[Milsurplus] Milsurplus Digest, Vol 146, Issue 1

Wed Jun 1 23:17:41 EDT 2016

Dave and Jim:

How about one of the BC546 series. hFE min 110 max 800?

https://www.sparkfun.com/datasheets/Components/BC546.pdf

Dave WA4OBJ

From: Milsurplus [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of
Jim Madden
Sent: Wednesday, June 01, 2016 8:08 PM
To: milsurplus at mailman.qth.net
Subject: Re: [Milsurplus] Milsurplus Digest, Vol 146, Issue 1

  Dave if you get any info on the High beta transistors let me know.

       I'm currently in the middle of building up a 42' T-17 metal body with
a Bell 500 phone element which i just measured @ 1170 Ohms. had to open up
the inside of the mic a smidge about .012 or so to get the element to fit
and had to turn down the Bakelite cover some what so it just touches the
element to hold it inplace I wasn't orginaly thinking of a booster but with
the small parts count I should be able to get it in there? did you take out
the original condensor?hope to get my GRC-9 on the air with it.Jim KB1PFL
> Message: 5
> Date: Wed, 1 Jun 2016 01:34:52 -0500
> From: "David Stinson" <arc5 at ix.netcom.com>
> To: <milsurplus at mailman.qth.net>, "ARC-5" <arc5 at mailman.qth.net>
> Subject: [Milsurplus] More on Bashless T-17 Microphone Resurrection
> Message-ID: <E45DB126B9974EDF9CAAE449B9749C30 at DaddyPC>
> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
> reply-type=original
> 
> So work continues on "resurrecting" 
> stone-deaf WWII T-17 microphones with little "booster" 
> circuits. I have 6 or 7 T-17s. One 1942 contract
> stock T-17 will adequately modulate a WWII radio
> (yes; I did double-check and it is actually stock, LOL). 
> The others are deaf as a post. Others have had luck
> whacking them around to "wake them up," but 
> I've beaten, bounced, bashed, baked and battered T-17
> elements for years. If I've ever gotten one to work 
> properly that way, I don't remember it. 
> 
> So far, I've been working-over mikes with missing 
> or unrecoverable elements (except the candle stick), 
> using at first an electret element and later using 
> a simple transistor "current booster" to amplify
> the currents through the weak carbon elements.
> Time to turn to a "stock" T-17 element 
> and see what can be done.
> 
> The subject T-17 is a stock 1944 contract with the 
> black plastic case. Used with the BC-669, held right
> to my lips and me saying "Foooooooore," this was the
> modulation waveform, which is typical of most stock 
> T-17 mics today:
> https://goo.gl/photos/ZeYyD6tjVKTvcDNn9
> 
> We've investigated and tested two very simple 
> circuits for "boosting" weak carbon elements 
> and both have worked well. 
> See my previous posts in this thread. Elements that
> measure low resistance, 60-250 Ohms or so, 
> should be installed in the Emitter lead of the NPN
> transistor. Those that measure high resistance-
> 2K Ohms or more- work best installed in the
> Base lead where they can be a large fraction
> of the total bias voltage divider resistance.
> 
> I removed and tested the stock T-17 element 
> with an Ohmmeter. It read around 1K- too much
> to go into the Emitter lead so I installed it in the 
> Base lead. Here is the circuit used to "boost"
> this element:
> https://goo.gl/photos/NSqmQrmimRvV9ySx5
> 
> Important: Note that the circuit above is almost
> the same one we used back on the 29th to boost
> the large "candle stick microphone" element.
> But there is an important difference.
> The carbon element in the Base lead
> is part of the resistive voltage divider coming from
> the Collector, bringing a positive bias voltage to 
> turn the transistor "on." The circuit works because
> the element acts as a variable resistor, varying that
> bias current and thus controlling the larger current flowing
> in the Collector-Emitter circuit, much like the small
> Grid current in a Triode controls the larger Plate current.
> 
> Because the condition of these weak carbon elements
> varies widely, the resistor "R" must be selected for 
> the individual element and for the transistor you use.
> The difference can be dramatic. 
> For instance- the resistor for the "candle stick" mike
> which used a lower-gain 2N2222 transistor (beta 
> tested 70) was 4.7K Ohms. This T-17 element 
> and a transistor with a beta tested at 110 required
> a resistor of 220K Ohms. The idea is to use as 
> high a value as possible and strike a balance between
> providing adequate Base current and loading-down 
> the microphone bias voltage provided by the radio.
> The easiest way to do it is to use a pot and tweak for
> best response and output.
> 
> With the little booster helping the original T-17
> element, this is what the "Fooooore" modulation
> envelope looks like now:
> https://goo.gl/photos/rUnhMAEixfyAx4Gp8
> 
> As with the earlier work, I prototyped the booster on
> a board and mounted it behind the T-17 element:
> https://goo.gl/photos/w6QUp3vy3C5rwZEdA
> 
> Boosting the carbon elements has been going
> so well, I've decided to set the electret subber
> aside for now. I get excellent audio reports 
> and I was never a fan of electrets anyway- 
> too noisy.
> 
> As I mentioned in a previous part of this 
> thread- the transistors I'm using are 
> unmarked NPNs that test with a beta of
> about 110. If anyone can suggest a common
> number transistor with at least this beta
> for this circuit I'd be grateful.
> 
> GL OM ES 73 DE Dave AB5S

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