I have this model that was in an orginal state with most of the components including the Top chassis filter caps still in place and the terrible deterioted rubber wiring. Not sure if some caps have been replaced there is a few mustard caps in place? The radio is a 1947/48 model we're mustard caps used then?

Anyway onto why I am stuck in identifying the cause of having almost halve the expect B+.when all valves are in place.

I have carefully replaced the bad wiring and the filter caps, as well as some of the other paper caps that could be leaking and dragging down the B+. I have tested and replaced valves,checked and replaced resistors.

The Output transformer and the Field Coil have the correct resistance and I am getting some audio.

The power transformer is operating as it should and with only the 5Y3 in place I am getting the correct B+.
As soon as I place the 6V6 in place, the B+ goes high on the insurge as expected but then drops to about 85V.

Unless I have missed something the most likely components have been replaced and checked the only thing I feel left is that the output transformer, or field coil is potentially failing somehow during power up?

So I thought I would put it to the great brain trust in this group on what else could be causing this or is the only way to find out what is going on, is change the electrodynamics speaker by finding a replacement and see if this corrects the problem.

Any thoughts or suggestions are most welcomed.

First suggestion is to check all the resistors in the grid one circuit of the 6V6.
And the coupling cap to that grid.
The 6V6 itself may be "gassy"
JJ

Mustards date from the early '70s.

Looks like the 6V6 is turning hard on. But it won't be due to the coupling cap if it's a mustard.
Measure the voltage on G1 of the 6V6. It should be -10 volts or so with respect to the chassis.

If it's nothing like this, when you replaced the electros, did you unintentionally connect the - side of the 1st filter cap to chassis and short out the back bias resistor?

Hi JJ,
I had checked all? the resistors and replaced R1, R11 and R 12, and the coupling Cap was replaced.

Interestingly the one resistor that I had missed was the back bias 50 Ohm 3 watt wire wound resistors which had gone open. Cannot believe I had missed it.

I replaced it with 2 x100 ohm 2 watt resistors in parallel and as you would expect the B+ settled and the Plate voltage on the 6V6 was 220V as expected, the radio is singing once again.

Do hate having to change all that old rubber wiring hoping that you did not mess up a connection and I was so focussed on checking and double checking it was correct that I had overlooked checking the back bias resistor.

Hi Ian,
Just saw your response after I had discovered that the back bias resistor earlier this afternoon was open.

I thought those Mustard caps, were more recent and they all checked fine.

I had not taken the -ve side of the electro directly to the chassis.

The Bias measures about 10V with the replacement of the 50 ohm resistor.

With a 5Y3 there is always a surge to near twice the running voltage, on start up; I will not use a cap of less then 500V on B+ where their is a 5Y3.

10V is pretty close to the -12.6V Grid bias of a 6V6. Do make sure its grid resistors have not failed high (normal).

I hadn't thought of the back bias resistor being open, but of course!

With it open, the B+ supply is also open, but then the very high negative bias on the 6V6 would effectively set it up as a current sink, where it would settle at a certain level, barely conducting at all.

Interesting to do the network analysis on this...

Having seen it more than once, The voltages are a dead giveaway. If referenced to chassis it is similar to a cathode resistor going open as that is pretty much what a back bias resistor is, only its usually for all tubes. If the control grid is tied to positive or becomes positive its a diode and that is done. A series 70 American Lyric (from memory uses a triode, diode wired to supply around -84V to the grid of a #50

As for a single tube cathode fail. All of the elements of the tubes tend to be at or close to B+. Do not heavy up that resistance. It needs to sail with the wind so that it burns up if there is a short. Its 50 Ohm 3Watt do not use 5Watt.

There should be an analysis made, as to why it failed? Was it C23, a heater cathode short, or just a HT dead short. That's a rare fail in a discrete resistor.

Sorry Marcc
I have been off the grid, so have only just seen your comments. I believe it was the C31 as I was doing a slow ramp up on the radio with all the original components in it. I sometime do this on a radio that has all or componets and clearly has not been switched on for a while. Doing this with a dim bulb set up and just the rectifier in place I attempt to reform the electrolytics. Once the voltage in got up to 180V the B+ started to behave differently and in stead of settling on the expected voltage it began to fall to much lower voltages. I think that is when the resistor possibly failed as C23 was failing.