Designing a new Rico Suave - 3-500Zs or 813s? Opinions needed.

[WD5JKO]

Tom,

  Perhaps you could start with a scale model run at reduced power. Many tube combinations come to mind, and here are two:

801 Triode RF PA, Modulated by a pair of type 46 tetrodes. This is what was in the BC-223, and good for 20 watts AM.

6S4B triode RF PA, modulated by a pair. This little triode is readily available and very inexpensive. Should be good for at least 10 watts Dc input, or 7 watts AM. This would be a QRO PW rig, and followed by an 8877 would make over 100 watts or so RF carrier.

   The idea is to play with different topologies without dimming the lights, or having 140 db ear shattering noise from large contactors "klunking".

Jim
WD5JKO

[KM1H]

Im not against the 3 common tubes I listed and in fact have them all in use here in a TX or an amp where they just keep running.

I just like to see something different once in awhile

[K1JJ]

Hi Jim,

Yep, there are a lot of PW possibilities to try.  My favorite is the 1930's 24G.  Wish I didn't tear it down years ago.


Mike/ZE:

Keith/ WA1HZK has a pair of 833As by a pair. He says they need BIG drive to get positive peaks. In fact he uses a 4-400A GG IPA driven by a DX-100 to get the necessary drive to his pair.

How much power and what grid current do you use for your pair?   I was thinking of using a single 813 in GG as an IPA for the single 833A..   It's slick that he uses no input or output tuning for his 4-400A. Just feeds the cathode and the output feeds the grids of his 833A where the tuning begins.



http://www.criticalradio.com/833%20Rig%20Project/Web%20Pages/Schematic%20Diagrams%20Page.htm

Look at "Article in Microsoft Word"

T

[kb3ouk]

That huge amount of drive power is only needed on the positive peaks, I suggest doing what Gates did in the BC-1T. They used a pair of 807s to drive the grids of the 833As. The 807s were modulated 20% by a tertiary winding on the modulation transformer. Since the driver and final are modulated in phase, there is a boost of power from the driver at the same instant that the final is putting out peak power. Since you are only planning on a single 833A, it should be even easier to drive like that.

[K1JJ]

That huge amount of drive power is only needed on the positive peaks, I suggest doing what Gates did in the BC-1T. They used a pair of 807s to drive the grids of the 833As. The 807s were modulated 20% by a tertiary winding on the modulation transformer. Since the driver and final are modulated in phase, there is a boost of power from the driver at the same instant that the final is putting out peak power. Since you are only planning on a single 833A, it should be even easier to drive like that.

Yep, I read that in Stu's thread.  Wish I had one, but there is no tertiary winding on this mod transformer.   The alternative is to modulate the FT-102 with some audio.  Though, I don't mind adding a simple GG class C 813 as an IPA for more drive.

It seems there are always trade-offs in every design. This is the price we pay for no screen / modulating choke.

T

[K5UJ]

That huge amount of drive power is only needed on the positive peaks, I suggest doing what Gates did in the BC-1T. They used a pair of 807s to drive the grids of the 833As.

807s seem like big boo boo.  BC1-T used them (Value Engineering).  read this:
http://www.oldradio.com/archives/hardware/Gates/1T.htm

BC1F used 813.  RA-1000 used even more scrote. 

[kb3ouk]

I'm not saying it's necessary to use the same tubes, I'm saying to modulate the driver like they did, that way the driver is putting out peak power when the final amplifier demands it. You wouldn't have to run an 813 for the driver, that's wasteful since you only require the huge drive power that it is putting out on the positive peaks. Or if you want to get really good peaks out of it, run the 813 as a modulated driver. It seems to me that by doing that it would improve the linearity, since the grid drive will be tracking along with the modulation (higher positive peaks=more drive, higher negative peaks=less drive).

[W2ZE]

Tom,

I use the same IPA in the big rig. I use a GG 813 IPA with a 2K dropping resistor, that drops the B puss down to about 1600v on the 813. Its simple, and 1 tuning control for the IPA. It's link coupled off the tank coil to the grid circuit of the 833a's.
Kieth is right to an extent. We have had this argument in the past. You need a lot of drive, but with the 813 GG, you get plently if you run a single tube. I ran a kenwood TS850 as the exciter with about 10-15 watts key down, well within the capability of the ricebox. The 813 at that voltage easily drove the pair of 833a's to 150 mils. You are going to need 75 mils of grid current. Use a combination of fixed bias of -80 volts and a 5k grid leak. The grid leak with drive will cut the tube off well past 4x times cutoff (needed for class C PM), and the -80 will let the tube conduct a small amount if drive is lost to provide the mod xfmr some sort of load before safety circuits cut in.
I can't wait to hear it actually. You might have the got the ol' juices flowing for me again.

Carl,

I have a HB single HF-300 modulated by a pair of 805's. Strange enough for you?

[KM1H]

807s seem like big boo boo.  BC1-T used them (Value Engineering).  read this:
http://www.oldradio.com/archives/hardware/Gates/1T.htm

Nope, the "value engineering" was the 6BG6G's used in early models and they had to go back to 807's which were fine. The sweep tubes didnt hold up well at 24/7 balls to the wall.

Carl,

I have a HB single HF-300 modulated by a pair of 805's. Strange enough for you?

Not strange at all, I like it. My very first attempt at an amp in 57 was PP HF-250's and 100TH audio. The HF's were from a scrapped diathermy machine and soft. The next try was PP 250TH's and 810's for audio, pretty conventional back then and used mostly WW2 surplus from AFMARS at Mitchel Field

These days I have a pair of HK-354's modulated by 8000's which was used often a few years ago before I started playing with linears.
The other amp is/was 250TH's with 304TL's which is apart for 6C21's and Im still contemplationg the audio. Too busy doing customer work these days.

Carl

[ka4koe]

Dern son, you must not have younguns running around the estate!!

Ever thought about building these suckers on commission?

[K1JJ]

Or if you want to get really good peaks out of it, run the 813 as a modulated driver. It seems to me that by doing that it would improve the linearity, since the grid drive will be tracking along with the modulation (higher positive peaks=more drive, higher negative peaks=less drive).

Yep, trying various techniques to improve linearity is on the list.  While monitoring for IMD and THD, adjusting the drive and/or modulation of the driver stage can make it cleaner.  From experience I've found that simple drive levels, screen voltage, loading and other tuning can change 2nd harmonic THD by as much as -15dB on a plate modulated tetrode, using a spectrum analyzer.  You should see the THD blips creep up and down. These are side splatter levels, really.

There's an old thread on this BB where Dino / WA1KNX adjusted the amount of screen modulation on his plate modulated DX-100. (6146s)  He was able to find the sweet spot and the rig did sound choice on the air.  I plan to do the same thing here with Fabio II and Rico Suave.

After all, whose to say that when sticking a 20H screen choke into the circuit is optimum? Chances are it could be better with some tailored R/C network work around the choke or screen dropping resistor.

Same with triodes - picking a "20%" driver modulation level. Maybe 13% or 26% is optimum. We don't know until we do some tests and tinker around with our particular parameters and situation.  I would think all users of plate modulated rigs would be curious to know if theirs' is optimized.

Mike/ZE:  I might do a GG 813 driver and then look to add some modulation to it. Either more drive or driver modulation will do the trick. It will be interesting to see which technique works best for THD.

T

[W3GMS]

Tom,
If your looking for a rig to give 200W out, how about a single 813 modulated by a pair of 811's.  If you want to go up a step, your old pair of 813's modulated by a pair triode connected would be great also. 

Remember much above 100% causes distortion to many that are not using sync detectors.  I  have always preferred a well processed 100% and then leave things alone. 

As has been mentioned, lots of choices and all have their set of merits!   

Joe, GMS   

[W2VW]

They got Joe too ''''''

[kb3ouk]

The 20% number seems to be mentioned often, that was how much driver modulation Stu was using in his GG amp, that's also how much Gates used in the BC-1T. From what I've saw of Dino's circuit, he was not using any screen choke at all, what he was doing was using a dropping resistor before and after the mod iron and then combining the voltage to get the optimum amount of screen modulation. I believe he was using 60% unmodulated voltage and 40% modulated voltage.

[K1JJ]

Dino's experiment would be something I'd like to try sometime if I ever used a screen dropping resistor.  Using a single 813 at about 2200V would mean about 70 watts of heat in the screen resistor. That wud be my heat limit before switching to a choke.


T

[K1JJ]

Ya know, second thought, Mike / ZE,  I'll bet you could try modulating your 813 GG IPA driver.  You already have some HV DC to the plate now.  Then find the in-phase tap off the modulation transformer and sum it in too.   A big 100 watt power resistor might do it. Maybe need less drive. Look for the best THD and linearity.

Would the connection between the mod xfmr and the 813 need to be thru a 2uf HV cap?

I may set this up myself once settled on a plan.

T

[W3RSW]

Thomas,
What happened to all those 4cx 350's with their very low IMD's?  ( I'm a comma man and proud of it!)

In the class C, Rf chain, one as a driver feeding a pair in PP.
In the audio class AB chain, one as a driver feeding another pair.

Plate Voltages on the drivers about half the finals or what ever yields the best transfer characteristics.

If you've got a ton of 'em, then double up everything, PP parallel, etc.

Yours,
-Ricardo (not really related to Rico this go-around, but oh so envious...)

[W2ZE]

Its something to think about Tom. Right now the rig is literally a shell of its former self. I still haven't assembled it back together from the move 6 years ago. The HF300 rig is on the air, but the antenna came down this winter and I haven't messed with it at all. Very little time these days, My main focus on hobbies is fly fishing. Have debated over the years whether to do an INR flush or just hang on to it all. In the end I always tell myself that I'm not hard up for the money and I will return to it someday.

For the big rig, the priorities are as follows:

get it on the air.
build low power exciter driven by QS1R/QS1E combo
build/install GFZ driver
then.....modulate the IPA

[K1JJ]

Mike,

Yeah, I hear ya - it comes and goes in cycles here too.  

Hopefully you'll get that 833A rig back up and running.

Cool on the GFZ driver and fly fishing.


The dream of designing and building is a lot of fun.  Actually, after the rig is done, operating can be anti-climatic sometimes.

What we need is for some of the old crowd to get back on and mix it up again. Still, there's a lot of new people on too. I plan to do some coast to coast in the future and also see what's happening locally on 75M.  Hope you hear the action and jump back in.

Spent a few hours up at 140' today getting up the rest of the WNW 2el 75M wire Yagi. Ready to hook up the coax.  Hot, sweaty and windy. It's getting scarier and harder to work up there these days. If we don't do it often, the mind plays tricks and can get us thinking like a wimp sometimes...  :-)


Later -

 
T