Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Swampertech on December 16, 2018, 06:15:18 pm
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I've got some 120Hz buzz, and was wondering: To troubleshoot noise in the preamp section section of a 5E3, is it acceptable to ground the grids of the 12AY7 to eliminate each section?
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yes
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Grounding the grids made no change. I'm running a pre PI master volume (1M) and I've got 41, 44, and 54VDC on the master volume. Is that normal?
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Maybe. Can we see a schematic?
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Sluckey, thanks for taking a look.
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That sure ain't no 5E3! But there should be dc voltage on that MV.
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:laugh: :laugh: Well, it started out as a 5E3. I'm building this for my son in law, and he was interested in an amp he could play at home, or take to a small club - thus the low power version (6V6) and the high power version(6L6). The cascaded inputs offer an interesting tube overdrive I thought would be nice going from rhythm to lead playing. The voicing of the two sections of V1 also gives a nice blend of Fender Tweed and Vox/Marshall tonality.
Anyhow, no it's not a basic Tweed. I was careful about grounds, especially of the input jacks and controls. I've got a star ground, one for Power amp ground and one for Pre amp ground. Both are bussed back to a common chassis point. All leads in the preamp section are shielded cable, grounded end at the signal source. With the Master vol up half way, the Lead channel volume control at 3 o'clock, there is minimum buzz, just sounds like a normal high gain amp.
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Could we see some gut shots of this amp? Layout becomes a big factor when you cascade for high gain.
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The guts
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Just saw the gut shot... not trying to be rude , it's just SO crowded in there
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I like the flashlight in the picture... gonna need it
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I suspect your layout is a major cause for your grief.
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I've measured the noise (40mV) after the coupling cap of the PI, but before the grid stopper going into V4, and the noise gain increased as the master vol control is increased. There also seems to be a DC component the noise rides on.
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In fixed bias mode there should be a negative dc voltage at that point. In cathode biased mode there should be zero dc volts at that point. Speaking of fixed bias... your bias cap should not be connected across the bias pot. The positive lead should connect to ground.
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There's 11mV at junction of C11 and R27. I haven't put it in fixed bias mode yet, but I can ground the + side of the cap. With the master vol at 12 o'clock, raising the bright volume control up doesn't change the hum, except when it is wide open, then the hum goes away.
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There's 11mV at junction of C11 and R27.
I call that zero.
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I think I'll re-wire around the relay, back to the original layout. If I can get one channel working, maybe it will reveal where the noise starts. Again, I don't see much noise until after the master volume.
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Just inserted a guitar signal into the return jack, hardly any noise. It's gotta be in the pre-amp stages.
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Hey sluckey, check this modified schematic. Relay and all related components are removed. Filter cap in bias circuit redrawn correctly.
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So was the bias cap actually connected as shown or was that just a drawing error? Is the amp behaving better now?
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The cap was connected that way. :w2:I had never gotten around to checking the fixed bias mode because of the buzz, but had initially adjusted the pot for max negative voltage on the grids. I'll start tomorrow gutting the relay and fixing the cap. I'll let you know, and thanks again!