1934 Gross TX - Could Use Your Input on Speech Amp and Oscillator Design

[W1UJR]

Hi Fellows,

I could use some help with a speech amp design on the 1934 Gross transmitter.
I've attached schematics so you can see the design I am working with.
The rig was built for use with a carbon mic, and I'd like get your suggestions on
using such, or modifying the circuit so it can work with a D-104.

For the sake of authentic, this is a 1934 rig after all, low power.
Just a type 57 as the 1st stage of audio, then a single 46, into a pair of 46s in the speech amp.

I thought the carbon mic might be the way to go.
But I have used a D-104 with the rig, however it needs to be the amplified version, and the speech amp gain has to be cranked right
up in order to get even a low level of audio out. I imagine that this is an impedance mismatch issue between the D-104 and
the speech amp design.

I can't recall hearing a carbon mic on the air, but I know that they are reputed to be very yellowy.
I have one of the older "candlestick" mics which I had considered using with this rig, thinking that
would be the way I'd like to go. I don't need "Hi-Fi" audio out of this rig.

What are your thoughts?

Next topic, the xtal oscillator stage is very finicky, wants to drop out of oscillation if
I load it too hard, and is sometimes hard to start with some xtals.
Wondering is this the nature of the design, or suggestions here?

Thanks,
Bruce

[W9GT]

Hi Bruce,

In order to be "period correct", you probably should stick with the carbon mike.  Those old ring-spring mikes are pretty cool.  Unfortunately, they certainly aren't hi-fi.  If you want to modify the audio to provide a little more soup for a D-104, you could add a stage utilizing a triode or a pentode, however, unless you built it as an external accessory, you might destroy the original character of the rig.  I would just go with the carbon mike and experience the 1930's audio that goes with it. 

73,  Jack, W9GT

[k4kyv]

The midtapped mic input transformer indicates that the original microphone was a double-button carbon.  Some of those old double-button ring mounted microphones actually sounded pretty good.  Not hi-fi by to-day's standards, but better than average communication quality and probably at least as good as the capability of the audio iron in the transmitter.  Some of the old ads tout frequency response from well below 100~ to beyond 3500~, and the push-pull configuration of the  double carbon button cancels out a lot of the distortion inherent to military type carbon mics and telephone mouthpieces. Carbon hiss may be noticeable, just as surface hiss is inherent to 78 rpm  records (some of which may not be surface noise at all, but hiss from the original carbon mics used at the recording session).

Not sure what kind of frequency response that transformer has, but I would keep it, and look for a real ex-broadcast double button.  They tend to be expensive on the "vintage" market, but one day you might eventually run across one at an affordable price or for something you are willing to trade.

In the meantime, I would build an outboard stage, using old buzzardly period components of the same era as the rig, something like a 6F5 triode or 57 pentode feeding a medium-mu triode like a 6C5 or 76, that works into a 15K to 500/600 plate-to-line output transformer, preferably something like the UTC A, HA or LS series.  You might not even need the first pre-amp stage at all and just feed the D-104 directly into the 6C5/76. That low-Z carbon mic input transformer should make a reasonably good line to grid input transformer.  In any case, avoid running unbalanced DC through it, since with the DB carbon mic, the button current was balanced via the common midtap.  Run the D-104 without the CB "power mike" attachment in the base; just feed the crystal element directly through shielded mic cable to the first stage of the pre-amp, using at minimum 5 megs of grid leak and preferably, 10.

[WU2D]

Bruce - You could put an old buzzard shield like a GOAT on that first MIC stage just for looks...

Mike WU2D

[KM1H]

Id just redo and keep the 57 stage as in any 6J7/6SJ7/6AU6 mike amp for a D-104, etc. The handbooks are loaded with examples and it should have sufficient gain. Leave the mike transformer in place for looks and enjoy decent audio. Carbon mikes sounded bad in the 50's and they havent got any better sitting in a basement for another almost 50 years.

Carl
KM1H

[Gito]

Hi Bruce

Since the audio input use low impedance input transformer used for Carbon mikes ,and you like to use a crystal mike has a high impedance output.you must use a Fet preamp to match this Impedance  (high to low).

A Fet preamp has a high input impedance and can be design with a low impedance output.connect this output directly to the input transformer .

I hoped it works.There' a picture of this Preamp.

About The crystal oscilator.

in theTT4 RCA tube hand book said.....when multigrid tubes having good internal shielding are used in crystal-oscillator circuit,It may be necessary to use external capacitive feedback to obtain oscillation.This feedback maybe provided by a small adjustable capacitor(ussualy no more than 2 or 8 microfarad)connected between  the grid 1 terminal and the plate terminal of the tube.
Under no circumstance should the external feedback capacitance be larger than necessary for oscillation,because even small excess values may provide feedback to destroy the crystall.

Regards

Gito

[N3DRB The Derb]

Bruce,

use a gimmick capacitor to do what Gito said re the xtal. take some period correct cloth covered wire and do a telephone linesman twist on it, creating a multi-value pf cap that can be trimmed downward as needed just by cutting the the twisted section off a bit at a time.

I did this on my 1939 Stancor 110-C and it worked fab. I doubled the number of xtals I could use.

No one looking at the rig could ever tell Gross didnt do it as long as you use the right wire. Manufacturers back then made all sorts of "adjustments" during a production run, undocumented most of the time.

on the microphone, got a National 'doghouse' power supply for the sw-3 laying around? makes a dandy period looking power supply and case for a single tube mic gain stage.     use the former power supply connector in front to plug the TX into the outboard pre amp.  Use another 57 inside.

[ka3zlr]

So Let it be Yellowy...That's "Class"....Don't Junk that thing up by chopping it to So Called Hi-Fi- Crapola  Specs.......Look at that mike stage, it's a Bute.. all Tranny coupled...front to back...Man,.... you build the the correct mike system to feed that first Stage the correct way..using todays standards and that thing will sing  and of course adding a little "Wick" behind to produce some Sig-in-nal in the wire... always helps...that's where you add in... the Caboose....


I just read the part on your oscillator Bruce, Check the Current drop on it and maybe Stabilization issue there on tune up.. Not enough Umpph there...Seperate Supply maybe...?...



73
Jack.

[Gito]

Hi Bruce

Crystal used for oscillator circuit has diferrent activity

a high activity crystal
a low activity crystal
a sluggish crystal or inactive

That's why most of the crystal oscillator tubes used the proper capacitor feed back to control the excitation of the tube to maintain oscillation.

See the extra C (feed back capacitor) in the picture

Regards

Gito

[k4kyv]

Most of the old pentode and tetrode crystal oscillator circuits used tubes designed for broadcast receiver audio service - 47, 59, 41, 42, 2A5, 6K6, 6F6, 6V6, 6L6, 6AQ5, etc.  The shielding provided by the screen grid on those tubes is not complete because there was no reason for the manufacturer to go to the extra expense when it is not needed for the service the tube is designed for.  But this "deficiency" makes them excellent for crystal oscillators without any need for external feedback capacitors.

If you use a tube like the 6AG7, 2E26, 802, 5763, etc. you probably will need a small feedback capacitor because they are very well shielded for rf and provide negligible feedback internally.

[Gito]

Hi

Yes .it's true what Don wrote.

By reading Bruce Article,He wrote sometimes it's hard to start with some Crystal.

Is it because a "Low Activity" crystal, Don ?

If it  so,maybe  it need a" little  push" to  make it Oscillate (by using a small C as feedback/parallel with internal C plate to grid of the tube),or it is the nature of the design as Bruce wrote ?

Regards

Gito

[Gito]

Hi

Here is a picture a Crystal Oscillator using beam tetrodes like 6L6 that used  a feed back condenser  (C5),of the lowest value for stable oscillation

As we know the shielding of the screen grid it's not complete(6L6),still it use a condenser (C 5) to stable the oscillator

Regards

Gito

[Opcom]

I've fixed some old carbon mikes by whacking them on the desk. gently. It unsettles the carbon, that has settled over the years and no longer was responsive.

[w1vtp]

I've fixed some old carbon mikes by whacking them on the desk. gently. It unsettles the carbon, that has settled over the years and no longer was responsive.

Yep -- did that when I was using "loop modulation" many - many years ago.   Had to whack it once in a while so I could be heard.

[W1UJR]

Sorry for the delay in answering your replies fellows, it has been a very busy spring at the biz.
Sleeping, working, sleeping, little ray-de-o.

I've got the Gross back together now, so the next stage is to work on the speech amp.
I like the suggestion on "dynamically stimulating", read pounding the table, the carbon mic element, will try that.

Before I deal with the modulator, one of oil filled caps has sprung a leak, drip, drip out of the seal on the bottom.
Guess those 70 year seals didn't appreciate the heat from my soldering gun.
Oh well, these rigs keep you busy.

Working on another project this week, just taking a bit of a break from the Gross.
Picked this up about 5 years ago, never got it on the air.
Interesting thing, the builder never hooked up the plate lead to the 47 in the xtal oscillator.
Everything else is there, but no connection.
It has inspired me to build my own version of the rig soon, on wooden breadboard.
See below.

[w1vtp]

Might this be what you're looking for?

http://cgi.ebay.com/Roaring-Twenties-Gibbs-Double-Button-Carbon-Microphone_W0QQitemZ220422354530QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item3352323662&_trksid=p3286.c0.m14&_trkparms=72%3A1205%7C66%3A2%7C65%3A12%7C39%3A1%7C240%3A1318%7C301%3A1%7C293%3A1%7C294%3A50

Al

[W1UJR]

Neat mic, very appropriate, but not sure if it works.
Figured I'd use one of the 30s candlestick mics.

Got the 1934 46 rig running over the weekend.

Original builder had not finished, numerous problems:
1 - No connection to the plate lead of the 47 tube oscillator.
2 - Missing two connections on resistors on the 47 tube.
3 - Meter for plate current for 47 hooked up backward.
4- No meter shut for 47 tube, pegs meter
5 - Schematic incorrect in CQ magazine, no connection between plate of the 47 tube to the paralleled 46 RF amps. Embarrassed how long that one took me to figure out. Yikes!

Now making soup, but running off my RX power supply, so only 200v on plates, wants to see 350 on osc and 500 on amp.

I need to build up a period appropriate power supply and then back to the Gross rig.

[Opcom]

the talk of mikes has me wondering if a carbon element from a telephone could be subbed into a D-104. Indeed, a couple of junk D-104's and a couple of carbon mouthpiece elements back to back inside one '104 casing, with a grille on both sides. Call it a D-103 and have a repro plate engraved as such.

[VE3GZB]

W1UJR, what a beauty!! That will always be the kind of stuff I call electronics!