[Jmeulemans]

I'm finally digging into my 1931 crown and it has been messed with previously. On the schematic, for the filament windings it shows the center tap on the 2.5V for the 45 tube(which is actually a 47) passing through a combination of a .5mfd cap and a 1.6k resistor and going to nothing. On the actual radio the center tap is connected directly to the center grounded terminal on a 500k pot and leaving out one side through a 0.1mfd cap going to the plate on the 47. Is accurate or am I just missing something?    

[Radioroslyn]

It would seem to me that you need the 1.6k and the 1/2mf cap to provide bias to the 47.
By grounding the ct it's the same as grounding the cathode of a indirectly heated tube. And ct on the hv winding is grounded also so there is no - bias voltage develop in the power supply.
The cap and pot connected to the 47's plate maybe a tone control.

[Jmeulemans]

Ok, so the center tap and bias network should be connected to the grid on the 47 to provide the proper bias correct?

[Radioroslyn]

No. Unground the ct of the 2.5v filament winding. Reconnect the cap and the 1.6k resistor as per the original diagram. The end of the resistor that was not connected ground it. This restores the filament bias network. As it was with the tap grounded there was very little - voltage being developed on the control grid. This causes the tube to draw an excessive amount of plate current.

[Jmeulemans]

Ok, got it. Thanks a bunch!

[Jmeulemans]

Ok so another question. The transformer is apparently also not original and the filaments for the 24a's as well as the 47 are tied together. If I put the cap and resistor from the CT on the transformer to ground will that have any negative effect on the 24a's?

[Radioroslyn]

No because the 24A's have cathodes and so any bias voltage applied to the filament is isolate from the control grid. The 47's are directly heated (no cathode) so any bias voltage applied to ct of the filament winding does effect the grid.

[MrFixr55]

Hi JMeulemans,

+1 on what Terry Radioroslyn said. Definitely lift the ground on the 2.5V Center Tap and add the Cathode bias resistor and bypass capacitor.

Re the center tap not being grounded affecting the other tubes, probably not. If you have any weird oscillations, place capacitors (0.1 uF?) between each leg of the 2.5V secondary and ground.

The Audio Power Output stage of any radio employing a class A or AB power stage must be biased. On most radios, this is done one of 2 ways, essentially the same but different.

One technique is that the B- (center tap of the Rectifier secondary winding) is grounded to the chassis and the cathode of the output tube connects to chassis ground through a resistor. This resistor is the cathode bypass resistor and usually has a capacitor in parallel with it to prevent low frequency phase shift. Since the output tube is directly heated (the filament is the cathode), the cathode bias resistor connects to a center tap of the filament (heater) winding of the transformer. This is what should be done on your set.

The other technique is that the cathode is grounded to the chassis, but a resistor exists between chassis ground and the rectifier secondary center tap. this dreates a "C- at the junction of the resistor and the rectifier secondary center tap. a resistor form the Output tube grid to that point provides the negative bias.

I looked at a few schematics. The AK55 has separate windings for the 24s and 45s with the 45 filament having a voltage divider between the 2 legs (transformer did not have a center tap; this resister served the purpose. from the center tap of this resistor the cathode bias resistor and bypass cap tied to ground. the AK82 has a single end 47 Output tube. the filament (cathode is powered by the same 2.5V secondary as the other tubes but the secondary is "center tapped" by 2 resistors like the 55. the center tapmis to ground because bias voltage is through a resistor between ground and the center tap of the B+ which is not grounded.

My RCA R7A has push pull 47s fed by the same 2.5V secondary as the other 5 tubes but this set has a ]n interstage transformer and the center tap is to a "C- developed in the same why as the AK82.

Bottom line is if the single 2.5V secondary is hefty enough to handle all 4 tubes, and since the original schematic shows no grounding at all for the 24As, I think that Terry's recommendation should cause no trouble at all. However, if Terry's recommendation is not performed, failure to bias the tube will cause the 47 tube, the output transformer, rectifier or something else expensive to burn up.

One other word of warning, make sure that the speaker is connected and that the output transformer is good before firing up the radio. If you see a bright glow coming from the screen grid, shut it down! 47s are somewhat expensive and somewhat rare. they were only used for about 2 years before the 2A5 was released.

Good luck. This looks like a nice simple 3 circuit TRF with biased "power" detector and while not HiFi, should work rather well unless you have a whole bunch of AM Stations close to each other and to you. This is a good representation of an early 1930s "Screen Grid" radio and is a historically important circuit. I restored a 1931 Gloritone 26 which is similar to your radio except for a 45 Triode instead of the 47 Pentode.

[Jmeulemans]

Thank you both for all your help! This particular radio does not have a detachable speaker and the output transformer tested good. I could just see that it has been messed with and a bunch of components replaced with incorrect ones and rerouted to different locations. I have every intention of rewiring as per the schematic and your recommendations. The other 3 or 4 radios I have worked on in the past were bone stock and all I had to do was replace caps and resistors so this one threw me for a loop.