860 as an RF final

[kb3ouk]

I was reading the datasheet on the 860 which is kinda interesting. First, the impedance of that tube is around 20k ohms. Second, according to that sheet, the efficiency of that tube is between 40 and 60 percent. why such poor performance?

http://frank.pocnet.net/sheets/049/8/860.pdf

[KM1H]

Thats an 860 shown here as my icon for a few years now.

Since they are a 20's tube and circuits were still in their infancy AND test equipment was prehistoric you can take specs with a grain of salt. There was no Class B audio yet and many output RF stages were run as "linears" in Class B or screen or grid modulated.

In Class C they are as good as any other tube within their frequency range.

Ive been slowly making progress on a 160-20 TX with an 860 final since I confirmed DXCC on 80 CW with the PP 211 oscillator. Parts are being recycled and the 211's will be PP audio.

Carl

[kb3ouk]

I think they are a neat looking tube, and seem to be rather common and not high priced, probably because the audiophools haven't found a use for them yet. But how would you find a transformer to modulate it since the plate impedance has to be one of the highest there is. Putting two in parallel would lower it down to more like 10k, putting it about the same as an 811. Could screen modulate it, that would probably give you output powers that are more in line with what the datasheet says.

[W7TFO]

Westinghouse used them in WW2 USN transmitters as oscillators and IPA's. More of them around than you'd think cause of that.

Google TBL schematics.

The triode equivalent is the 852.

No worse matching it than doing a 100TH or 254W, mod wise.

They will sail up past 10m easily, you can coil the wire leads and make parasitic suppressors sorta that way.

Easy to cool, too.

73DG

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[kb3ouk]

yea, the Navy used them in the TBM and TBK as the lower level RF tubes, with their big brother, the 861, as the final, and the TBL used them as the low level RF and modulator tubes, with 803s for the finals.

[W7TFO]

The TBL's 803's were suppressor modulated straight from the separate speech amp.  The 860's only did RF in it.

Here is a link to where my TBL wound up:

http://www.qrz.com/db/KD6TKX



73DG

[kb3ouk]

Was there any particular reason that they mounted the 860s base up in those transmitters?

[W7TFO]

The tube didn't care and it made the filament wiring easier.

73DG

[kb3ouk]

ok, i wondered. the only bad thing about those tubes that I've heard is the filaments are brittle.

[W7TFO]

Only on used tubes.  New filaments are pretty tough.

Once they are lit, the tungsten takes a set and gets the consistency of pencil lead.

Same thing on most any large TX tube, current production or vintage.

Same reason I'll drive 300 miles to pick 'em up, rather than pay for a shelf queen.

73DG

[KM1H]

I worked on all thse rigs in the Navy which is why I went with the 860, a real funky looking tube.  Anything that worked as a stable oscillator at 1000V had to be an interesting tube. Never remember having to replace them or an 861 even after gunnery exercises. Been looking for an 861 or two for years now with a good filament.

The 803 is another good tube and consider it an 813 on steroids at least to 20M. It was purpose designed for the Navy and they wrote the specs just for that; it laughs at 3000V and 300ma and some have gone more than that as SSB GG linears.

[kb3ouk]

If i have this correct, the 803 is a true pentode, while the 813 is a beam power tetrode, and the internal leads in the 803 are longer than in the 813, right? What i liked about the 860 is its old buzzardly looks, plus i like tubes like that with the plate and grid connections coming out the side and top. 808 is another tube like that, I wanted to try and build something around one of those if I could get my hands on one, but they seem to be hard to find and expensive, while the 860 will do about the same amount of power out and it's a lot more common.

[KM1H]

Yes, the 803 is a true pentode with long leads, especially the grid which limits frequency response but was fine for the USN where its top end requirement was 18mc. I havent tried any above 20M but they were easy to drive and completely stable in GG.

The 803 has a seated height of 8 5/16" vs 6 5/8" for the 813. Both are 2 9/16 wide and have a T-20 bulb.

Your correct about the high plate impedance of the 860 and I had to dig up my notes to realize that I cast aside the 211 audio idea and have yet to chose an alternative that fits the pre 1930 requirement. Any ideas or will I have to live with well under 100% modulation and the 211's? Finding good, non gassy alternatives is not going to be easy. I could still scrap the pre 1930 idea and use 808's as Class B wasnt even invented yet and just use the TX on CW for pre 1930 events. I DO have a NIB pair of 808's from pre audiohole days that will do much better as mods.

This project keeps getting more complicated than anticipated

 

[AB9ZG]

Pre-1931 the 852 and 860 were typically Heising modulated by the 845 for low power and by the 849 for high power and high modulation.

https://lh3.googleusercontent.com/-JgfGLNKdc1w/T-Ir3H-UNRI/AAAAAAAABCw/ej9VYpaY9c8/s800/DSCN1362.JPG
In the above table, the 860 is being run above its maximum current, which is fine for hams. But RCA had to follow their own rules so they modulated parallel 860's with the 849 in their 1kw broadcast transmitter.

https://lh5.googleusercontent.com/-b-jJaFYqzJU/T-IrxbdBDoI/AAAAAAAABCo/EV0P0e37Ifs/s800/rca.png

[W7TFO]

Gotta love that UV-207 water cooled linear final in the 1-B!

73DG

[Opcom]

Correct about them not being scarce but at one point I thought they were. I have maybe 2-3 of them, and one of the 852's, buried in storage.
They turn up at antique radio auctions in boxes of other large unusual tubes e.g. 3-200's, and no one wants them. Got a box of 5 or 6 good used 4-1000's that way. Auction form said "big old tubes min. bid $10", but the antique radio folks are looking for ancient receiving tubes and old 40's and later common ham stuff. They usually do not have the right facilities or support parts to build or work with tubes like the 860. Different hobby!

As for using the 860's, I don't build to that era so I don't have a good reason to do consume them, and would as soon save them for the next guy. We are all custodians only..

I believe they would be a great match to the BC-610 modulator iron with one or maybe two 852's as the class C stage. I offered them to my friend Dennis W5FRS but he declined as he prefers triode modulators.

Don't forget also the 200-600W multimatch transformers out there that claim to go as high as 20,000 Ohms.

[kb3ouk]

Or if I can't find the right iron I could just get 2 of them and run in parallel at about 120 ma for the pair with 1500 volts on the plate would put it at 12k ohms and allow for 180 watts in with about 125 watts out.