10-02-2023, 03:27 PM
I am puzzled also about that GND that does not work.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
54C Baffle
10-02-2023, 03:27 PM
I am puzzled also about that GND that does not work.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
10-02-2023, 03:38 PM
Hi Ron.
The 12A8 and the 12SK7 both have pin 1 connected to chassis ground. The 12SQ7 DID have pin 1 connected to the chassis also. Just like yours. But the sound was garbled. I disconnected pin 1 of the 12SQ7 from chassis ground and instead connected it to B-. Now it works fine. I did use a PM speaker. Actually, the same one you used. I'm in the process of converting another 54C right now. We'll see if it acts the same.
10-02-2023, 03:41 PM
I see the confusion on that post. It should have read "instead of ground", not "instead for ground".
10-02-2023, 06:29 PM
Hello Patrick,
Sounds like you got winner and good luck with the second one I sure the team here would like to know how it turns out . Sincerely Richard
10-03-2023, 07:31 AM
Patrick,
You know, connecting pin 1 of that 12SQ7 means the metal shell of that tube is now connected directly to one side of the AC line! That, to me, is not a safe configuration. Are you using a polarized AC cord and plug? Is the plug and cord configured so that neutral is connected to the wide blade, and the hot side to the narrow blade? -- Ron Ramirez Ferdinand IN
10-03-2023, 07:52 AM
No. It's not a polarized plug. But I guess I should use one, with B- connected to the wide blade?
10-03-2023, 08:03 AM
And I do have the back panal safety switch installed which ties to both hot and neutral.
10-03-2023, 08:53 AM
Sorry for the wordiness. My humble $0.02:
I never understood why just about every AA5 radio has the power switch between "B-" and the power cord. Is this to reduce hum? In radios of this vintage, before metal tubes, the chassis was usually connected to B- via a cap or cap and resistor to get the chassis at B- potential for RF / AF but not for power. This is to reduce the possibility of electrocution before the advent of "double insulation", interlocks, etc. In act, on the latest AA5s I believe that the chassis is connected to B- because of double isulation and interlocks. Have to check the schematic for a late model. (You experts know all of this; I make this comment for Pholks new to the hobby.) Prior to metal tubes, the IF and 1st AF tubes had metal can style shields that connected electrically to chassis GND. If the 1st AF tube had a grid cap (75, 6Q7, etc.) the grid wire was shielded or was also inside the can shield. Otherwise, the 1st AF stage was very unstable. Even in AA5s with miniature tubes (12AV6, etc.) that tube requires a shield connected to chassis. Verify the following:
Hope this helps. I have gotten my "U know what" kicked by simple issues on simple radios on more than one occasion! "Do Justly, love Mercy and walk humbly with your God"- Micah 6:8 Best Regards, MrFixr55
10-03-2023, 09:06 AM
Patrick Wrote:But I guess I should use (a polarized plug), with B- connected to the wide blade? Yes!!! Safety first! It may well be that by using a polarized plug, and ensuring B- is tied to AC neutral and not hot, that then you may be able to once again connect pin 1 of the 12SQ7 to chassis, as my 54 is. Also please take note of John's (MrFixr) comments - and John, thanks for weighing in here. John, yes, manufacturers used the AC switch in the neutral side of the line in an effort to reduce hum. They also did not use polarized plugs back when these sets were being made. I use a polarized cord and plug on any AC/DC set I work on, whether it is modified (like my 54C) or otherwise kept stock. I also change the wiring so the AC switch is in the hot lead instead of the neutral. I have not noticed any increase in hum in the sets I have changed the switch wiring on in this manner. -- Ron Ramirez Ferdinand IN
10-03-2023, 09:19 AM
John (MrFixr)
FYI, here is the schematic for the modified 54C: https://ronsradios.com/images/54_schem_Rev5.jpg Note that the cathode of the 12SQ7 connects directly to B-, and the grid is biased with a 3.3 meg resistor, also connected to B-. -- Ron Ramirez Ferdinand IN
10-03-2023, 10:49 AM
"It may well be that by using a polarized plug, and ensuring B- is tied to AC neutral and not hot, that then you may be able to once again connect pin 1 of the 12SQ7 to chassis, as my 54 is."
I'll try that.
10-03-2023, 02:02 PM
Honestly, I am not a fan of using ANY pin of the AC outlet, Hot or Neutral, as a GND for a tube case.
What I do not like about this whole situation is there is an unanswered question to a problem, whose solution requires some safety compromise. Especially considering there is an implementation (Ron's) that is known to work. People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
10-03-2023, 04:38 PM
> Honestly, I am not a fan of using ANY pin of the AC outlet, Hot or Neutral, as a GND for a tube case.
I totally agree. The only connection the chassis of my modified 54C has to the neutral side of the line is through a 150K resistor, as was typical of many five tube AC/DC radios. -- Ron Ramirez Ferdinand IN
10-04-2023, 08:11 AM
We have success! Using a polarized plug, and making sure B- is tied to neutral, I was able to tie pin 1 to gnd. No more garble, and we are as protected as possible. Of course, with the safety switch in, it disconnects both hot and neutral when the back is removed. Thanks to all.
10-04-2023, 08:31 AM
Better.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
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