BC-348-O Filaments

[WB4YVO]

ALL, How can you tell if the filaments are wired for 24 or 12 volts.

Skip

[n8fvj]

You could measure the filament resistance. Add up the tubes resistance with all in series of two tubes in series each, then all tubes in series with 4 tubes in series each. Another way if a little risky but needs no math is power up on 12 volts and look at the filaments. If 24 volt wired the tubes will barely light up vs being normal brilliance if wired for 12 volts.

[KA3EKH]

The BC-348 is wired for 24 volt and the BC-224 is wired for 12, that being said most BC-348 that are out there today have been change or rewired so you need to check for yourself. I have a web page about the BC-348 that I put together years ago and it shows the CQ Conversion manual schematic for changing the filament string over for 6 volts. there is also a link for the video I did of the process back then too.

[KA3EKH]

Da! here is the link:

http://staff.salisbury.edu/~rafantini/bc348modifications.htm

[n8fvj]

I did a power supply simulation on the power supply in the link and it produces about 300 volts DC at 80ma current. The BC-348 does not like more than 220 volts B+.

[Pete, WA2CWA]

I did a power supply simulation on the power supply in the link and it produces about 300 volts DC at 80ma current. The BC-348 does not more than 220 volts B+.

Rather then just making a blanket statement like you did, why don't you explain to the readers what is your "power supply simulation" and, what parameters did you apply, and how you came about your conclusion.

[KA3EKH]

That is  the schematic from the 1948 CQ Surplus Conversion manual, just used as a example. think the transformer I used in that conversion was only rated at something like 275 on each side and by the time you added the choke was maybe 240 or so. I have a lot of older radios and have a isolated AC strip with a auto transformer on it and feed all the old tube radios around 110 volts and that tends to reduce the overall output too. Back in the old days you would use what ever power transformer you came across. Now day a lot of people look at things like overall B+ and that's not a bad thing. Its just an example of what most Hams were doing back in the day, way before simulations!

[KA3EKH]

Dam, did it again! second time today I screwed up on the same thread! just looked at the radio and the one on that page is one I did using a old power transformer from a URC-35 receiver, its rated at 350 Volts and center taped so that comes out to around 175 volts on the B+, hell maybe 165 by the time it squeezes thru the choke. Maybe if I had a simulator and knew how to use it I can say for certain. Or can just go old school and pop the cover and use a meter.
Ten or twelve years back there was a huge controversy about "whats the minimum plate voltage" that can be used to prevent any damage and there was an entire group of people claiming that they had there ARC-5 and BC-348 receivers working on +24 volts and some as low as +9 volts but that always sounded like a stretch to me. The radio works well on my reduced B+, 220 volts may be ideal but life is too short for ideal.
Don't know, who can say. maybe you are better off not reading my threads?

[KA3EKH]

Once I get started just too lazy to stop. On the subject of reduced plate voltage and tubes have to beat the drum about the Collins R-392 receiver. It’s a all tube HF receiver that was used in the field for AM, CW and Teletype, all tubes, maybe a thousand or so. Anyway that receiver used only the applied 28 volt source. The tubes were arraigned in series parallel being all six volt filaments but they also took a part of the incoming 28 volts and filtered it and used that for the B+, long story short there was no voltage higher then the applied voltage in the radio. The did eventually come up with a solid state replacement for the audio output tube, think it was something like a 26A6 or something weird like that but the little receiver worked surprisingly well. The transmitter it was paired with was a T-195 with the entire set being called an AN/GRC-19 The T-195 has the reputation for being one of the worst transmitters ever being they were first generation auto tune and often would not and they put out 100 watts of AM or RTTY but would consume enormous amounts of current to start there dynamotors. When they were installed on a M-38 they required a three belt generator and would stall the engine if you turned it on at idle, driving down the road transmitting would slow the vehicle down fifteen miles an hour.
Sorry, know it has nothing to do with the BC-348 thread just once I get started it’s hard to stop.

[n8fvj]

I did a power supply simulation on the power supply in the link and it produces about 300 volts DC at 80ma current. The BC-348 does not more than 220 volts B+.

Rather then just making a blanket statement like you did, why don't you explain to the readers what is your "power supply simulation" and, what parameters did you apply, and how you came about your conclusion.
/quote]

Duncan PSUD2 power supply simulator. Pete, this does not remove the 220 volts DC voltage limit on the BC-348.

[kb3ouk]

Back when this stuff was a dime a dozen, guys ran what they had and often times pushed it past what the rated max limit was. Especially with military radios, which were often overbuilt for reliability and run at lower voltages, particularly aircraft transmitters which often had the B+ ran at a lower voltage than what would be considered typical on a particular tube due to most planes in WW2 being unpressurized, they would start having issues with arcing and discharges at the higher voltage at high altitudes.

[n8fvj]

Most used BC-348s were converted to 120 volt with the power supply. If you buy an original dynamotor model, this is the cheapest power supply one can build using new parts. Two transformers are needed. Antek AS-0506 at $20 and AS-1T175 at $42. Wire all four 6-volt windings in series for 24-volts filament.  24 volts at 3.6 amps is available and more than needed.
Wire the two 175 volt windings in series and use a full wave SS rectifier with two 1N4007. Use a CRC filter network with two 270uF @ 350 volt capacitors and a 220 ohm 10 watt resistor for a super low AC ripple of only .03 volt at the 80ma load. A choke is not need. You will have 215 volts DC of almost pure DC. Buy a chassis on eBay for lowest price. About $100 total.

[WB4YVO]

Ok  mine was wired for 6 volts.   I rewired it for 24 volts ..  Next question  i also rewired the panel lights in series.. but they had 6 volt lamps in it  what bulb did they use for 24 volts?

It says use Lamp  LM27  .

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[KA3EKH]

Obvious question, why change it back to 24 volt? You plan on running a dynamotor? They make noise so you end up just using headphones if that’s the case. Of course the other thing is that the stock audio output transformer was 300 or 3000 Ohm so it did not lend itself to driving a speaker directly.
I have done the old WW2 stuff quite a bit back in the old day and always opted for AC power supplies. Although there is an entire school of thought that somehow you don’t get the “real” experience with out running the dynamotors, logical extension of that thinking is that your going to need a B-17 or B-24 to house it in. Some people have gone as far as building mock flight decks in there basement and have there ARC-8 (BC-348/ART-13) set up installed to recreate that WW2 experience. 
It’s all bass ackwards for me. I was not around in the old days but do remember when the BC-348 was used by many Hams and SWL operators with AC power supplies, low impedance output transformers, coaxial inputs and such. Some may argue that the BC-348 saw more and extended use in that world then in its short WW2 life.
The weird thing about the ARC-8 is that hey were used in cargo and transport aircraft well into the sixties so that family of radios was around for many years in both military and Ham service.
Ultimately its your radio, what ever you do with it is up to you, saw one once where someone had removed all the tubes and solid stated it.

[WB4YVO]

Well you wouldn't want to see what other things were done  to it..  jumpers shorting out parts  wires disconnected, filaments wired for 6 volts.. his power supply was a real mess..  etc.  missing parts.  So i decided to make it as close to original that i could..

That's why it was rewired for 28 volts and all mods?? taken out..I did recap it all. But with those looking like mica caps  You cannot re stuff them.

I guess I could have left it 6 volts  but with everything else messed up  The 6 volt mod was probably messed up also..



Oh well.

Skip

[KA3EKH]

Think the big thing today on restoring these things is the face plate. If you have a receiver that has a clean front panel without holes or other modifications its worth restoring. Fifteen years ago, I did the video on a 348Q that I picked up at a Fest for $20 that was already hacked and did the three most common Ham modifications, AC power supply, coax connector and S meter. Last year picked up another 348Q that had a perfect face plate but was hacked to death on the insides. Did a face transplant to the receiver that I had done years ago. Dropped the S meter and coax thing but kept the AC power supply. Also threw in the original shock mount. Attached are a couple pictures. One of the radios with the face removed and another of the finished radio in the background along with a BC-224 twelve-volt version.
The BC-348 receivers are not as common as they once were but they are still out there, have fun restoring yours and although it may not be a perfect example yet there will be more around so the experience you get will be worth it.

[n8fvj]

All the BC-348s and no 9 to 1 Unun installed for the lBC-348 1 to 5 ohms antenna input.

[WB4YVO]

Ok  Rewired for 28 volts  I am getting about 11.2 Ohms resistance across the 24 volt input  .. that's the filaments and the lights..

Does that seem Right?

Skip

[KA3EKH]

wont be a lot when the tubes are cold. hit it with +24 or so and watch what happens! remember when putting it back together the B- from the power supply is held above ground by a fifty Ohm resistor to develop a negative bias on the audio tube. If you just ground the B- from the power supply the tube gets real hot! That was a common Ham mistake.
Also the stock audio output transformer is weird having something like a 300 Ohm and 3000 Ohm secondary, I used an old output transformer from a TV set to get 3 Ohms for driving a speaker.

[WB4YVO]

Ok  Doubled checked my wiring.  Started to apply 24 volts just filaments and lights Measured current  It started to climb to about 500 ma..  So I turned it off.

Question is  Running the filaments and lights  What current should it draw  NO HV connected..

Skip

[KA3EKH]

Think the O has like ten tubes in it. A good rule for old tubes like that is they draw around 300 Ma per tube for the filament. That comes out to about three amps for the entire filament system. Also you will find it will draw more current when the tubes are lighting up. would expect that you will be seeing a total current drain of around two and a half to three amps under normal operation.

[WB4YVO]

Ok  Here is what I get..  At 28 Volts input  I measure about 920 MA.. That's counting the lights too.  Measured voltage at a few tube and they are right at 6 volts..

So now on to the HV 

[KA3EKH]

Sorry about yesterday, CloudStrike recently pushed an update that been wreaking havoc amongst a number of workstations and been tied up trying to resolve that. So now realize that I had several mistakes in my post of yesterday, that along with my fixation on always running the filament string at 6.3 volts AC.
Let’s look at the basic design of the radio when used at 28 volts DC, there are seven tubes, all of them except the 41 tube are 6.3-volt 0.3 amps for a dissipation in watts of just under two watts, let’s just say two watts each (P = I X E 6.3 X 0.3 = 1.89) So, 6 tubes at 2 watts each is 12 watts total. The 41 sucks down 0.4 amps so that’s (6.3 X 0.4 = 2.52) or about 2.5 watts. That gives us 14.5- or fifteen-watts total.
The dial lamps may be a watt or two for both of them so now we are up to about seventeen watts for lighting up the bottles. If the receiver was wired for 6.3 volts we can figure for 17 watts we would need around 2.5 amps of current (17/6.3 = 2.69) but you are running the original 28-volt system.
Let’s look at what they do in the receiver! There are two filament strings each with four tubes in it, one string has the 1st Det, 2nd RF, 1st RF, and Oscillator that are all 0.3 each. Ohms law tells us that current is constant in a series circuit so as long as 24 volts is applied to VT-91 everything is cool, the other four tubes have an issue. V-48, the 41 tube wants 0.4 to light that bottle up. The other tubes only want 0.3 and if current is constant in a series circuit how do you resolve this? That’s where R-501B comes in by supplying a little extra juice to the 41 tube so its happy. This entire mess is happy at 24 volts but you are feeding it 28 volts if you happen to have your radio in a B-29 so in order to drop from 28 down to 24 R501A will do that but in the process, that’s dissipating power too. (4 X 0.7, both filament strings together = 2.8 watts) now our total power consumption is up to just about 19 watts not including the dial lights. Let call it 20- or 21-watts total.
Your measurement of 0.92 amps at 28 volts comes out to about 26 watts (28 X 0.92=25.7) and that looks close enough to me, ideal would be 0.75 amps but I think we are close enough to say you got it nailed.
Ok, that’s what I think. Certain any number of smarter people will tell me the error of my math or description but that’s how I see it. Thanks for the opportunity for letting me have a chance to review my basic Ohms law, it’s been a while!