lotsa questions about stuff i should already know but dont

[N3DRB The Derb]

how much current will 4 triode audio drivers of the normal 2A3 etc. variety want at full strap?

single plate to PP grids interstage - the idea is to use the most step down you can while still getting enough munky on the grids to drive the next tubes,correct?   I got 2:1 and 4:1 choices. This is a UTC S-2 little bitty thing just to split you into P-P
after your mic amp tube & gain control. gets you into P-P quickly and sets up the rest of your voltage stages.

Any law against running a common supply for yer low power RF and lower level audio? I have a UTC S-44 which is a low voltage hi current monster (500 ma) along with a matching S38 550ma swinger. It would be nice to only have to make one common supply for both low level rf and speech amp/driver decks. 470 volts on a 807 buffer amp and 430 divided down as needed in the audio. I only need very few watts to drive either the 4-400s or the HK's.

The S-44 makes either 470 or 430 dc volts after rectification depending what secondary you use.

What tube rectifier would pass enough ma to work well at the low voltage high current point this xformer operates at?  IIRC correctly 5U4 only goes to 220 ma or so. I guess I could run separate 5U4's on each secondary tap.

Any other choices? besides diodes?

[KE6DF]

single plate to PP grids interstage - the idea is to use the most step down you can while still getting enough munky on the grids to drive the next tubes,correct?  

I don't believe this would apply to voltage amp stages that run in Class A (or AB1) since there is no grid current. Having a big step down only works against your efforts to amplify voltage. 

Any law against running a common supply for yer low power RF and lower level audio? I have a UTC S-44 which is a low voltage hi current monster (500 ma) along with a matching S38 550ma swinger. It would be nice to only have to make one common supply for both low level rf and speech amp/driver decks. 470 volts on a 807 buffer amp and 430 divided down as needed in the audio. I only need very few watts to drive either the 4-400s or the HK's.

The S-44 makes either 470 or 430 dc volts after rectification depending what secondary you use.

What tube rectifier would pass enough ma to work well at the low voltage high current point this xformer operates at?  IIRC correctly 5U4 only goes to 220 ma or so. I guess I could run separate 5U4's on each secondary tap.

Any other choices? besides diodes?

I don't think there is problem with using this big transformer for both voltages. A couple things to keep in mind.

1. The voltage rating of the S series UTC transformers assumed 115 on the primary. Many places have a higher line voltage than this. This probably doesn't cause a problem for the transformer, but you will get higher voltage out so take that into account. My QTH has 123 v service.

2. 5U4's are speced for up to 550vac on each plate. The transformer you mentioned would exceed this with current line voltages. The 5AU4 can handle higher current than a 5U4, but it's limited to 500 vac.

3. You probably are best off using either two 5R4's with the plates tied together to form a single diode. They have higher voltage ratings. Or you can use two 836's which have plenty of voltage and current capability, but require 2.5v 5a on the filaments and use a plate cap.

4. Perhaps some of the damper diode tubes would work, but be careful about the cathode to heater spec. The 6de4 is one what might work, but it doesn't have quite the current spec you need. It's limited to 175 ma if I remember right, so you could get out twice that from a FWCT circuit or 350ma.

I've used 836's before. They are a good reliable tube. Last forever.

[N3DRB The Derb]

I don't believe this would apply to voltage amp stages that run in Class A (or AB1) since there is no grid current. Having a big step down only works against your efforts to amplify voltage. 

ya know what you be right. Sometimes I dunno if I should still be trying to do this stuff. 

they scraped off the radio knowledge and stuck it in a jar down at Georgetown U.

I'll look at the 836, sounds promising.

[flintstone mop]

Hey Tim
I'll try to scan the scheezo for the Raytheon audio and voltages needed, then you can see what makes up a quad of 6B4's or 2A3's to drive some big pubes

Phred

[k4kyv]

how much current will 4 triode audio drivers of the normal 2A3 etc. variety want at full strap?

single plate to PP grids interstage - the idea is to use the most step down you can while still getting enough munky on the grids to drive the next tubes,correct?  

That's basic it for class-B drivers, whether single plate to PP grids or PP plates to PP grids.  The idea is to make the class-B grids see a driving voltage from as low impedance source possible, while still maintaining enough voltage swing to fully drive the modulator.  Since continuously variable class B driver transformers are non-existent, you have to settle for the greatest step-down ratio you can find in an available transformer that still achieves the minimum grid voltage swing you can get away with.

Grid swing to the modulator tubes that exceeds what it takes to drive the modulator tubes into saturation is wasted.  If the driver stage hits the saturation point before the modulator stage, the full capability of the modulator stage is never realised.  Ideally, the driver stage and modulator stage would flat-top at exactly the same time, but chances are you will never hit this ratio exactly, and you will have to settle for one or the other of the above.  You are better off wasting a little bit of the class-B driver capability than running the modulator tubes below their maximum output capability.

[WA1GFZ]

Derb,
Maybe they removed some smarts but they left behind the best part about knowing you have to be careful with high voltage. You wouldn't want it the other way.

[N3DRB The Derb]

I do worry a lot bout getting hit. HV is the lifeblood of big fat mauls, cant get around it, but I'm more scared of it now than ever. I'm still taking heavy doses of anti seizure meds and they do make one's head a bit fuzzy.

[K5UJ]

I do worry a lot bout getting hit. HV is the lifeblood of big fat mauls, cant get around it, but I'm more scared of it now than ever. I'm still taking heavy doses of anti seizure meds and they do make one head a bit fuzzy.

Derb, maybe when you have to do something critical you should get someone in there to spot you, someone local.  Maybe W3HM could come over for a few minutes.

Rob K5UJ

[N3DRB The Derb]

yeah, he's only 3 miles away.

I've thought about inviting Mike 2ZE over for some radio action. He's out of the game right now due to apt living.... lives about 30 minutes away. but not much fun to be had when you cant get on at all, and no fun just sitting around watching someone else build. When I get on with some power I'll see if he wants to partake.

I did invest in a Fluke HV probe model 80K-6 which takes the range of my Fluke DMM to 6KV and keeps my hand out of range. Very nice probe, has a built in 1000:1 voltage divider and 75 megohms input impedance.

My new feedline is going to be another 4 to 6 days arriving yet due to labor day. I have a good feeling about that stuff, I think it's going to work real well for simple dipoles. cant wait to get the antenna back up.

[K5UJ]

I should get one of those probes.  What's the feedline?  Ladder line?  Coax?

R.

[The Slab Bacon]

Timmy,
          There is no problem with using the same "low voltage" power supply for both the RF and audio low level stages. I've been doing it in my 4X1 rig for years. Just be sure you have adequate filtering / bypassing in the RF stages to keep it there. And be sure that the supply is adequate to handle the required current draw. A lot of class - A low level audio stages can suck down a lot of juice.

Typically, class A or AB1 (no grid current) driver / interstage transformers are usually always a step up transformer, where class B or AB2 are just about always a step down transformer to obtain the leverage needed to handle a current sucking low impedance load.

When I wrung out that transfoma that I have for you, I remember that it was a step down, I just dont remember the ratio.

[The Slab Bacon]

<snip>   What tube rectifier would pass enough ma to work well at the low voltage high current point this xformer operates at?  IIRC correctly 5U4 only goes to 220 ma or so. I guess I could run separate 5U4's on each secondary tap.
Any other choices? besides diodes? 
<snip>

816s would prolly be usable, if not, consider good old faithful 866s. They would handle the current with no problems, and at that low voltage they will NEVER arc over. And besides, they look pretty kool when they are glowing blue.

[N3DRB The Derb]

This transformer is a beast. 500 ma but really low voltage. Weighs a metric ton. I have a 500 volt dc meter and a 1000 ma FS meter that will be peachy keen for it. Gonna be the first deck I make I think. Wanna start with easy stuff.

feedline is new on the spool Belden 8210 KW 72 ohm parallel transmission line. It's hi power 72 ohm twinlead basically. 13 awg wire.

[KE6DF]

816s would prolly be usable, if not, consider good old faithful 866s. They would handle the current with no problems, and at that low voltage they will NEVER arc over. And besides, they look pretty kool when they are glowing blue.

The 836, 866A, and 3B28 are all similar in that they have the same base, pin-out, and are roughly the same size.

The 836 is a high vacuum tube as has a max continuous current of 250ma and a max PIV of 5000v.

The 866A is Mercury vapor and has a max continuous current of 250ma at 10,000 PIV, or, if the PIV is less than 2500v the current is 500ma.

The 3B28 is Xenon and also has a max continuous current of 250ma at 10,000 PIV, but if the PIV is less than 5000v the current is 500ma.

No worries about long initial warm up times for 836s or 3B28s. Can operate in any position.

The 3B28 is very nice for medium voltage, high current supplies due to it's higher current ratings up to 5000v.

You can build your supply with a four pin socket and probably use any of the above and it would work fine -- which ever you can find at the right price.

I have a box or 3B28s and 836s, but don't have any 866As -- but that means I'm missing the glow (although 3B28 glow a little).

[k4kyv]

I wish none of my 866A's or 872A's ever arced over.  3B28's and 4B32's are much tamer but the glow isn't as pretty.  But handled carefully, a good MV rectifier can be made to last a long time.

The 836 is a high-vacuum equivalent of the 866 (not the 866A).  Its PIV is less and the voltage sags under load because of the internal voltage drop in the rectifier tube.

[N3DRB The Derb]

well, in this instance the voltage is pretty low in the scheme of things. Current handling is more important.

Don, you're the source of the choke I'm going to use, so TY and mazel tov. 

at 500 volts AC I'm really tempted to go ahead and use 866's. Unless I didnt warm them up right, I dont think they would ever
arc back.

plus the slab gave me a bitchen 866 fil transformer needs to be used.

[ke7trp]

I vote 866s or just diode it. 

I never work on my Big rig without another person here.  Normaly, I have to wait days to work on it but thats ok. I have another ham come over and we agree on what we are about to do before we make the change ect.. So far, I am alive


C