Ceriatone Overtone Special -- FET input sounds crap *SOLVED*

[xtian]

A client traded me this 50w Ceriatone Overtone Special, the first Ceriatone I've met in person. Gotta say, the lead dress and soldering is excellent. And it sounds great, too. All except for the FET input, which is too quiet, and goes BLAT when I strum hard. There is a 10K trim pot, which appears to be the output level adjustment. Input B+ is 320v, and I measured 10.6v across the 100uF cap. How do I troubleshoot this thing?

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[martin manning]

Check the FET bias per the guide in the Dumble files section: https://ampgarage.com/forum/viewtopic.php?f=5&t=18389

[xtian]

I read your guide, Martin, but was confused. When you write, "Verify Vdd is 18-20V," is that voltage across Rs?

[martin manning]

Vdd is the supply voltage at the top of the drain resistor.

[bepone]

"Vdd" -in short- voltage on 100uF 25V blue cap on the fet board, derived with that 150k from Ub+ V1

[bepone]

i never liked that Vdd derivation..i was always worried here in case of short on 150k for some reason, you can kill/damage your self on guitar strings very fast, FET wil burn like a popcorn and 350V+ will be on the strings

[xtian]

Thanks, Martin, got it sorted. Rs needed to be increased from 2K7 to 3K3 to get closer to center bias and stop the gating effect, and I increased R1 from 8K2 to 15K to allow Vdd to rise from 10.5v to 17.2v for better headroom. Sounds good now.

[sluckey]

i never liked that Vdd derivation..i was always worried here in case of short on 150k for some reason, you can kill/damage your self on guitar strings very fast, FET wil burn like a popcorn and 350V+ will be on the strings

Hmm. My strings are solidly connected to chassis ground.

[bepone]

350v on pickup, hands on strings, and.. (insulation)?

[Mark]

Bear in mind some of the newer FETs are crap. I found this out building a pedal with a FET in it. J201’s especially come to mind.

[Richard1001]

i never liked that Vdd derivation..i was always worried here in case of short on 150k for some reason, you can kill/damage your self on guitar strings very fast, FET wil burn like a popcorn and 350V+ will be on the strings

I frequently worry about pigs learning how to fly.

[bepone]

I frequently worry about pigs learning how to fly.

it is just a meter of risk assessment done and how safe is to play with guitar cable plugged in FET input connected to high voltage and how safe is to play with guitar cable to tube input connected to high voltage. i will be happy to listen your findings in this two cases

[Charlie Wilson]

I noticed that Dumble always uses a 1/2w resistor for the 150k. Maybe to insure that the resistor opens up if there is a surge.
CW

[sluckey]

350v on pickup, hands on strings, and.. (insulation)?

It is highly unlikely that 150K resistor will ever short, but just for shits and grins, let's say it does. And the FET also burns like popcorn. That 350V still has to go through a series 1M resistor to get to the input jack. That 1M will limit the maximum current to 350µA (.35mA). Certainly not a lethal situation. In fact, the electronics inside your guitar will create a voltage divider that will reduce this current even further. Heck, you may not even get a tingle. Might get hurt more by licking the terminals of a 9V battery. I think your little scenario was poorly thought out and now you're just floundering around trying to validate your statement.

[bepone]

why poorly. what i'm saying i'm always also supporting. situation, i dont blindly follow any schematic, let say nor do some other builders. i never use 1M for grid stopper in FEt input, something normal instead in tenths of kiloohms.. if i don't convert FET circuit to *triodeizer* before.
and 150k if is small wattage can burn also if you have dust and humidity accumulated (normal situation over the years) this can lead to slow carbonisation on the surface and with time short on 150k.

tube INPUT stage is galvanically isolated with electrodes in vacuum and chances that player will be electrocuted are close to 0.. but with FET input they are not 0..
it is better instead to supply FET stage with 12VDC rectified with some regulator LM7812 from low voltage secondary usually present in this type of amps. this was the point.