The AM Forum

Triode Connected Beam Pentode Class C Amp

I never noticed this before...
The RCA 1614 datasheet shows typical operation for triode connected plate modulated class C telephony (as well as the usual pentode connection). I haven't seen this on other beam pentode datasheets, why the 1614? What would be the advantage of operating it in this manner?

Don

Not seen that before but hope someone has - this is extremely interesting!
Love the high plate voltage rating. Is the 1614 a selected 6L6 or just made a bit better?

As Class C operating triode:
Plate+screen efficiency: about 6% less than the tetrode value
Bias required: 1.8 times as much as the tetrode value
Peak grid voltage: 1.7 to 2 times as much as the tetrode value
Grid current: 2 to 3 times as much as the tetrode value
RF Drive: 4 to 5.3 times as much as the tetrode value
RF Output: 77 to 86% of the tetrode value
Efficiency: 89.6 to 93% of the tedtrode value

These are pretty close in terms of power output, and the drive requirement is rather trivial even in Triode mode (0.8W), although it could be a consideration in some sets where a few percent efficiency, a few pennies of cost, or dual use of tubes in RX/TX functions is important.

This brings to my mind a question of: What natural but possibly obsolete or directly heated triode operates in a manner close to the 1614 triode conditions?
To rephrase: Are the 1614 triode conditions such that it could replace a small triode in a specific transmitter model without much rework of the set? Which Triode?

Otherwise, seems like an option for designer convenience.

   -The 1614 was a metal 6L6 intended for RF power operation and tested to give 25W at 8MHz.- Source: radio Museum.org

So it is a selected or 'binned' 6L6? Had this suspicion but never seen the insides of the two compared mechanically to understand better.

Like a 1620 is to a 6SJ7.

The entire 16xx line was an effort on the part of RCA to offer popular tubes with socket-compatable, better noise and service linearity types at a modest cost increase.

I can tell you a 1612 was a lot better than a 6L7 in old audio limiters.

73DG

wonder if there's a cross reference chart. Post it in Latin so the audio folks won't see.

Hope someone may comment on the original post. I'm no expert on triodes vs beam tubes.
My opinion may be off base, with only the data sheet to go on.

Probably for the same reason a 6CL6 has a triode rating, fewer components for screen biasing with slightly lower stage gain.


Phil - AC0OB

I wonder which market RCA was targeting to test and publish data for triode connected plate modulated service of the 1614, but no others tubes (that I'm aware of). I also wonder if any manufacturer fielded such a transmitter.

Presumably, wired as a pseudo-triode, it will take modulation the same as a natural triode would. Multi-band neutralization may become an issue. Basically, it's back to the Pentode/Tetrode vs Triode thing.

I may have to give this concept a try sometime. It would make an interesting exciter for a linear and it would be a unique lash-up on the air...
"Rig here is a homebrew triode connected 1614 modulated by a pair of triode connected 1614's driving a leenyar. Over."

Don

That would be pretty cool. People would talk!

Was trying to compare those conditions to similar sized triodes in the same voltage range. The 2A3 is smaller at 15W plate. Is there a 19-21W Triode that runs at 325-375V on plate?

No curves for 1614 triode mode, maybe the 807 which has the same curves can be extrapolated, with the 1614 as triode and the 2A3 both at 250V. -which might be acceptable for the 2A3/6B4.
The difference inbias for a given current  - this means the mu is quite different, right?
ahhh this may be pointless but it was interesting to look.

About the curves - the data says the 1614 user should use the 807 curves. But are they the same tube indeed? The 807 screen is rated 300V, but we see the 1614 running 400 ICAS. The 807 as a modulator can take 400V CCS in triode connection.

6L6, 1614, 807.. like three bottles from the same fine vinyard, of close vintage.

Patrick,

This thing seems to have really piqued your interest. Thanks for putting all that data together.

According to tube lore, the tube manufacturers engaged in 'creative bottle stuffing' at the end of the tube era. Apparently not doing much more than marking different designations on what was internally the same tube. If they didn't invent the phrase 'close enough for government work' it seems that they repeated it often enough. So, tubes are like a box of chocolates? :o

Some of the different ratings on the datasheets may be a result of specifying the tube in different services.

If there's any advantage to triode connection of a 1614 modulated stage, the entire 6L6 'family' become candidates. I've had a hankering to build an 807W exciter rig for years. If I ever do, I'll try the triode connection just for fun. ;)

Don

One thing I could never figure out is why E. F. Johnson engineers in their infinite wisdom chose to use 1614s as modulators in the Ranger when they're RF tubes. Metal 6L6s for RF shielding makes sense, 6l6GCs are excellent audio tubes and are the compact ones of the 6L6 line. Again in their infinite wisdom they used 6146A modulators in the Valiant. They are lousy audio tubes, 807s make great modulators.

"6L6, 1614, 807.. like three bottles from the same fine vinyard, of close vintage" doesn't make a whole lot of sense, the 807 is odd man out. 6L6 and 1614 are directly interchangeable, with a socket change 807 and 6BG6 are interchangeable. I figured that out early on and since I had a ton of 6BG6 pulls I used them in the 807 modulator in a Radio Amateur's Handbook. A few years later I read the history of those tubes, early RCA B&W TVs used 807 H output tubes. Then it was decided that all octal sockets were used for uniformity, the 807 was unnecessarily rugged for that purpose and mica cutouts are cheaper than ceramic spacers, put an octal base on it and the 6BG6 was born.

"6L6, 1614, 807.. like three bottles from the same fine vinyard, of close vintage" makes sense to me because I use those mainly for push pull audio around 40-60W (except 1614 which I never had a pile of), along with the 6CD6, 1625, and AB2 6146s in a mobile tube audio amp and that unit sounds fine.