’d leave it wired for 6.3v, although it wouldn’t be original.

Don’t worry if you’ve destroyed one of the valves, none of the types used are difficult to obtain.

Good luck
Pete
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Old 8th Jan 2012, 10:07 pm   #4
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Default Re: Marconi CR300 PSU (and other things).

Are you going back to 24v [presumably ship voltage?] for authenticity Mike? Seems a lot of trouble-I think most CR series got rewired at 6.3
over the years but could be wrong. Never saw an article where it's reversed before. Is there a tech reason? Just curious.
Dave W
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Old 9th Jan 2012, 8:42 pm   #5
2E1CIH Mike
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Default Re: Marconi CR300 PSU (and other things).

Sean, Pete, Dave,

Thanks for your advice.

Yes, a replacement valve makes sense. Thanks for that Sean.

Dave, I was thinking of going back to 24V for originality. However, this is my first attempt at a restoration and it's definitely not going to be a museum show-piece, so I'm wondering if I should leave the heater wiring now. Maybe I need to just get on with replacing/re-stuffing the caps.

Can anyone recommend a supplier for caps for such a beast please?

Thanks.....Mike
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Old 9th Jan 2012, 9:08 pm   #6
Alan Stepney
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Default Re: Marconi CR300 PSU (and other things).

There are two schools of thought regarding replacing capacitors.

I favour only replacing those that are faulty.

Others prefer to change all of them, (apart from ceramic or mica types). Whilst it is personal choice, I feel that the latter can introduce faults unless you are especially careful.

The BVWS does a nice line of the typical values you will need, both electrolytic and "paper" types.
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Old 10th Jan 2012, 2:17 pm   #7
2E1CIH Mike
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Default Re: Marconi CR300 PSU (and other things).

OK thanks Alan.

I can see three types of capacitor:

1) "Canned" electrolytic....I intend re-stuffing all of these (one or two look suspect just from a glance).

2) Light brown, wax coated, rectangular, about 50mm x 22mm x 6mm thick. I'm thinking these are paper and of the unreliable variety. There aren't that many, so I'm thinking of replacing all of this type. Could you please tell me if my ID is correct?

3) Mid brown, plastic, rectangular, about 17mm x 9mm x 4mm thick. I can see the value of one of these: 0005 micro Farad. Could you tell me what type these are please? Is it best to replace them? Does it mean 0.0005 micro F (ie 0.5 pF)?
Also, there's a dab of black paint at one end of these. Does this indicate that they are polarised?

Sorry.....more questions again.....

Many thanks for your patience and time.

Regards.....Mike
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Old 10th Jan 2012, 3:53 pm   #8
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Default Re: Marconi CR300 PSU (and other things).

5000pf.

David GM8JET
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Old 10th Jan 2012, 5:22 pm   #9
2E1CIH Mike
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Default Re: Marconi CR300 PSU (and other things).

OK thanks David.
So, I take it it's 5 mF = 5000 pF?
Cheers Mike
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Old 10th Jan 2012, 5:47 pm   #10
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Default Re: Marconi CR300 PSU (and other things).

.0005 is 500pf

Jim
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Old 10th Jan 2012, 7:30 pm   #11
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Default Re: Marconi CR300 PSU (and other things).

The rectangle capacitors are generally very reliable - The waxy ones are Mica, the bakelite cased ones are moulded mica.

There are numerous metal canned type capacitors - any that are rated at 0.01uf or above might be suspect, but arent worth restuffing if you arent bothered with total originality - remove the cap - if it has a threaded end, then replace with a nut and bolt with a solder tag attached - then fit the replacement cap to this point.

Otherwise there will be a few bias capacitors - these are larger can types, they can be restuffed if you really want to, but with modern equivalents being so small, you may get away with hiding them near the valve base.
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Old 10th Jan 2012, 8:05 pm   #12
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Default Re: Marconi CR300 PSU (and other things).

Sorry!one digit too many.

David
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Old 10th Jan 2012, 8:17 pm   #13
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Default Re: Marconi CR300 PSU (and other things).

Quote:
Originally Posted by 2E1CIH Mike View Post
OK thanks David.
So, I take it it's 5 mF = 5000 pF?
Cheers Mike
5000pF = 5nF (5mF is 5000uF, strictly, though m is sometimes used for microfarad as in mfd)

but it's 500pF as already noted anyway.

Also sometimes seen uuF (micro micro) for pF.
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Old 10th Jan 2012, 9:10 pm   #14
2E1CIH Mike
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Default Re: Marconi CR300 PSU (and other things).

Gentlemen,
Many thanks for clarifying things for me.

Sean, I appreciate your tip about not necessarily restuffing. I'm being realistic and realising I have a long way to go before being able to complete a "top-notch" restoration and so, will take your advice.

I know I may make some forum members cringe with my lack of knowledge and experience, but I am very eager to learn and view the CR300 as a vehicle for me to do this.

I really appreciate you chaps spending your time answering questions on things you have probably known about for decades.

Anyway, just to make sure I have understood your comments about what should be replaced, and what shouldn't; Are you saying the darker/smaller plastic caps are usually OK to leave and also the larger wax coated ones are also usually OK to leave? I'm wondering how difficult it would be to get to them once I put it back together if I find problems when I eventually power up.

Thanks......Mike
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Old 10th Jan 2012, 9:37 pm   #15
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Default Re: Marconi CR300 PSU (and other things).

You would be unlucky to have trouble with any of the "square" caps, although not completely unknown, it is rare, and you could well upset the alignment of the RF circuits if you were to change these and not use exactly the same replacements.
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Old 10th Jan 2012, 11:47 pm   #16
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Default Re: Marconi CR300 PSU (and other things).

Quote:
Originally Posted by 2E1CIH Mike View Post

...and the extra resistance to ground will reduce the chance of an arc occurring within the power transformer in the event of a speaker being unplugged.
Hi Mike, I'm not sure where you heard that, but it is the output transformer that is at risk of damage in the event of a speaker being unplugged. Adding a humdinger to the heater circuit will not prevent this happening.
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Old 11th Jan 2012, 1:17 am   #17
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Default Re: Marconi CR300 PSU (and other things).

Retstorers golden rule:-
If it isn’t broke don’t fix it, if you do it will be broke.

Run the set up on a Variac for a few hours to settle the caps , usually they will reform.
Just check then to see if the set works, you will be, we hope surprised, these sets are tough.
You could then check for unwanted dc on grids, listen for undue hum, check the voltages and only then start “restoring"??
So many good sets lying around spoilt by over eager restorations.
Do not re-pot caps unless these prove to be at fault, most will be fine.
If you are new to the game just replace one component at a time and check the result.

The power supply is very easy to create , especially if you keep the 6.3 configuration
Xmer, rectifier, choke and two caps, perhaps also a thyristor if you have one.
These sets do not require fancy regulated supplies, any decent DC source will do for the ht and a AC supply for the fills.
Several times recently we have seen discussions re power supplies for these type of sets and at times the proposed complexity is crazy and not at all required.

Good luck

James
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Old 11th Jan 2012, 7:11 pm   #18
2E1CIH Mike
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Default Re: Marconi CR300 PSU (and other things).

OK. Many thanks for all comments and advice. I will consider things carefully before diving in.
Kind regards.....Mike
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Old 12th Jan 2012, 8:58 pm   #19
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Default Re: Marconi CR300 PSU (and other things).

Located an article I have about the CR series with info on the 300. Apparently there were two versions. Are you aware of this Mike?
As for the heater voltage, I was only going off what I've seen so don't be influenced by my comment too much. You will probaly [and definetely Sean] have much more practical expertise than myself.
Dave W
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