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Author Topic: Class A Audio Amp for Drake R-4C  (Read 15123 times)
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aa5wg
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« on: December 01, 2010, 10:42:15 AM »

Hi to all:
Do you think this class A audio amp could be implemented into the Drake R-4C receiver?  Here is the link to look at the class A amp:
At present I don't have the schematic diagram.  The tube/valve in use is a Russian 6922.

http://cgi.ebay.com/Valve-Class-Tube-Headphone-Amplifier-preamp-audio-amp-/260697180301?pt=LH_DefaultDomain_0&hash=item3cb2c3488d#ht_5847wt_1140

73,
Chuck - AA5WG
Cedar, Michigan
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aa5wg
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« Reply #1 on: December 01, 2010, 10:55:44 AM »

Here is another class A amp with a different tube, 12AU7.

http://cgi.ebay.com/Valve-Class-Tube-Headphone-Amplifier-pre-Bravo-V2-/260697180184?pt=LH_DefaultDomain_0&hash=item3cb2c34818#ht_5958wt_1140


And, do you think this 3 band Graphic Equalizer could be implemented into the Drake R-4C?  Here is the link:

http://cgi.ebay.com/Bravo-Tube-Headphone-Amplifier-V3-EQ-Equalizer-valve-US-/260677485333?pt=LH_DefaultDomain_0&hash=item3cb196c315#ht_3975wt_1176

73,
Chuck - AA5WG
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Ralph W3GL
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« Reply #2 on: December 01, 2010, 03:26:24 PM »



   Why?  What's the point?

   The R-4C is a communications receiver with appropriate built in
   audio (although crappy sounding by limited pass-band due to its
   50kc IF stage !)...  In its day it was highly rated.

   Actually, out of the box, the R-4B was a better receiver than a bare
   bones R-4C... I have one of each here...

   You want to improve its utility, do a Google search for mods. There
   are lots out there that are useful and don't degrade its remaining
   value if it has a full complement of filters...[Think Sherwood (SP?)!]

   Actually, as I recall, the power transformer is fully loaded by the receiver
   as is and would be overloaded by a class A stage -eq's added load!

  

    
    
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73,  Ralph  W3GL 

"Just because the microphone in front of you amplifies your voice around the world is no reason to think we have any more wisdom than we had when our voices could reach from one end of the bar to the other"     Ed Morrow
WQ9E
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« Reply #3 on: December 01, 2010, 03:43:44 PM »

Ralph is right on target.  The R-4C has a "goofy" power supply system and the combination of deriving regulated low voltage for the SS stages from the B+ and using a inefficient and heat generating output stage are major weak points.   I presume these class A amps are outboard so they wouldn't load the internal supply further but they also wouldn't do anything (without modification) to reduce the existing problems.  In any case, HiFi is not something you will get from a C line without major changes to the IF and detector system.

With some fairly minor modifications the R-4C is still a very good DXing receiver and is fine for contesting except it doesn't provide rapid QSY and memory so necessary today.  I have a couple of C lines and the R-4C with an outboard Autkek QF-1A is still my favorite CW receiver for 160 and 80.  The crystal filters and electronic passband tuning provide good selectivity but are not as audio friendly as the early L/C electromechanical passband system used in the B and earlier lines.   The only time my C line has been on AM was to check into the local 160 meter net and to listen to WWV for time/frequency checks.

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Rodger WQ9E
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Brrrr- it's cold in the shack! Fire up the BIG RIG


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« Reply #4 on: December 01, 2010, 05:54:32 PM »



Better idea - take the audio out at the appropriate point before the internal audio amp (or perhaps even use a high performance AM detector circuit) and use an external audio amp and EQ if you wish...


                    _-_-bear
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_-_- bear WB2GCR                   http://www.bearlabs.com
aa5wg
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« Reply #5 on: December 01, 2010, 06:20:04 PM »

Hi WQ9E, Ralph and bear and all:

I was thinking of not using the audio section of the R4C.  Instead home brewing a class A approach.  What changes to the if and detector stages should I do to help obtain quality class A push-pull AM for the R-4C?

Thank you.
Chuck - AA5WG
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N5KAT
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« Reply #6 on: December 02, 2010, 07:52:52 PM »

Try dialing up the bias a little. They are really running the snot out of that AF power amp. When I reduced the current in the stock power amp, I got a far more acceptable result. Less distortion and less mechanical sound. Liked it more than IC chip amp replacements.

I forget the numbers but mess with it and see.

It will not be audiophile grade by any stretch. I had a lot of R4Cs and tried everything, including picking off the audio and running it to an outboard 2A3 amp and high grade speakers. It is what it is. Communications grade audio.

In a walk down memory lane, I recently picked up an R4B and recapped it. Now THAT radio sounds good!

Nowdays, I'd hesitate to chop up stock R4Cs and get them to somebody who appreciates them for what they are.

If you are not in a contest or pileup situation, the R4B (or A) might be a far better choice in terms of casual operating enjoyment.

Even in terms of audio amp mods, the R4B might be a better place to start than the C.

Today you can beat an R4C for difficult and crowded operating situations, but you still can't beat an R4B for armchair relaxation.

Joe N5KAT
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Ralph W3GL
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« Reply #7 on: December 02, 2010, 10:16:11 PM »

  

      If you read between the lines, I said basically the same thing back there in
      my original post! Huh

      If you really want to trick the R-4C out, acquire the Sherwood mods package.

      Sell the "C" to a DXer and invest in a "B" and go from there... You might have
      enough left over for a pack of gum or something... Roll Eyes

      Actually, there is a way to make the receiver perform like a Pro plus more.
      Build a SoftRock SDR unit on the output  frequency of the first IF stage of
      the  R-4C and  feed that to a computer running Power SDR Software.  That
      will give you every mode available on hf shortwave, including  panadaptor
      and waterfall displays...(LSB, USB, CW, AM, FM, Sync Detector, etc)     

      To prevent overloading the first mixer in the receiver use an 102 fet isolation
      stage between the mixer plate and the SoftRock board. Power the added board
      from the receivers 12vdc source and use the open hole on the back of the
      R-4C to run the SR output to the audio in on the computer. Connect HiFi amp
      to computer AF output...

      The above would work with any of the R-4's as well...

     

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73,  Ralph  W3GL 

"Just because the microphone in front of you amplifies your voice around the world is no reason to think we have any more wisdom than we had when our voices could reach from one end of the bar to the other"     Ed Morrow
Rob K2CU
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« Reply #8 on: December 02, 2010, 11:12:50 PM »

Take it from someone who has really "re-engineered" the R4-C. The original class A audio amp has to go! it overheats the rig and over stresses the funky power tranny/supply. Later serial number units have a little more voltage on the secondary for the nominal 12VDC. once I changed over to to a bridge type IC class AB amp the reduction in heat from this change, as well as changing over to Blue LED lamps, has made the PTO stability rival my hot rodded TR-7.  The bridge output allowed me to use the same output transformer so that the anti vox line to the T4X-C worked correctly.

But...the biggest improvement in audio was to replace the single 4 inch oval cheapo speaker in the MS-4. A buddy fabricated a new panel with two four inch high performance drivers. Now it sounds really good in all modes!
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aa5wg
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« Reply #9 on: December 02, 2010, 11:37:27 PM »

Joe, Ralph, Rob and all:

Thank you for the very good information regarding the Audio improvements for the Drake R-4C.  Bias, Softrock and Driver Speakers all seem to be good ideas to me. 

(1) When you say two drives do you mean 2 full range speaker drivers?

(2) Would it not be better to gut the stock audio board and go with something else?  My preference is to go with a class A tube system.

73,
Chuck - AA5WG
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Rob K2CU
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« Reply #10 on: December 03, 2010, 02:14:23 PM »

We used two 4 Inch wide range speakers, such as you would mount in a car door. they are 4 Ohm each, wired in series. My friend Don had built them and I don't know where he got them.

I had redesigned a number of the PCBs in the radio. One was the audio/PTO low pass filter board. the boards were designed to be drop in replacements, not just add-ons. The audio output was done with a pair of LM 380s though any IC amp at the 4 watt or more level and bridge output will do. I connected these in the bridge configuration, and, as i said earlier, connected the output directly to the original output transformer.

Second board replaced the power supply board. Each supply was designed with much higher filter capacitors and two three terminal regulators for +12V and neg 12 V were added.

Third PCB replaced the more complex Third IF/Detectors/AGC/BFO board. It has a 1496 product detector and the precision detector for AM. The high voltage tube circuitry for the third IF was just rebuilt.

All were designed to be nearly direct drop in replacements, and used the original harness connections for the most part.

A last fix had to do with the IF filter outputs. I have a full load of filters and was irritated by the different S meter readings for each selected filter. The problem was caused by failure to correctly terminate the filter outputs to 50 Ohms. I added a tiny perf board to the top of the filter switch with a MAR3 MMIC from mini  circuits. It provided the 50 Ohm load and a little gain. In fact, to keep the gain distribution and hence the proper  operation of the already complex R4-c AGC profile, I had to introduce 6 dB loss after the filters by putting a small shunt cap on the grid of the third mixer. This also allowed me to remove the stupid JS T7 tap bandaid that Drake had ginned up. Changing filters now amounts to at most a needle's thickness in S meter change.

My friend Don had offered to get blank PCB made up and we did that, though I had bread boarded all using prototyping board stock and anyone interested in doing same can send me an email for details, including schematics, pix, and part layouts. All parts are through hole.


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aa5wg
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« Reply #11 on: December 04, 2010, 08:24:43 PM »

Rob, yes I would like to see and read your material regarding the modifications to your Drake R-4C. 
Thank you.
73,

Chuck - AA5WG
Cedar, Michigan
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