The AM Forum

I think this may be a timely issue for D104 users ... replacement OEM elements are essentially unobtanium .... existing stock elements have a WIDELY varying sensitivity and frequency response ....

well anyway Kobitone makes several elements that Mouser stocks in a past (2006) catalog ....I could not find it in their most recent (2010) catalog .... low sales volume tends to make things dissapear !

go to Mouser.com and type in 25LM024 as a part number .... a 2 inch crystal cartridge with tech specs is available for a very reasonable price ....the Collins Collectors have reported good results from Kobitone products

I have just ordered 10 of them and will report results ... oh yes, I have NO connection with Mouser except as a satisfied customer

dunno about them... last heard people were saying the kobitone was not quite up to the better Astatic cartridges... maybe the new model is better, we can hope.

But there were so many made that i'd just look for one at a hamfest or online I think...

             _-_-bear

Or a Bob Heil Element and you'll have that Heil Sound...  :)

Look there's a HC104 Element it's on this web page:

http://www.heilsound.com/amateur/products/hc104/index.htm

Why yes the HC104 for the D104 and only $70 Dalla and be part of the Heil Zound Team. 8)
So Don't Delay Call Bob Today.. :D

73

Jack.

Happy Campers here:

http://www.aralb.org/Heil/

Even Chipper was there

For a few Dallas less you can Rob an HC5 element out of your "Heil Goldline" and get the job done qicker an cheaper.

You would think now that Astatic has discontinued the replacement D-104 xtal element, that someone would be producing an after-market replacement that would be close to identical, both electrically and physically, to the original.

I think the original crystal element has better low end response than the Heil replacement, based on the Heil spec sheet.  With a high enough load resistance, the original microphone element is nearly flat to below 100~, and I recall reading claims from Astatic that its undefined "response" went down below 50~.

I wonder if anyone has ever seen one of the original early 30s crystal elements.  It apparently used a mounting screw in the back instead of the foam pads and glue used in later models.  The very early D-104 heads have a big screw in the middle of the back plate, but inevitably when you open one up, there is just a nut inside to hold the screw head in place, its only function being to stop up the hole, and the mic has a standard replacement element with the foam pads.  The original D-104 was designed to mount in a ring, suspended by springs, just like the old double button carbon mics, so the crystal cartridge must have been hard-mounted to the case, and the acoustical isolation was provided by the springs.

The early 30s ads from Astatic mention an "improvement" in the original cartridge a year or two later after the mic went into production, that was supposed to have better frequency response and fidelity than the original.  That may be when they made the change in the mounting configuration, since later "vintage" heads that still have provisions for spring mounting do not have the big screw head in the back plate.  But I'd like to see an original just to learn how they did it when the mic first came out.

BTW, the one I picked up on ePay a couple of years ago has been refurbished and provided with a stock Astatic crystal element that I salvaged from a junk mic that still worked.  I wired it up for high-Z balanced output, and now use it with the push-pull preamp I built last winter.

But I'd bet real money if you placed that HC5 element in there along with the rest of your collection of Mic's at the stand at your Station you'd get even betta response..an it'd B noticeable I betcha...These elements aren't brickwalled cuttoff for the headroom you enjoy Don you might B surprised.

You will notice a brightness  :)

Jack

I've tested a number of Kobitones into a very high impedance load and can report they are not up to the task.
Several very vocal X-perts report they are. Too bad the big lie doesn't work for frequency response.

There's just something unhappy about turning a D-104 into just one more Heil mic.  Like taking a vintage Rolex shell and sticking a quartz movement in it -- even if it does tell time better than the original ..   ;D

Grant/NQ5T

I think if any of my three D-104 heads fail, I'll convert them to a condenser element.

One thing I have notice that all three of my mic's have a different sound into the same rig and all three have different level outputs as judged by the gain control. I think I'll look at them again for the most pos peaks of each for the same gain setting.

yup Terry .... the 3 wire head connector makes using electrets ok for both interface schemes .... hard to run an electret as balanced output and they are very breath pop sensitive (need lots of foam)...

I've experimented with using walkman earphone / ear buds elements as microphones .... put 3 or 4 identical ones in series, properly phased .... reports received are that they work quite well and sound good ...will generate breath pops too (more foam)...

can even place a series capacitor as a high pass with the corner freq at 500 Hz to 1 kHz and it gives the rising high end response but with phase shift .... still has less voltage output than the crystal element eebts (everything else being the same)

I think the Kobitones got a 'good rep' more as being an available substitute than actually sounding as good as a D-104. The old 'something is better than nothing'. They work, just not as well.

It's been a couple years, but at one point Astatic was still repairing mic heads if you sent them in, but had stopped selling the replacement cartridges in favor of keeping the remainder for their own service. No idea if they still do, IIRC they got sole 4-5 years ago. Might be worth looking into.

Naaa the HC104 is a perfect fit he sends all the foam and components needed, and for those Ickems that just have to have eletrec condenser feed
he supplies the needed parts.

That man got it covered read the web site information is all there.

Bob is the man....

http://www.heilsound.com/amateur/products/hc104/index.htm

73

Jack.

I bought a handful of the various Kobitones from Mouser several years ago to try in an old Astatic 200 shell I have.   The results were pretty bad to my ear.  I tried the higher impedance 3/4" and the 2" and didn't like the sound of either as recorded without EQ on my son's Yamaha home 4-tracker.  I also put one of the small elements into a dead Peavey shell with the same unsatisfactory results.  It's been a while, but I think the sound was  very thin with almost no low end.

I bought these because at the time the buzz was that they sounded just as good as the real thing.   I can't see it but YMMV.

Harry, KT4AE
Maryville, Tennessee

I have had great success buying junky looking D-104's on ebay for like 10 to 20 bucks....

so far, out of the 3 or 4 I bought the original cartridges inside were fine!

Just a thought...

John KX5JT

That's a cool way to do it, I couldn't believe what the guys were asking for 104's up at Butler Haa I just walked on.. :)

But ya know I'm just as Hooked on antiques as most of the group is I'm thinking, but sooner or later the 104 mystique is
a Fad man they bring in backround noise some of the more better condition ones your fighting with sibilance and ya know this
is 2010 audio has marked it's ground along the way, a guy offers a chance to upgrade the thing I have done this prior to
Bob's offer for myself and a few friends and I've really had no complaints, I mean ya have quite a choice of elements.

I know there not cheap..but hey weight the difference mystique or better performance an keeping the Desk Mic look. :)

Time changes try new things.

73

Jack.

Sounds like you have not experienced a good working D-104 yet.

No I'm an Experimenter.. We Can dig it.

Congratulations.

How much is a box of Wheaties these days anyway?

Gotta Snack some Wheaties the man says:

Are you concerned about my Fiber Intake I eat a balanced diet that the Doctors give me to follow.

Is there anything else ..??

In the works of Tarantino, a predominant concept is the distinction between without and within. It could be said that the premise of neodialectic deappropriation suggests that language serves to disempower the underprivileged. If semantic precapitalist theory holds, we have to choose between the neosemanticist paradigm of context and textual rationalism.

The primary theme of the works of Tarantino is the failure, and eventually the absurdity, of subdialectic truth. Thus, Foucault suggests the use of neodialectic deappropriation to deconstruct hierarchy. Reicher implies that we have to choose between Debordist situation and subdialectic materialist theory.

Yes, I looked at several pages of google for a frequency response graph of a Heil HC-5 so I could post it next to that of a properly terminated D-104 and only found some specs which said 300-8K with a rise at 2K. Run-on sentences are my speciality.

Do you have access to such a graph?

I use air for my diaelectric material.


klc

Yessum. these catz got the answer for ayaething...if you can't build it or scared to it ain't my problem.
Dig that Quoting tarantino I feel important now Elated....

I refurbished the early model D-104 I got off the ePay auction that was posted on this site a  couple of years ago.  The element was bad, but I salvaged a good one from a junk D-104 I picked up from an antique shop.  I wired it for high-Z balanced output, using a 2-conductor shielded cord to feed the push-pull pre-amp I built last winter. Reports tell me that the balanced output D-104 standing alone sounds almost as good as the two mic combination I have used for decades.  The total resistive load on the mic is 20 megohms (10 megs of grid leak at each tube), but the plate current in each of the push-pull high mu triodes seems balanced and stable enough although this far exceeds the maximum recommended grid resistance per the RCA manual. The pre-amp unit has a built-in pre-emphasis curve in addition to the acoustical peak that comes with the D-104 crystal.

The early D-104 heads are 3/16" thicker than the ones made after 1937.  I have tried the same element in both heads, and the old thicker one seems to have slightly better low end, maybe due to the larger air cavity surrounding the element, or it may just be a placebo effect. But I would like to see one of the original first edition elements just to find out exactly how it was constructed and mounted. There may not be any still in existence (pre-1935) that actually works.

I use a slightly elastic fabric cord to suspend mine, instead of the springs.  I have a spring mounted desk mic (but I hate desk mics) and you can hear the springs vibrate whenever the mic is tapped.  It actually sounds kind of cool to pluck the springs while transmitting. The fabric cord (per PA0ASD) does a better job of acoustical isolation.

PA0ASD 1935 video at 2:15 (http://www.youtube.com/watch?v=qUE04hpwQ1Q)

Don,

 I can get through to you I built up a few mics with Bobs elements I have one an I passed out three to a few friends
of mine and they were surprised. So I make mention here an some guy gots ta axe me about my wheaties big Deal.
The worlds gona stop if I don't produce graphs so let it stop. I don't care it was an experiment I Made it turned out
well and there's some happy Ops out there with upgraded D-104's NO BIG DEAL.

But they wana dig on me man...Keep it up...I gotta get sum Wheaties man..Wooooooo

73

Jack.



 

I think you missed the point. It's not about building or being scared. The point is the Heil cartridge frequency response will lack on the low-end compared to a good D-104. If you don't want that low-end, then rock on. If not, your mod is of no use.

Now shut up and eat your Wheaties!

Who's busting your balls? Pointing out a technical difference is not busting your balls. It's getting info out there and letting people decide what's right for them. Choices. Kinda like having Wheaties and Corn Flakes in the cabinet!

All I can say here is that I have never worked a guy who sounded bad when he was using one of the old xtal D104s into a high Z tube rig.   

Unless the first pre-amp stage uses something like a 50K or 100K grid resistor.  You need at least 5 megohms.

I have that Heil kit. Its horrible. It was a waste of money. This might be "ok" if you where to hack up a D104 and install this for use with an modern SSB rig. Other then that, Its horrible.  Trying to match it to an old rig was again a waste of time even when I used the Matching transformer. Maybe you guys will have better luck then I did.  IN the end, I had a flat bass-less tone that I hated. I just got another D104 head at a ham fest, put that back on and life was good again.  Its in the Drawer if anyone wants it.

Bob is a real nice guy.  He is happy to help anyone.  But I just did not care for this product.

Also be aware of the replacement cartridges sold on ebay. These are nothing more then the EAR piece from an old Telephone. I have pulled them out of the ear section of junk phones, Its exactly the same as whats being sold.  The only reason these are good is your original element is GONE and you used the replacement to save the mic from the trash.

C

The grid resistor on the Eldico TR-1 is 10 Megs  Somewhere around here I have an Astatic JT-30 I got many years ago.  I hope I find it in the course of my on-going summer house clean out project (what I'm doing now while the bands stink).  I've looked at the xtal mics on ePay and there is simply no way to know what you are really getting.

If something ever goes wrong with my nuts I'll consider a mic with a 300 hertz corner frequency. Otherwise a properly terminated D-104 is the best mic ever made for voice over radio without needing tone control.

Seems some other radio service information is being applied here. Why would a D-104 pick up any more background noise than a Heil replacement element? Maybe if it were using that awful preamp marketed to part 95 people jacked into the front end of a good buddy rig.

I'll wager 10,000 hi hi fine buisnesses I can do a blind transmit test with a D-104 into an MPF-102 verses any so called replacement and get at least 90 percent on the listening end to prefer the tin can mic.

 

this space for rent

ditto

I have to admit, I'm flummoxed   ???

A D-104 is a D-104, and it's a classic, and an excellent mic, and you can still get good crystal cartridges (Astatic, not the imports).  A Heil something or other, is a Heil something or other -- and they're fine, too.  Except even the "wide range" HC-6 chops the low end (you'd be better off with a PR-40, that is if you bought it before the price was jacked).

I also own one of those "in the case" last production D-104's (that was sneered at in the "ebay and other" part of the forum, which just wasted a lot of disk space to make some kind of a point).  And I'm happy I do.  It's in the case, and in the box, untouched.

Why is this discussion even happening?  It seems to have turned into a Heil marketing thread.  If I want to sound like a parrot hitting the PTT switch I'll buy an HC-4, if not, I won't ..  ;D

The Kobitones aren't hiding at mouser, just search for 'crystal microphone'

Here's the data sheets for 2 models..... WITH GRAPHS!   ;D

http://www.mouser.com/catalog/specsheets/KT-400025.pdf

http://www.mouser.com/catalog/specsheets/KT-400023.pdf

What tube does it use in the first stage?  I can tell that it is a triode, but that's about all.

I looked at the schematic by following the link in the previous thread, but the folding crease in the original paper as it was scanned makes the tube type illegible.

In a day when D-104s are still plentiful as dirt, I don't think anyone needs to panic. Alternatives have always existed, some prefer something new and shiny from a wrapped. I bet I see 20-50 D-104s at the average 'fest. Most are cheap.

Never met Bob Heil, but from what I've gathered over the years he's a decent sort. And he offers products for the amateur community, which gets him more points in my book. Whether his products suit a specific need or not is up to the buyer to decide, obviously. I have to say that, to my ear, the Heil mics I've heard tend to sound thin or pinched and strike me as better for SSB work than AM, out of the box. When he started making visual knock-offs of the old Shure and EV mics from the 50s, it was confirmed in my mind. Most AMers would prefer the real thing, not a copy, even if they rebuilt the old mic with new guts.

D-104s always had a intentional presence rise which gave them a somewhat peaky sound. And, while not as 'fat' sounding in the low end, still sound better than any Heil mic I've heard to date. Outboard processing notwithstanding, you can make a carbon mic sound better with processing. ;)

Hello,

 The statement from the Left coast came in About Muddyness to much bass on the Air here hard to understand the Brothers on the East Coast.

OK.

Now at the same time Question comes in about a replacement head for the D-104 I'm thinking I put in my Mod that Bob offers it kills two birds it cleans up the audio for DX and isn't a bad replacement for the crystal head keep on hand for the DX contacts or change your settings on your processors this is whats on my mind then some Cat starts talking about Wheaties Huzman playen Father Bob an all hell breaks loose.

See I Know the Heads are removable put one on for DX put one on for Local isn't that so easy.. :)

 if Someone has an Idea that's bad VooDoo to the Bass Mongers...LOL it's Hiliarous in here..

But stilll no one has mentioned how good it would be for the DX to the coast it's Plus I'll say it again an again an again..
No We can't let Zed.L.R. put down our God D-104 that we grew up with Oh Nooooooo.. ;D ;D it's a CB Mic an that's it.too bad.


We can talk good mics here an we have many times the 104 eh left over...

Some of the Heil mics are designed to have good bass response.  I visit the Heil booth every year at Dayton, and talk with Bob on 75m AM from time to time.  One year he demonstrated a mic (don't remember the model #) but it had a switch that changed it from restricted to wide range.  I thought the wide range sounded much better, but of course, in a rig that inherently can do little or nothing better than space shuttle audio, the restricted mode would sound as good or probably better.

Heil also makes broadcast microphones.  Check out some of the current broadcast rags for his ads.

I might consider Bob's replacement element for the D-104 if it had the same low end  response as the stock Astatic crystal version. But I am not interested in the version that cuts off the lows below 300~.

Astatic elements for the D-104 are like many of our common transmitting tubes.  They don't make them any more, but there are enough good ones out there in existing mics that it will be a long time before the supply dries up, but used ones may get more expensive as time goes on.

Actually, we can probably thank the good buddy crowd for the abundant supply of these microphones.  As far back as the 1960s I  remember it was a common belief amongst hams that the D-104 "doesn't sound good on sideband".  So, if they hadn't come out with the CB version that has that crappy pre-amp in the base, the mic and replacement elements would no doubt have been discontinued many years before they were.

Not that long ago, almost half of the cars on the road sported a CB whip, and every other house had a rooftop CB antenna (probably one of the reasons why, by default, HOA dictatorships now prohibit outdoor antennas of any kind).  So there must be jillions of D-104s kicking around, and eventually many of them will find their way to garage sales, junk shops or kerbside on trash pick-up day, and of those, a significant percentage will have been stored in an environment that doesn't destroy the crystal.  Crystal elements go bad when kept in excessively hot and/or humid conditions, such as damp cellars or hot attics for extended periods of time.  Whenever I find one for a few bucks or can pull one out of a dumpster, I grab it just for the xtal element, which is good in about 65% of them, roughly the same rate of functionality as tubes picked up at hamfests.

I think the reputation for sounding poor on slopbucket is that many if not most pre-90s SSB rigs already had narrow, restricted, peaky space-shuttle audio, and when the resonant peak of the D-104 was added, the harsh, tinny result was almost unbearable to listen to.  Kind of like listening to a female voice on slopbucket.

If an AM rig has a muddy, bassy sound with a D-104, it has serious audio problems that extend far beyond the microphone, and would likely sound even worse with another mic, except perhaps with a military surplus or telephone handset carbon mic.  That was one of the intentional designs of the D-104 response curve: to avoid a muddy sound, yet still have some bass capability. Rolling off everything below 300~ doesn't cut it (no pun intended). Not the best microphone by any means nor guaranteed to be "high fidelity", but something that produces acceptable voice quality through a reasonably good audio chain.

Well I can't say alot of what you said I'll get looked at as making trouble again But That's Cool I'd rather be on the outside
I got nobody to suc up too..LOLOL... but Your right we've always agreed on these things I'm very Glad you understand it's
Great.... :) As far as the 300 cutoff you can ask Bob for the better one you like. the HC-5 ain't that bad it's not to brick wall.

Me thinks someone is into zeig Heil  ;)

Hi Mikey   ;D what a day.

<<<What tube does it use in the first stage?  I can tell that it is a triode, but that's about all.>>>

it's a 6SL7GT.  That drives a 6V6 which drives the 811As

I was guessing if I ever wanted to use low Z line level audio into the rig I'd clip on to the grid of the 6V6 (pin 5) but I'm just guessing.  I have not read about the tube and as a noob don't know much about it at all.

I have all kinds of plans don--the mod trans. secondary is 7500 ohms and i have a 65 H mod reactor I can try just to experiment, but after I get the rig working stock.  I also want to drive the rig Rf with a VFO--some browsers have a click to magnify feature that helps see the schematic but I don't want to hijack the thread--i was gg to say there are all kinds of D104s out there but they are not all the same quality.  check eBay to see what I mean.

R.

You make an excellent point there, Don. I thought of that after I'd left, the fact that, like using a restrictive receiver that limits high fidelity reception, you can do the same thing to any mic using a marginal rig. In fairness to the Heil mics, I've heard some RE-20s with outboard bass being inflated to the point they sound muddy and crappy overall. Also heard a few guys running old broadcast mics on SSB. If you ever thought SSB couldn't sound any worse....

The D-104 has been around as the standard for years, a baseline to measure other audio against. Not because it was the ultimate or best sounding mic, merely prevalent and as you point out, very acceptable audio.  Its place in radio history was sealed long before it became a favorite of the CB crowd, though the lack of 11 meter buyers was a main reason cited by Astatic when they discontinued the mic years bak (has it been 10 years yet?). That little 'power mic' amplifier in the base really gave them a bad wrap in later years.

Regardless, if you've got a full range voice, you really don't need anything beyond a rig with a clean, unrestricted (within reason) audio chain. If you don't, all the processing and fancy mics in the world won't change it.

Yes, Todd, it's been around since about 1933.  Decades before CB. Quite a fad, since by definition, most fads are short lived.

Mine is on a boom.  I have one on a desk stand, but rarely use it.  Neither one has an amplifier in the base.  Where would you put it? The desk mic base is solid cast iron.  ;D

Rob, the 6SL7GT is a high mu triode, not too different from what I use.  My single ended pre-amp uses a 10 megohm grid resistor, too, and the 1st stage is a 12AX7.  My push-pull pre-amp uses a pair of 6F5s in the 1st stage (electrically the same as a 12AX7) and each grid has a 10 meg grid resistor, loading the mic with a total of 20 megs.

That's a slick set up, Don. Your boom looks remarkably like the kind used for navigator's lamps in WWII aircraft.

My early D-104 has the nifty script tag and the screw in back, which IIRC like yours, has a nut on back and nothing more. Unfortunately I didn't get the eyelets with mine, so I need to figure out the thread size and install some. Then locate a large ring for it.

Haven't checked the element in it yet, though I recall the spacing material had dried up and was rattling around inside. Another project for later.

That could be what it was originally. It came from an old timer's shack, and it used to hold a lamp, but it was a regular 110v ac lamp with standard outlet plug and cloth covered twisted pair cord. I used it over my workbench for a long time, but I was always banging my head on the lamp, so I made a mic boom out of it before I managed to lose my cool some day and smash the damned thing with a sledge hammer.


They all had that tag with "Youngstown, Ohio" on it, along with serial number.  Wonder what serial number yours is.
I have another one with the big screw in back and the Youngstown name plate, but no holes for eyelets.  The early ads indicated that a version without the spring mount was available.

I forget the screw size, but I did try some small screws and found a size that fits the eyelet threads.  I think it was a #3 screw, maybe 3-40.  If you can't find any ready made ones, it should be easy enough to find a piece of brass rod the proper small diameter and use a die to put threads on one end, and then bend it into a hook shape to form a set of eyelets.  You could even chrome plate them.

The desk stand with the other microphone was originally used with a double button carbon mic, which I still have but doesn't work.  The springs were very rusty, but I ran into a mic collector on the internet who had some extra brand new springs that he had ordered custom made.

One of the eyelets in the boom mic is held in place with JB Weld.  While the mic was in the care of a previous owner, some idiot managed to strip the threads in one of the eyelet holes.



Don't toss it without checking it out first.  It could still be good.  The rochelle salt wafer appears to have been dipped into some kind of wax compound to hermetically seal it from the humidity in the air.

I won't toss it, Don. Just haven't gotten to it yet. I at least is here now, made it down in a box of mics in June. Once I dig it out I'll open it up and take a few shots, maybe try it out some night when the static subsides a bit more. Last night wasn't too bad, worked Long Island 20-40 over.

What do any of you know about the amplified D-104's? I took the plunge and purchased one on eBay.

If it still has the stock crystal element and the element is good, it's about the same as any other D-104 element. I don't think they changed anything about the crystal part when they added the "power mic" feature for the Good Buddies.

The amplifier in the base is a piece of crap.  Best to remove it and use the mic with high-Z output, or else build a homebrew one that doesn't distort the audio.  I think there are a few schematics and circuit descriptions floating round the net.  I also recall that Timtron came up with one.

The D-104 cartridge has a reputation that no two sound exactly alike.  Some sound good while others have been reported to sound crappy, brand new from the factory.

I was planing using it on a line in on some pro equipment for use with Skype.

Thanks I have a D-104 with a home brew amp in it and the schematic. That's what I'll do ;D

The line-in impedance of most modern equipment is on the order of 10-20 kOhms (sometimes even higher). Make the output Z of your preamp 1/10 this is all is FB.

I have a UTC Linear Standard Series audio input transformer that is rated at 100,000 ohms unbalanced primary to 500 ohms balanced secondary.  The catalogue says it is designed to match a crystal microphone to 500Ω line.  I don't know how well it would work with the D-104, but most crystal cartridges require at least 5 megohms load for any kind of decent low frequency response.  I need to try it out because I am not sure how a transformer load would compare to a purely resistive load as far as low frequency response is concerned.

A crystal microphone is said to be equivalent to an ideal a.c. generator with about a 500 pf capacitor in series.  That's why the high resistance load (>5 megohms) is required for good low frequency response. And I know from my own experimentation that working a crystal microphone into a 100K grid resistor results in Space Shuttlesque audio.

At least on pro gear, line inputs are designed to be bridging. Max voltage transfer is the goal so the line out Z on most pro gear is much lower - like 100 Ohms or less. This way, the output is not loaded and the frequency response is unaffected too.

This is why when swapping out interconnects on hifi gear can have a huge effect on the sound and volume level.  None of this gear is standardized. I realized this years and years go when I was into hifi audio.  Some guys think its the expensive cable but many times, Just experimenting with the length of that line will change the sound.


I would not believe what is written in a manual.  "600"ohms is just telling you its low :) 

I got a decade or multi resistor box at a ham fest. Plugged it in place of the Grid resistor.. WOW!!!!!  does that effect a D104's sound!. I could make that mic sound very low and bassy or very high pitched.  If you hear a D104 that is very high pitched, Its the rig and not the mic.. For example. Plug one into a Dx100 :)  I use it to match the D104 to the rig, Then I measure it at the radio and put that resistor in. Its often times very close to the sound you had with the box inline.

C

That 10M maximum has to do with the rig, not the mic.  If you use too much resistance in the grid circuit the stage becomes unstable and the plate current may wander, as a minuscule grid current adds an additional bias voltage to the tube. In the early days of radio, this was often done intentionally to provide "grid leak" bias, but proved to be too unstable for mass produced equipment. Sometimes the first amplifier stage in a low-level preamp may use "contact bias", with no cathode  resistor or grid bias resistor at all.  I seem to recall that circuit in some of the speech amplifiers described in mid-50s Handbooks.  The grid just floats, and the negative charge that builds up on the grid it what biases the tube. BTW, it is a misnomer to call the grid resistor of a class-A amplifier stage a "grid leak resistor" unless grid leak bias is actually used to bias the circuit.

For most tubes, the RCA Receiving Tube Manual actually recommends something like a maximum of 0.5 megohms of resistance from grid to ground, but this would be too low for a crystal microphone. When you go above 5 megohms or so, it may be necessary to select tubes for the first stage to find one that is stable. The way I do it is, with no signal input to the grid, measure the voltage across the cathode resistor.  Now short the grid to ground and measure again. Find the tube that shows the least cathode voltage change, thus the least plate current change, from the shorted to unshorted condition.  My push-pull preamp uses 10M from each grid to ground. Instead of a 12AX7, I use a pair of 6F5s, which have nearly identical characteristics to one section of the 'AX7, but the single triode allows individual selection of each tube in the first stage. The  push-pull configuration puts a total load of 20 megs across the mic. There is no need to use a grid blocking capacitor at the crystal mic input, since the crystal mic itself forms a capacitor and should measure infinite resistance across its output terminals.

If the mic sounds too bassy with the high grid resistance, that means the speech amplifier has insufficient high frequency response.  You need to do something to boost the highs, not cut back the lows with less grid resistance.  I use a simple method to generate a "presence rise", which consists of a specifically calculated low value of cathode bypass capacitance based on the time-constant formula.  This  low bypass capacitance causes the cathode to appear bypassed at higher frequencies, but at lower frequencies it acts as an unbypassed cathode resistor, and the degenerative feedback lowers the gain of the tube.  Using two tubes in cascade with the special cathode bypass gives me about 10 dB of boost at the high frequency plateau beginning at about 2 kc.  The rising response begins at about 800~ and rises steadily until it reaches that plateau. This presence rise is in addition to the resonant peak of the D-104.  By limiting the peak to the upper midrange, beyond 2K, this doesn't make the audio sound tinny as long as there is sufficient bass response to balance the treble.

Thanks Don for the succinct post.   I may try a version of your PP Speech Amp.  Thanks for the detail.   

73  Vic

I'll try to remember to draw up a schematic of mine and post it.

That would be helpful.  Thanks again !     Vic