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Author Topic: 3CX1500A7 in Henry 2K  (Read 6619 times)
K6CKD
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« on: August 26, 2005, 08:21:53 PM »

I installed a 3CX1500A7 in a Henry 2K.  Everything went smooth.  (I also have an AL1500 so I got some ideas from that)  The Henry had 3-400Zs which I believe are zero bias.  When I put the 8877 in,  I changed nothing except I lowered the filiament voltage a little.  My problem is that the static plate current is about 300 MA.  It should be 250 MA.  I guess I have to put a little operating bias on the tube to bring it down.  I'm not sure how to do that.  Any ideas???  By the way,  the amp. works great and I get great signal reports on it.  THANK YOU VERY MUCH FOR ANY HELP....BOB
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K0BG
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« Reply #1 on: August 27, 2005, 05:50:40 AM »

300 ma? If it is in standby, it should be practically nil. On key down with no drive, 250 is about right. I'm not too familiar with the way Henry does their biasing, but somewhere in the mix is a zener diode. At least that is where I would start looking.

Alan, KØBG
www.k0bg.com
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K6CKD
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« Reply #2 on: August 27, 2005, 12:42:23 PM »

To: K0BG  I'm sure there is a zener,  but that would be for the standby mode.  I draw no current in standby....that is the good part.  When I key down...300 MA.   I may have to ad a relay that puts some operational  bias in the transmit mode.  That's the part I'm not sure of....Where to put it and how much???  T H A N K S      B O B
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K0BG
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« Reply #3 on: August 27, 2005, 03:02:00 PM »

Like I said, I'm not too familiar with the Henry. However, how most GG amps are designed; a zener provides operating bias, not standby bias. The zener is usually in series with a meter resistor (of what ever size but usually small in value), and a larger resistor typically about 20K. When the amp is keyed, the large resistor is shorted out, and the zener then does its job. The zener can be anything from 5V to as much as 11 volts depending on design, and the tube in question.

There are other ways to bias a GG amps, but this is the most common, or at least a variation of it.

Alan, KØBG
www.k0bg.com
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KA5N
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« Reply #4 on: August 27, 2005, 04:05:05 PM »

Maybe you just need a slightly higher voltage zener diode.
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KA5N
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« Reply #5 on: August 27, 2005, 04:16:16 PM »

I should have looked at the AL-1500 schematic first.  The AL-1500 uses two 7.5 volt 10 watt zeners in series for CW, i.e. -15 volts and one is shorted out for SSB operation for -7.5 volts.  I don't have a schematic for the Henry amp.
Allen
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K6CKD
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« Reply #6 on: August 28, 2005, 11:04:35 PM »

I have the 2K schematic and I don't even see where they are coming up with any bias at all.  I think this was before the days of the zener diode..I do appreciate all of your comments.  H E L P .........
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WB2WIK
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« Reply #7 on: August 29, 2005, 09:54:54 AM »

You replaced a pair of 3-400Zs with an 8877 and are asking these questions?  Something about that situation seems unusual.  Did you do this modification, yourself?

The 8877 has a separate cathode and heater.  It is normal to provide about +3v bias on the cathode to bring the tube to about "normal" operating parameters for transmitting, and then to add a big resistor in series with that to bring the tube down to "standby" conditions, which would be zero idling current (tube cut off).

Instead of using a zener, I usually use a string of standard rectifiers, forward biased, so I can "adjust" the idling current based on plate voltage and tube condition, which of course vary from amp to amp.  If the Ep is 4 kV and the tube is new, you might need higher cathode bias voltage; if the Ep is 3 kV and the tube is old, you might need less.  Each forward-biased silicon junction provides about 0.7V or so of cathode bias.  I usually install 5-6 diodes in series, and then jumper the ones not needed, to adjust the bias for Ep and tube condition.  In series with that, I install a 10K, 25W resistor which is shorted by NO contacts on a relay for "TX," and the opened by those contacts for "STBY," thus reducing tube heating and possible noise generated by the tube during standby/RX periods.

This is standard operating practice for an 8877.  The cathode needs to be decoupled from the DC biasing stuff I just described using an RF choke and bypass capacitor.  The choke has to handle the full cathode current of the tube, so I usually use a 1.2Adc(min rated) RF choke.

WB2WIK/6
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K6CKD
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« Reply #8 on: August 31, 2005, 12:41:30 AM »

To:  WB2WIK,  Actually installing a 3CX1500A7 was easy.I have a schematic of a Henry 3K,  AL1500 plus one from the ARRL hand book,  all using the 3CX1500A7.  Everyone shows a different way to do the bias.  You said disconnect the cathode from the filament.  In the AL1500,  they tie everything together.  Only one filament lead is standing alone.  That is the configuration I went with.  So your fix is different from the AL1500.  If you can work with that,  would you PLEASE e/mail me.....bobzimmer@inreach.com  Much thanks...Bob  PS
If anyone is knowledgeable of this situation.  feel free to respond by E/mail.  In respect to WB2WIK  your idea almost gets me there...I just need to refine it a little more...
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WB2WIK
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« Reply #9 on: August 31, 2005, 11:29:48 AM »

The 8877 pins 1 & 5 are the filament.  Pins 2-3-4-6-7 are the cathode.  They have no internal connection.  Pins 1 & 5 should connect to the filament transformer, and the pins should be bypassed at the socket.

Pins 2-3-4-5-6-7 are strapped together at the socket normally (but not connected to pins 1 or 5 in any way), and both RF signal input and cathode DC bias are applied here.  I usually use an RF choke (~100uH, 1.5A) at the socket, followed by a ferrite bead over the cold end of that choke wire to add more high frequency decoupling, and a .01uF/.001uF to ground as a bypass on the cold side of that choke.  Then, to that "cold" side RF choke is applied DC bias which comes from a string of rectifiers in series (anode to choke, cathode towards ground) and also in series with a 10K, 25W resistor that gets shorted out by the PTT (T/R) relay on transmit.  In series with that diode string and 10K resistor is a 1.5A cathode fuse, then a 0-100mAdc "grid current" meter to ground an also a 0-2Adc "plate current" meter whose other terminal goes to the B- return of the HV power supply.  This allows safe and easy monitoring of both grid and plate (cathode) current in the return wiring, so nothing is above ground by more than millivolts.

This is standard, conventional 8877 biasing.  In lieu of a string of forward-biased rectifiers, some use a single power zener instead.  Doesn't matter, except the string of rectifiers allows some "adjustment."

WB2WIK/6
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K6CKD
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« Reply #10 on: August 31, 2005, 12:43:04 PM »

The only thing that is not clear is your statement that pins 2-3-4-5-6-7 are strapped together at the socket.  (yes)  But not connected to pins 1 and 5 in any way. Well,  pin 5 is connected in series,  that's the way they do it.  How do I get pin 5 out by itself when Ameritron leaves it in a series connection??  I hope I have a position on my T/R relay that is not being used so that I can use it to short the 10K resistor.  Once I have that pin 5 question anwsered,  I think I am ready to roll.  Mucho thanks...Bob
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AB5Q
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« Reply #11 on: September 03, 2005, 02:06:07 PM »

The 3-400Z is designed with a directly heated cathode; the 8877 is designed to be indirectly heated. You cannot indirectly heat the 3-400Z however I am not aware of any reason why you can not run the 8877 in either configuration. Be sure to allow enough time (2-3 minutes) for the indirectly heated cathode to heat up before operating.

It's perfectly logical to run the 3-400Z at zero bias below certain anode voltages. So that explains why there isn't any bias circuit in the amplifier. Biasing for directly heated cathodes typically occurs through the center tap on the filament transformer with associated low pass filter.  

Although it would be handy to have a third contact set on the TX/RX relay to accommodate a bias circuit, it's not necessary. Simply add a SPST reed type relay with coil parallel with the TX/RX relay in place of the third contact set.  

It sounds like you're very close to achieving the desired result. Good job on the 8877 conversion although it would have been more cost effective to use a GS-35B.  Sounds like you had a spare 8877 sitting around collecting dust($$).
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