Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Captain chunkulus on August 19, 2019, 08:41:27 pm
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So I have maybe a stupid question but I still need to ask because I know one of you fine fellas know the answer. I was re reading aikens article on grounding and one thing stuck out to me.
"The ground of the second filter capacitor, after the choke or filter resistor, is the star ground point for the preamp stage grounds. Use a local common point for each preamp stage ground, and run a wire from this common point back to the second star point. If two stages are out of phase with each other, they can share a common local ground, but don't use more than two stages per local common ground."
He says to use a local common point for each preamp stage unless two stages are out of phase.
Question 1, would you star each individual cathode on a 12ax7 since there are 2 stages in each tube?
Question 2, how do you determine if stages are out of phase? J
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Aiken says "If two stages are out of phase with each other, they can share a common local ground". Simplified: out-of-phase preamp stages share a common local ground. You attempt to state the inverse of this and get it wrong.
NYC parking signs state when you can't park. My wife translates this into when you can park; get's it wrong; we get parking tickets.
Suggestion: use the Doug Hoffman grounding scheme.
The output of a tube is a mirror image of its input. This is because the incoming AC signal voltage is a waveform wiggling on the input grid G1. It impresses a negative image of itself on the stream of electrons flowing between the cathode and the anode in the vacuum inside the tube. I.e., the output signal of a tube is 180 degrees out of phase with its input signal. Hence, the the output of stage 2 is out of phase with stage 1's output; the output of stage 3 is out of phase with stage 2, but back in phase with stage 1's output. Let's say the phase is either A or B:
A > 1 > B
B > 2 > A
A > 3 > B
B > 4 > A
Successive gain stages are out of phase. Every other gain stage is in phase. Odd numbered gain stages are in phase with each other, and out of phase with even numbered gain stages; and vice-versa.
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The output of a tube is a mirror image of its input. This is because the incoming AC signal voltage is a waveform wiggling on the input grid G1. It impresses a negative image of itself on the stream of electrons flowing between the cathode and the anode in the vacuum inside the tube. I.e., the output signal of a tube is 180 degrees out of phase with its input signal.
Successive gain stages are out of phase. Every other gain stage is in phase. Odd numbered gain stages are in phase with each other, and out of phase with even numbered gain stages; and vice-versa.
That's true for common-cathode stages, but not for, for instance, cathode followers of grounded-grid stages (in phase). Cathode followers are common in guitar amps. Phase inverters create both an in-phase and out-of-phase signal.
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So, if I had say a 2 tube preamp with the 2nd tube being a directly coupled stage and the output was taken from the cathode, those stages would need to have separate ground nodes?
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If by preamp tube you mean gain stage, then yes (according to Aiken). My personal opinion is that obsessive star grounding is usually an unnecessary pain in the rear end. More conventional grounding schemes (look at some Fender, Marshall, ... layouts for inspiration) do the job just fine. But hey, that's just me...
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Here's more on grounding from Merlin's web site. (If you haven't read it yet.)
http://www.valvewizard.co.uk/Grounding.html
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What about multiple channel amps or preamps that share one half of the first tube but that then have their own individule stages like a sokdano x88? How would you go about grounding one of those?
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me, I take the simpleton approach, eliminate 2 ch's and make 1 work until all 3 work "stand-alone" then a let them co-habitat n see what gets reproduced :icon_biggrin:
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What about multiple channel amps or preamps that share one half of the first tube but that then have their own individule stages like a sokdano x88? How would you go about grounding one of those?
Aiken's speaking of "preamp [gain] stages". It doesn't matter how many gain stages are in a single tube, and with which other gain stage they share an envelope (a tube). Treat every gain stage as independent.
In your example, they could all share one "star leg", because stages one and two are out of phase and the two stages following it (one for each channel) won't be active/heard together.