I have just started out learning about valve radios. After quite a while searching the usual auction sites I came across a deceased estate sale offering eight old radios for next to nothing. I really only wanted one to learn on but it was too good a bargain to refuse. So I now have eight radios wrapped in bubble wrap in a huge cardboard box under my bench. I picked the worst looking one to start on. The plastic cabinet was busted in several places and not all the plastic was with the radio. After piecing the plastic together there is a two inch hole in the back of the cabinet which will need to be filled somehow. The radio turned out to be a HMV 61-51. it was missing it's speaker but otherwise looked complete and original. Much reading ensued and a signal tracer/injector and dim bulb unit were knocked up. I did the preliminary inspection and voltage and resistance testing as suggested by so many of the repair guys. Well most of it anyway. I know. shoulda done more. Huge thanks to Dave Tipton. I have been devouring his you tube videos and they are a great help. The power transformer tested OK and visually the caps looked alright so I started it up on dim bulb. All good but no sound from the radio. I found a faulty resistor (R8) and replaced it. To my great surprise the radio came to life. The volume was very weak and when I turned it up the sound got very distorted and not much louder but at least I got reception of a sort. A huge step forward for me and I went in for dinner a satisfied customer.

More reading followed and I soon discovered that there was good voltage going to the output transformer but nothing leaving it to the plate of the output tube. How did I miss that? All other tubes seemed to be getting voltage albeit a bit high. Anyway it turned out that the output transformer seemed to be shot so I came to a grinding halt. Then just as an afterthought I decided to power up the radio after I had removed the output transformer. To my great surprise the radio worked just as it had before. Is this even possible? A quick scan of the schematic reveals a possible signal path direct to the speaker but I really have no idea if this is correct or not. But it does seem to be the only way the radio could work. If there is anyone out there familiar with this radio that could help me understand whats going on here I would love to hear from you.

I should mention that before powering up I removed the original two pin cable and added a three pin cable and grounded the chassis. Not entirely sure but I think that was the right thing to do from a safety perspective. I have a couple of other questions that have cropped up during my stumbling around this circuit over the last week or so.

Q1. When I fitted the three prong plug I traced the input wiring. The neutral (blue) wire is connected to the on/off switch which then goes to the top of the transformer winding. The active(brown) wire is connected direct to the bottom of the transformer winding. This seems to put the switch in the return path after the transformer. That seems wrong to me but the schematic is not clear so I have not changed it yet. Any advice would be appreciated.

Q2. I have looked on the internet and it looks like finding a replacement output transformer is not going to be cheap or easy. There was some discussion on using one of those PA System line transformers (which are cheap and easy to get ) but plenty of advice not to use them as there is some sort of core saturation problem that could damage the radio. Do I put this one in the parts pile and pull out one of the others ? Or do I press on and get this one working properly? What do you reckon?

Thanks for reading this novel.
Cheers
Ray.

Hi Ray

Grab one of these from Jaycar https://www.jaycar.com.au/5w-line-speaker-transformer/p/MM1900

They work fine and are less than $10.

Good luck

Ben

Thanks Ben I will pick one up tomorrow.

Don't operate the radio without a working speaker transformer because you can damage the output valve and the power supply section.

After repairing about a dozen of this series and owning one. Beware of R5 & R6 feeding the screens of V1 & V2. Both 22K in parallel. In later models, probably organised by the bean counters. That pair of resistors was replaced by a single 10K with a different number. If that single resistor is one watt it will often cook.

Active switching is preferred. Do make sure that the cable is wired correctly at the plug top. I do tag and test as well & its amazing as to how much I find especially in extension cables, wired wrongly.

Thanks for the suggestions guys. This morning I picked up a couple of the line transformers that Ben suggested and wired one up. Pleased to report that the radio came to life with good volume and not much background noise. Before I hooked the transformer up I tested the voltages across the different taps and worked out the impedances for a 4 ohm load. From memory tap 1 was 10000 ohms and tap 2 was 5600. The tube data for the 6M5 suggested a 7000 ohm impedance is what it would like to see. So I experimented with both taps and I cannot discern any difference between the two. They both seem to work equally well. I was a bit worried on startup as Robbert posted not to start the set up without the output transformer. I only ran it for maybe 30 seconds in that state and I must have got lucky with not doing further damage. But I have filed that away for future reference.

Marcc I tested R6, R7 which was just a single resistor on my set. It tested good so I left it alone. And thanks for the info on the wiring. I am yet to reverse it but will do that next. I tested the plug and it is wired correctly.

I have some caps coming and I will change the old paper ones out when I get them. I am on the lookout for a 7"x5" speaker but for now the one I have sounds just fine. I may have to make some sort of plate to mount it in the case. I don't think the case is in original condition as the speaker grille has been glued in and will be impossible to remove without damaging it. I can see no sign of how the speaker was originally mounted. I am guessing there would have been some plastic tabs that must have been removed. The case is in poor condition with cracks and a couple of holes in the back piece. Some of the plastic was inside the set but not all. The other funny thing about the case and the set in general is that it has no tone control pot. There are only two knobs on the front and the other two have been blanked off with white button like pieces. I have not seen another one like that in all my internet searching. I wonder if this was done to save money in the later sets.

Anyway thanks to everyone that responded with help and tips. Much appreciated. Feel free to make further suggestions. Especially with how to maybe mount a speaker inside. Cheers Ray.

A serious error especially, but not exclusively with speaker plugs, was to leave the Screen of a Pentode or Tetrode, with power and the Plate open circuit.

In that scenario the screen becomes the plate: It cannot handle the current and is often destroyed, often causing further damage.

The figure of 7000 is the minimum recommended, you can have more if that's the only choice. So use tap 1, 10000. The more windings gives more output and more load for the 6M5 so it runs cooler.

The reason for not using the set without a speaker transformer is because without the anode connected, G2 becomes the de-facto anode for which it is not designed, so it glows red. It also has no load at all, so excessive current will flow, possibly damaging the 6M5, the rectifier valve, the power transformer, and whatever components are in the HT filter (choke or resistor). If the power transformer melts, you could have a fire. So, please don't do it.

Tap 1 it is. Thanks