No gain, unresponsive eq, and squealling on new build

Discuss our latest amp kit.

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ivan H
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Post by ivan H » Mon Mar 04, 2013 11:07 pm

Hi, looking at pin 2 of V1a, it looks like u have a cold solder joint. Notice it is a dull grey rather than shiny silver color & the solder hasnt wicked into the pin. Clean the socket pin with some kind of sharp implement & resolder with a clean, tinned iron. U might do several of the socket pins so the solder wicks in properly. This may not be causing the sqealing (tho pin 2, V1 may, its a bad connection) but should be addressed Also trim excess wire on pin 6 & 7 of V3, notice they are almost touching. Have u checked V1 for microphonics by tapping with a pencil or something like? The reason why Marshall puts the input into V1b first is because it is the quietest half of the tube. BTW, how close r u standing to the amp with ur guitar? Does it still squeal with the guitars volume on zero? Hope u get it sorted. Cheers

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neikeel
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Re: No gain, unresponsive eq, and squealling on new build

Post by neikeel » Tue Mar 05, 2013 10:06 am

You are describing classic blocking distortion, secondary to parascitic oscillation, if it happens with the gain knob being turned up it is almost certainly an issue in and around V1a. Basically as you turn the gain up there is electrical interference with your signal waves initially causing high frequency positive feedback type interference (the squeal) ultimately cancelling each other out (the loss of output).

Using grid stoppers and shielded wire helps, as does keeping your wires short as possible whilst keeping the plate wires (blue) down close to the chassis and the green grid wires up in the air (ie away from the plate wires).

Doing what you have done with the layout/spec you have (ie 2k7/0.68 and 10k cathode resistors) you should be ok.

I would check over my components and joints carefully.
Neil

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