Re: {Collins} 30L-1, keyed down current @ no excitation.



It seems like this line of discussion is tapering down, and there is one issue that has not been cleared up to my satisfaction. So.... here goes. The bias supply voltage has been described as -157VDC and -170VDC, measured at the antenna relay connector (RCA) on the back. That voltage is the 'bias to cutoff' voltage to keep the four tubes (811A) from drawing any current when NOT transmitting, right?? My look at the Collins schematic indicates the bias supply to be -120 VDC, which I suspect is supposed to be the voltage when the bottom of the voltage flow goes to ground (like when we ground the Antenna Relay connector, causing current to flow from the filtered rectifier output, through the relay coil and the parallel resistor (2K) and then through the 39 ohm resistor we have all been agonizing over, to ground and back to the bias supply. Idling current on all of our 811A's is determined by the bias voltage coming from the 'top' of the 39 ohm resistor, establishing a bias of about (-120/1700+39) or 69 milliamperes through the 39 ohm resistor, or -2.7 volts. That's the idling bias, and the idle current through the tubes will be related to that.

The fact that grid current will flow from/to each grid through 47 ohm grid resistors will give the circuit analyzers fits and result in a different effective bias voltage when 'driven' with RF, but all of this talk of idle current does not relate to that circuit condition.

Did I mess up this math?? or should we be looking for the tiny bias voltage at the grid pin rather than worrying about 157 or 170 volts, or the size of the filter cap in the bias supply??

Or should we care?

John
K0KHZ
----- Original Message ----- From: "W9HAK" <w9hak@xxxxxxxxxxxxx>
To: "Bent" <bjoe@xxxxxxxxxxxxx>; "Collins" <collins@xxxxxxxxxxxxx>
Sent: Monday, February 20, 2012 3:10 PM
Subject: Re: {Collins} 30L-1, keyed down current @ no excitation.


Hello Bent,

Yes I think you should get about the same current reading on the Ip meter of the 30L1. I happened to check mine yesterday and it is 110 mills, I am using 572B's. But the original 811's also were running 110 mills.

I would try one final tube at a time and see what current each draws. As posted on the reflector about 27 mills per tube is what you should see. No drive is required, just key the amp and read the Ip meter. Even if the meter is not very accurate each tube should have the same reading.

Do be careful while doing this test. I have done this in the past by just putting the top back on the final cage and tightening the screws around the interlock.

Please respond to the list serve as well as to me. Many may get information from our discussion.

Smith
W9HAK


On 2/20/2012 14:43, Bent wrote:
Hello Smith

My 30L-1 RE has a serial no. 40800.
I think it is equal to yours.
R28 on mine is 39 Ohm.

Some pictures on //oz5zd.dk.

As you see I have some nice Collins equipment, but because of my ignorance and trouble with the bias and current on the 30L-1, I have not yet made one QSO at all, even I have owned it for some years yet.
And that I worried about it.
Quit honestly.

If you are able to obtain 110mA with your 39 Ohm, then I should be able to do the same.
Right ?

What do you think about it ?

I can inform you I use the picture of your nice Collins Line as background.
I see on the left side the WE 30L-1.
Is it from 1970 - 1974 ?

Br.
Bent


----- Original Message ----- From: "W9HAK" <W9HAK@xxxxxxxxxxxxx>
To: "Bent" <bjoe@xxxxxxxxxxxxx>
Cc: <collins@xxxxxxxxxxxxx>
Sent: Saturday, February 18, 2012 9:54 PM
Subject: Re: {Collins} 30L-1, keyed down current @ no excitation.


Hello Bent,

I hope the following information will help in making a decision about your 30L1.

My 30L1 was made around 1970-1974 and R-28 is a 39 ohm 1/2 watt carbon composition resistor. This is the original resistor installed by Collins. As I mentioned in an earlier e-mail it acts as a fuse if a 811 shorts grid to plate. You don't want 1800 volts on your key line.

Your -157 volts sound slightly low, mine runs -170 vdc. My minus supply uses a silicon diode and a 20 uf filter capacitor. The 20 microfarad is a change from the original 10uf as initially installed by Collins. Don't use a larger capacitor, 20 microfarads is fine.

Smith
W9HAK


Bent wrote:
Hello All

Until now thanks to all who had responded on this issue.

I do not know whether this theme is fully clarified.
I am not quit sure.

Based on the responses am not able to conclude something.

It seems for me, based on different generations of instruction books that R28 should be 39 Ohm. Maybe this is still an open question.

Is someone able to draw a final conclusion about the correct key down current if R28 is 39 Ohm ?
In my case I measure -157 VDC on the ANT Relay connector at 1800 VDC.

Is 200 mA instead of 110 mA OK for normal SSB use ?

If the current increases to the doubble the power increases four times as far as I know.
And that generate heat.
Where does this extra heat occur in the circuitry, tubes ?

Thanks again.

Br.
Bent
OZ5ZD




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NETS: Tues & Thur: 3.805 MHz-2000 Central + Fri: 3.895 MHz-2000 Pacific
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