The AM Forum

Has anyone ever tried triode connected 807s in AB2 linyeeear service? I'm thinking about rigging a pair up for a small amp to use at the cabin.

Before you ask why;

Because I got's a dozen 807s of all kinds and wanna do this totally outta junkbox parts. That's why!

Buddly,
          there were a few circuits for that in some of the older ARRL handbooks.
But........................ There is a problem doing it that way. According to the handbook articles, you had to "modify" the tubes to disconnect the beam conforming plates from the cathode to keep it stable. (remove and reinstall the base)

IIRC, 1625s had it piped out the bottom making them a much easier choice to do that with.

An 807 is basically a 6L6 with a plate cap, and I dont ever recall anyone making a grounded grid amp using 6L6s.

How about tying G1 and G2 together and running the 807s "triode connected" with a grounded cathode, this way the beam conforming plate would be grounded? I know they do it that way for class B audio service.

Some brands could be modified as you describe but not all.  I remember an article in QST about cutting a hole in the base and removing that connection and moving it to a pin.  In other brands the connection is in the envelope.

I thought that was for 1625s. Is my memory going now too?!?

That sounds like a good idea, Frank. I'm making up a fixture to try different configs and settle in on voltages. That scheme looks like a gooder start.

You might be right  ???  ???

"Is it the memory that goes first or is it the hair?"  I forgot.............. ;D  ;D

I don't  recall ever hearing of that modification for 807s.  One problem is that the 807 has only 5 pins, which are all used up, so you can't bring out the BF plates to their own pin. You would have to double up with one of the grid pins.  I supposed the way to find out would be to bust the base off a defunct 807 and see if the plates come out with a separate wire. 

I'm sure it would work only with some tubes, since they changed them (and cheapened them) considerably before discontinuing to manufacture them altogether.  I'd say the least likely candidate would be one of the later versions with only mica insulation (original ones had ceramic supports inside the tube) and lacking the little shield below the plate structure.

807s are a pita at RF. Heck 4 of them might match a 4-125.

1625 needed the mod not 807.

I think it was, yes. But it's been 40 years since I read that (and was building things with 1625s), so I can't say for sure.

But I do remember that it didn't work for every brand of tube, as some brands made the connection in the base and others inside the tube itself.

Like I said, Frank, I'm trying this cause I have a sh#! load of'em. Ifn I had a 4-125 I'd use it.
But alas, I don't.
Besides, Ifn I can get in the 100W carrier class from the cabin w/o bulk I'll be happy. Right now the 718 only does 35W carrier.

Now I am wondering where my mind is, anyone seen it?  You are right.

I've left.
I've gone to find myself,
But if I should return before I get back,
Please ask me to stay...................

YUP! My left brain is out and my right brain is looking for it.
Leave a message and they'll get back to ya.

Wow, Buddly, I called my Primary Care doctor this morning and that was pretty much
the same response I got from his secretary  ;D  ;D

Hey Buddly,

Gonna take more than a sxxt load of 807's to get you to a 100
watt carrier in linear service...
 

yea, I figure about 5 or 6

Buddly,

You might as well go for putting enough in parallel to achieve a 50 Ohm output impedance!

Characteristics and Typical Operation
Push Pull Class B Amplifier - 30 MHz max.

Plate Voltage ................................. 750
VGrid No. 1 Voltage ............................ 0 VPeak
Grid No. 1 Voltage........................ 555 V
Plate Current (Zero Signal) ................... 15 mA
Plate Current (Maximum Signal) ................ 240 mA
Driving Power ................................. 5.3 W
Load Resistance ............................... 6.65K Ω
Power Output (approx) ......................... 120 W

It's called making'em sing for their supper!

Yea an 807 TWA class A no tuner required

Does anyone remember the OTL (output transformerless) audio amps that were from the 1960's?   Tom's idea of using a whole bunch of paralleled 807's reminded me of that. 

How about a whole bunch of 807's in parallel using a "cathode follower" output design?   With 50 or so 807's you may have an output source impedance around 1 ohm.  Then modify an output network/filter like those used on solid state RF amps to step up the 1 ohm source to the 50 ohm load.

Hmmmm.  maybe you can make a whole bunch of paralleled 807's work without any tuned circuits.  Use only an output filter to keep unwanted crud out of the antenna.

Or  hook all those 807's up in series and use a 7.5 kv power supply.  Lets see... with 500 volts drop across each tube in the series, you'd need 15 807's.  But how would you drive them?  PDM?  Or a whiz bang digital series clocked sequenced pulsed BILdulator?

Sorry, Buddly, Its a Friday and my mind hasn't had a weekend to recover from the grinding down of the weekday workloads.........

73
Ted  W8IXY

Anybody remember an article from the 60s that used an old TV chassis, about a dozen 1625s (when they were 19 cents each) and some kluge of a power supply to make close to a kilowatt. I think it only worked on 80 and maybe 40 because of the paralleled capacitance.

It seemed to work but it was ugly looking.

Now you guys are on track to replicate the Galaxy amp built in the early 70s using sweep tubes.

I still have some 1625's I modified in the early 60's for a one driving 6 GG linear. It worked OK on 80-20M with 850V. Only 3 brands had the suppressor/beam lead brought out of the envelope.

P&H sold an amp that could use 837's or modified 1625's.

Carl

That's OK Ted. I understand. Been there myself.

Hey, they laughed at Tesla too so I'm in good company!
But with that class B push yank data it might be doable.
I guess we'll see. If not, I'll at least lighten my sh@! load of tubes!

A variation here is the EL-34, a true Pentode with a separate suppressor pin at the base. One version, the E34L from Penta (Teslovak) boasts a 32.5 watt plate dissipation. I use one in grid driven service on a modified Central Electronics 20A that make ~ 55 PEP on 80m in linear mode. It has 700V plate, and 300v G2 on it.

http://www.pentalabs.com/pdfs/EL34S.pdf

If I were to try this as a GG amplifier, I would include the filament as being driven like is often done with the 8877. This minimizes any heater to cathode leakage problems. I bet this series of tubes would do well as a RF amp when cathode driven.

Jim
WD5JKO