2nd Gen FET question (Solved)

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martin manning
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Re: 2nd Gen FET question

Post by martin manning »

Looks ok, Paul. Can you measure voltage at the top and about half of that across the drain resistor with a battery powering it? If not, I'd suspect that the FET is bad.
Guitarman18
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Re: 2nd Gen FET question

Post by Guitarman18 »

Hi Martin,

With 14.9V (2xflat 9v batteries!), I now get 7V across the drain resistor, having changed it from 1.6K to 2.4K.

Does that sound correct. Do I need to get closer still to the half way point?

Thanks for your help.

Cheers,

Paul.
'Beauty is in the ear of the beholder'
Guitarman18
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Location: UK

Re: 2nd Gen FET question

Post by Guitarman18 »

The FET is actually working!!!!!

I'm amazed that the biasing needed so much adjustment. On my previous builds with FET, they have worked with the prescribed values. This one, however, showed no signs of gain boosting at all(there was signal, but no added gain), which made me think is was a more serious situation.

Thank you Martin.
'Beauty is in the ear of the beholder'
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sonicmojo
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Re: 2nd Gen FET question (Solved)

Post by sonicmojo »

I have a bunch of 4416A FETs. I usually use a socket and try different ones for the best voltages. They can be very different.
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martin manning
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Re: 2nd Gen FET question (Solved)

Post by martin manning »

Good to hear you got it working! Once you get it back on the amp's power supply check the voltage at the top of the drain resistor to see if you need to adjust the voltage divider. I think shooting for the 15V you had on battery power is a good target.
Guitarman18
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Location: UK

Re: 2nd Gen FET question (Solved)

Post by Guitarman18 »

Apologies for the slow response. Gigging has kept me away from my amps!

Having replaced the 8.2K with a 12k resistor I'm now getting just over 14v, with 7v across the drain resistor.

At these voltages I'm getting significant boost from the FET.

I now plan to add a Ratio control to the overdrive channel, to bring my stock 2nd Gen in line with the Hybrid spec.

I'll report back when I've managed to complete the changes.

Thanks Martin, your help was invaluable.

Best wishes,

Paul.
'Beauty is in the ear of the beholder'
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