когда кручу ручку гейн поначалу он прибавляется а затем начинается жуткий свист и прочяя хрень в чем дело подскажите

Дело в паразитных связях. Смотри монтаж, длину проводов, экранировку, соединение земли звездой и прочие меры по устранению паразитки.

соединение земли звездой это как

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Star Grounding Amps traditionally use the chassis for signal ground. This is not the best choice since it can create ground loops and bad ground connections may develop over time. It is better to use star grounding in which all of the local grounds are collected at a single "star ground" point. With star grounding there is only one connection between the chassis and signal ground. Here are some rules for laying out a star ground. More information on grounding can be found in the Tube Amp FAQ and the Tech Info page of Aiken Amplification (see the Links page on the AX84 website). (1) Connect the power transformer center tap directly to the negative terminal of the first power supply filter cap then run a separate wire from the negative terminal to the star ground point. (2) Collect the ground points of each tube and its associated resistors and capacitors to a local ground point that is not connected to the chassis. Run one wire to the star ground point from each collection. (3) Run exactly one wire from the star ground point to chassis. (4) Insulate the input and output jacks from the chassis. The safety ground wire from the mains is separate from the signal ground. Run a wire from the AC ground to the chassis near where the AC power enters the chassis.

Обязательно посмотри, заземлено ли шасси и все ли металлические части заземлены...

SPASIBO VSEM