12-27-2022, 11:33 PM
Hi Radio Rich,
I looked at several other radios with 4" or 5" speakers that have "good tone". (The 2 RCAs are certified by their Chief Engineer to have the tone of the "Golden Throat")
The variations in the coupling caps between the volume control and 12SQ7 Grid (C4 on your schematic) and between the 12SQ7 plate and 50L6 grid (C6 on your schematic) vary greatly among different brands of AA5 radios. This may be due to voltages on the tubes, Grid leak resistors, cathode bias, etc. Following is a list of cap and resistor values translated to your schematic: (edited, apparently, the spacing does not translate, image of table attached)
C4 C6 C7 R6 R7 R8 R9
Your Mitchell 0.005 0.01 0.01 6.8 Meg 220K 480K ?
Emerson 330 0.002 0.024 0.024# 15 Meg 500K 500K 140
RCA 8X541 0.02 0.002 0.05* 5.7Meg 220K 470K 150
RCA 8X71 AM FM 0.01 0.001 0.02* 10 Meg 680K 820K 150
What does seem to happen is that if the coupling cap from the volume control to the 1st AF is high, the coupling cap between the 1st AF and Output Tube is low and visa versa.
* These radios have an output transformer with a tap for B+ to the other tubes. The Output Bypass Cap is
between the 50L6 Plate and the tap, not between the plate and output stage B+
# This seems a little high.
Do also check or replace C3 and C5. These are usually 100 to 250 pF and are used to filter remaining RF in the audio signal.
3 Mods I think may help:
Up C6 to 0.02
Add a 25uF 50V Cap parallel to R9 (the cathode resistor). The RCA 8X71 has it.
If there is room for a higher compliance speaker with a modern magnet, replace it. I know that this may be difficult.
As far as the really unclear upside down tubes, several manufacturers did this in the 1940s. I have seen this on Crosley and even some RCA schematics (for RCA built units).
I agree with Terry, John and Rod who recommend checking out the speaker. I will hazard a guess that most of these were around 4 Ohm. What diameter is the speaker? Is there decent bass response? you should see the speaker cone move with bass notes or deep male voice. If you press dead center of the speaker, it should move relatively freely. Compare to other 4" radio speakers. Sub the speaker or test the speaker on a known good radio.
Good luck with this unique unit.
I looked at several other radios with 4" or 5" speakers that have "good tone". (The 2 RCAs are certified by their Chief Engineer to have the tone of the "Golden Throat")
The variations in the coupling caps between the volume control and 12SQ7 Grid (C4 on your schematic) and between the 12SQ7 plate and 50L6 grid (C6 on your schematic) vary greatly among different brands of AA5 radios. This may be due to voltages on the tubes, Grid leak resistors, cathode bias, etc. Following is a list of cap and resistor values translated to your schematic: (edited, apparently, the spacing does not translate, image of table attached)
C4 C6 C7 R6 R7 R8 R9
Your Mitchell 0.005 0.01 0.01 6.8 Meg 220K 480K ?
Emerson 330 0.002 0.024 0.024# 15 Meg 500K 500K 140
RCA 8X541 0.02 0.002 0.05* 5.7Meg 220K 470K 150
RCA 8X71 AM FM 0.01 0.001 0.02* 10 Meg 680K 820K 150
What does seem to happen is that if the coupling cap from the volume control to the 1st AF is high, the coupling cap between the 1st AF and Output Tube is low and visa versa.
* These radios have an output transformer with a tap for B+ to the other tubes. The Output Bypass Cap is
between the 50L6 Plate and the tap, not between the plate and output stage B+
# This seems a little high.
Do also check or replace C3 and C5. These are usually 100 to 250 pF and are used to filter remaining RF in the audio signal.
3 Mods I think may help:
Up C6 to 0.02
Add a 25uF 50V Cap parallel to R9 (the cathode resistor). The RCA 8X71 has it.
If there is room for a higher compliance speaker with a modern magnet, replace it. I know that this may be difficult.
As far as the really unclear upside down tubes, several manufacturers did this in the 1940s. I have seen this on Crosley and even some RCA schematics (for RCA built units).
I agree with Terry, John and Rod who recommend checking out the speaker. I will hazard a guess that most of these were around 4 Ohm. What diameter is the speaker? Is there decent bass response? you should see the speaker cone move with bass notes or deep male voice. If you press dead center of the speaker, it should move relatively freely. Compare to other 4" radio speakers. Sub the speaker or test the speaker on a known good radio.
Good luck with this unique unit.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
