813 maul note to K1JJ

[W3RSW]

Tom,

I and I'll bet several others, sure want to thank you for posting a great circuit; simplified and condensed from years of AM experience.  I utilized many of its features, not the least of which was the -75 volt fixed bias.  A lot of literature says that biasing method coupled with the remaing from 'grid leak' generates the least distortion.  Several places I deviated including adding an additonal capacitive load on 160 with separate ganged band switch section, switched input turn links for each band in the grid circuit, utilizing a slight amount of cathode bias via a zener, modulating directly through BC-610 iron (no Heising for now, plan for later when I can obtain a decent choke and HV, 2uf cap.) and other smaller changes.

But overall I stuck with the plan; what a great circuit.

I keep forgetting but several recent references to your 'maul' finally reminded me.  Someone's probably already brought it to your attn. but the screen and grid current meter polarities are reversed from what they ought to be on the schematic that's credited to W2WDX.

Thanks again & 3's.
Rick 

[K1JJ]

Hi Rick -

Thanks so much for the kind comments.

Yes, there are a few changes I would make to the original schematic, though they are minor.  Steve/HuzMan wud have to do it on his site.

OK on the meters being reversed in polarity... :-)   Gotta fix that.

Another change is the neutralizing variable cap to ground should be more like .001, not 300pf.  In fact if could be a fixed cap and the neutralizing adjustment would then be the distance of the sampling plate from the 813 plate.  To set it, just turn the fils on ONLY, key the rig and sample some rf with the scope off the loading cap. Then adjust the sampling plate distance (or size of the plate) until this feedthru voltage is minimum on the scope - that's it, all neutralized and set for good.   Many 813 rigs don't even bother to neutralize, but get away with it, but on the higher bands I feel it's needed.

I would also add Lentz chimneys and air to the 813's for better cooling.   I have them on my latest 813 rig, as you know.

BTW, on the fixed biasing... I usually put a zenor to regulate the fixed supply nowadays to keep the RF rectified drive from charging up the caps. On my 4X1 PDM rig, I was going from -90 volts up to over -400  from the charging effect!  It was blowing out caps before I knew what was happening.  A stiff, lightly regulated zenor solved the problem. I use the fixed supply simply as protection in case of lost RF drive to prevent the finals from dissipating heavy power cuz of no self-bias normally produced by the RF drive.

Probably a tune up procedure as well as basic troubleshooting should be added to the schematic. I get at least an email a month from someone one has built one and needs help setting up the various parameters. Though, everyone who has built one has been successful in the end, as far as I know.  Bob, W1RKW, as a new big rig builder, is the latest one I know of and has done a great job getting his 813 rig built and tested.

I wonder if you could post the parameters you run your 813 rig at?  Like all cuurents, power out, drive power in, modulation positive peaks, method of audio limiting, problems, etc. and any other info about tuning, etc. We tend to assume everyone knows this stuff, but it is often the first time for many.

Later, OM.

T

[W3RSW]

Sure, be glad to.  FWIW, your schematic added impetus to get the darn thing started.

-It will take a litte while to write up something.  I usually make up an op. 'manual' for the rigs I build.
btw, the rf input circuit bypass I used was 500pf/1kv  (2ea. 1000pf/500v micas in series.)  I figured the 300pf you orig. thought of came from an old single 813 circuit somewhere. My neut. cond. is a full plate scrote 2 to 15pf Johnson, I think. Had it so used it.

Where to begin.. so much to designing a class C stage. In the neutralizing example,
C2_neut./C1_{grid ground bypass} = C_gp/C_in.
For two 813's in parallel, C_in= 32.6pf and C_gp= 0.5pf
so if I pick C1 as 500pf, C2 or C_neut works out ot be 7.65pf.
The actual neut. capacitance, of course, was found by feeding rf into a loaded up amp without plate and screen voltage and adjusted until the feed through was a minimun but with the above calc's I new the Johnson 2 to 15pf cap. was ok.

As for the pi-net I used WA2WHV's java calculator, (air variables) which I found to be surprisingly close, even the turns calculator by him for the inductor.  I have sufficient extra turns (last 4 or 5 now strapped together) to take the plate voltage up to 2200 later on from the 1600 existing .  Interesting that his calculator includes a choice for air var. or vac. var. tuning capacitor.

Just looking at these two items mean a detailed description will be lengthy.  I've gotten two whole check off sheets of things I had to re-visit, fix and /or finish after first HV turn-on.  Relays didn't receive, neut. retuned, final taps to be made on coils, input link turns made variable, .....you name it.   Not to discourage anyone but once you take the plunge half the fun is getting it working well.

But as you and many have mentioned before, half the fun is in using what you have as long as the values exceed voltage ratings, etc., e.g., my plate coupling cap. to the pi-net is two 680pf/15kv doorknobs in parallel rather than the 500pf /5kv shown. Reactance at 160 seems to like the extra capacity.

Well, I guess we could go on and on but the object is to collect parts, call for help when needed, be open to suggestion and get started.

[K1JJ]

Excellent info, Rick!

Yes, the neutralizing cap formula spells it out nicely once it's thought out..


What are the operating parameters you run the rig at full strap and medium strap... (currents, voltages) drive required, and modulation % peaks obtained, etc?

I would like to put together all the threads related to the 813 rig into one area on this site for future builders reference. Yours will be included, of course.

T

[W3RSW]

Tom, more stuff in the musoft write file attached. Same stuff originally written 'here' so the subscript stuff is 'out lout.' Have to make up my mind as to a format and stick with it.

So I'm running the amp at the "lower" voltage setting now, aout 1600 volts. Same voltage on the plates of the 811 modulators. I've done calculations for 2000 volts, pi net, etc. and allowed extra coil stock (now strapped) for the higher voltage. I probably also later will use triode connected 813's or 572B's as modulators and feed them 2000 volts from same supply.  Main pwr. xfor. is a P-Dahl rated at 3000v. 1 A, CCS. I'm currently feeding 120 into the 240 primary, choke in negative lead. Way overkill but originally bought it for my old 4x1 "lineeearr."  I may have to dig up something else to get 2000.  I think 3000 will be a bit much...

Here are op. conditions on 3715 kHz.

P_dr, input drive to 813's at 12 watts.^*
Plate current is part of cathode current as follows and I_p = I_{k - Is} - I_g.
I_k, cathode current meter, 400 ma.
I_s, screen current, 59 ma.
I_g, grid current, 25 ma.
I_p, calc. plate current, 316 ma or 158/tube

E_s, screen voltage, 345
E_p, plate voltage, 1590
P_in, plate input power calc., 502
P_out, power output on Heath guessmtr., 375
Eff = approx. 74.7%

I'm feeding the 811's with the Grammer surplus 20 watt output transformer backwards, of course.  This tranny has multiple taps on secondary, 4 through 500 ohms so some combo yields about 2k ct to the grids of the 811's.  The audio amp is a RS 30watt xistor amp with EQ sliders, 2 mike inputs.  'Mike' mainly used right now is a D-104 w/ hb MFP102 source follower for inpedance matching. The gate of the MPF 102 has 16 meg. load from crystal element.  With that and the EQ, I'm seeing furnace rumble modulation when the damn thing kicks on here in the basement.  Looks to be about 20 to 30 cps.  I also have an EV 'speedball' of some sort, I think Shure, that has a 10k or 600 ohm transformer built in and XLR connector. It sounds way to bassy for my tastes but haven't screwed much with EQ for it.  The CBS volumax that I used in the 813 shoebox rig is currently out of circuit.

Modulation envelope kicks at least 150% on peaks. very brassy. Oh and I flipped the plate caps on the 811's for biggest spikes, (polarity.)  Did same on the 32V2 so that the D-104 matches both rigs.

Only problem is the audio feedback from the BC610 iron. I may have to sound insulate it some way. Fibreglass?  I have to angle the mike element just so to prevent high pitched squeal.  A lesson there, with our older ears and good practice anyways to always run the scope. Supersonic stuff shows up right away.

* Same as old single 813 shoebox, 'cause it had damping resistors across grid input coil/cap.

3's for now.
..still writin' away Rick and will send as stuff is typed up.

[N3DRB The Derb]

I remember the one concession I made to modern times when I built the Derb TX was zenering the bias to the 2A3  mod drivers at -62. If I could build something now, I'd use the grid leak/zener combo for sure. It's about fool proof. If you lose yer drive, the zeners clamp down and save your bacon.

could someone point me in the right direction to the D-104 MPF fet circuit? I'd like to build it into my G stand and be able to switch it in out.

 

[K1JJ]

Hi Rick,

Wow, that's quite the comprehensive write-up for 813 rig parameters!

The more detailed into like this, the more diversified examples for new builders to pick out what they need for their specific situation.  Nobody builds the rig the same with the same voltages or transformer ratios, etc,  so lots of examples are needed.

When we eventually condense some of these threads down, we'll have to put this info into one area on the site.

Way to go, OM.

73,
T

[W3RSW]

thanks Tom; I'll publish my exact coil stuff if someone wants it.  Hard to find pre-wound coil stock these days ; didn't feel I needed 1/4" Cu tubing at this point but using the java calculator I mentioned, it would be easy to find the no. of turns for tubing for a really strappin' rig as you've shown on your schematic.

Derb.
Pretty sure the MPF102 circuit is on Huz's page, "AM Window."  Also here's a quick verbal rundown.   Oh, and do not plug in the mike while rig's running. It tends to zap the JFET pretty quickly.  I bought a 10 pack for a dollar or two from Mouser. Have used 3 already cause they're so damn sensitive. Keep forgetting and do stuff like touch the PTT line to audio input by sloppy waving around the connector while rig is energized, etc.  -way stupid... if'n you know what I mean.

I didn't bother to make up the DPDT polarity reversing switch that's in TimTron's MPF102 circuit since I just switch mod. plate caps as req'd.  Saves a switch and an add'l 10uf output capacitor.  Also since my drain has about twice the load resistance as the source I get a little gain. I think Tim recommended 10k resistors in both source and drain.

So here it is.

The necessary unused PTT switch is already built into the D104 stalk for the 9volt battery. Batt. is off unless PTT switch is gripped.

Crystal element through 22k into gate of MPF102 and 16 meg. to ground there.  A 47pf mica to ground at gate too.  Source follower is 4.7k to ground, tapped off with 10uf/35v to audio output.  Drain is simply a 10k to +9 volts and 100uf/35v 'peak supply cap./filter' on the +9 rail, after the switch of course.

-thinking about a couple of paralleled diodes across input to keep spikes from popping the JFET but not sure about audio distortion above (& below) +/- 0.7 volts.  A crystal element may actually have peaks above this.   -Or so say some of the golden ear boys.   

Anyway as you know, a cathode or source follower has the beauty of very high input impedance and very low output impedance.  The low out imp. makes for little and rf pickup, etc.