Hi Team,

I am new to refurbishing valve radios and the project I am undertaking is my first restoration and I have already come across a knowledge gap. I have searched the internet and this forum seeking an answer but haven't been able to find one.

I have started the process fo replacing the paper and electrolytic caps. I found what I think are the two electrolytic caps - one a Ducon 8mfd 600V that is clearly labelled and the other is a Ducon 16mfd can rated at what appears to be 525V. The 8mfd when tested shows a 10.5mfd reading and the 16mfd shows 24.4mfd.

Neither are marked with a positive or negative identifier. I have checked continuity to earth from the two terminals leads to each capacitor (the 8mfd removed from the circuit). The can has continuity form one terminal to earth so I can identify the negative there. The 8mfd capacitor does not have continuity to earth on either lead so I can't use that method to determine the polarity. The only differentiator I can see is one end of the capacitor (axial) is a red composite material and the other appears to be aluminium. I feel like a bit of a numpty having to ask but :

1. Am I right in my assumption that the aluminium end of the 8mfd axial capacitor is the anode (+ve) and the red is -ve?
2. Should one of the leads to the axial electrolytic capacitor have continuity to earth the same as a the can? If not can you please explain why.

Thanks in advance for your advice.

Cheers
Craig

Hi Jols, the Ducons I see in radios with wire solder leads are usually an alloy can fitted into a cardboard tube.
The alloy can is negative so the lead welded to the that end is negative.
Theother end of the can has a red 'plastic' plug and that is the positive wire.

Electros you buy now are usually in black plastic with markings showing + and - wire ends.

To keep the look of a radio the same I push the dead alloy can out of the cardboard tube and slide in a new replacement.

Fred.

Thanks Fred,

That makes it clear for me. Now to find some 600V replacements!

Cheers

Craig

Hi Craig, you will find local suppliers of caps and bits on ebay.
I buy from aussie suppliers who import in bulk and sell in small quantites.
I have had no problems with the quality of electro caps this way.

We dont promote any specific sellers on this site it just leads to all sorts of argy bargy.
You will note the lock on the for sale part of the site, no commercial sellers allowed.


Cheers, Fred.

Thanks Fred,

I have sourced the caps I need so now just need them to arrive in the post. I was lucky and got a batch online from just up the road plus JayCar for everything except the 8 and 16μF electrolytics. I found some 33μF and 18μF 350V electroytics at a reasonable price so will wire them up in series to meet the need.

While I wait for the to arrive I can check all of the resistors given I can access them now the capacitors have been removed.

Cheers
Craig

I found some 33μF and 18μF 350V electroytics at a reasonable price so will wire them up in series.

Connecting capacitors in series is not a recommended procedure except for voltage doubling circuits. Voltage balancing resistors are needed across them. Also, the total capacitance of series-connected caps is the total of the reciprocals of the individual values -- so 1/33 + 1/18. Not sure what value you are trying to achieve.

The rule of thumb with caps us that you can replace a cap with a higher capacitance, but never a lower voltage. A higher voltage is OK, too, but not too much higher.

These days capacitors (and resistors) are made according to a preferred series. The common preferred series for 10% tolerance is known as E12.

E12 values: 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2

GTC notes balancing resistors are a good idea when putting caps in series.
You might add a 100K across each cap.
The 100K must be a 1 or 2 watt size body like the Soanar resistors that Jacar have.
Don't use the little 1/4 or 1/2 watt or "1 watt" common body size they will just heat up and go poof.
The balancing resistor is a theoretical thing, essential if you were a mass producer of a commercial product.
I have series caps in some projects making up 600 volt replacements and not bothered.

The thing about resistor body size goes for most replacements in an old radio chassis.
Some resistors wind up with hundreds of volts across them and a fair amount of current so little resistors will overheat, turn brown or catch on fire.

Fred.

Thanks GTC

What I am trying to replace are an 8μF and 16μF 600V electrolytic capacitors in a Mullard Radio. I have not been able to locate these so my plan was to create them by wiring 350V capacitors in series.

The concept I had in mind is based off the following:

To replace the 8μF 600V with two 18μF 350V.

Ctotal = 1/(1/C1+1/C2) = 1/(1/18+1/18) = 9μF
Vtotal = V1 + V2 = 350+350 = 700V
I max leak = 3mA (use 20% for calculation)
Vheadroom = 100V
Rbal = Vheadroom/0.2xI max leak = 100/(0.2 x .003) =167kohm
Resistor Current I=V/Rtotal (ignoring internal resistance) = 600/334000 = .0017A (1.02W) or 0.5W ea therefore two 167kohm 0.5W resistors to balance.

To replace the168μF 600V with two 33μF 350V.

Ctotal = 1/(1/C1+1/C2) = 1/(1/33+1/33) = 8.5μF
Vtotal = V1 + V2 = 350+350 = 700V
I max leak = 5mA (use 20% for calculation)
Vheadroom = 100V
Rbal = Vheadroom/0.2xI max leak = 100/(0.2 x .005) =100kohm (in parallel with each capacitor)
Resistor Current I=V/Rtotal (ignoring internal resistance) = 600/200000 = .003A (1.8W) or 0.9W ea therefore two 100kohm 1W resistors to balance.

I am completely new to this - perhaps I have completely missed the mark on how to get the job done without breaking the bank.

Dont overthink the capacitors too much.
The originals were + and - 50% or worse.
In a valve radio so long as there is something in there that acts like a capacitor it will be ok!
Fred.

You can obtain brand new axial 600V 8μF and 16μF capacitors from fellow Vintage Radio member Taller Carl.

I don’t think his contact details are hidden.

Thanks MonichromeTV I will look him up.

Fred/GTC

I appreciate the advice but now I'm a little confused - and probably because I am coming from this from experience as a sparky as opposed to a technician.

As I couldn't find the right sized capacitors I ordered what I needed to create the desired output in V and μF and calculated a balancing resistance to meet the design load and replicate a single capacitor. On one hand the advice is doing this is not a good idea and on the other it has been done successfully, sometimes without balncing resistors, and I shouldn't overthink it.

So at the moment I'm not sure what the takeaway is: don't do it unless creating a voltage doubling curcuit, crack on with what I had planned using 100kohm 1W resistors to balance the voltage across the sub circuit, use series capacitors with no balancing resistors or contact Taller Carl and get the right components.

I expect that using the correct capacitors as the obvious choice; but is the originl plan a poor option. If so why - simply trying to understand the fundamentals.

Please don't take offence - the advice is greatly appreciated; however, to a newbie the advice seems aligned but slightly contradictory.

Cheers

Personally buying cheap inappropriate capacitors is really wasting money creating extra work using more parts & wasting time. I fix I get parts. Placing electrolytic capacitors in series without balancing resistors introduces unreliability and as far as I am concerned, is to be avoided.

In sets with #80 or 5Y3, or anything that behaves like them, remember that the old GP Red Ducon electrolytics had a surge or peak voltage rating of 525V for sets that had 250V on the plates, higher 600V. It is rare to see PV or SV on modern caps so I will not go below 500V.

450V caps in that application I have found unreliable and I have serviced hundreds of sets. It has also reached the point where I will place "new" caps and older stock on a reformer, to ensure they have "form" and are unlikely to fail. This is also why sets need a regular run.

With resistors I predominantly buy 1 watt to get the voltage rating and keep the inventory down.

Do not touch Mica and Ceramic capacitors at this point. removing the wrong ones can have catastrophic ramifications.

Marc

Tallar Carl: https://vintage-radio.com.au/default.asp?member=Tallar%20Carl

Using two identical electros of double the capacitance in series with (typically) 100k balance resistors is perfectly OK.
This technique is regularly used where you need higher voltage and/or ripple current ratings.

Hi Craig, no offence taken!

With old radios the circuits had a wide tolerance of component values.
The parts available at the time were at best 10% of value.
Components can wander 10 to 20% and the circuits worked fine.

You can use exact value replacement electrolytics or you can make up to size with series or parallel combos.
As Marc notes the important thing is the peak voltage rating, some sets will generate 500 to 600 volt off the rectifier before the other valves draw current and load the voltages down to 400 or so.

If you series up 300 volt rated units put some 100k resistors across them to share 500/600 volt evenly.
Capacitor value only needs to be "about" the original.
On those old caps they were pushing to get 8 or 16 μF in the cans.
Nowadays 30 or 50μF is easy in much smaller packages.
I have bought caps from Taller Carl with no problem of all sizes, they are in heaps of my repairs and projects.

There is a line between being exacting engineering and being practical, i'm a practical person.
Just look at my body of work on this site, being practical gets things done arguing about exact science is a waste of effort and does not get the job done.
Buy or make up caps to suit the voltage rating with μF close enough and that job is done.

Cheers, Fred.