April 1, 2022 at 6:18 pm #18402Doug ParkerCVRS Member
I have an Electrohome Roland 6T8 AA6 radio that has a 33 ohm resistor that is connected between the 35W4’s anode and the dial lamp filament tap at pin 6.
I have gotten 2 schematics for the 6T8 radio. One schematic has the R15 – 33 ohm resistor in the circuit and the other schematic does not.
My chassis was untouched and it came with the resistor in question installed from the factory. I would assume that this was a later revision.
As far as saving the 35W4 for eventual filament failure during radio power on which is the better of the two circuits to run with?
Attachments:April 4, 2022 at 5:54 am #18405Ralph SpracklinCVRS Member
While I am by no means yet an expert on these things, I will attempt to explain it.
This 35W4 Tube was designed in the mind-set that it could be used in a circuit using a Dial Lamp or without a Dial Lamp. That is why the the filament on this tube is Tapped. Because your Radio uses the Dial Lamp, a a Shunt Resistor is needed. Obiviously the Design Engineer, did not include it in the one schematic as it was not needed because that schematic was for a model that did not have a Dial Light. Therefore your Radio, because it has a pilot light and the resistor, it will be OK, with no added failure dangers to the 35W4 tube itself. The resistor acts as a fail-safe (not certain if that is the correct word to use). should the Dial lamp go out. Let’s see what others on this site think of my explanation. See data provided on Franks Radio Data Site for the 35W4 tube. And by the way this Dial Lamp should be a #40 or a #47. And the 33 ohm R15 resistor according to the schematic is correct.
RalphApril 5, 2022 at 2:59 pm #18410John GreenlandCVRS Member
Attached is the recomended way to use a 35W4 as taken from the latest RCA tube manual.
I beleive that the resistor in question from the plate of the 35W4 is to limit the current though the 35W4.
You will notice on the attached sheet that the caps in the chart are in the order of 30 UF or so, not the 250 that is currently in use in this radio.
This size cap would take a higher initial current.
Also, according to the diagram in the attached sheet a shunt resistor is across the panel lamp and not in series with it.
I would keep the resistor in place.
John G. VO! CAT
Attachments:April 5, 2022 at 5:35 pm #18413Ralph SpracklinCVRS Member
John. I am now wondering if the Shunt Resistor on Dougs radio is in series or is it shunted across (in parrallel) wth the dial lamp, which to me seems to be the logical place for it. Maybe Doug will advise as to its actuall location.
RalphApril 7, 2022 at 4:22 pm #18428Doug ParkerCVRS Member
John and Ralph. Thank you for your input. The R15 – 33 ohm 1/2 watt resistor is factory installed in my chassis from pin 5 (plate-anode of the 35W4) to the pin 6 and one side of the lamp just as the schematic shows. Obvious there was two different schematics for the same radio. The untouched chassis had the two (50 ufd – 150 volt) filter capacitor sections. I would agree that the R15 is used to limit the current flow through the 35W5 as it is in series with the anode to the rest of the circuit. I measured the voltage drop across R 15 when there was no signal applied and the voltage drop across the resistor was almost 1 volt. That works out to 3 milliamps approximately. The voltage at pin 7 of the 35W4 was 94 volts DC and the line voltage was measured at 120 VAC. I swapped in another 35W4 I had and the voltages were the same. I will install the chassis back into the radio as everything was working properly. Again, Thank you.
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