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My new project, 2 x KT88 60W amp
My new project, 2 x KT88 60W amp
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Old 27th January 2007, 05:22 PM   #31
kevinkr is offline kevinkr  United States
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My new project, 2 x KT88 60W amp
Definitely twist all heater wires, this is particularly critical the low level input stage and driver stage. There should be no heater wire loop around the socket, arrange things so that the wires pass entirely around one side or the other of the socket, not both sides.

The filament circuit(s) should not be floating, if there is a center tap available it should be grounded or elevated above ground with a bypassed voltage divider. Elevating the filaments above ground will help with filament/cathode breakdown voltage ratings in LTP and cathodyne (split load) inverters by reducing the voltage difference between the two. Usually this also results in much quieter operation as it reduces leakage in the insulation from filament to cathode. Just be careful not to exceed the insulation rating of any tube filament sharing that filament supply.

There should only be one connection between the chassis and the amplifier ground. Use a star ground arrangement or buss grounding with only one point connected to chassis. There should also be a connection for mains safety ground to chassis right at the point where power enters the amplifier.

You should use rubber grommets anywhere wires pass through the chassis. (Output and power transformers)

Since you say it is relatively easy to rotate the output transformers 90 degrees I'd do this. This will virtually eliminate the mutual inductance between the power and outputs and greatly reduce coupling between them.

Grid stopper resistors should be used on all tubes - 1K should be sufficient for most types. These must be mounted right at the socket to be effective.

220 - 1K resistors on the screen grids of your KT88 will save you a lot of grief by preventing parasitic vhf oscillations in the output tube.
A number of use have had reliability problems with some small metal films in this location (me included) so I would recommend a 1W resistor and I generally used carbon film for this purpose.

"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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Old 27th January 2007, 10:37 PM   #32
zafira1981 is offline zafira1981  Argentina
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Join Date: Jan 2005
Location: Argentina
thank you very much!!!!! i will follow your advices. This means almost re-asemble all the circuit but is a good moment to polish the aluminium

thanks again,

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