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Author Topic: xmtr gain on Drake TR-4C  (Read 591 times)
WU3D
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Posts: 135




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« on: November 12, 2017, 12:38:11 PM »

Im running a D-104 non amplified mic on a Drake tr-4c radio. I have to run the xmtr gain up all the way to get good audio response from the mic
and to get good power output on modulation. I suspect I need to replace at least the Mic Amp tube { 12ax7 or 12ax7a} tube. What other tubes
deal with audio output that may need replaced with stronger tubes ? The final tubes are in very good condition !!.
Thanks
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WW3QB
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« Reply #1 on: November 12, 2017, 06:59:26 PM »

I use an unamplified D-104 with my TR-3. It could be the audio amp, but before swamping tubes I would try another hi-impedance microphone. When running barefoot, my gain control is set to 12-1 o'clock for full modulation.
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WU3D
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« Reply #2 on: November 12, 2017, 07:05:29 PM »

Yes my D-104 is also unamplified. I will look for another audio amp tube and also a new driver. I wonder if I should use new Chinese tubes or NOS like RCA or another American brand.
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KE4OH
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Posts: 131




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« Reply #3 on: November 13, 2017, 07:13:00 AM »

A few thoughts ... The crystal element of your D-104 might be at the end of it's days. Definitely worth trying another high impedance mic. FWIW, Heil sells a replacement dynamic element with an impedance transformer.

Before you do tube swapping, get your ohmmeter out and check all the resistors in the audio chain. The old carbon comp resistors tend to drift high.

As to the audio tubes (12AX7 in your case), there are Chinese, Russian, and Czech factories making these. Some years ago, quality was a problem. That has largely been addressed. No need to pay big $$$ for new old stock (NOS) US, European, or Japanese tubes. Good used tubes are also a viable choice.
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73 de Steve KE4OH
K1DA
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« Reply #4 on: November 15, 2017, 03:06:19 PM »

You don't need an amplified mike but there are many sources for FET "source follower" circuits which will match the 104
mike element impedance to the input if a TR 4.  Most can be built right into the base of a D 104 ...which will hold a 9 volt battery as well.  With proper matching you get a LOT of gain.   
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WU3D
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« Reply #5 on: November 16, 2017, 04:58:10 PM »

Thanks guys !
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WA3SKN
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« Reply #6 on: November 17, 2017, 08:15:59 AM »

Before replacing tubes, I would apply a 2-5 mV signal at 50,000-100,000 Ohms impedance to the mic input jack and see how the modulation is  Those crystal mics were known to get moisture in the and have low outputs over time.
Any test eqpt available?

-Mike.
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WU3D
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« Reply #7 on: November 17, 2017, 06:52:40 PM »

Ok Mike. I have some tubes here tho that I can try. I still think I will look to buy new 12AX7A Chinese tube as well as a new 12BY7A Chinese driver tube.
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WU3D
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« Reply #8 on: November 18, 2017, 06:08:07 AM »

Can I replace the 12AX7 tube with a Electro Harmonix 12AX7 EH tube ?
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WA3SKN
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« Reply #9 on: November 18, 2017, 07:36:59 AM »

I lost my tube reference a while ago.
The 12ax7s should sub OK.
The 12ax7 is a high mu dual triode while the 12by7a is a sharp cutoff pentode... as I recall.
73s.

-Mike.
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