Homebrew resurrection from the ground up

[w5hro]

Hello All..

Well after months of waiting the big homebrew rig is slowly being put back together from the ground up with some finishing touches. This has been a real project reminiscent of past projects years ago during those cold winter months back east.

At the very bottom are two Peter Dahl KWS1 transformers and two KWS1 chokes. One side is for the final modulator amp and the other side is for the final RF amp. Above that is the filter deck consisting of oil filled caps, bleeder resistors, and two current meters on the front panel converted to read the HV of each supply.

Above the filter deck is the final 810 modulator deck. I’m currently working on adding the 3 diode negative peak limiter with a separate DC supply for the keep alive. The parts are on the way. The final step is to finish the T-368 exciter and make the necessary changes to the 4-400CG RF deck.

Any suggestions or comments would be appreciated.

Brian, W5HRO / 6


                       Transmitter Front



                       Transmitter Back

[ke7trp]

Looks great Brian.  Are you using one 4-400?  Or two? If using Two, Why not look into adding the Bias tracking system like that was posted on the Audio amp? 

C

[ke7trp]

Real nice.. 810s are plenty then.  The ratings of the CG tube seem outrageous.

C

[KC4VWU]

Man, I'm really envious! That looks great Brian! You are going to leave it as-finished; no painting, right? I love wrinkle black just as good as anyone else, but that bright, clean look is really refreshing.

I hope to get started back on my HB Tx. before Christmas.

Phil

[W2NBC]

Brian,

The polished panels and the thought-out symmetry of components is VERY nice..

Hope to hear that maul in New Joisey..

73,
Jeff

[KA8WTK]

That is very nice work Brian.

[N2DTS]

Real nice looking!
Love it when everything is nice and clean, and new.

If you ran 3000 volts on the 4-400 you could likely get much more out of it.

Brett

[N3DRB The Derb]

as most here know, there's nobody that dig's the black wrinkle more than I, but I gotta say that transmitter looks fresh, yo.  

when are you gonna put some good double race ball bearing wheels under that rack? Guy on ebay is unbeatable for cost and quality. Lemme go find him.

huh, ebay appears to be crapped out. cant do any searches or find anything.

[ke7trp]

Just wow... I love the polished, clean panels.  Such a simple and elegant design.. Cant wait to hear it one day.

What is the Exciter?  What Drives the audio?
C

[N3DRB The Derb]

I might have to polish up my cad steel stuff and use it for the P-P 4-400 rig. 

I dunno about the front panels tho.

[VE3GZB]

Any suggestions or comments would be appreciated.

Dear Santa, can I have the above transmitter for my Xmas present!

LOL!!! But WOW, what a beauty!

[KC2IFR]

Brian.........your work is 1st rate without a doubt.
Nice job.....

Bill

[kc6mcw]

Very nice work!

[K5WLF]

Absolutely beautiful work, Brian. I'm planning to start a homebrew project for AM soon and you've certainly set a standard that'll be hard to match.

73,
ldb
K5WLF

[ke7trp]

Love the videos.  Keep em comming Brian. That Transmitter is a work of art. I love that Limiter..  Huge diodes.  The time you have invested must be huge.

CLark

[N2DTS]

Looks great.

I had a long qso a while ago about the 3 diode peak limiter setup.
It was about the switching times needed in the diodes, the bricks are just strings of diodes in series, lots of diodes in series, the 14kv ones I use need something like 14 volts (or more, cant remember) before they conduct, 14 volts divided by .7 volt drop per diode is 20 diodes in series!

Two issues, I used to blow out one of the three diodes regularly, plus got reports the signal was real wide.
From what I have been told, 20 diodes in series would all turn on and off at the same time, and much faster than needed for audio frequencies, but I am not sure....
TV damper diodes were mentioned as a very fast, high voltage and current diode to replace the bricks.

My bricks are 14kv, 1 amp, 250 amp surge, hard to see why one would blow out at 2000 volts on the plates, that might be 6kv on peaks, without the speed of the switching having something to do with it.
Nothing in the trans arcs, even though the vacuum variable plate tuning cap is only good for 5000 volts....

Perhaps if the audio high end is not strictly limited it gets beyond the diode response time, although experts say no way.
Or what happens if RF gets on the B+?

Be sure to test the rig to see how wide it goes when the ncl is operating.

Brett

[W7TFO]

Absolutely stellar engineering & workmanship. Big, built to last, and easy to run.  Bravo!

[k4kyv]

Nice looking mod transformer.  I like the way you mounted it on standoff insulators. Did it come from a broadcast transmitter?  Are you using a reactor with it?

Simply adding another pair of 6A3s in parallel with the first pair wouldn't drive the 810's significantly harder.  It would reduce distortion, but for substantially greater audio voltage swing on the 810 grids you would need to change the turns ratio of the driver transformer to reduce the step-down.  Ideally, the 6A3's and 810's should saturate simultaneously as the audio peaks exceed a certain point.

[k4kyv]

Actually, you can get more audio with less distortion by using fixed bias.  I recall that RCA says 10 watts for a pair with cathode bias and 15 watts with fixed bias.  Part of the problem with cathode bias is that the tubes run AB1, and the varying plate current causes the bias to shift with the audio signal.  One solution that would avoid an additional bias transformer would be to run about 370 volts total on the plates of the 6A3's, and then cathode bias them using a stack of zener diodes to adjust the bias to 40 mills per tube, and make sure the filament-to-plate voltage on the 6A3's ends up close to 300.

Or you could use a separate half-wave rectifier with reversed polarity on one half of the winding of the plate transformer that supplies the 6A3's (or any other low voltage stage in the transmitter for that matter).  Follow this up with a resistive voltage divider to pull off the 6A3 bias voltage.  Since you are not driving them into grid current region, the bias supply need only supply a few milliamperes through the bleeder, and you tap the bias voltage off the bleeder.

[N2DTS]

Seperate fixed supply in one rig (2X813) variac in the 3X4d32 rig.
If its adjustable, you can select the neg mod percentage.


Brett

[K5UJ]

That's looking like a beautiful rig, an example of how you only get top quality if you build it yourself.

[ke7trp]

Real nice Rig man.  Love the Neut cap and the BIG Chinese variable.  Where did you get that variable?

C