Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ToneJunkie on October 24, 2025, 01:52:24 pm
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Hey everyone,
I’m working on a Princeton Reverb (No Trem and moding the PI for more headroom with node C) build and just finished notating a schematic showing how I plan to ground each section of the circuit using a multiple-star / bus-wire approach. I’ve been reading Merlin’s article and tried to apply what I’ve learned — I find creating a layout fun and engaging even though I'm sure I'm trying to reinvent the wheel.
I’d really appreciate it if some of you could take a look at my schematic (If you see any screw ups tell me) and let me know what you think — whether my star points make sense, if anything looks like it might cause hum or ground loops, or if there’s a better way to handle certain returns.
Thanks in advance for any feedback! I’ve attached my annotated schematic below.
I was thinking I would put ground GD near the input on my chassis and then ground GA, GB, GC all to the same spot near the transformer.
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You may need to ground the NFB tail to GA.
Also, if you're open to moving the PI to node C and optimizing the ground scheme for the sake of noise, I would recommend also moving the reverb driver to the C node, decreasing the 18k resistor feeding that node to 12k to compensate for the additional load, and reducing the driver cathode resistor to 1.5k to buy a bit of headroom. The hum from the reverb stage vanishes to nothing and the reverb is cleaner at higher volumes when this is done.
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You may need to ground the NFB tail to GA.
Also, if you're open to moving the PI to node C and optimizing the ground scheme for the sake of noise, I would recommend also moving the reverb driver to the C node, decreasing the 18k resistor feeding that node to 12k to compensate for the additional load, and reducing the driver cathode resistor to 1.5k to buy a bit of headroom. The hum from the reverb stage vanishes to nothing and the reverb is cleaner at higher volumes when this is done.
Can you help me understand the NFB to GA? It seems that is attached to the 3rd stage of the preamp fed by the D node. Don't you try and ground that with the GD I'm just trying to apply what the Merlin article suggests. I may have missed something.
Yeah the reverb driver option sounds good. Is it quieter due to being fed by a less "dirty" node?
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From Merlin: "The secondary side of the output transformer (if one is used) should always be wired directly to the speaker jack using heavy gauge wire. This is true no matter what ground scheme is used. A separate wire (which does not need to be heavy gauge) should then run from the negative connection of the speaker jack back to an appropriate star. If global feedback is not used then this speaker ground wire should be returned to the power-amp star. If global feedback is used then the speaker ground should be returned to the local star of whichever stage the feedback happens to be applied to, which is usually the phase inverter."
https://www.valvewizard.co.uk/Grounding.pdf (pg. 273)
Doesn't mean you have to do it that way, certainly Fender didn't, but it appeals to me to follow a system, and Merlin's is at least as good as any other.
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I definitely missed that one... thanks so much!
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Yeah the reverb driver option sounds good. Is it quieter due to being fed by a less "dirty" node?
Yes. It sounds great and retains reverb integrity even when cranking up. Move the ground to the appropriate PI node.
If you add Merlin's PI gridstopper you'll have addressed the bulk of the common complaints on the PR circuit.
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Having only a single input channel, the PR is an excellent candidate for Merlin’s galactic ground system - if you use separate filter caps (instead of the cap can). Your marked up schematic is the way I did my current one. It’s a great studio amp which I’ve had for 15 years.
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Recently finished up a PR no-trem myself--basically a AA1164. I used only two ground circuits: one for the power stage and PI, and another ground for the preamp ground/control pot ground. Both tie directly to the main circuit ground at the transformer bolt. It's been a very quiet amp so far. Probably just luck on my part... :laugh:
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Having only a single input channel, the PR is an excellent candidate for Merlin’s galactic ground system - if you use separate filter caps (instead of the cap can). Your marked up schematic is the way I did my current one. It’s a great studio amp which I’ve had for 15 years.
Awesome I will check out the Galactic and I am using separate filter caps. Did you do any mods to the circuit you ended up liking?
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Recently finished up a PR no-trem myself--basically a AA1164. I used only two ground circuits: one for the power stage and PI, and another ground for the preamp ground/control pot ground. Both tie directly to the main circuit ground at the transformer bolt. It's been a very quiet amp so far. Probably just luck on my part... :laugh:
Did you do any mods you ended up liking in the amp?
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I built a 6G2 Princeton using a cap can (because of space limitations), but added a separate filter cap for the preamp section - it turned out very quiet.
My next plan is to build an AA1164 Princeton with 1-tube reverb and no trem (along the same lines as Lectroid), and so all these tips in this thread (and Lectroid's) are much appreciated.
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Having only a single input channel, the PR is an excellent candidate for Merlin’s galactic ground system - if you use separate filter caps (instead of the cap can).
Yes, don't use a cap can, use separate individual filter ecaps.
It will make grounding much easier.
And mount each cap close to the part of the circuit it will feed B+ to. Don't put them all over where the cap can would be by the power transformer.