Question about Grounding

[dude]

I'm using a star grounding point, in a cathode bias 18 watt 6V6 layout with one grounding point as in layout below.

Things are tight, any problem with using the other leg of the PT for some of the grounds (the grounding bar, cathode bias resistor and filaments grounds?

I did run a ground wire from the both legs of the PT  (3" apart).

I don't think this is a ground loop and not a problem but checking.

Thanks as always,
al

[HotBluePlates]

Things are tight, any problem with using the other leg of the PT for some of the grounds (the grounding bar, cathode bias resistor and filaments grounds?

It won't be a problem.


I would personally ensure the output tube cathode resistor is grounded at the same spot as the first filter cap and the power transformer center-tap, but that's more an abundance of caution, since 2 points on the chassis a couple inches apart is "the same spot."

[dude]

Yeah, I'll a little over cautious, I build this design with el84s and it's got some gain to it.

With the 6V6's and resistor changes in the PI this amp should be more Plexi like, I should have way less gain.

I can run the bias ground to the filters.

The only thing about this built is the OT I used, MM Tweed deluxe, I thinking it's not going to give me  the headroom I want. But on the other hand, I never turn the gain up that much and would mic the amp at gigs.

I'm getting too old to lug equipment around, the Crate VC20 cab with a good ten and this circuit should be a good little blues amp, hope it's not boxy sounding...?

Thanks,
al

[labb]

That looks a lot like Zaphod Phil's 18 Watt Super Lite. I have built that amp in both the 6v6 and the EL84 version. I think it is a good sounding amp with either tube. One thing for sure it is going to break up early. I'm trying to remember how I grounded it. I think that I put the PI, pre amp cap, pots, pre amp and input jack grounds on a buss and grounded the buss on a terminal right at the input jack. The rest of the grounds I grounded with the 1st filter cap right at the first filter cap. I know that is two ground points but it worked for me.

[PRR]

It would be a wee bit better to take your rectifier return (PT HV "0") directly TO the filter cap -. Then strap filter cap - to main circuit ground.

Also.... what is that 220K in series with rectifier +?

[labb]

I think that the 220K resistor is suppose to be 150 to 220 ohm resistor used as a sag resistor. Some versions of the schematic show a 220K resistor to ground in parallel with the 1st filter cap. It is a cap bleeder resistor. That layout with the 220K resistor as shown is wrong.

[dude]

Yes, the layout is wrong. Your right labb it's a sag resistor simulating a rectifier tubes sag and should be 150 to 250 ohms. I think who did the layout was thinking bleeder resistor as 220K is the usual value. It's usually placed at the first filter B+ to ground.

That layout is indeed from Zaphoid Phil over at 18 watt dot com many years ago. He came up with the TMB for a 18 watt lite and I build the first one, at least I think with his help. Took off from there with many people changing things a bit.

The version I build is a 6V6, plexi style. I just followed Mark Huss's plexi for a lot of the resistor values. I did play the amp for the first time last night, very nice. The presence works great, each turn of the knob give a whole new range to the tone stack. But the PT I used from the Crate VC is low B+, 302v with tubes in and warm.

On the Crate schematic the voltage is posted at 320v w/tubes and SS rectifier. I wonder why I only get 300v, but I'm using 6V6s and the Crate used EL84s, is that the difference? My 6V6s are old used tubes and may be weak but they sound good. I'll tey a different pair and see if the voltages raise a bit.

I plan on making a MP3 and posting with some pic's after a few tweaks maybe.

al

[J Rindt]

It would be a wee bit better to take your rectifier return (PT HV "0") directly TO the filter cap -. Then strap filter cap - to main circuit ground.

Also.... what is that 220K in series with rectifier +?

What advantage does that offer.?
Thank You

[HotBluePlates]

It would be a wee bit better to take your rectifier return (PT HV "0") directly TO the filter cap -. 

What advantage does that offer.?

The rectifier draws big current pulses when charging the main filter cap, especially if the amp is Class AB (or B).

The charging path is PT wire  ->  Rectifier  ->  Main Filter +  ->  Main Filter -  -> PT Center-tap  -> PT wire.

The concern over ground loops is not so much a physical loop, but that large ground currents could create a voltage drop when 2 places that are "ground" have resistance between them.

There's no such thing as truly-0Ω at room temperature; maybe the wire between two points is 2Ω. If 3mA passes through that, there is a voltage drop of only 6mV, which won't disturb your circuit. If there is 3A passing through that resistance, your circuit is bouncing up & down on 6v when it should be close to 0v.

Since the loop of PT, rectifier and main filter cap carrying the largest currents in the B+ supply, trouble is prevented when the wiring is as short and direct as possible.