| Author |
Message |
    
Alsworld
| | Posted on Tuesday, December 31, 2002 - 9:57 pm: | 
|
I have a friend who is seeing power losses in his Maverick 250 base amp and is thinking of replacing the tubes. The losses have come over time and not suddenly. He suspects tired tubes but is not sure. Here is what it has, as I've seen various tubes in these throughout my searches on the internet for him:
(1) EL500
(4) 6JG6A's
(4) 6KG6A's
I know nothing of tube amps but am trying to find replacement tubes and figured I would ask here. I have the RFParts catalog, and have found some sources on the web but does this sound sane? Slow losses of power over time indicate tubes kicking the bucket? Best sources for these (or proper replacement) tubes? Any help would be appreciated.
Alsworld |
2600
| | Posted on Thursday, January 02, 2003 - 4:59 pm: |
|
Tubes are high on the list of causes to suspect, but don't forget the filter capacitors in the high-voltage power supply. If they look suspect, or if they are original, they will have to be replaced. The servicing textbooks from the tube era would stress that you pay attention to statistics. The things that fail most often should get looked at first when doing a repair. Tubes are the least-reliable component. Duh. Electrolytic capacitors were always the second one on this list.
D&A did NOT put any resistors across each string of three capacitors. They used three parts rated at 350 volts each in a series string. This adds up to 1050 volts. On standby, the High Voltage is about 950. Gives you a 100-Volt "margin" between the total rating and the actual working voltage. Nobody much stocks 350-volt parts any more. The 450-Volt rated parts are the routine replacement. You can't get away with just two of them. They only add up to 900 together, and 950 will make them fail prematurely.
Adding a 470K 2Watt or so resistor across EACH of the three capacitors in series insures that the 950 Volts is split evenly three ways. I'm not sure just how D&A was able to get away with leaving them off, but they did. I never try. New filters get bleeder/equalization resistors. Every time.
If anyone has looked at what comes out of this Maverick with a 'scope, bad filters would put a "sawtooth" pattern on top of the modulation. If you see that, or if folks at the other end hear a noticeable "hum" on the signal from it, it's filter time. Odds are that both tubes and filters are causing trouble, but the filters can hold the thing back when they get old and weak, even if the tubes are still fairly strong.
73 |
Jyd
| | Posted on Saturday, February 01, 2003 - 8:44 pm: |
|
it can be converted to 12jg6a and 24lq6 alot cheaper to buy |
Jyd
| | Posted on Saturday, February 01, 2003 - 8:44 pm: |
|
it can be converted to 12jg6a and 24lq6 alot cheaper to buy |
|
