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Author Topic: Low output on SSB with Drake TR-4C  (Read 16826 times)
KE0BKQ
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« on: October 27, 2014, 08:48:24 AM »

I've recently been lent a Drake by a local radio club to help "get on the air". I'm a fairly new licensed General. Tube radios are a bit before my time and I have had some help in learning how to tune the radio itself. 

Basically, I believe I have it tuned ok, the output power is about 80-100W if I use a CW key. I found a high-Z microphone that is supposed to be compatible with the Drake as well. When I key the mic and talk, I can see the watt-meter track with my voice between 5-10 watts or so. Turning up the xmitter gain seems to have little effect.  I don't want to do anything wrong that might damage the rig, for obvious reasons, so I'm a little hesitant to mess around with much more on my own.

It could just be I don't know what I'm doing at this point. Am I doing something wrong? or could there be another issue that needs to be address? If so, where might I start looking. 

Thanks!
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G3RZP
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« Reply #1 on: October 27, 2014, 10:01:57 AM »

Unprocessed speech has a peak to average ratio of around 9:1. Do you get around 80 to 100 watts on a whistle? If so, what you are seeing is probably about right - if you had a peak reading wattmeter or an oscilloscope, you could check with that.
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KE0BKQ
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« Reply #2 on: October 27, 2014, 10:45:01 AM »

Thank you for the suggestions....
1) if I whistle or use a tone generator, I get max out at maybe 20watts
2) if I blow right into the microphone I can get about 30-35watts peak
3) cw key output 120 watts
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KH2G
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« Reply #3 on: October 27, 2014, 12:19:44 PM »

Be sure you're using a peak reading meter and you can try running the mic gain up a bit but shouldn't need to max it out by any means.
Dick KH2G
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WB4SPT
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« Reply #4 on: October 27, 2014, 12:36:45 PM »

I kinda just went thru this with a T4X, which I believe has a similar mic gain circuit.  
It rather sounds like the mic might be a bit soft.  Curiously, I have a couple of Shure mics, each with a HighZ/LowZ switch.  Both mics required "full up" mic gain to make a QSO about normal.  Turned out, both mics had been modified to disable the HighZ function.  
So:  can you actually measure the audio voltage out of the mic?  It doesn't need to be plugged into the TR4.
AND, what mic do you have?  Some are fairly sensitive to shock and age (crystal).
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KE0BKQ
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« Reply #5 on: October 27, 2014, 12:50:02 PM »

I don't have a lot of test equipment, but it appears the mic puts out about 50mV at its highest from a quick reading on a voltmeter.

It's a low impedance mic with a low to high impedance transformer added. Here's the actual mic:
http://www.ebay.com/itm/321408938899?_trksid=p2060778.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Someone has also suggested the pre-amp tube might need to be replaced (I think it's about a $10 part)
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WB4SPT
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« Reply #6 on: October 27, 2014, 05:10:36 PM »

My Shure 444 develops a bit over 300mV rms when a loud "four" is just an inch or so from the grill.  Even so, my Drake mike gain is about 1/2 to 3/4 up. 
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G3RZP
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« Reply #7 on: October 28, 2014, 12:39:53 AM »

If you can't get the full output off a whistle, the first tx AF amp probably needs looking at. Worth checking the voltages on the tube socket to start with.
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KD1I
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« Reply #8 on: October 28, 2014, 12:11:04 PM »

I think WB4SPT hit it for you.   50 mV is really low for this vintage equipment.   Of course, looking with a meter is not the best way but I have an old D-104 that puts out an easy 150 mV peak speaking normally - into an O'scope so your mics output is probably a bit low. Can you borrow another high impedance mic and try that?    73, Jim
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KE0BKQ
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« Reply #9 on: October 28, 2014, 04:57:23 PM »

I'm going to try to put some of my General studying to use  Wink There's a 90% chance I'll mis-calculate something though..

Let's say I have an audio mixer that puts out about 2.4V peak of line level audio. I want to get that down to 150mV peak, so I need to reduce it to about 6.25% of the power.  If I do a power calculation, that's about 12dB of attenuation. So if I make a 12dB pad  ( I think I can use a 3.3K resistor in series with a 1K resistor in parallel) , would something that that work?   Will the pre-amp be happy with any 0-150mV source ? 

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KE0BKQ
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« Reply #10 on: October 28, 2014, 05:09:55 PM »

Ok.. It just hit me in the shower (like all ideas seem to do) that I was confusing "power" and "voltage" so I probably did mess up the calculations.

But also I'm wondering if it is significant that turning up the transmitter gain knob while talking seems to have no effect on the power output of the radio.

I don't know exactly what tubes do when they go bad, but I guess it's plausible their output can be limited, but still functional, in a bad one.
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AC2EU
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« Reply #11 on: October 28, 2014, 06:09:28 PM »

I'm going to try to put some of my General studying to use  Wink There's a 90% chance I'll mis-calculate something though..

Let's say I have an audio mixer that puts out about 2.4V peak of line level audio. I want to get that down to 150mV peak, so I need to reduce it to about 6.25% of the power.  If I do a power calculation, that's about 12dB of attenuation. So if I make a 12dB pad  ( I think I can use a 3.3K resistor in series with a 1K resistor in parallel) , would something that that work?   Will the pre-amp be happy with any 0-150mV source ? 


The simplest way to do it is this:
Put two silicon switching diodes in opposing directions across the rig  input for protection/limiting. 1n4148???
On the mixer, turn the mike padder ( if you have one), channel pot, and main to zero. Now you got nada.
bring the main to 25%
bring the channel to 25 %
adjust the mike pad for best results, this may mean all the way up on a low output mike.
continue to adjust the channel as needed.
Lower mixer settings = less noise let the mike do as much of the work as possible

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KE0BKQ
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« Reply #12 on: October 28, 2014, 08:14:39 PM »

Sure... why try to make a theoretical calculation when turning a knob can do  Wink
I started experimenting with it and I think things are in the right direction. It definitely is outputting at a more reasonable power now. I ran a PC with RTL dongle on the other side of the house and recorded some of the audio and it didn't sound bad except for some static right when I keyed up.  I'd much rather use the mixer as I can use a nice condenser mic I have for some pro-audio stuff I did a while back.

I'll have to play with it some more and see if I can find a local radio buddy to help test and give me an audio report.
Thanks for all the help!
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WB4SPT
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« Reply #13 on: October 29, 2014, 01:28:10 PM »

It will be near impossible to damage the TR4 mic circuit, no matter the input voltage.  There is a 47k ohm res. in series with a .001 uF to a tube grid.  So, inject away! Wink  Welcome to the tube world.
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AC2EU
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« Reply #14 on: October 29, 2014, 07:06:55 PM »

Sure... why try to make a theoretical calculation when turning a knob can do  Wink
I started experimenting with it and I think things are in the right direction. It definitely is outputting at a more reasonable power now. I ran a PC with RTL dongle on the other side of the house and recorded some of the audio and it didn't sound bad except for some static right when I keyed up.  I'd much rather use the mixer as I can use a nice condenser mic I have for some pro-audio stuff I did a while back.

I'll have to play with it some more and see if I can find a local radio buddy to help test and give me an audio report.
Thanks for all the help!


If you are using a modern condenser mike, you won't need a preamp, but you will need a battery, resistor and an isolation capacitor. I doubt that you will find 5v to run it on that rig!
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