David don't forget that the original twin electro had a common negative, don't make the mistake I made once, when replacing it with individual electros I accidently soldered the second electro's negative to ground, both go back to the centre tap.

OK, thanks Marc.

Looking at the schematic for the 11-29, shouldn't there be a resistor between the 8mfd and C53 to apply a bit of 'backpressure' to the 8mfd?

If you don't mind Marc, can you tell me why on some radios they connect the negative side of the filter cap to the centre tap rather than to ground? I guess it's just going between B+ and B- but what is the advantage?

Our house supply varies between 230 and as high as 243 depending on the time of day.

David

Hi again Rudolf,

Yes, I twisted the two negatives together and both back to the centre tap. I have to pull it apart again so I will just check that though. What was the effect when you did it the way you described?

When I got the radio, someone replaced the double electro with one. The radio had excessive hum so I flashed one across B+, it didn't like it at all, that's when I checked the circuit and discovered what I mentioned above. Fitting the missing electro in its correct place gave me almost negligible hum.

Ahhh... thanks for that Rudolf

BTW... I checked mine and it was wired correctly

David, worth reading about biasing. http://www.aikenamps.com/index.php/what-is-biasing

Thanks Rudolf, I've read it a few times before but as I gain more knowledge it makes more sense and tends to stick. Thanks for the link I'll read it again, very much appreciated

Cheers, David

The Centre tap of a tapped transformer is the most negative point, or the negative of one not tapped. Most sets with back bias will have the first filter cap across the higher voltage then the second to ground across the back bias, otherwise you get hum modulation impressed on the cathodes (which amplify)

Philips & Pye however, are ones that will actually put both caps on the higher voltage & then use a third bypass cap across the back bias, however, if the cap is electrolytic the positive goes to chassis.

The biggest confusion is when the choke / field is in the negative and both positives are common.

Marc

Thanks Marc,

I think I see now, because the cathode is directly connected to ground, connecting the filter caps directly to ground also would induce a hum in the cathode. By connecting it to the centre tap it is essentially in parallel with the back bias resistor. That would explain why when Rudolf momentarily connected a filter cap from HT to ground it produced more hum.

I have done a few Phillips sets and seen the filter cap positive connected to ground and correctly installed the replacement Capacitor.

I don't believe I've come across the choke situation as you describe

Cheers Marc and as always thanks for your help

David

David, I'm nearly finished restoring the STC 141 which I bought from Facebook Marketplace, if you check the circuit you'll see the choke going from the centre tap to ground.

The STC 141 is a classic example. Its choke is actually acting as the "Back bias".

The general theory was that keeping the body of the coil and former at the lowest potential between them, placed less stress on the windings & also reduced corrosion. Back bias was a common way of applying a fixed bias without the need or a lot of cathode resistors. In the main it was the current of all of the cathodes passing through the back bias resistor which developed the required voltage drop. The voltage between Negative & chassis was usually that of the control grids of the OP tube/s.

If you see an OP cathode grounded always look for back bias. Never heavy that resistor up: Better it burns in a fault situation than the transformer.

The traps, ISO OP transformers: Most had a grounded insulated internally metal cap (so the can may cut the wires) The transformer body within, is "alive". PYE / Astor were not alone in putting on their High End sets an open OP transformer, inside the pan & mounted on a thick Bakelite Board. If you see that, the transformer is "Live Frame" and will bight. Same theory as the choke in the Negative.

Thanks Rudolf and Marc,

I appreciate you both taking the time to explain all this. I hadn't seen the layout in the 141 before or worked on one like it but understand it now you have explained it, I would have had trouble figuring out the theory there.

I have done two radios where the the choke case was live (Healing 401, can't remember the other one) and isolated from the chassis and understood it was to reduce the potential between the windings and former as you say. I stayed well clear when it was running

Thanks for the tip with the BB resistor, I probably would have "heavied it up" as you say. I can see now that most/all of the sets DC current draw is through the back bias resistor and it can act as a slow blow fuse, if that's open suspect problems elsewhere, got it.

As I said, I very much appreciate you helping me get my head around this, it's difficult finding out the little things so thanks to you both.

I hope your STC comes up good Rudolf, I like the 'caravan' style radios, are you doing this one for yourself?

Cheers, David

David, my mistake it's an STC A140, same back bias system as the 141. I'll post photos of the finished product as there are many already on what it looked like when I received it. The radio can be seen on "Buying from Facebook Market place" in General Discussion. Yes it is mine part of what I call my Orphanage collection, restored bakelite cabinets that are of no value that only I enjoy.

Here is a picture from Radio Museum https://www.radiomuseum.org/r/stcaus_a140f_a140.html

I always advocate that if you are willing to learn how a radio works; (or anything else works for that matter,) In the case of the radio: If you gain a good grounding in how it all works, then you have a better chance of understanding why it won't.

On the subject of differential voltages etc. Note on the circuit of the 141 a dotted line within the laminations which will find its way to chassis: Normally that is an earth symbol. This is a shielded transformer. They are different: As the name suggests they have an internal metal shield. Any set I find with one, will automatically be fitted with an earthed mains cable. Why?...……..

On my bench is a Kriesler 11-81A. It had paper caps, but the principal fault was a pulley fault in the dial drive. It was also reported by the owner to be down in performance. At this point it still had its two wire cable and no earth leakage on the primary. However, during calibration (the main deafness issue) it was noted that there was voltage on the chassis. The high impedance meter put it at 55V AC.

This is the sort of "charge" that a shielded transformer will put onto a non grounded chassis and is quite normal. I have not re-tested (Test & Tag) the set with its new cable, however I expect it to pass as it was not faulty before and no RCD's had tripped (no faults mains side).

Rudolf, by your definition all mine are orphans

I looked at the photos of your A140 and you have your work cut out there. That's exactly the restore I like doing but a lot would just use it for parts.

I use JB Weld too, I tested it against other two part mixes on Bakelite and it was by far the best.

Cheers, David