The AM Forum

I've been in process of building a... well it's been different things at different times, but it's a two-holer GG Amp for the moment.  Some day it will evolve into a pair of something moded by another pair.

Home Brew Relay Rack construction, Wound the filament transformers by hand, cobbled the B+ supply using salvaged Microwave oven parts, two MOT's parallel-series into a Full Wave Bridges using 866 MV rectifiers (cause they are cool that's why) I put in a timer so the filaments will warm up for 90 seconds before the B+ is enabled. I am going to stick in another to run the fans for a bit after turn off. I can "adjust" the output from 2800 - 4000 volts using a simple Bucking transformer arraignment
. 
Plugged in a pair of 4-250s in the sockets, lit the filaments and let everything sit for about 15 minutes,

Hit the B+ and there was no smoke! She sat there cut-off just like she's supposed to, very nice.  Keyed the amp by hand (a relay takes a 25k/10 watt resistor out of the fil center tap) and the no-signal idle current was ~50ma.  Still no smoke. Let her sit a few minutes at idle and all was good, no smoke, no funny smells, no sparks!  B+ was solid under just the idle load and bleeder, but that as it should be.

Thought this was a good place to stop, so shut everything down, and I'll put on the finishing touches and run her into a dummy load with drive applied to see if I can tune it up.

When the testings done, I'll put in the 4-400's and I'll be able to "Drop the Maul"!

Feels good and took a long time. Still ain't done, but she's working!

A special thanks to those of you who have supported me with parts and advice here on the board and over the air. 

" Wound the filament transformers by hand, cobbled the B+ supply using salvaged Microwave oven parts, two MOT's parallel-series into a Full Wave Bridges using 866 MV rectifiers (cause they are cool that's why)"

I've got a surplus 5V, 20A  filthament xfermer I've wonnered about rewinding to 10V .....  just havent got around to doin it .....


klc

Congratulations Ed.  Lotta satisfaction there!

Bill

Kool Beanz! ! ! ! ! ! ! ! ! ! !

Time to pour on the coals!!

Yes, I'm feeling good about that!!  It was quite a feeling to see the filaments alight and the meters actualy moving.  A definite plus was that they were going where I thought they would too!


the low voltage stuff isn't bad. Use good heavy wire for high current apps.  I used microwave oven trannys (MOTS) for all of em.  Remove the HV winding and magnetic shunts, then wind on enough wire to get the output V you want.  Insulation for the voltage is applied too.  Fine adjusting was done by adding turns to the primary, which also helped the tranny not saturate (I think).  Multiple outputs are done the same way, although it's takes a bit of work sometimes to get all the voltages right.

Congrats Ed! It's been a long road for you indeed, 2-3 years now?

I used the same approach as you when bringing the big rig back online after 10 months down. Test a little, count your blessings, move on later. If you do it in stages, you're less likely to meet with nasty, potentially damaging surprises.

Bet I'll hear you down here in Fo' Land just fine. Sounds like a good fall and winter ahead for AM.

If it's to be class C plate modulated, I would recommend changing the grounded grid to grounded cathode, grid driven, when the time comes.

My main transmitter, the HF-300 rig, slowly evolved from what I first built in the early 60's.  It is completely different from the rig I started out with then, but there was never a specific moment when I put something new on the air and then retired a previous rig.  The thing has gradually morphed from what it was then to what it is now, and it's still a WIP.

run her at high idle 20 minutes to lap in the cam.

Like we used to say in my drag racing days:

Git her loosened up and legg it!! "If it blows, it goes" 

us poor boys built them loose

First testing with RF was successful, sort of.  Well I did get power out, but before to much time had passed, I got That Smell and then plate voltage began to go funny, then quit.
  Seems that even though the "book" says bucking transformers can be significantly smaller than the transformers they are feeding, they should still be Big enough.  Oh well at least I have a replacement that is 24v at 8 amps.  HA that should do it. The now deceased bucking unit was much smaller, and of unknown ratings. It sacrifice will not be in vain. Some smoke must be made, and I'd rather it be cheap smoke.

On a different note, The plates of the tubes were glowing Orange for a few seconds.  This was a bit disconcerting. I really didn't have time to verify the tuning as the failure occurred too early. 
How much color is TOO much color?  I know many power tubes are happy showing red or even dark orange plates.  Is it all just based on what the meters say?


Don = yes it will be converted to a grid driven class C rf final when the time comes. Right now, GG is teaching me enough.

measure the temperature of the exit air. Warm is ok but hot means short tube life

Well, ran some RF through into the dummy load, Tunned up nicely at reduced input from the TS-520.
Ran it up to full power and things looked ok.  Looks like the PS sags pretty bad, and there seems to be some 60hz coming in from somewhere (both are probably related to insufficient power supply filtering- I used what I had but I know I need more) but it works!  The plates light up, but there isn't a lot of difference between idle and full scrote.   I was running just under a 800 watts input, and getting just over 350 watts out on CW.

Neat.  The air coming out gets pretty warm, I added a muffin fan on the plate connectors.  I do have to do some chassis work to get more air out around the tubes/sockets.

800 in and 350 out sucks, sounds like the pi network is wrong.
Try reducing the L which will increase c of both caps.

    As I recall, this is a pr of 4-250s' running GG...  400 watts dissipation...
    NOT biased for class C, idle current @ 50 or so mills (no applied bias
    voltage)...  Some class in a linear mode for those tubes... 800 watts in,
    350 out =  40%         Not all that bad!

    Run in class C and of course the efficiency will go up but looks like
    Ed has close to what the circuit needs right now...

You are correct Ralph.  even to the 50 mils idle current! 

I'm pretty happy over all.