matchless lighting build ?

[chuckb]

Hey guys, i just finished a matchless lighting build and it sounds pretty darn good except that when I turn the treble up full it cuts down the volume .When I turn the treble back the volume comes back to normal. What seems to cause this? Also the schematic shows there's a cap on lug 3 of the treble pot going to pin 8 of the PI but it doesn;t state what value it is. Anyone know what the value might be? I'm using a 220pf there now but I'm thinking maybe a 50pf would be better. I've switched the wires around on lug 1 and 3 thinking that the treble was inverted on the schematic, but no luck.I've checked and rechecked my wiring against the schematic and can't seem to find any mistakes. The amp sounds great when the treble is between 0 and halfwaybut if I go pass halfway the volume drops considerably. Anyhelp would be greatly appreciated. Thanks in advance.

Chuck.

[chuckb]

Here is the schematic for the matchless lighting.Prowess Amplifiers - Matchless - Schematics - Matchless Lightning

[Chuck H]

Only three things I know can cause what you describe. One is a mistake in tonestack wiring. I don't think this is the problem. Another is a bad tube that breaks into a microphonic fit when too much gain or top end is applied. I don't think it's a bad tube either. The last thing is a parasitic oscillation. When the amp is adjusted with enough gain in the treble frequencies it begins to oscillate at a frequency the speakers can't recreate. Possibly even above your ability to hear even if a more high fidelity speaker were used. The amp is working so hard at this oscillation that there seems to be a volume loss in the audible frequencies.

The most likely causes for oscillations are layout, lead dress and ground scheme or some combination of these things. You want to be sure the amp is layed out so that inputs and outputs aren't in close proximity to each other. You want to be sure that any leads that carry current from the amps output aren't overly long or have pathways near preamp circuitry. Also vice versa, you don't want preamp leads to be overly long and have pathways near power amp or output circuitry. Long ground leads that share output and input signals are bad. Grounds should be run seperately for the preamp and power amp to different termination points. OT leads inside the chassis should be routed away from the preamp and twisted together. Sometimes an amp will oscillate only when the chassis is removed from the cabinet. The added shielding (or possibly ground plane) of the foil or plate in the cabinet can stop the problem. So if you haven't tried the amp in a cabinet yet, you might try shielding the chassis open side.

Chuck

[chuckb]

Hey Chuck, yes I believe you to be right on the OT and PT wires being to close to the preamp. I'm going to reroute these and also make the wires coming off my pots a little shorter. I do have lots of distance between the inputs and the outputs,so I don't see that as a problem.I may add another terminal strip for all my preamp ground connections as well. I do have my preamp grounds and my poweramp grounds going to the the same terminal strips although none are on the same lugs. Thanks Chuck, I really appreciate you advice and I will chime in and let you know how things turn out with the changes.

Chuck.

[axpro]

+1 on osc problems.... when you dime the treble there is a lot of brightness in this circuit (from my experiences) and lead dress becomes CRITICALLY important.

great sounding amps though.

[Bruce / Mission Amps]

Lighting

What's a matchless Lighting
Is that like an electronic starter on my BBQ grille?

[chuckb]

Hey Bruce, OOOOPS! That's a "Matchless Lightning" and she sounds great thanks to all the help I get here on the this forum.

Chuck.