Home-Brew 250 Watter?

[W9ZSL]

  Hello and Help!  My original idea was to use a 4-125A modulated with a pair of 811As.  I have the plate transformer from a Gates BC-1T 250 watt output job.  It's good for a solid 3600 VCT.  I used 120 volts in a rectifier breadboard and got exactly 900 volts no load.  See pix.  With 240 I'll get 1800VDC no load at an amp.  I have a pair of 350MA chokes , 1 swinging with a 12, 8 and 4MFD at 3KV.  That's the 8 in the pix.  I'll use the Gates bleeder.  1500 solid volts loaded.  I want to use just 1 supply here.  The 811A Modulator is in.  With -9 volts bias, the mod should be capable of over 200 watts.  I have a Hammond 175 watt Mod iron with 15K primary and 4 secs so 150 watts would be loafing.

Problem is at that plate voltage I don't think a 4-125 is a good choice.  813???
Suggestions?  Pair of 811's as a PA?  That would be great but I don't have any diagrams for such an animal.  My plan was to build an "all purpose" amp in the '62 West Coast Handbook replacing the 4-250A in that schematic.  Exciter will be a Kenwood TS440S. 

Soooooooo   73 and TNX!

Mike - W9ZSL

[W7TFO]

That is a good, well proven power supply.  Plenty of poop for a linear or modulated service. 

The Gates 1T is a kW output TX, not 250W.  Was the transformer from a BC-250?  Looks like one from a "1" series, but ca't really tell from the pix.

Find a bigger tube to be able to use that oomph, things will be a lot happier.

One suggestion is an 833, cheap & available.  810's. Or a set of 4-400's.  Lots of Russki ceramic tubes available, too.

73DG

[KA2DZT]

Not sure I'm following what you're asking.

3600vct with 120v on primary or 240v??

The mod xfmr is 15K primary and what's the sec impedances.

With two chokes (LCLC) you'll be lucky to see about 1500 VDC after the filters.

Run the 811As with about 4 volts bias.  I use a 4.2V Zener in the CT lead of the filament xfmr.

Those chokes I guess are 19C36 and (IIRC) 19C43, they're rated for 300ma.

Run the 811As off the first choke and the final through both chokes.

Try to get 10-15ufd on the first choke and more on the second choke.

At 1500 volts maybe a 813 would be better.

You won't get 250 watts at 1500 volts.

Are you building an amp or a plate modulated xmtr??

So, what was the question again??

Fred

[KC2ZFA]

I understood that there's a PS that can do 1500VDC@1A. Now, if 250W is the PA output being sought then there should be at least 384W input (at 65% efficiency) which will require .256A at 1500VDC to achieve. Obviously, the PA tube(s) must be able to dissipate 134 watts.

On the other hand, the 811A modulator at 1500VDC can deliver 340W audio at .313A current draw (if we believe the datasheet), and this is way too much audio (so speak softly and adjust the modulator xfmr ratio).

It seems to me that the single PS, with the chokes listed, and connected as Fred described, will be able to handle this (assuming the choke that feeds the modulator doesn't care if its rating is "slightly" exceeded). However, the available audio power will be too much.

Now comes the tube selection...It turns out that 2 813's will do 250W out with this single PS but they'll be near the edge of their rated CCS dissipation at 1500V.

How did I do master Fred ?

[KA2DZT]

You did just fine,  I'm also looking at the mod iron he has and it's not going to handle two tubes (813) in the final.

The plate xfmr has plenty of current reserve, the chokes are little light but they'll work.

Fred

[W8ACR]

If you have 1500V @ 350mA available, I'd say a single 810, a single 8000, a single 250TH/TL or a pair of 100TH/TL's in the class C final modulated by 811A's. As K9ACT and I have said before, "triodes are nice". Run the RF final at about 200mA. The modulator would barely have to work. Put a window in the front panel so's you can see the final glowing. You'd have a very buzzardly vintage transmitter. Avoid ceramic tubes at all costs.......they emit no photons. This degrades the buzzard factor exponentially.  I think of them as very large 2N2222's.

Even better, find a modulation reactor, take the plate current off the mod transformer secondary, and run the final plate current at 250mA.

Even better, build a separate power supply for the modulator at about 1250V, run the modulators zero bias, and run the RF final at about 300mA. You'd still have plenty of audio to modulate 100% at 450W RF input.

I'd also get a vintage exciter - a Johnson Ranger or Viking comes to mind - but that's just me.

Ron

[W7TFO]

[quote author=W8ACR link=topic=29307.msg226916#msg226916 date=1319433925

I'd also get a vintage exciter - a Johnson Ranger or Viking comes to mind - but that's just me.

Ron
[/quote]

Or a Meissner Signal Shifter.

73DG

[KB2WIG]

  """   I'd also get a vintage exciter """


this is whatt u need

[KX5JT]

  """   I'd also get a vintage exciter """


this is whatt u need

or THIS

[W2ZE]

I think an 813 would be an excellent choice if you have them on hand. It would require low drive, and they are very plentiful. I personally am a fan of troides, but given you have ts 440, the best you could hope for is about 25 watts of drive key down. An 813 would only need 5 or so watts, 10 at the most. An 810 would require alot more drive given the Mu, and are becoming much harder to find.

It pretty much comes down to what you have, and how easy it is to get a spare. Just my 2 cents.

Mike

[KC2ZFA]

There are three conditions placed on the topic by the original poster: 250W out, a 1500 VDC@1A xfmr, and "I want to use just 1 supply here."

Peter

[KA2DZT]

Peter,

By the time we hear back from the original poster, the thread will be 3 pages long.

Your three conditions are correct, that's about what his questions were.

Fred

[KM1H]

Something doesnt sound right as a BC-1T PS runs 2500VDC at full load.

What bands is this for?

Id certainly go with a tetrode in the ouput as you wont need the hassle of neutralization at least thru 20 or 40M. The 2500V is fine for an 813, 4-125A, or 4-250/400.

If you variac it down or actually have a Gates 250W transformer then the 813 is an ideal choice. Ive had no problem getting 200W out at 1150V with one from 160-20M and at 1800V its right on the money to match that mod iron power rating. Keep the grid current at the high end of spec, 320-350 Eg2, and it easily does 200ma Ip as well as nice upward modulation. A 4-125A likes more HV before it wakes up.

I wouldnt run Chinese 811's over 1200V, they blow daily in the Ameritron and Collins amps; 572B's will plug right in and work fine to around 2500V.

Since youre limited to about 300W input with that mod iron you now have to decide what the output impedance is and then dig thru the tube manuals for a Class C tube that comes close at whatever voltage you have there.

Maybe provide us wit a bit more info.

Carl

[W2VW]

I agree with 2ZE.
The TS-440 drive power will drift especially at higher levels needed for a triode final.

[W9ZSL]

  Wow.  Good stuff.  I just tested the system with 120 volts.  The transmitter will be working with 240.  Yes, that transformer came with a BC-1T.  Here's the scoop.  I was cleaning toilets out there around the time the transmitter was put in service.  The manual is dated 1960.  The station was on 1240 Conelrad with a post WW2 Gates 250 watter.  Then there was a possibility of going to a KW day and 250 night.  Being cautious the owners bought a BC-1T 250 watt version that could be upgraded to a KW by replacing the plate iron and the filter cap.  I have both of those.  The other iron was already to go to a KW.  I don't know why they didn't go with a KW/250 rig since that is what they ended up with (cheep, cheep).  I got the parts left over after the conversion.  The 900 volt reading shown is with 120 on the primary of that same iron.  With 240VAC I'll have 1800 VDC no load. 

[W9ZSL]

Just a P.S. here.  I scored another plate transformer to use for the modulator.  It's 3150 VCT at 300 ma so it should work well with a pair of 811As.  I have another one rated at 820 volts and is made for a solid-state rectifier.  With a doubler I should have close to 2KV to keep the 4-125A happy.  Thanks all for the info!

Mike - W9ZSL

[The Slab Bacon]

Be careful using a transformer designed for full wave center tapped service in a FWB or voltage doubler circuit. Quite often thay will zorch out between the center tap and either ground or the primary winding.

(just ask me how I know)   

[Detroit47]

Be careful using a transformer designed for full wave center tapped service in a FWB or voltage doubler circuit. Quite often thay will zorch out between the center tap and either ground or the primary winding.

(just ask me how I know)   

They also are not designed to handle the higher Volt amp of a capacitive input. This is even worse on a voltage doubler.


N8QPC

[The Slab Bacon]

True, but,
If you are running a transfoma way below it's current / duty cycle rating and have a step-start setup, that is not as big a problem as the insulation breakdown. I have been running a plate transfoma from a BC-1 into a cap input filter for some time now without any problems. (I needed the extra voltage outpoot) However the input cap is rather small in value, just enough to kick the DC out up a few hundred volts. (keeps the 4X1 a little happier)

[Detroit47]

The lord knows that 4-1000s like lots of voltage. In used to have an amplifier with one in it, I had 5 kv on the plate it was a real performer. Great tube there are 3 indstuctible tubes as far as I'm concerned. The 4-1000a ,3cx3000a7 and 4cx5000a.

John N8QPC

[KM1H]

Add the YC-156 and YC-243 (socketless 3X6). Both will run forever as AM linears at as little as 2500V for the QRPers  and driven with a 50-100W plate modulated rig. A 25W ricebox doesnt cut it unless the final has 3500V + where the gain goes up.

[WD8BIL]

A 25W ricebox doesnt cut it unless the final has 3500V + where the gain goes up.

That's no problem with a kicker in the middle!

[W9ZSL]

Actually I'm aiming for about 300 watts input to the 4-125A.  The plate iron coming will be operated with a pair of solid-state 866 replacements and will be totally conventional.  I should end up with around 1200 VDC loaded to the P-P 811As.