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




 
The AM Forum
April 04, 2020, 01:27:35 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] 2   Go Down
  Print  
Author Topic: 813 amp project  (Read 4986 times)
0 Members and 1 Guest are viewing this topic.
n4joy
Member

Offline Offline

Posts: 194



« on: February 17, 2017, 10:41:15 PM »

I am building a 813 HF amp as described in a February 1969 issue of 73 Magazine: https://archive.org/stream/73-magazine-1969-02/02_February_1969#page/n11/mode/1up

Just for fun and curiosity, I am building exactly down to the exact uuf and winding count.  I completed the tank coil and filament choke.  I will admit that making the tank coil spacer was rather unpleasant and this was my first attempt.  My Boonton L/C meter shows just over 15 uH.  I had also had difficulty "threading" the tank coil as it wanted to bind--several hours of work were spent just on the tank.   I secured all other parts with exception to the chassis and cabinet.  I will post my progress!  



* 102_2685.JPG (1563.91 KB, 2848x2144 - viewed 358 times.)
Logged
KD6VXI
Contributing
Member
*
Offline Offline

Posts: 2133


Making AM GREAT Again!


« Reply #1 on: February 17, 2017, 10:49:13 PM »

It may have taken hours,  and it looks it,  and it's going to be a really good looking rig I'm betting!

What did you use for the coil spacer?

--Shane
KD6VXI
Logged
n4joy
Member

Offline Offline

Posts: 194



« Reply #2 on: February 17, 2017, 11:02:21 PM »

Thank you.  I wanted this to be a true "junk box" home brew amp but some substitutions were necessary.  I used acrylic rod for the spacer but did originally try pieces of plastic and glue (epoxy) as the author noted.  It was a total mess and I simply couldn't get the spacing correct (I also tried tile spacers).  Here is a photo of some of the other components.  Finding a decent chassis and cabinet is proving to be difficult and potentially expensive.  I even wound the L3 choke on the HV with a wooden dowel and the exact AWG wire as did the author.  I am experimenting by following the article as closely as possible--final result should be interesting!

The power supply is also coming from an other article and I will also replicate: https://archive.org/stream/73-magazine-1965-05/05_May_1965#page/n39/mode/1up


* 102_2686.JPG (1542.12 KB, 2848x2144 - viewed 287 times.)
Logged
K1JJ
Contributing
Member
*
Offline Offline

Posts: 8104


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


« Reply #3 on: February 17, 2017, 11:12:19 PM »

Looks like a neat linear amplifier project.... simple and will work well.

One suggestion:  To achieve a 1:1 swr input on all bands AND improve the IMD products by about 5 dB, add a small variable pi-network to the RF input.   C-L-C  or even an L-C will work.  There is very little voltage there at 50 ohms, so small spacing will work fine.  Initially you can clip lead the parts in and test to see what works best.

And, a little breeze from below chassis will help. I have also used lantern glass as chimneys.   Even air directed down from above the tubes using a muffin fan will produce a nice, even air flow across the whole tube.

Tom, K1JJ

Logged

Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz,  +-6.0 KHz or  +-8.0 KHz when needed. 

"You know my name"  https://youtu.be/noGjJyEDm5s?t=135

There's nothing like an old dog... a puppy... a dog in its prime... or ANY dog!
n4joy
Member

Offline Offline

Posts: 194



« Reply #4 on: February 17, 2017, 11:39:14 PM »

Great suggestions, Tom.  I have a 100uf variable I can use and will wind a coil for the LC.
Logged
K1JJ
Contributing
Member
*
Offline Offline

Posts: 8104


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


« Reply #5 on: February 18, 2017, 01:05:10 AM »

Great suggestions, Tom.  I have a 100uf variable I can use and will wind a coil for the LC.

Hi Chris -

Any small coil that can be tapped with a small rotary switch will work fine.
I usually use a small roller inductor for the job - this makes precise tuning easy.

That 100 pF cap will be too small in value. This is a 50 ohm circuit in and out, so you will need at least 300 pF and even 500 to cover 160M.  I usually use one of those  common multi-section (3 sections paralleled) broadcast receiver variable caps. But the best thing to do is build the amp and then clip lead in some  C-L-C  or L-C parts and see what works best. Then mount them permanently.

You can roughly test the input tuning on all bands when the rig is off using an MFJ-259 meter or equivalent connected to the input and a non-inductive 50 ohm resistor from fil to ground. (output)

Yes, since this is a linear amplifier, the tubes will really appreciate the extra cooling, especially on AM. The carbon plate 813s will actually go from a glowing red plate (bad) to a dark plate when using a lantern chimney and some air from below. Highly recommended for linear AM.  Run as much plate voltage as you can get away with (up to 3500V)  for best positive peaks, efficiency and best IMD. Load it heavily. (more C2 plate loading) to drop power 7% after peak tune for best IMD.

Add a string of regular diodes in the filament CT lead to increase bias until the tubes idle where you want with a given HV, if needed.

T
Logged

Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz,  +-6.0 KHz or  +-8.0 KHz when needed. 

"You know my name"  https://youtu.be/noGjJyEDm5s?t=135

There's nothing like an old dog... a puppy... a dog in its prime... or ANY dog!
n4joy
Member

Offline Offline

Posts: 194



« Reply #6 on: February 23, 2017, 05:57:56 PM »

Thank you again, Tom.  Your recommendations are greatly appreciated.

The author indicates he neutralized the 813 with an Arc-5 antenna connector in proximity to one of the 813s.  I have limited experience with the 813 and am wondering if this is necessary or recommended--say for 20 meters?  He does question the necessity of neutralization of 813s and my research online suggests it is not required.

Chris
Logged
w7fox
Member

Offline Offline

Posts: 74


« Reply #7 on: February 23, 2017, 09:47:38 PM »

Chris,

I built a similar amplifier and it works great.  I used a pi net at the input so the exciter, an SB-101, sees a 50 ohm load.  That way, I can switch back and forth with the amp, or barefoot, without retuning the SB-101 final.  One thing about the 813 amplifier though, it only takes about 65 watts to drive it.  I had trouble keeping the drive down, and the grid current would run too high.  I solved the problem by lowering the drive to the control grid. This is an old trick that has been published a few times.  Instead of connecting the control grid to ground, it is connected to a tap on the filament choke.  I get about 600 watts out using a 2500 volt 400 ma power supply.  Grid current runs 100 ma.  Good luck.

Best regards,
Fox
Logged
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1450


« Reply #8 on: February 23, 2017, 10:44:10 PM »

You might also want to look at the Electric Radio article of June 2016, issue #325 by Bohn, The 2X-813 Linear Amplifier


Phil - AC0OB
Logged

Charlie Eppes: Dad would be so happy if we married a doctor.
Don Eppes: Yeah, well, Dad would be happy if I married someone with a pulse.NUMB3RS   Smiley
n4joy
Member

Offline Offline

Posts: 194



« Reply #9 on: February 25, 2017, 11:20:40 AM »

Thank you, Phil and Fox.  I will look for the article.  I plan to use this amp with my Lettine so the drive will likely be 30-35 watts or so.  My power supply (500 ma transformer) should deliver 3KVDC no load.  

My chassis arrived today... will start mapping out components and drilling/punching.  Photos are pending!

Chris, N4JOY
Logged
W4AMV
Member

Offline Offline

Posts: 583


« Reply #10 on: February 25, 2017, 12:06:28 PM »

Hi Chris. A tip of the hat on the tank coil. I did the same construction tank for a pair of 811a's. It did not turn out as nice as yours but it worked. However, not real pretty, so I found a 3 inch ceramic form ribbed at a hamfest for a couple of bucks and swapped out my ugly one.  Smiley

Tnx for the reference to the 73' paper. One note not mentioned by the experts here, the plate choke. I see you mentioned the choke on the supply side of the bypass cap, but not the primary choke at the plate. What do you plan to use there? This one is somewhat critical to any SRF present and the 73 article identifies a unit I am not familiar.Care on the choke and it needs to be checked out ideally in place in your final layout. Tnx again!

Alan W4AMV
Logged
n4joy
Member

Offline Offline

Posts: 194



« Reply #11 on: February 25, 2017, 02:09:02 PM »

Thanks, Alan.  I have given up on perfect and am more tolerant to imperfections when working on my projects; but I likely spent more time than needed on the tank.  I used a micrometer to measure spacing but winding the forms too the most time--totally unexpected.  In the spirit of the article, I am calling this a "junk drawer amp" so it won't be a show queen.  Smiley  

Good question on the plate choke.  You can see the ceramic core plate choke in the photo above, which I had in my parts box.  My Boonton LC meter says 105uH and the wire size appears right.  I will double check the gauge but it looks to be about 22 AWG.
Logged
W4AMV
Member

Offline Offline

Posts: 583


« Reply #12 on: February 25, 2017, 02:49:04 PM »

Great Chris. That is quite sufficient inductance. The real key issue is the self resonant frequencies, there are at least 2 key ones, present. Ideal, they should be out of band. You can search the internet on the topic and there are excellent articles on using a GDO to find the series and parallel resonant points. As a lesson on the topic, I picked up the famous National R154 choke which had a significant problem operating on the 15 meter band. It was soon modified by National and became the R154A. I think those are the right numbers. I have both and for fun I measured both with the GDO and sure enough verified the issue that was discovered by folks back then...  Smiley
Logged
K1JJ
Contributing
Member
*
Offline Offline

Posts: 8104


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


« Reply #13 on: February 25, 2017, 02:58:30 PM »

Thank you again, Tom.  Your recommendations are greatly appreciated.

The author indicates he neutralized the 813 with an Arc-5 antenna connector in proximity to one of the 813s.  I have limited experience with the 813 and am wondering if this is necessary or recommended--say for 20 meters?  He does question the necessity of neutralization of 813s and my research online suggests it is not required.

Chris

Hi Chris -

I see the author fed some negative RF feedback into the fil choke via a winding.  Hopefully he cold-neutralized it by adjusting the pickup's proximity to the tube plate. Unless we adjust the feedback carefully using a scope, it can actually make things worse.

No, there is usually no need to neutralize most linear grounded grid amplifiers.  The grounded shield that the grid and screen form between the plate and filament (cathode) is enough to minimize RF feedthrough that can cause instability on the higher bands..   Just be sure to employ good practices that keep the input components from seeing the output.   The antenna relay can become a troublesome point of coupling in/out, so look it over too.

813s in GG and grid driven have always run well for me. It is a monster of a tube - very under-rated.

In contrast, a grid driven (cathode grounded) 813  (tetrode/pentode)  may need neutralization when run class C, class B or whatever on the higher bands. There is no natural shield action by the grid. Though, with a super good layout, many hams have gotten away without neutralization since the screen is at RF ground.  

BTW, when using a a grid driven triode final stage at shortwave frequencies, it is mandatory to neutralize .

T

Logged

Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz,  +-6.0 KHz or  +-8.0 KHz when needed. 

"You know my name"  https://youtu.be/noGjJyEDm5s?t=135

There's nothing like an old dog... a puppy... a dog in its prime... or ANY dog!
n4joy
Member

Offline Offline

Posts: 194



« Reply #14 on: February 25, 2017, 10:01:08 PM »

Great information as always--thank you!  I will use the grid dipper and double check the plate choke.  I am fairly certain I purchased it as a 1-30 MHz plate choke, 1 amp.  I appreciate the clarification on neutralization.

I am getting ready to punch holes in the chassis--lots of careful measuring.  I picked up the radio chassis punches on eBay and they appear unused or rarely used.  My so called "chassis" is actually a 14x14 industrial cake pan.  It is thick, very sturdy, and perfectly square (not tapered).  I could probably stand on it!  There is enough depth/clearance underside for components and wiring.  I simply wasn't going to spend an arm and a leg for a large Hammond or Bud chassis.  I don't have sheet metal tools and/or experience to fabricate my own.  I will use angle bar and pre-cut aluminum panels to construct the front, side, and top panels.

Work in progress... Roll Eyes



* Resized_20170225_214502.jpeg (265.07 KB, 1600x900 - viewed 232 times.)
Logged
w8khk
Member

Offline Offline

Posts: 924


This ham got his ticket the old fashioned way.


WWW
« Reply #15 on: February 25, 2017, 10:21:32 PM »

Hi Chris,

That is a good looking collection of hardware for your amplifier, and the chassis should work fine for you.

As I look at your simulated layout, I would like to offer one suggestion.  The angle brackets to mount the plate tune capacitor may be left on the stator side of the capacitor, mounting the stator close to the chassis, or they may be moved to the top, or rotor side of the capacitor.  If you mount the capacitor the other side up, with the stator and connections up, away from the chassis, you will realize three benefits:  1) lower capacity between the capacitor and chassis ground, allowing a greater degree of control of the minimum capacity of the tune cap.  2) better protection against high-voltage arc between stator and chassis, due to increased spacing, and 3) shorter, lower loss (shorter wiring) between the plate circuit of the 813 and the tank coil and tune capacitor.  You may be able to do the same thing with the loading capacitor, but that is not clear in the picture.  Hope this helps....

GL on your project!  73, RIck
Logged

Rick / W8KHK  ex WB2HKX, WB4GNR
W4AMV
Member

Offline Offline

Posts: 583


« Reply #16 on: February 26, 2017, 02:06:48 PM »

To add to Rick's comments. I wrestled with layout for days! However, a combination of what is in the books combined with folks who built them and they worked perfect is worth it's weight. I ended up with the tank band switch close to the plate tune cap if for no other reason than keeping the coil taps as short as reasonable. The plate load cap wound up towards the right side of the chassis obviously close to the antenna port. There are comments in the handbooks about keeping the High Q tank inductor at right angles to masses of metal so as to keep the Qul as high as possible. Frankly, I have seen it done following and NOT following that rule! Need to keep the tube socket grid connections as close to chassis ground as possible so as to minimize the lead length of the bypass cap and or the ground connection. I sub mounted my sockets about an inch or so below the chassis. So to solve that issue, I built a large Aluminum plate L bracket to nestle up to the socket and keep the inductance small. Final note, I did not see it in the article, but adding a T/R relay to bypass the amp may be useful. Although at higher power levels this might be more trouble than an aid. And as Tom added, a pi match at the cathode input is used. The pair of 813's have a published Zin for GG. Can't recall, perhaps 120 ohms or so. An easy match to 50. I placed my pi match in a small shielded enclosure above chassis. My 2- cents. Alan.  

Oh, I forgot to add... A glitch fuse on the HV supply line near the bypassed plate choke. I used a single strand of wire from 50 ohm coax shield. Mounted on a HB piece of pcb. Other's can chime in on what they have used.
Logged
n4joy
Member

Offline Offline

Posts: 194



« Reply #17 on: January 11, 2020, 01:05:07 PM »

I am providing an update on my 813 amp project, which was started almost three years ago!  Well, I was inspired to finally get it done and hopefully have it ready in time for the AM rally.  Again, this was intended to be a junk box amp where most components would be scrounged or made including the chokes and tank.  The chassis is a cake pan!  I very closely followed the amp and power supply design from a February 1969 and May 1965 issue, respectively (links are earlier in this thread).

All that is left is:
1) Secure a plate transformer.  Finding one has been impossible (probably many are in garages and sheds hoarded by old hams), so I am leaning toward the Antek 800 VA toroid).  Guess I will have to buy a new one! Sad  
2) Install the front panel, drill for meters and bandswitch, and tap tank coil
3) Finish some wiring

I know it is crude looking, but this was my first amp build and it was built in the spirit of "use what I have."  There are several changes I would have made, but I will move forward and get it done!


* 20200111_122550.jpg (1319.7 KB, 4032x3024 - viewed 118 times.)

* 20200111_122559.jpg (944.61 KB, 4032x3024 - viewed 85 times.)

* 20200111_122610.jpg (1030.98 KB, 4032x3024 - viewed 133 times.)
Logged
W4AMV
Member

Offline Offline

Posts: 583


« Reply #18 on: January 11, 2020, 02:32:12 PM »

Hi Chris,

Looking good. Is the HV transformer on another chassis? I see what appears to be
6-500 V stacked cap bank. So that would be 3kV and I assume you are looking at 2.5 kV. I see 3- of the voltage divider R's for equalization, assume the other 3 are hidden from view. However, what is their R value and wattage? They look like flame proof R's but their wattage seems small from their physically size. For the typical case I tend to go with 5x the bleeder power on that voltage divider. So I would end up with 20 W R's give or take. Just to keep the heat in check. Yours look a bit small.

Alan

P.S. I visited the PS reference. He goes with 2 W on 100 k (600 k bleeder). Chris, I would use 10 W. I have seen these equalization R's heat, shift values, no longer properly equalize the HV and BOOM. The HV bank caps go. My 2 cents.
Logged
n4joy
Member

Offline Offline

Posts: 194



« Reply #19 on: January 11, 2020, 05:04:22 PM »

Hi Chris,

Looking good. Is the HV transformer on another chassis? I see what appears to be
6-500 V stacked cap bank. So that would be 3kV and I assume you are looking at 2.5 kV. I see 3- of the voltage divider R's for equalization, assume the other 3 are hidden from view. However, what is their R value and wattage? They look like flame proof R's but their wattage seems small from their physically size. For the typical case I tend to go with 5x the bleeder power on that voltage divider. So I would end up with 20 W R's give or take. Just to keep the heat in check. Yours look a bit small.

Alan

P.S. I visited the PS reference. He goes with 2 W on 100 k (600 k bleeder). Chris, I would use 10 W. I have seen these equalization R's heat, shift values, no longer properly equalize the HV and BOOM. The HV bank caps go. My 2 cents.

Hi, Alan.  I appreciate your review and input!  Yes, you dispense good advice.  I had yet to solder and install all of the 100K 5w resistors.  I need to place a Mouser order, so I will go ahead and install 10 watts or more.  Thanks!
Logged
n4joy
Member

Offline Offline

Posts: 194



« Reply #20 on: February 06, 2020, 09:30:32 AM »

I am still moving forward with the amp project.  I am doing a little bit every evening after work and family time.  Wiring is almost done, a few more components need to be added, and a fan will be installed for cooling.  I picked up a real nice cabinet on ePay and it is at the powder coater--pics to follow.  I appreciate everyone's guidance.  There are likely a million things that the experts could nit pick about, but this has been a fun project.


* 20200202_220357.jpg (1131.01 KB, 4032x3024 - viewed 95 times.)

* 20200201_163618.jpg (1385.2 KB, 4032x3024 - viewed 99 times.)

* 20200202_220338.jpg (1635.81 KB, 4032x3024 - viewed 105 times.)
Logged
w7fox
Member

Offline Offline

Posts: 74


« Reply #21 on: February 06, 2020, 10:08:09 AM »

Chris,

It looks like you are making good progress.  I have to throw in my two cents though.  I am uncomfortable with the appearance of the high voltage capacitor string.  It may be the photo's perspective, but it looks like the caps are touching the chassis.  The cap at the top of the string would have 2000 volts between the case of the cap and the chassis.  It seems unlikely the plastic skin of the cap would withstand this for long.  I mounted my caps on a 3/8 inch sheet of plexiglas for this reason.  Also, your equalizer resistors are underneath and close to the caps.  This will tend to cook them and may lead to short life.  Carry on and enjoy your good work.

73,
Fox
Logged
n4joy
Member

Offline Offline

Posts: 194



« Reply #22 on: February 06, 2020, 10:16:54 AM »

Chris,

It looks like you are making good progress.  I have to throw in my two cents though.  I am uncomfortable with the appearance of the high voltage capacitor string.  It may be the photo's perspective, but it looks like the caps are touching the chassis.  The cap at the top of the string would have 2000 volts between the case of the cap and the chassis.  It seems unlikely the plastic skin of the cap would withstand this for long.  I mounted my caps on a 3/8 inch sheet of plexiglas for this reason.  Also, your equalizer resistors are underneath and close to the caps.  This will tend to cook them and may lead to short life.  Carry on and enjoy your good work.

73,
Fox

Thanks, Fox.  I appreciate your input.  I can lift the string with spacers and put a thin sheet of fiberglass PCB board under it.  In regard to the equalizing resistors, the original PS design from 73 had 2 watt resistors.  I figured the 10w resistors would stay relatively cool (they are not touching the caps).  I can snip the leads, extend, solder, and recover the leads with fiberglass sleeves.     
Logged
n4joy
Member

Offline Offline

Posts: 194



« Reply #23 on: February 06, 2020, 11:20:07 AM »

I fortunately found a sheet of phenolic board and will cut to size and drill holes to match the existing chassis holes for the terminal strips.  I can then slip the board under the HV cap string and use slightly longer screws to mount the strips.  I had originally planned to do the string turret board style but had so many long terminal strips laying around.   
Logged
KK4YY
Member

Offline Offline

Posts: 565


Your best isn't as good as you can be.


« Reply #24 on: February 06, 2020, 11:47:46 AM »

Given the same resistance value, the 10W resistors dissipate the same amount of power as the 2W resistors do, generating the same amount of heat, albeit spread out over a slightly larger area. Spacing from the caps helps, as does forced air movement below the chassis.

What larger wattage resistors do is to provide a safety factor to reduce the possibility of their failure. However, if one of the resistors in the string were to fail "open", that capacitor will see a higher voltage that may cause the cap to fail - quite dramatically. Also, bleed-down is compromised. This is a serious safety concern.

My solution to this failure mode is to use multiple paralleled resistors across each cap. This way, if a single resistor was to fail, the voltage division of the string would only change slightly and bleed-down would still occur when switched off. As a bonus, lower wattage, less expensive resistors can be used. The voltage division of a failed resistor in the string can be easily calculated to stay within the voltage rating of each capacitor.

Electrolytic capacitor life is dramatically reduced by high operating temperatures. The life of a cap rated at 85 degrees C is very short when operated at 85 degrees C, perhaps just a matter of months. Granted, that's pretty darn hot, but the cooler the better.


Don
Logged

All your worries won't add a day to your life, or make the ones you have any happier.
Pages: [1] 2   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 0.115 seconds with 18 queries.