So I've been making the Mesa/Boogie Mark IIc+ inspired preamp from Bancika's schematic (link below), on my first self-designed turret board (link below too). I'm not very experienced on tube amps yet (built a KT88 single-ended power amp before) and I've come to a problem. The grid bias on v3b is just ~9V instead of ~57V. Probably because of this I get only a tiny signal out of the amp.
The schematic
The layout
I've checked the layout and the wiring multiple times, but have no idea what could be causing this. I have swapped tubes around, but it has no effect on the problem. I was hoping that somebody had even a tiniest hunch on what the reason for this could be?
-Lassi
The schematic
The layout
I've checked the layout and the wiring multiple times, but have no idea what could be causing this. I have swapped tubes around, but it has no effect on the problem. I was hoping that somebody had even a tiniest hunch on what the reason for this could be?
-Lassi
Your meter could be loading that point to ground, since it's a high impedance point. What meter do you have?
Make sure that the 470k is not going to ground, and the resistors are the right values in Ohms.
There is also a mistake on the schematic, the voltage at the grid should be 133V, not 57V.
There's no voltage drop across the 470k resistor.
Is the voltage drop across the 1.5k resistor about 2V?
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The DMM I have is a plain old and cheap one that really needs a professional replacement. I've checked everything around v3 multiple times, and they should be correct. Otherwise I need a pair of glasses 😀 Is there any other way to measure this high-impedance voltage? I'll post the drop on the 1.5K in the morning.
Since this is a guitar pre-amplifier related thread it has been moved to Instrument & Amps.3b should be about 100V as shown as 133V in the schematic. You must have some value of the resistor wrong.
BUT the more important question is why don't you connect the grid of the 3b directly to the plate of 3a? The plate voltage of 3a is 242V, connect the grid of 3b directly to plate of 3a will give you 240V at the cathode of 3b. Save all the extra components in between. You AC couple, then you use self bias to get 133V!!!
BUT the more important question is why don't you connect the grid of the 3b directly to the plate of 3a? The plate voltage of 3a is 242V, connect the grid of 3b directly to plate of 3a will give you 240V at the cathode of 3b. Save all the extra components in between. You AC couple, then you use self bias to get 133V!!!
Well, as wired, there is over 30dB of attenuation between 3a and 3b in that resistive divider + pots + relay...
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That drop sounds correct. As pointed out you have a lot of attenuation between stages 3A and 3B, you may want to reduce that considerably. The current biasing arrangement is fine.
Then there's likely no problem with that stage. Just a measurement problem, making the grid seem too low when it's actually ok.
Good to know, thanks for that! I'll have to inspect the front panel wiring once again. It's weird though, because every knob and switch has an effect on the signal...
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