BC 375 transmitter using a 6L6 tube instead of the VT 25

This is the BC-375 world-war 2 transmitter, which was used in the American "flying fortresses" during WWII.
The BC-375 transmitter normally uses the VT-25 tube (10Y) as a microphone pre-amplifier that drives the push-pull power amplifier, which on its turn plate-modulates the RF power amplifier.
Problem is; these VT-25 tubes have become ridiculously expensive on the used market, because of these audio nutcases who have been driving up the prices for the VT-25 like mad men. I see used VT-25 tubes go for as much as $200.
As I am not willing to pay that ($200 actually bought me FOUR of the large VT-4C tubes that you also see in this video), I had to come up with an alternative. One important criterion for the alternative was that mods to the transmitter are NOT allowed.
So I am now using a 6L6 audio penthode that I wired for triode mode (and also wired for directly heated); this means the 6L6 screen grid is tied to the anode (the "plate"), and the 6L6 cathode tied to one of the filament terminals (which is then the ground pin in the BC-375).
I added a 5 ohm 5 watt resistor in series with the filament, so that the 6L6 tube "sees" the specified 6.3V filament voltage from the 7.5V that the VT-25 normally gets.
As you can see in the video - it works PERFECT! Modulation sounds as good as with a VT-25, maybe even better than that!

[@robertrockwell7581]

you are a happy man i can tell. congrats on the build and tube mod. very nice.

[@LifeIsTooShortForQRP]

Me a happy man? My wife might disagree with you :-)

[@jaut-76]

Im trying to persuade a dealer in the uk to part with the one of many he has but it’s hard since these seem to be a target for audio guys to strip the output tubes and some other parts and scrap the rest so the dealer is apprehensive about selling them.

[@CodfishCatfish]

What’s sad is that Rosmallen amateur radio rally a complete BC375, BC348, several tuning units, antenna trailing cable and all interconnects, canvas bags, spare valves, power supply all the ancillaries including mics and spare hook up cables, webbing and more.......€400 the lot. Guy wanted €500 and I couldn’t ship it to the UK and it wouldn’t fit in the car. I went to arrange shipping and when I returned the guy sold it for €400..... I was in tears as the BC348 shock Mount was worth €200 on its own. This part was normally left in the aircraft when it was scrapped and only a very few ever made it to market. A hunk of metal with 4 rubber cushions.... silly prices. I know someone with the BC191 which was the 12 - 14v ground version of the BC375 and naturally the rotary converter was designed to run from aircraft generators and 24 - 28v. If you don’t have the correct PSU then you need to supply 500v obviously. This is a joy to watch and pure genius....kudos

[@LifeIsTooShortForQRP]

Yes, those original equipment mounts are very expensive. Like I mentioned in the video, I did get my hands on the original BC-375 mount (not shown in the video). Paid $200 for the thing. But they are incredibly rare, so I thought: What the hell. Yes, that was a bummer; $500 for the whole with all those accessories is a steal. I bought this one at Dayton for $50, without any tubes and all the black wrinkle paint had chipped of, weird enough. Must have been sitting in someone's barn for many years, then the ants eat this kind of paint (no joke, this is really true!) . But the unit had no corrosion, no damage, no mods and was therefore very restorable. All the black painted surfaces you see here were repainted by me using black wrinkle varnish, except for the TU box. That box was added later and had its original paint. Cheers.

[@VK2UAL]

Well done old mate , and yes the audiophiles see things just a little differently ie old valves are the refer madness for an audiophile lol Richard vk2ual

[@joefox7303]

Excellent work! Which side of the filament did you connect the cathode of the 6L6 to? Also do the electrophoresis power supplies generate any RFI in your receiver?

[@LifeIsTooShortForQRP]

It doesn't matter, connect the 6L6 cathode to either pin 2 or 7 of the 6L6 filament pins, but make sure the cathode ends up connected to the VT-25 socket terminal that goes to ground (thus NOT the VT-25 filament side that goes to R1191 and R1192, see the BC-375 diagram downloadable from the web). The electrophoresis power supplies are very RF quiet, no switching hash noticeable on my receiver. Check eBay, they go anywhere from $50 to $200 I have noticed, depending on condition. Cheers.

[@joefox7303]

Thanks very much, I’ve had my BC-375 operational for several years, but have never been happy with the power supply regulation (10%) I’ve ordered a couple of the electrophoresis supplies. Thank you !

[@357Shakey]

Um, ah, um...wow! I have some old tube gear. I wish I had "valve" (as the English say) work as part of my repertoire. Just transistors. That's a nice rebuild and a great modification to keep it pretty stock and looking period correct. BTW - for the person who posted the thumbs down - get a life!

[@LifeIsTooShortForQRP]

We call the thumb-downers "vinegar drinkers". When they get up in the morning, they drink a bottle of vinegar first. Helps for a sour outlook on the rest of the day.