Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubenit on December 05, 2010, 11:13:57 am
-
I typically follow Hoffman's grounding scheme for grounding. However, he doesn't cover a typical passive effects loop grounding.
Question 1: Do you advise connecting the send receive in passive effects loop grounds to one another and then the
buss wire for the preamp? In the same fashion that Doug recommends for the guitar input jack.
Question 2: I am trying to figure out if I should ALSO connect the send and receive for the effects pedal AND the
input and out of the outboard effects loop to the busswire on the FX-Reverb box?
In other words, ......... do I want to have the outboard effects loop buss wire and jacks connecting up to the amp head
buss wire using Hoffman's grounding scheme?
Would appreciate your input and thoughts particularly if you have direct experience with this?
Thanks, Tubenit
-
Question 1: Do you advise connecting the send receive in passive effects loop grounds to one another and then the buss wire for the preamp? In the same fashion that Doug recommends for the guitar input jack.
I've done 3 amps with preamp out and power amp in jacks (same as your passive loop jacks). One amp has the jacks on the front panel and they are grounded to a preamp buss just like the pots and preamp board grounds. Then that buss is actually grounded at a single point ground on the chassis. The other amps have the jacks mounted on the back of the chassis with a wire running to a single point ground.
I believe that the important thing is to NOT use the chassis as a conductor for any power amp grounds. IOW, connect the PT center tap (or negative side of a bridge) to the chassis at a point near the PT. Also connect the plate node filter cap and screen node filter cap directly to the PT center tap. This keeps all that high power trash from flowing through the chassis and minimizes the chance of it getting into your more sensitive circuits. Once you do this, it doesn't really matter if you use a buss or star or a combo for the rest of the amp.
Question 2: I am trying to figure out if I should ALSO connect the send and receive for the effects pedal AND the input and out of the outboard effects loop to the busswire on the FX-Reverb box?
I'd give that a try, but I'd wire it such that it would be easy to separate those grounds if needed. That's just an educated guess. I don't have any experience with outboard FX loops.
-
I believe that the important thing is to NOT use the chassis as a conductor for any power amp grounds.
I connected the PT center tap to a seperate ground.
IOW, connect the PT center tap (or negative side of a bridge) to the chassis at a point near the PT. Also connect the plate node filter cap and screen node filter cap directly to the PT center tap
Regarding the outboard FX &Reverb box:
I currently have all the filter caps connected to the buss wire thinking about them as though they are "preamp" type filter caps ......... but if I am understanding you correctly ........... I should be treating them like "poweramp" type filter caps and giving them a separate ground such as Hoffman recommends for poweramp filter caps in his grounding scheme.
Q: Am I understanding this correctly?
Thanks Sluckey! I always appreciate your excellent help. It sure has made a huge difference on a bunch of my projects.
With respect, Tubenit
-
I currently have all the filter caps connected to the buss wire thinking about them as though they are "preamp" type filter caps ......... but if I am understanding you correctly ........... I should be treating them like "poweramp" type filter caps and giving them a separate ground such as Hoffman recommends for poweramp filter caps in his grounding scheme.
Well, it's not exactly like a power amp, but the reverb driver is a very small power amp. Make that verrry small since you're using 1/2 12AX7. The more important reason to separate node A filter cap ground is because it has very high 120hz ripple current flowing thru it just because it's the first filter cap after the rectifier. So, use a separate wire to connect the node A cap negative back to the same chassis point that the PT center tap is connected. BTW, think of the PT CT just as you would the negative battery terminal on your car.
Node B filter cap also has a lot of 120hz ripple current present since you only used a 1K decoupling resistor. I'd probably bump that resistor up to 5 to 10K. There's so little load current involved that the B+ voltage should not decrease much, but the higher resistance will filter the B+ better and will give better isolation (decoupling) from node A. So, I'd rather use a separate wire to connect the node B cap negative back to the same chassis point that the PT center tap is connected.
Now you have the first two nodes connected directly to the CT and none of that 120hz ripple current is gonna flow thru the chassis. Nodes C and D will be much cleaner than nodes A and B. I'd probably increase those 2.2K dropping resistors for the same reason as mentioned above. Then I'd connect the negative leads of node caps C and D to your preamp ground buss. Since your buss is soldered to the backs of your pots you don't need to run a separate ground wire back to the PT CT ground point. But remember, the small currents flowing in nodes C and D ultimately must return to the CT (battery negative post). It will flow thru the chassis to get there.
I'm not advocating star grounding over buss grounding, but all the equipment I work on uses star (single point) grounding schemes with separate signal and power grounds. I use a combo scheme in these simple guitar amps and it works fine for me. I believe the most important thing is to separate nasty grounds from quiet grounds and if you examine Doug's scheme or a good buss scheme I think you'll find that principle at work.
And having said all that, it may not make a detectable difference in your FX box. :grin:
EDIT... I struck thru a comment above because I just realized you did not use your usual 1 tube reverb circuit. I didn't change, delete or add any text.
-
I will follow and try your suggestions!
I can confidently say that I have found at least ONE source of the buzz. One end of the .22 cap going to the 220k/500p
was NOT soldered. The angle of the tag on the tagboard made it difficult to notice that it wasn't done.
So, I am anticipating this being much quieter and working just fine.
Thanks again for all your help!
With respect, Tubenit