Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
Another 813 Build




 
The AM Forum
December 12, 2019, 09:45:56 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: 1 ... 7 [8]   Go Down
  Print  
Author Topic: Another 813 Build  (Read 19946 times)
0 Members and 1 Guest are viewing this topic.
w9jsw
Two shots of Whisky
Contributing
Member
*
Offline Offline

Posts: 284



« Reply #175 on: December 01, 2019, 07:16:53 PM »

I got tired of waiting for Lehman's to get more stock of the lantern globes so went searching. These popped up. I mounted the sockets under the chassis with a 8-32 nut to make a 1/8 in space between the socket and the bottom of the chassis. I cut the hole 2.25in which gives a 1/8 in space around the base of the tube. I mounted a small 12v fan that I will run at 9V so keep it quiet. I will fully cover the base of the chassis so the air only has one place to go.

While these lanterns are only 1/2 high I figured it will still properly cool the base of the tube. I will be running the tube at 2000-2500 volts. So I am hoping this give it adequate cooling. These tubes are advertised as not needing cooling under normal conditions. I am near/at the max ICAS rating so a little bit of cooling should be enough, I hope.

I got the lantern glass at a company called American Mantle. They were $7.50 plus shipping. I ordered 4.

John
Logged
K1JJ
Contributing
Member
*
Offline Offline

Posts: 7874


"Let's go sailing, Tommy!" - Yaz


« Reply #176 on: December 01, 2019, 08:29:26 PM »

Very nice, John.  Professional look.

I would suggest some improvements: Remount the 813s  with 1/4" socket sub-mounting -  and drilling 3/16" holes around the rim (between the mounting screws) to add air flow. (Drill 16 holes total, 4 per section between screws)  This will make a huge difference.  One small 12V fan on the back of the RF or Mod chassis will hardly do the trick the way it is.

If the 12 V small fan doesn't do it, try a small squirrel cage blower that can handle some back pressure using a Variac to slow it down. You should feel a moderate breeze coming thru the chimneys.

In addition, usually the chimney goes all the way to the top of the tube and bends inward to push air past the plate seal.  You will find no air hitting the plate cap with a shorty chimney like that.  I wonder how much difference it would make if you placed another chimney on top of the existing one?  You could always order the correct chimneys later on from Lehman when they are available

But all in all, it's true that an 813 needs no pressurized air when run within limits, though there still needs to be some ventilation and heat radiation flow.  IE, just think of how hot an 813 would get inside a sealed metal box with no air flow at all.

T

Logged

Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
KL7OF
Member

Offline Offline

Posts: 2105



« Reply #177 on: December 01, 2019, 11:43:22 PM »

A friend got a  2 x 4x1 HB  old school contest amp that has cut down pickle jars for chimneys..regular glass...how cool is that?
Logged
w9jsw
Two shots of Whisky
Contributing
Member
*
Offline Offline

Posts: 284



« Reply #178 on: December 02, 2019, 07:04:16 AM »

Thanks Tom. All good suggestions to implement, as always.
Logged
KB2WIG
Contributing
Member
*
Offline Offline

Posts: 4329



« Reply #179 on: December 02, 2019, 11:18:41 AM »



Coleman lantern chimbleys have been mentioned in the past.

Just a thought,

 KLC
Logged

What? Me worry?
kc2we
Member

Offline Offline

Posts: 76


« Reply #180 on: December 03, 2019, 09:15:22 PM »

At 1250 VDC plate u need -160 bias in AM Plate modulation, class "C" mode. I'd use a fixed screen supply with a small 10 hy series choke for complete modulation. I like that better than no fixed bias and using a clamper tube. If you use a series resistor for screen voltage and no fixed bias, tube wastes a lot of heat and never really cuts off if keying in CW.

The bias can be derived from a VR105 plus a grid leak resistor of about 4.5 K @ 1 Watt to make -160 volts. The fixed bias is more than 2 X cutoff and resting current with no excitation should be zero. Grid current runs around 12 - 13 ma. with a drive of about 3 watts.


Seth KC2WE

Note: Been doing this since 1959 L o L
Logged

Seth Taylor
K8DI
Contributing
Member
*
Offline Offline

Posts: 69


« Reply #181 on: December 03, 2019, 09:50:19 PM »

I'd use a fixed screen supply with a small 10 hy series choke for complete modulation. I like that better than no fixed bias and using a clamper tube. If you use a series resistor for screen voltage and no fixed bias, tube wastes a lot of heat and never really cuts off if keying in CW.

I've seen the choke in the screen supply in a couple schematics. Can anyone explain how this works, what the pros and cons are?

Ed
Logged

Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
w9jsw
Two shots of Whisky
Contributing
Member
*
Offline Offline

Posts: 284



« Reply #182 on: December 04, 2019, 06:31:08 AM »

Looks like Seth's comment got duplicated across 2 threads...
Logged
K8DI
Contributing
Member
*
Offline Offline

Posts: 69


« Reply #183 on: December 04, 2019, 06:37:44 AM »

Looks like Seth's comment got duplicated across 2 threads...
It did, and he may have only meant the other, but it may apply here as well. The 813 data sheets say that the screen should be supplied from a separate modulated source (eg an additional winding on the mod transformer) or a resistor from the plate, for best linearity. Your design has it from a separate unmodulated source.  I see a choke in your schematics. Is the choke a good add on? What will it do in terms of the screen being modulated? And so on?

Ed
Logged

Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
w9jsw
Two shots of Whisky
Contributing
Member
*
Offline Offline

Posts: 284



« Reply #184 on: December 04, 2019, 06:29:35 PM »

I will let Tom opine. I am mainly a kit builder here. My schematic doesn’t show that i am now planning a regulated FET 350v supply with a clamp circuit for that. I do have a winding available on the mod transformer that we could use if I get help on how to implement it.
Logged
K1JJ
Contributing
Member
*
Offline Offline

Posts: 7874


"Let's go sailing, Tommy!" - Yaz


« Reply #185 on: December 04, 2019, 08:36:37 PM »

  The 813 data sheets say that the screen should be supplied from a separate modulated source (eg an additional winding on the mod transformer) or a resistor from the plate, for best linearity. Your design has it from a separate unmodulated source.  I see a choke in your schematics. Is the choke a good add on? What will it do in terms of the screen being modulated? And so on?
Ed

Hi Ed,

You can use a 10H choke in series with the screen, a resistor in series with the screen or a resistor from the plate to screen of appropriate values.  It won't make much difference in modulation level or linearity which technique you use.  The amount of screen modulation compared to plate modulation is small.

Hook up a scope and SDR spectrum analyzer and run some tones thru. You can try each technique and adjust for best performance, (IMD and audio peaks) though just using a choke works out fine. The choke's impedance will vary based on audio frequency while the resistor remains constant thru the whole range.

The advantage of the choke is its DC resistance is low and the wasted power is small. When running a big tube, this can generate a lot of heat when using a resistor, especially from plate to screen.  A small tube makes it easier heat-wise.

Dean, WA1KNX wrote an article in the archives that shows how to optimize the resistor/capacitor screen network for a DX-100. But again, just throw in a 10H choke and you will be done with it for down to 30 HZ or so.

When homebrewing, I never liked a separate screen mod transformer winding to modulate the screen because you are stuck with whatever the winding ratio is and is more of a specific design for a specific tube line up, like a pair of 4-400As modulating a pair. If you use a different tube line up, then it may produce too much or too little screen modulation.

I have always used a self-modulated choke in the screen for my plate modulated 813s, 4-1000As and others rigs. I notice most of the big guns do the same thing over the years.

Tom, K1JJ
Logged

Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
KD6VXI
Contributing
Member
*
Offline Offline

Posts: 2057


Making AM GREAT Again!


« Reply #186 on: December 07, 2019, 11:57:24 AM »

Peek A Boo

I see two!

4-1000 that is!

--Shane
KD6VXI


* 20191123_143652.jpg (2565.17 KB, 4160x3120 - viewed 27 times.)
Logged
w9jsw
Two shots of Whisky
Contributing
Member
*
Offline Offline

Posts: 284



« Reply #187 on: December 07, 2019, 03:50:51 PM »

Show me the back side when you get it presentable.
Logged
Pages: 1 ... 7 [8]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone © 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.041 seconds with 18 queries.