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Old 1st Mar 2016, 11:21 am   #81
ms660
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Default Re: HMV 653 Broadcaster all wave.

The power rating of the resistors poses an interesting question for you and one you should be able to answer with a bit of simple maths, if you've not done it before it's quiet easy, there two ways, the one described below is used to show how current comes into the equation so to speak:

R5 together with R6 form a potential divider to supply V1 & V2 screen grids, If the current flowing though R5 and the voltage developed across R5 is known then you can work out the power dissipated by R5 or indeed any resistor, the screen grid voltage for V1 & V2 are both the same and is given in the Trader sheet, as is the HT voltage that supplies R5, subtracting one from the other will give you the voltage developed across R5, now you know the voltage developed across R5 the current flowing through it can be calculated by using the simple Ohms Law formula I = V/R, once the current is known then the power in Watts that's dissipated by R5 can be calculated because you now know the current flowing through it and the voltage developed across it, this is the formula: W = IV.

Once you know the power dissipated all that's needed is to select a resistor that will happily cope with that power dissipation, general rule of thumb for me is 2 x as a minimum.

So... With that sorted you might then be asking the question as to why the screen grids are supplied via a potential divider? It's to do with the type of valves used, if you look at the schematic you will notice an arrow shown through both valves (V1 & V2) The arrow denotes that the valves are of the variable mu type so that AGC can be applied, when the gain is reduced the screen grid voltage will rise, as it rises the valves internal resistance will be reduced, this will reduce the selectivity of the tuned circuits in the anode circuit, to help keep the screen voltage to a more constant level when the gain is varied (by a varying AGC voltage) a potential divider is used.

Hope this explains a few things, any errors then guilty as charged.

This post might cross with others.

Lawrence.
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Old 1st Mar 2016, 1:22 pm   #82
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Default Re: HMV 653 Broadcaster all wave.

Working out the power dissipated in resistors from sparse information can indeed make for an interesting mild exercise in maths and deduction sometimes with a few steps along the way. In the case of these two resistors (I'm working from the Trader sheet), substituting the working voltages quoted suggests about quarter of a watt dissipation in the lower 23k, slightly over a watt in the higher one. <However>, with a set like this that uses a directly heated rectifier and lots of series resistance in the power unit from the likes of the field coil choke, the HT will rise far higher than usual for the possibly 20-30 seconds that it takes the indirectly-heated valves to warm up thoroughly and take their normal current. If we assume that the HT is about double its running value for this period of time, then that suggests that both resistors would dissipate nearly 3 watts each. As it's only for a few seconds, they won't heat much- but a typical modern 0.6W resistor, which would seem to be fine for the lower component of the potential divider, will be very small and therefore fast-heating. Moreover, if the lower resistor did eventually go open-circuit, the two valves would get much higher screen grid voltage- this might possibly cause instability, but it would definitely make them draw unnecessarily higher anode and screen current. Even if it didn't exceed the ratings, it would still be a waste and possibly erode their lifespan. I'd play it safe and use a 1W type for the lower one and 2W for the higher one.

I've actually been going through the same thought process with these same function resistors (23k upper, 35k lower) with a Marconi 559, a not-dissimilar topology set but with RF stage. There's an identifiable strand of circuit philosophy running through this family of pre-war HMV/Marconi sets all the way to the big-brother CR100. Note that there's a pre-war version of the E6-series capacitors and resistors that goes 10, 15, 23, 35, 50, 75, corresponding to the familiar modern 10, 15, 22, 33, 47, 68 series.
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Old 2nd Mar 2016, 1:35 pm   #83
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Default Re: HMV 653 Broadcaster all wave.

Ok I've digested both last two posts and done my maths. Just want to run it by the experts as it's been a while since I've used ohms law.
Ok, both screen grids of v1 and v2 are 76 volts. The HT feeding R5 is 238 volts ( I got this figure as the HT that feeds the screen grid of v4 also is on the same HT rail that feeds R5. ) So, 238 - 76= 162 volts being developed across R5. So, 162V 23000 ohms = 0.007 amps.
Wattage required = 0.007 amps 162 V = 1.14 watt resistor needed. As you rightly say the HT can be much higher at switch on for a few seconds at switch on till the set is drawing the correct current. So I've ordered 3 watt 22000 ohm resistors for R5 and R6. I couldn't find 23k resistors but with tolerances I'm sure this won't make a huge amount of difference. The resistors I've ordered also have a 20 second kind of built in overload feature which would suit this application for overload at switch on as you describe. But being 3 watt I'm sure they will cope without this overloading feature anyway. My only concern is that they are rated at 200 volts and peak 400 volts so hopefully these are still suitable as I couldn't find any others rated at 240 volts. I've attached the detailed information on the resistor for you to see. If someone could okay my maths so I know I've got it right for future regardless of whether the resistors are ok or not. Many thanks.
Dave.
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Old 2nd Mar 2016, 2:46 pm   #84
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Default Re: HMV 653 Broadcaster all wave.

The maths looks ok.

350 volt rated resistors are available from Farnell and probably other suppliers.

Lawrence.
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Old 2nd Mar 2016, 2:56 pm   #85
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Default Re: HMV 653 Broadcaster all wave.

These resistors are from maplins and are rated 200 volts and 400 v peak. Would they be a definate no no to fit as a replacement for R5 and R6? Surely if it's peak is 400 v then it should be ok with a constant 238 volts? Many thanks for all your replies.
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Old 2nd Mar 2016, 4:07 pm   #86
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Default Re: HMV 653 Broadcaster all wave.

From the limited info in that data sheet I would interpret it as 200 volts DC/AC rms as the rated voltage, they'll probably be ok for what's needed.

Lawrence.
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Old 2nd Mar 2016, 4:45 pm   #87
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Default Re: HMV 653 Broadcaster all wave.

They sound like they'd be fine, as both the 200V continuous and 400V short-term ratings will give a good margin over their use here. It's raised a useful point, though, and one that often crops up with re-furbishing vintage valve kit (I'm ticking myself off mentally for not mentioning it earlier in post #82 when I should have!). One favourite fault with switch mode power supplies for a few decades now has been that the small, high value (often high tens to hundreds of kilohms) low-power start-up resistor can go very high value or open circuit with time as a result of voltage stress, even though well within its power rating. It's important to check voltage ratings when purchasing components for our old stuff.
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Old 2nd Mar 2016, 8:53 pm   #88
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Default Re: HMV 653 Broadcaster all wave.

Yes, I'm a stickler for making sure the correct rated parts are used. Can be easily over looked. Yes the vintage radio may work with say underated caps and resistors and all will be fine but at the same time things may be overUn, overheated and damaging other components and shorting lives of valves and making things work harder when they don't need to. Great, got my new 3 watt caps soldered in to replace R5 and R6 and got some more rotten wiring replaced.
I also need to replace some wiring from the volume control that is screened. Can anyone advise where I might get some screened suitable lengths of wire? Or is it totally necessary to have it screened?
Also I now need to replace carbon resistors R7 and R3, V1 anode smoothing resistors. I also need to replace R4, V1 anode feed resistor. I need to do my maths again and work out the voltage developed across these resistors and current to work out what wattage I need for R7,R3 and R4. The trouble I'm having unless I'm being completely stupid, I can't see how this HT line is connected to V1 anode. Unless it's done internally inside the valve? Probably a stupid question I know. Many thanks for your help.
Kind regards,
Dave.
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Old 2nd Mar 2016, 9:44 pm   #89
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Default Re: HMV 653 Broadcaster all wave.

Yes, if replacing that cable it needs to be screened cable, I'll have a Google
for some when I have time.

R3 & R7 supply the LO (local oscillator) anode via R4 and L11, the LO anode is actually a grid that functions as an anode, so far as I can see that's all R3 & R7 supply, LO anode current given in the Trader sheet is 4ma, you can work out the power dissipated by squaring the current then multiply by the value of R3, ditto for R7, ditto for R4, choose resistor wattage rating accordingly, I normally go 2x.

This post might or might not cross.

Lawrence
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Old 2nd Mar 2016, 9:47 pm   #90
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Default Re: HMV 653 Broadcaster all wave.

Are these the string of resistors associated with the oscillator HT feed? I suspect that these end up at the second grid from the cathode of V1, via the feedback winding?- if so, this grid acts as the anode of the oscillator circuit, a sub-anode if you like, and the valve's combining of this oscillatory electron stream with that produced by the aerial signal at the signal grid results in the mixing action with the intermediate frequency component appearing at the main anode (that's an enormous over-simplification, by the way, this is a subject where some (not me!) could fill many pages with explanation). This is what's referred to as multi-grid mixing, sometimes this circuit is known as a "converter" as both oscillating and mixing are done by one valve and electrode structure.

Yep, crossed!
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Old 2nd Mar 2016, 10:03 pm   #91
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Default Re: HMV 653 Broadcaster all wave.

RDH4 Chapter 25 covers mixers in general, here's the link for anyone that might be interested, I've glazed over it several times over several years, for me, what you want goes in and what you want comes out t'other end but with a different name

http://www.paleoelectronics.com/RDH4/CHAPTR25.PDF

Lawrence.
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Old 2nd Mar 2016, 10:26 pm   #92
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Default Re: HMV 653 Broadcaster all wave.

RE: Screened cable:

http://www.maplin.co.uk/p/miniature-...er-metre-xr88v

There's other stuff on that site that might also do, I'll leave it to the OP to decide.

Lawrence.
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Old 3rd Mar 2016, 8:53 am   #93
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Default Re: HMV 653 Broadcaster all wave.

Thankyou, Lawrence and turretslug for your most helpful replies. Maths not being my strongest point, why do we square the anode current before we work out the wattage of the resistor needed. Why did I not have to do this when working out the current draw for R5 and R6 in my post #83? Is it because the anode current varies?
Thankyou for the link to the screened cable. The screen on the cable I removed was connected to earth with a terminal. Is there a way of tidily doing this and soldering the braided screen without it looking like a pigs ear?
The cable you have kindly located for me is cut to length so the screen will not be terminated ready to solder to my earth tag. Not sure if there is a standard process for doing this correctly? With this suggested coax being 75 ohm, would this not cause an issue? I've had a look at the site too and found some audio screened cable that is not 75 ohm coax. Trouble is Maplins don't give any voltage or current specs for the cable. Many thanks.
Dave.

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Old 3rd Mar 2016, 11:34 am   #94
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Default Re: HMV 653 Broadcaster all wave.

RE: Working out wattage etc, there are various ways, it's all down to the info that can be gleaned from the manual (or actual measurements) If the voltage developed across the resistor is known then that voltage squared divided by the resistance will give the power dissipated, as will determining the current through the resistor then multiplying that current by the voltage developed across the resistor, as will squaring the current flowing through the resistor and multiplying that current by the resistance.

In the case of R3 & R7 no voltage difference information is given in the manual but the current is given so in that case the quickest way to determine the power dissipated is to square the current then multiply by the resistance.....So now we've gone through the three methods of arriving at a result (power) with the information that's been available, all basic stuff but well worth remembering.

Soldering wires etc neatly is down to practice, for removing wires components etc I use a desoldering pump, desoldering wick, usual tools etc in whatever combination, for doing screened cable strip off the outer insulation to whatever length is required and unpick the braid, twist the braid tight but not tight where it contacts the inner insulation, the solder tag(s) should be well prepared, they need to be clean and tinned, some folks tin the braid and the inner conductor some don't, I never do unless it's old stock stuff that's oxidized, prepare the inner conductor, feed the ends of the braid and inner conductor through the solder tags by about 3 mm or so then bend them over and under, make sure everything's sitting proper and not under strain and that nothing's going to pop off, that way both hands will be free during the soldering operation, a quick solder with a hot iron and the jobs done.

The characteristic impedance of the cable (75 ohm or whatever) will make no difference in this case. The characteristic impedance of a cable is given in ohms but it can't be measured with a ohmmeter, for audio use the main parameter, signal wise, is the cables capacitance per meter but only on longer runs where the source impedance is high, the lack of voltage rating for the screened cable wouldn't affect anything in this case.

For RF work things can change, transmission line is a whole different subject in that respect, but if you are interested then the following video is well worth watching:

https://www.youtube.com/watch?v=I9m2w4DgeVk

As is this:

https://www.youtube.com/watch?v=DovunOxlY1k

Both are classics.

This post might cross.

Lawrence.
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Old 3rd Mar 2016, 1:16 pm   #95
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Default Re: HMV 653 Broadcaster all wave.

Slightly more mathematically......

P = V * I

for a resistor,

I = V/R

so P = V * V/R which is Vsquared over R.

Similarly

V = I * R

so P = (I * R) * I which is Isquared times R.

A couple of useful visual aids are triangles

..V...
I * R

..P
I * V

Cover up the value you want and the formula to get it is left visible.
__________________
....__________
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.=.| _---\__|__|_---_|.
.........O..Chris....O
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Old 3rd Mar 2016, 1:22 pm   #96
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Default Re: HMV 653 Broadcaster all wave.

The triangles... I remember them by remembering to be with her, cockney style that is... wivvir.

Lawrence.
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Old 3rd Mar 2016, 8:09 pm   #97
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Default Re: HMV 653 Broadcaster all wave.

Lawrence, you have been golden, thanks for the maths lesson. I knew of ohms law regarding volts amps and resistance but not power and not how to derive the power from only being given amps and resistance. Thanks Chris for the triangles I will jot down in my radio repair note book. I will go do my maths and see what I need. Lawrence, thankyou for the mentoring and patience with me. I like to know how things work and how to solve these problems for myself. Not to be just given answers so I can have a deeper understanding with each set I repair. I do have a radio repair book with lots of math that some I understand and then it goes way off on a tangent for me. I don't want to become a circuit designer but it's important to grasp and understand the basics and what each resistor/ component is for and what function it serves. Otherwise diagnosis would be guess work. Many thanks for everyones replies.
Dave.
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Old 3rd Mar 2016, 8:13 pm   #98
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Default Re: HMV 653 Broadcaster all wave.

Keep at it

Lawrence.
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Old 3rd Mar 2016, 8:47 pm   #99
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Default Re: HMV 653 Broadcaster all wave.

Ok, I've had a go at the maths. I'm using the Marconi trader sheet 732.
Not sure I've got it right but please feel free to correct me.
Valve 1 anodes current draws are 4.0 ma and 1.9mA.
This gIves us a total draw of 5.9mA.
Squaring this is, 0.00590.0059=0.00003481 mA
0.00003481 5000 ohms (R7) = 0.174 watt resistor needed for R7.

For R3 0.0059 0.0059 =0.00003481 15000 ohms (R3)
= 0.522 watt resistor needed for R3.
As I say, not entirely sure I've done this correctly?
Dave.
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Old 3rd Mar 2016, 9:06 pm   #100
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Default Re: HMV 653 Broadcaster all wave.

The 1.9 ma mixer anode current doesn't need to be entered into the calculation as that supply is from a different feed.

Just the 4ma which is the oscillators anode (G2) current which is supplied via R3 & R7.

Don't forget that the calculation will give the power dissipated in the resistor(s) The wattage rating of the resistor chosen should be higher otherwise it might run hot.

Lawrence.
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