Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: frpax on March 21, 2017, 02:45:21 am
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Hi guys. I have a question (yeah, only ONE question... so far!) about the AB763 2 channel layout.
Should the buss wire on the "Bright" side be grounded? The other one is, so I'm just wondering. See the pic below.
(http://img.photobucket.com/albums/v376/fpaxton/groundbuss_zpsjlolanza.jpg)
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No. That buss is grounded through the normal channel input jack on the far left. Likewise, the vibrato channel buss is grounded through it's input jack. That ground symbol you see on the far right end of the buss is just to let you know the buss is a ground buss. It's not actually connected to chassis on that far right end.
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No. That buss is grounded through the normal channel input jack on the far left. Likewise, the vibrato channel buss is grounded through it's input jack. That ground symbol you see on the far right end of the buss is just to let you know the buss is a ground buss. It's not actually connected to chassis on that far right end.
Thanks, sluckey! I was hoping you'd chime in.
You were an enormous help to me on my first amp, that Hammond M3 / Plexi conversion.
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No. That buss is grounded through the normal channel input jack on the far left. Likewise, the vibrato channel buss is grounded through it's input jack. That ground symbol you see on the far right end of the buss is just to let you know the buss is a ground buss. It's not actually connected to chassis on that far right end.
I thought I'd read somewhere that counting on the input jacks as a source of ground was a bad idea because they always tend to come loose over time. Is that really a big deal? I've always wondered and on some amps I couldn't get noise under control unless I ONLY used the jacks for the preamp ground.
~Phil
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I thought I'd read somewhere that counting on the input jacks as a source of ground was a bad idea because they always tend to come loose over time. Is that really a big deal? I've always wondered and on some amps I couldn't get noise under control unless I ONLY used the jacks for the preamp ground.
Input jacks can come loose. I've seen some loose ones. But I would not say they "always tend to come loose". I've never had one come loose on my stuff. I use a lockwasher and ain't afraid to crank the nut down using a 1/4" drive ratchet wrench. :icon_biggrin:
Hoffman has a separate ground for the PI and PA. That ground buss on the pots is basically just for the preamps. Hoffman counts the reverb and tremolo circuit as part of the preamp. Something worth noting... Hoffman's preamp buss is securely soldered to the back of ever pot on the front panel. So, every pot body, bushing, lockwasher, and nut provide a ground just as the input jacks provide a ground. The entire front panel becomes a ground plane for the preamp. This was the case for the original AB763 amps as well, except Leo used a brass plate under the pots and jacks. Leo's ground plate and Hoffman's ground buss both work pretty well. I would lean toward Hoffman's buss simply because I have seen odd problems arise from using the brass plate due to the galvanic reaction of dissimilar metals. The brass plate problems are usually corrected by using a big soldering iron to solder the brass plate to the chassis.
I don't solder my buss to the back of the pots, but it's not because I'm afraid of having multiple chassis contact points for the preamp ground buss. I simply don't want to deal with the hassle of replacing a pot later on, therefore, I float my ground buss above the pot body and run a short jumper from the pot terminals when needed, just as my layouts show.
The important thing to remember is the high power, poorly filtered, noisy power amp grounds do not share this preamp ground plane.
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Gotcha, and yeah I don't solder to the back of the pots anymore either, as I've read it can be really bad on the pots themselves. but still, the vol pots have a wire from t heir ground terminal to the buss anyway, so I'm getting multiple chassis connection points, so I guess that all makes sense.
Thanks,
~Phil
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No. That buss is grounded through the normal channel input jack on the far left. Likewise, the vibrato channel buss is grounded through it's input jack. That ground symbol you see on the far right end of the buss is just to let you know the buss is a ground buss. It's not actually connected to chassis on that far right end.
I had the same question and was glad to find the answer here with a quick search. I am kind of dense though so I would just like to be sure: Are there two separate front panel busses which ground through jack/pot connections with the chassis? One on the normal channel gear for the preamp/reverb 'chunk' and a separate one on the other channel for the tremolo 'chunk' ?
Bonus question - does the ground for the cap doghouse run to the ground lug on the PT?