810's Drive

[W4RFM]

I plan on building a simple transmitter to simulate a GATES 250GY.  It will use a pair of 810's driven by a 4-125a (because I have a ton of new ones) and a 6AG7 oscillator.  Acording to the introductory blurb that RCA put out in the 1930's or 40's it takes 40 watts of drive, however in the Gates design it looks like that 813 driver they used is throwing 135 watts or so at the grids.  Anyone out there (in radio land) have a real 250 or 500 GY and can tell me what the driver is actually doing.

[w4bfs]

hi Bob .... I have a 250gy which is in the process of rebuild/restoration ...I will take a look at schizmatic and see ...iirc the 813 is at 1100V or so of B+ .... need to see what screen V is ....I have seen a 813 removed and it was very chocolatly colored ... not so usual for that tube...Cliff said they lasted a long time in there

[W4RFM]

Hey Beans,
I got a copy of the book on the 250 and the 500GY, it shows 1400Vdc at 130 to 160 ma. and 330v on the screen, of the 813 driver. That seems like a lot, maybe its brute force to compensate for a mis match.  I dont know, thats why I am polling the forum.

[Steve - K4HX]

It's overkill typical of broadcast transmitters. No need for an 813 or 4-125 to drive a pair of 810s for Amateur Radio service. IIRC, there are some designs in old handbooks using one or two 807s as drivers.

[W4RFM]

My first thought was a 6146B or the 807 pair, like you said Steve. I just wanted to be sure it had enough drive.

[Steve - K4HX]

Here's the dope from the November 1938 edition of RCA "Ham Tips."

[KD6VXI]

Here's the dope from the November 1938 edition of RCA "Ham Tips."

I wonder what the authors of  Ham Tips,  GE leaflets,  etc.  Think of us still gleaning info from their writings.....   Bet it would be a real ego boost to some.

--Shane
KD6VXI

[w1zb]

I built a homebrew transmitter using a pair of 810's in push-pull configuration and was able to use a Johnson Ranger at about 40 - 50 watts drive power to obtain legal limit.
73 Jerry W1ZB

[W4RFM]

Jerry that is what I needed to hear / see! Thanks, I will down size the driver.
(Course I already cut a perfect 2.75 inch hole in the chassis.)  

[w4bfs]

Jerry that is what I needed to hear / see! Thanks, I will down size the driver.
(Course I already cut a perfect 2.75 inch hole in the chassis.) 

well maybe a 4d32

[W4RFM]

I'm gonna start over and build a WRL Globe Chief 90 clone.
*UPDATE*  No, I ain't!  I got to looking at the diagram and good grief what a weird power supply idea. No ground on the power supply! and the B- feeds the cathodes, do what?
I resorted to the ARRL handbooks for a 6AG7 and dual 807 plan.  Found it in the 1957 book.

[PA0NVD]

That's not too weird. I desigened several two tube balanced RF generators of 25 kW and amplifiers with 3CX6000A7 of 10 kW at 27 MHz and used this set-up. It allows to make a real ground to the grid and put the RF leak between the cathode and ground. Grounding grids using caps at higher frquencies is sometimes difficult in order to prevent VHF and UHF oscillations due to simple mechanical dimensions of parts at those frequencies and power levels. Many amps of RFgrid current

[KD6VXI]

I built my last 4cx5k that way as well.

Grounded screen grid.  All voltages return to the cathode.

Look at Rich Measures site,  he gives a good description.

Makes for a very stable design.

--Shane
KD6VXI

[flintstone mop]

Here's the dope from the November 1938 edition of RCA "Ham Tips."

I wonder what the authors of  Ham Tips,  GE leaflets,  etc.  Think of us still gleaning info from their writings.....   Bet it would be a real ego boost to some.

--Shane
KD6VXI

During that time; the people involved were young educated wild minds that could do anything and came up with some outrageous inventions and circuits. The 500,000 watt WLW transmitter was just a small part of the thinking power of these pioneers

[W3RSW]

I suspect that the authors have pretty much expired. They'd definitely need that boost, way more than just egos.  

Seriously, looks like it'll be a fun project.  I like the 4d32 driver idea. It's a really rugged and way underrated tube easily good for 40 to 120 watts output.  The 7 pin socket matches a bunch of other neat tubes too, RK 829, 5894, and believe it or not an 8877 (3cx1500A7, Waaay overkill, urk )

With the 829 or 5894 you could have a PP driver stage too.

[John K5PRO]

RCA BTA250L used 810s driven by a single 828, an old timer. 1200 volts at 95 mA on the plate of that tube. 1500 @250 mA for the 810 pair in the final stage.

[W4RFM]

Seems like I remember that if a triode will work in the circuit, an 811 A is just be an equivalent tube and much easier to find.

[Steve - K4HX]

The GE BT-20-A used the same line up.

RCA BTA250L used 810s driven by a single 828, an old timer. 1200 volts at 95 mA on the plate of that tube. 1500 @250 mA for the 810 pair in the final stage.