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Roberthorne
| | Posted on Sunday, January 12, 2003 - 9:27 am: | 
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307 this amp will key but the 6146 tube will get red. no watts out and the choke next to the 6146 was burned up and the ground wire from transformer to chassy ground was burned up too. |
307
| | Posted on Sunday, January 12, 2003 - 7:03 pm: |
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I would guess that the "-" bias is "+" due to the negative power source going south. Tubes cherry when the bias is to high. In most cases , not all , the bias is a negative voltage in the MOHAWK line...
307 |
2600
| | Posted on Monday, January 13, 2003 - 12:00 am: |
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If this is a model 5B, there are two basic places to look.
The 6146 driver in the Mohawk is grid-driven, with a fairly high negative DC bias voltage on the control grid. Looks a little like a Black-Cat JB-150 circuit. There are two gas-filled regulator tubes that hold the screen grid voltage on pin 3 down to around 200 volts, I think. If they are missing, that voltage will be double or more the normal 200 volts or so, and the tube will run really hot, unless some other change was made to the circuit that takes their place. A really large zener, or a string of small ones could take the place of those two tubes. Zeners are kinda fragile, so if it has them instead of the two skinny regulator tubes, they can still be blown.
The control grid on the driver tube, pin 5 connects to three parts: a RF choke and two capacitors. If that choke is blown, the tube will cherry the very moment you key. If the 1.5k resistor on the other end of the choke, or the bridge rectifier fed from the yellow wires, or the filter caps that it feeds have gone down, there will not be enough bias voltage to keep the tube from "cherrying". I'm pretty sure it takes around 50 volts or more.
Either way, you will be looking for too much positive voltage on pin 3, or not enough negative voltage on pin 5.
Or, dare I say it, a bum 6146 tube? After the first few times they cherry up, they are just never the same again.
73 |
307
| | Posted on Monday, January 13, 2003 - 3:55 pm: |
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I love it when it gets down to "wire colors"...2600 is the man...
I remember a long time ago working on a Mohawk that used a Zener and it was bad. I can't recall which Mohawk amp , however it fixed the problem. I seem to remember that the Bias was like -60VDC or close. |
Roberthorne
| | Posted on Monday, January 13, 2003 - 7:55 pm: |
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mohawk amp THIS AMP IS A MODEL 8A SERAL#30 THIS AMP STILL HAS THE 2 GLASS TUBES IN IT #OA2 BEHIND THE METER.A 6146 DRIVER AND 2572B TUBES IN IT. ON THE HIGH VOLTAGE 3000 V BOARD I HAVE 4 GREEN GREEN PLASTIC DIODES #5 DSI D1-850-C THE 8 IS EATHER A 250 OR AN 850-C WHERE CAN I GET THIEM? THIER IS ALSO A BOARD UNDER NEITH THE 6146 TUBE,THAT HAS A RELAY ON IT, ON ONE SIDE AND ON THE OTHER SIDE IS WHERE 7 WIRES PUSH ON TO IT. THIER ARE 4 WITE DIODE W/2 RED BANDS ON EACH .ARE THIES DIODES?,AND THE 2 HIGH VOLTAGE CHOKES, ONE NEXT TO THE6146 TUBE AND THE ONE UNDER IT ON THE BOARD WHAT MH-MA ARE THEY? CAN I PUT ALL 2.5 AMP 1000 VOLT PIV IN THE BRIDGE, LOCATION C-R3 ON THAT BOARD.I REPLACED THE 8 CAPS AND THE TWO ON THE BRIDGE BOEAD TOO.ALSO WHEN THE AMP IS ON THE TUBES HAVE A SLIGHT BLUE TINT IN THIEM. |
Roberthorne
| | Posted on Monday, January 13, 2003 - 11:55 pm: |
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MOHAWK AMP P.S THE VOLTAGE ON PIN 3 IS TOO HIGH OVER 300V AND PIN VOLTAGE ON PIN 5 NO READING AT ALL. E-MAIL HORNE150@AOL.COM |
2600
| | Posted on Tuesday, January 14, 2003 - 12:42 am: |
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Well, the board with the relay on it should have one-piece bridge rectifier, with the legend "CR4" printed near it. The yellow wires from the transformer go to the two legs marked either "AC" or "~". The "+" leg of this bridge rectifier is grounded. A meter probe to the "-" leg should show around 70 volts DC NEGATIVE POLARITY. This feeds into R27, a 1.5k resistor and a 25 uF (or so) filter cap C37. You should read about the same voltage on both ends of R27. A small choke coil L14 is the next component in line with the "bias" voltage that we are following downstream here. If you don't have at least 60 Volts NEGATIVE polarity on BOTH sides of this choke coil, the driver tube is guaranteed to cherry, and in a hurry. My prediction is that you will read the negative DC voltage on one side of the choke coil L14, but not on the other side. If BOTH sides showed no voltage, you would have an additional problem. The 572-B tubes would be pulling current on standby, and get really hot before you key. So long as you have zero volts on pin 5 of the 6146, it will start committing suicide from the moment you key it. If L14 is bad, the replacement is supposed to be 56 uHy, I think. The current rating on it doesn't need to be more than 1/10 of an Amp.
If pin 3 on the driver tube is showing 300 volts, then that little OA2 tube is either blown, or has been disconnected, like to make the wattmeter read more.
From your description, this amplifier has more than one problem. Fixing only ONE at a time just won't do the whole job.
73 |
Monk
| | Posted on Tuesday, January 14, 2003 - 3:21 pm: |
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2600,
Do you have schematics on the Mohawk amps?
Thanks
Monk
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Roberthorne
| | Posted on Friday, March 07, 2003 - 7:44 pm: |
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MOHAWK AMP # 8-A DOSE ANYONE HAVE ANY PAPER WORK THIS AMP? |
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