Late 70's v4 hum kicking my hiney

[danb435]

Hey guys, I'm bumping this, I really do not want to beat this horse to death more than it already has been but my v4 has been badly mucked around with in the past and I could definitely use some help from those knowledgeable. Does anyone possibly have an actual wiring diagram that would show how and where the grounding goes in this thing ('76 distortion model), or alternately, can anyone give me an idea as to how the grounding is supposed to go? Everything I've researched leads to having one and only one ground going to the chassis to avoid ground loops. That said, the schematic indicates that both input jacks and the pre/power interface jacks are all individually grounded to the chassis. That's 3 chassis ground connections to the preamp board which I understand is not a best practice. I'm also confused with the electronic "system" ground vs. the chassis ground. Where are the power tube cathodes grounded to? Where does the OT secondary ground go to? So many brown wires (at least I think that's what they're "supposed" to be), so many PCB soldering pads to choose from. Any and all help really appreciated.

[smacbride]

So I'm currently working on a VT-40 distortion model re-build myself.

On my amp the input jacks are actually insulated from the chassis so it's not actually grounded at that point. The shield on the input wiring is actually grounded on both end-points, therefore grounding the input jacks onto the PCB. I have completely traced all the wiring in my amp and it is indeed only grounded at the pre-amp/power-amp input/output jacks. The power tubes are also grounded to that point as is the OT.

So it does actually use a star grounding scheme.

My amp has a lot of loud crackling and popping noises which as been narrowed down to a PCB issue. My plan is to replace the PCB with a hand-made turret board, replacing all the components.

[Hangman]

If the jacks are supposed to be grounded only at the preamp/poweramp input/output jacks, that would indeed cause some problems. I don't think I've ever noticed that. Thanks for that info!

[danb435]

Hey guys, I'm bumping this, I really do not want to beat this horse to death more than it already has been but my v4 has been badly mucked around with in the past and I could definitely use some help from those knowledgeable. Does anyone possibly have an actual wiring diagram that would show how and where the grounding goes in this thing ('76 distortion model), or alternately, can anyone give me an idea as to how the grounding is supposed to go? Everything I've researched leads to having one and only one ground going to the chassis to avoid ground loops. That said, the schematic indicates that both input jacks and the pre/power interface jacks are all individually grounded to the chassis. That's 3 chassis ground connections to the preamp board which I understand is not a best practice. I'm also confused with the electronic "system" ground vs. the chassis ground. Where are the power tube cathodes grounded to? Where does the OT secondary ground go to? So many brown wires (at least I think that's what they're "supposed" to be), so many PCB soldering pads to choose from. Any and all help really appreciated.

I'm going to revisit this one more time, I've finally obtained a scope to do some serious digging. I've traced the signal path from the guitar input through all the tone shaping and gain stages of the preamp and found negligible noise right up to the phase inverter tube 6AN8A. The exception is the midrange control, when bumped up much past 2:00 it gets very noisy. Back to the PI tube, the control grid of the pentode side on pin 8 looks OK, however when I check the control grid for the triode side on pin 2 the noise turns really ugly. If I plug a shorted dummy plug into the power amp in jack, there is little to no hum. Anyone have any ideas why the PI tube is introducing so much noise? And the midrange control as well? Regarding the VT40 rebuild below, the isolated inputs thing is interesting, those jacks are definitely grounded to the control panel on my head and the original schematic with the amp seems to verify this as well. I may try lifting some stuff to see what happens. I would sell my soul (almost) to know exactly how the lead dress and grounding on this amp is supposed to be laid out! Thanks all.