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Silvereagle
Junior Member
Username: Silvereagle
Post Number: 12
Registered: 4-2003
| | Posted on Friday, January 05, 2007 - 4:28 pm: | 
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I am trying to lower the dead key on a 2 tube blackcat amp.I am putting 1.5-2 watts in.It swings to 65 watts and hold steady when talking.I have turned the coil adjustment and the lowest I can get it to key is at 20 watts.What can I do. |
2600
Advanced Member
Username: 2600
Post Number: 579
Registered: 7-2002
| | Posted on Saturday, January 06, 2007 - 2:52 am: |
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One quick, dirty trick is to increase the voltage of the zener diode that sets fixed negative grid-bias voltage on the tubes. Higher negative bias voltage, less carrier.
Trouble is, the exact layout and parts placement in the separate power supplies changed from one year's production to the next.
It might have just one 30-Volt 1-Watt zener, or a pair of 15-Volt in series. Adding another zener in series, like 5 Volts or 10 Volts will reduce the carrier power.
You could also try to optimize the value of two disc capacitors, both hooked to that adjustable coil. Much trickier and time-consuming.
Lacking a diagram that's known to match yours, or helpful pictures, you also need somebody with the skills to identify a zener diode and make a simple mod without torching your two expensive tubes.
When 8417 tubes got up to around a hundred bucks, we started checking that bias voltage before the tubes are put back into the sockets. If that voltage is missing, for whatever reason, the tubes will cherry up and commit suicide just as soon as they warm up, before you can key it the first time.
Good way to ruin a perfectly fine day.
And if you've substituted some cheaper type tube for the 8417, this usually throws off the setting of that carrier-set coil in the rear. The two disc caps on that input coil are chosen to match the tube type in use for that production run.
If the carrier level seems to "peak" at one end of the slug's travel, or near the middle, one or both of those caps isn't quite right for the tubes in use. If it was built for 8417s and has, say, 7581s in it, those caps won't be optimized for the substitute tube type, but for the previous type.
73 |
Silvereagle
Junior Member
Username: Silvereagle
Post Number: 14
Registered: 4-2003
| | Posted on Saturday, January 06, 2007 - 6:35 am: |
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This amp has a 47 ohm 2 watt and a small like .001 in parallel leading from the coil to the keying relay.I did put in a new 8417,will try 6L6 and see what that will do.Will try adding another zener in series and go from there.Thanks for the info. |
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