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Collins 32V-2 power Modification




 
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Author Topic: Collins 32V-2 power Modification  (Read 9558 times)
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Mike/W8BAC
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« on: January 04, 2005, 01:02:55 PM »

I want to limit my 32V-2 to about 120 mills @ 600 volts input so I can run it into a 10 db amp. It has been suggested to me to use a voltage divider on the screen of the 4D32 rather than lowering the plate voltage. This will allow the modulators more head room.

Have any of you done this? Also I'm wondering if any audio mods are needed. Thanks in advance

Mike
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W2VW
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« Reply #1 on: January 04, 2005, 02:03:36 PM »

Quote from: Mike/W8BAC
I want to limit my 32V-2 to about 120 mills @ 600 volts input so I can run it into a 10 db amp. It has been suggested to me to use a voltage divider on the screen of the 4D32 rather than lowering the plate voltage. This will allow the modulators more head room.

Have any of you done this? Also I'm wondering if any audio mods are needed. Thanks in advance

Mike


The way to wind up with more modulator headroom is to lower the plate and screen Voltages. If you just lower the screen Voltage then you will draw less plate current (still at high Voltage). This will change the plate load and modulating impedance. Your mod iron will no longer present the modulators with the same load impedance. Same for the output tank.
Try to maintain the same ratio of plate Voltage to Plate current when reducing power. The rig will load up pretty much the same way and you will have better modulation.
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Mike/W8BAC
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« Reply #2 on: January 04, 2005, 02:41:31 PM »

I did a Google Search and came up with this WA1HLR audio mod for a 32V-3. A quick look at the V-2 schematic shows it to be almost identical. I'll give it some thought. http://www.amwindow.org/tech/htm/32v.htm

Someone has to have invented this wheel already. It would be nice to come up with a proven power reduction layout. Thanks for the advice Dave.
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W2VW
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« Reply #3 on: January 04, 2005, 03:07:26 PM »

What you really want when running this thing into a linear is continuous power adjustability. Not just reduction either. You want a way to be able to run your exciter power up while tuning the amplifier. An outboard D.C. supply just for the final would be ideal. The best way to adjust the linear properly is to drive it with enough carrier power to make the amp put out it's rated max P.E.P. Of course you will want to do this quickly. You then reduce carrier drive to 1/4 or even less depending on percentage of positive modulation. You said the amplifier was capable of 10db. If you want to run max legal power then you will be driving it with 37.5 Watts carrier (assuming 100% positive modulation). Your peaks will be roughly 1500 Watts P.E.P. out. Ideally, you'd want to drive the amp with 150 Watts while quickly tuning it for max. You can do the same thing with a few other methods but this one works best. I'm assuming (yes I've seen the old odd couple re-runs) that your amp is closer to 13 db which puts you right in the ballpark as far as exciter max out.
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Steve W8TOW
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« Reply #4 on: January 04, 2005, 03:44:22 PM »

Mike, I did the "HLR" af mods to my 32V3 and you hear how that sounds.
I also built solid state plug in rectifier replacements, added some new caps and the such...

As for reducing power, I echo the issue of reduction of B+ and Plate Current results in a change of the Class C load Z for the modulators (& MOD iron)
not something I suggest in doing...
I so run my 32V3 at 100 mA on low B+ tap, BUT in my case, the modulation trany is driving a 500 ohm load to the driver iron of my higher power Class B  mod deck and the 4D32 is driving the Class C RF deck...(thus the mod iron has no B+ going thru the secondary).... :cool:

vri 73 steve w8tow
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Always buiilding & fixing stuff. Current station is a "Old Buzzard" KW, running a pair of Taylor T-200's modulated by Taylor 203Z's; Johnson 500 / SX-101A; Globe King 400B / BC-1004; and Finally, BC-610 with SX28  CU 160m morn & 75m wkends.
73  W8TOW
Mike/W8BAC
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« Reply #5 on: January 04, 2005, 03:55:12 PM »

Interesting about running the DC supply seperatly. I wonder if this could be done by running the transformer AC supply for the B+ and screen on a veriac outside the cabinet?
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W2VW
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« Reply #6 on: January 04, 2005, 05:52:58 PM »

You would want to keep the full B+ on the modulator.
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W8ER
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« Reply #7 on: January 04, 2005, 06:26:58 PM »

Guy's

What was suggested to Mike about lowering the screen voltage to control the power output is not all bad. I did this when playing with an Eico 730 attached to a DX60. Simply lowering the screen voltage to the 6146 did indeed lower the RF output, nicely as a matter of fact! It also, as you suggested, changed the impedance of the load that the modulator saw but the result was not disaster or even bad. It would affect the efficiency of the transfer of the audio power to the final but since I had way more audio power than was necessary, I didn't care. It didn't affect the frequency response or distortion either. A purist may argue that it's a bad thing to do but I also did it to my Ranger, which I am using on 160 now and I'll be glad to demonstrate the effect on the air.

I will say that when the power output of the DX60 or the Ranger got excessively low, say maybe below 5 to 8 watts, I did start to hear some slight audio problems but since it takes nearly 20 watts to drive a pair of 3-500Z's, the problems were of no concern to me.

I'd say .. try it Mike! I think you will be pleasantly surprised.

--Larry W8ER
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Steve - WB3HUZ
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« Reply #8 on: January 04, 2005, 11:09:46 PM »

I'm sure it can be done. KD2XA has been doing it for years, but I don't recall his power reduction technique.

[/i]
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Mike/W8BAC
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« Reply #9 on: January 05, 2005, 01:15:33 PM »

The schematic shows the screen supply running on the low voltage (filament) section. Two OA2 regulators in series hold the screen of the 4D32 at 300 volts.

Question; If one OA2 is pulled and jumperd will it have a linear effect on the final by dropping the output by half? Some resistance could be added in front of the first OA2 to adjust.

The plate supply could be lowered if necessary by using a variac on that transformer.

Comments? Thanks
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W8ER
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« Reply #10 on: January 05, 2005, 01:58:41 PM »

Mike so eloquently asked:
Quote
Question; If one OA2 is pulled and jumperd will it have a linear effect on the final by dropping the output by half? Some resistance could be added in front of the first OA2 to adjust.


No Mike, the relationship is not a linear one.  Take a look at the circuit that Gary Blau W3AM used to control the screen voltage in his Ranger.

http://www.w3am.com/ranger.html

He basically uses a 20K 4 watt pot in a voltage divider to supply the screen. I am sure that readjusting the values of the divider would work just as well on the 4D32. The only concern that I would have is that with a regulated supply as you described for the 32V transmitter, that you did not come up with a divider that drew excessive current as it may upset the regulating capabilities of the OA2's.


--Larry
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Mike/W8BAC
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« Reply #11 on: January 05, 2005, 02:54:21 PM »

After making my last post I went back and reread the responses. The light is starting to shine a bit brighter.

Dave I now understand the variac on the plate supply is not a good idea since it will lower the modulator plates as well so if needed the 4D32 plate voltage will have to be lowered separately and in proportion with the screen supply on the final

The amp is an Alpha 374A running a pair of 8874's  grounded grid. With the plate supply in low tap (CW, FSK and SSTV) the amp is very close to 10 DB. I would like to run this amp at between 25 and 35 watts dead carrier drive. It should last a long time that way.

Larry, wouldn't it make more sense to lower the voltage before the regulators? Ideally it would be nice to have the modification switched so the exciter can run at rated power and use it as an exciter when needed.

Thanks For The Help
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W8ER
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« Reply #12 on: January 05, 2005, 04:16:00 PM »

Mike said:
Quote
Larry, wouldn't it make more sense to lower the voltage before the regulators? Ideally it would be nice to have the modification switched so the exciter can run at rated power and use it as an exciter when needed.

Actually Mike, you really can't lower the voltage into the regulators.  By installing a resistive divider on the output of the regulators you WILL be able to turn the pot up for full output to run barefoot and simply turn it down to provide a lower output to the amplifier when you choose to run it that way.

Looking at it reasonably, I don't know why you want to drive the amp with a 32V.  You are looking at probably 200 watts or so of carrier from the amp and the 32V will give you 100 watts of carrier all day long. I think the difference would be very small to the person listening on the other end.  Now if you were driving a 76CA with 3 8874's and could be talking about 300 watts plus from the amp .. well I might be more inclined to understand that.

Hope that helps.

--Larry
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