Hi,
I have found a pair of RCA 7094 NIB tetrodes in my loft (I bought them years ago as part of a large lot - didn't know they were in there).
I'm a novice in the tube amp stakes - currently just building my first (a nice version of the Bevois Valley amp from Morgan Jones - Sowter transformers arrived yesterday! may be ready for xmas... or maybe not)...
Anyway, do the 7094s have any audio use? Has anyone done anything good with them? As I only have 2, I was wondering if they would make a good SE tube (though I was going to do a 6C33 SE amp next).
Should I just put them on eBay for the radio amateur lot?
Thanks
Nick
I have found a pair of RCA 7094 NIB tetrodes in my loft (I bought them years ago as part of a large lot - didn't know they were in there).
I'm a novice in the tube amp stakes - currently just building my first (a nice version of the Bevois Valley amp from Morgan Jones - Sowter transformers arrived yesterday! may be ready for xmas... or maybe not)...
Anyway, do the 7094s have any audio use? Has anyone done anything good with them? As I only have 2, I was wondering if they would make a good SE tube (though I was going to do a 6C33 SE amp next).
Should I just put them on eBay for the radio amateur lot?
Thanks
Nick
I have never heard of these before, but after looking up the data, they have a set of curves for "Hi Mu triode connection". Here you connect G1 to G2 and use it for the control grid. In this mode it looks like a really pumped up 811A, which makes a good audio amplifier in A2 operation. These tubes would likely do well in a A2 SE amp, but that requires a really good driver circuit. They will also need a lot of plate voltage, maybe 600 to 1000 volts. Since there are no obvious circuits for this, you are forging into uncharted teritory. If you are new to this hobby, I would try a different tube first, at least the 6C33 doesn't need a lot of voltage.
If you decide to put them on Ebay, let us know here. I am crazy enough to try this (as if I don't have enough projects already), but I don't watch Ebay very often.
If you decide to put them on Ebay, let us know here. I am crazy enough to try this (as if I don't have enough projects already), but I don't watch Ebay very often.
https://frank.pocnet.net/sheets/049/7/7094.pdf
There are a seller on eBay but he got four from a collector.
Also I never hear about this tube, in A2 could be a good thing.
There are a seller on eBay but he got four from a collector.
Also I never hear about this tube, in A2 could be a good thing.
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The 1500V B+ CCS ratings to 400W has a plate to plate load rating of 8300. Roughly 32:1 turns ratio. A dual primary power transformer for 120/240 operation with an 8V secondary could do it. If it's subwoofer only a 500w power transformer should work fine. Might need some feedback. This is with a 4 ohm sub though. A 2 ohm will need a 4v or 240-480v primary which will be more expensive.
Will need something good to 2000V+ isolation which might be the hard part. Industrial automation systems use 8V and I can see a few of them online.
Not exactly Hi-Fi but with some feedback and tweaking I imagine it would sound decent.
Will need something good to 2000V+ isolation which might be the hard part. Industrial automation systems use 8V and I can see a few of them online.
Not exactly Hi-Fi but with some feedback and tweaking I imagine it would sound decent.
Been there, blown that! If you have 1500 volts of B+ the OPT will see something like 3KV of signal voltage across it. A 240 volt primary will NOT like this, especially for low frequency duty. It will likely not have enough inductance to eat large signal voltages below 50 / 60 Hz. The lack of inductance will cause high tube current and unhappy tubes.
To look at this from the other direction, you want to put 400 watts into a 4 ohm load.....you need 40 volts RMS of signal voltage. How are you going to get that out of an 8 volt winding.......not happening.
I tried a 600 VA 50 / 60 Hz toroidal isolation transformer that had 4 X 120 volt windings, wired them all in series, and wrapped a new secondary over that and fired it up with 4 X 35LR6's (big TV sweep tubes) on 650 volts. This was OK down to about 40 Hz at 300 watts. Any more power, or lower frequency caused the plate current to skyrocket. At these power and voltage levels things tend to go wrong quickly, so I did not push into the red glow of death zone.
When I was a kid making guitar amps I would use power transformers from old TV sets as OPT's. Wire each plate to a red wire on the HV secondary, and feed B+ to the center tap. I wired all the heater windings in series and connected up as many speakers as I could find. I got pretty loud for free, but I didn't have the means to measure power or distortion back then. If you could find a surplus 120 to 3800 volt CT power transformer, it might be suitable for a low frequency OPT. For the primary to be usable as a speaker secondary winding it would need to carry 10 amps. This would be a big transformer.
I decided a while ago to give up on the big transmitter tubes. Available OPT's are very limited. My big amp will use TV sweep tubes on no more than 700 volts......but I already have the OPT's, rated for "400 watts at 20 Hz." 1250 ohms CT to 0-2-4-8. I got them cheap over 10 years ago.
" If you could find a surplus 120 to 3800 volt CT power transformer"
I think I may have something like that. A couple different ones with 120V primaries. One is in a 3CPX1500A7 linear I built a few years ago. Had a dedicated 30A circuit for it. I went through a lot of sweep tubes as a kid but used them to make RF amps for CBs. Had I got caught I'd probably still be paying on the FCC fine.
I think I may have something like that. A couple different ones with 120V primaries. One is in a 3CPX1500A7 linear I built a few years ago. Had a dedicated 30A circuit for it. I went through a lot of sweep tubes as a kid but used them to make RF amps for CBs. Had I got caught I'd probably still be paying on the FCC fine.
Who would make such a thing........Mine ran a 4-400 on 3500 volts. It needed a big blower to cool that tube. It glowed a pretty bright red on AM. The only thing that I have ever built that actually scared the $%# out of me.
Then there was the Gates HFL-3000........slightly north of 3KW. You can tell the time frame from the pictures. Smokey calling Bandit.......
Note the picture of me holding a fluorescent light tube near the antenna on the Road Runner. It's lit without wires.
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The one I built is roughly 2KW input and around 1500 out. 3900V at half an amp. I have a 220 primary transformer for it so I may experiment with the 120v that's coming out of it. 3KW is nothing to sneeze at! It commands respect or you're dead in a flash literally. I may tap into your knowledge when start to mess with this thing.
As you can tell by the pictures the Gates transmitter was put on the air in the mid 70's. A friend in the surplus business got it and asked me to power it up for him. It turned out that the Gates needed 3 phase power and the warehouse had single phase 240 volts. The biggest breaker in the panel wasn't big enough to power a 3KW transmitter.
I had already put the 1KW Collins amp in his Road Runner. It used a pair of large boat batteries in the trunk feeding a mechanical motor generator set 24 volts which it converted to 240 volts 400 Hz. The Collins ran on 400 Hz with a swap out of the cooling fan. It was driven by a Pride 100 inside the car. He plugged the car in Tesla style to charge the batteries by day, and played at night. We could light up an abandoned gas station by keying up the radio.
I told him to find me a 7000 volt 1 amp transformer with a 240 volt primary and call me back, really never expecting to hear from him about that again.
About a week later he had a transformer the size of a file cabinet setting next to the Gates. I wired it into the power supply of the Gates with steel core spark plug wire, and connected it to the breaker panel with a pair of automotive jumper cables. The first power up blew the 100 amp breaker which killed power to the entire warehouse.
After exploding a Simpson 260, I found the shorted diode, and the transmitter was live. Intended for commercial shortwave broadcasting, it covered 1.6 to 30 MHz with a fully automated mechanical auto tuning system. You set the frequency on a crank dial and hit the tune button. Motors whine, lights blink, then the "ready" light comes on. 100 milliwats drives it to 3 KW out, more input makes a little more output, too much input shuts it down. We had a switchable attenuator between the radio and the Gates. Any power level from a few watts to over 3 KW was available by turning the knob.
This setup was in a warehouse next door to a CB shop on the south side of Miami. I lived about 40 miles north, with an interstate road in my back yard. I could barely hear him when he had that thing running at full crank due to the CB traffic on the interstate. I had about 1200 watts and he could hear me just fine.
My favorite trick.....crank it up on channel 19 and announce" Attention, this channel will be closed for repairs until further notice." It would get me one or two minutes of quiet before the noise rose back to 20 over.
Those days are gone, and I gave my amp away about a year after I made it. The fun had worn off and it was too dirty for ham use. Still cleaner than the junk found on CB. I could often hear truckers on my Pioneer stereo at home even when it was off. They got picked up on the speaker wires and demodulated in the output transistors.
Despite my user name, all of my ham radio transmitters have been solid state. I was a transmitter engineer at Motorola for 41 years, and silicon was free. I'll put something together and be back on the air some day, but I haven't got an antenna up yet. I left south Florida 4 years ago. I currently live in a "holler" with 200 to 400 foot tall ridges on either side of me. There is no TV or cell service here, and not much RF seen on a spectrum analyzer with an antenna on it, so I haven't been in a hurry to put up an antenna.
I had already put the 1KW Collins amp in his Road Runner. It used a pair of large boat batteries in the trunk feeding a mechanical motor generator set 24 volts which it converted to 240 volts 400 Hz. The Collins ran on 400 Hz with a swap out of the cooling fan. It was driven by a Pride 100 inside the car. He plugged the car in Tesla style to charge the batteries by day, and played at night. We could light up an abandoned gas station by keying up the radio.
I told him to find me a 7000 volt 1 amp transformer with a 240 volt primary and call me back, really never expecting to hear from him about that again.
About a week later he had a transformer the size of a file cabinet setting next to the Gates. I wired it into the power supply of the Gates with steel core spark plug wire, and connected it to the breaker panel with a pair of automotive jumper cables. The first power up blew the 100 amp breaker which killed power to the entire warehouse.
After exploding a Simpson 260, I found the shorted diode, and the transmitter was live. Intended for commercial shortwave broadcasting, it covered 1.6 to 30 MHz with a fully automated mechanical auto tuning system. You set the frequency on a crank dial and hit the tune button. Motors whine, lights blink, then the "ready" light comes on. 100 milliwats drives it to 3 KW out, more input makes a little more output, too much input shuts it down. We had a switchable attenuator between the radio and the Gates. Any power level from a few watts to over 3 KW was available by turning the knob.
This setup was in a warehouse next door to a CB shop on the south side of Miami. I lived about 40 miles north, with an interstate road in my back yard. I could barely hear him when he had that thing running at full crank due to the CB traffic on the interstate. I had about 1200 watts and he could hear me just fine.
My favorite trick.....crank it up on channel 19 and announce" Attention, this channel will be closed for repairs until further notice." It would get me one or two minutes of quiet before the noise rose back to 20 over.
Those days are gone, and I gave my amp away about a year after I made it. The fun had worn off and it was too dirty for ham use. Still cleaner than the junk found on CB. I could often hear truckers on my Pioneer stereo at home even when it was off. They got picked up on the speaker wires and demodulated in the output transistors.
Despite my user name, all of my ham radio transmitters have been solid state. I was a transmitter engineer at Motorola for 41 years, and silicon was free. I'll put something together and be back on the air some day, but I haven't got an antenna up yet. I left south Florida 4 years ago. I currently live in a "holler" with 200 to 400 foot tall ridges on either side of me. There is no TV or cell service here, and not much RF seen on a spectrum analyzer with an antenna on it, so I haven't been in a hurry to put up an antenna.
Wow! Sounds like some wild times. I've been known to give the truckers a hard time. I think I may give solid state a try in the future. I've got an HP 48VDC 3kw SMPS to power it. Some folks on a popular auction site sell boards ready to go less the hybrid modules and heat sinks. Filters are mandatory also. Here is a photo of the amp I built that contains the 120V primary transformer I mentioned. The 220V primary transformer going in it with a triode it uses is in there too. Next photo is my go to Henry amp that was converted to the same triode before I got it. I put a much beefier Peter Dahl transformer in it. Upgraded the power supply and blower among other things. Last photo is one row of radios including a 1966 Drake set. I've also got some Collins stuff including a 50's AM station. Most of the HF bands have been dead hence me dabbling in the tube audio stuff.
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