Re: Hoffman Plexi50 Turret Board

[Rp3703]

I just have a question about the filter cap grounding when using multi-section 50-50's. Marshall ran a wire from every cans ground to the chassis through one of the mounting brackets nuts. I have read all sorts of things online telling you not to connect ground to the chassis except at the power supply at the first filter cap as well as at the input jack. Your layout drawing is kind of vague as the whether or not each cap should be grounded or not, any thoughts as to the preferred method?

[sluckey]

Your layout drawing is kind of vague as the whether or not each cap should be grounded or not, any thoughts as to the preferred method?

Page two clearly shows all three cap cans grounded to a ground lug at a PT bolt. Also connect the PT HT CT to this same lug.

I would personally connect the can for nodes D and E to the pot ground buss which should be connected to chassis at the input jacks.

[EL34]

I just have a question about the filter cap grounding when using multi-section 50-50's. Marshall ran a wire from every cans ground to the chassis through one of the mounting brackets nuts. I have read all sorts of things online telling you not to connect ground to the chassis except at the power supply at the first filter cap as well as at the input jack. Your layout drawing is kind of vague as the whether or not each cap should be grounded or not, any thoughts as to the preferred method?

It's not vague
As Steve pointed out, the PDF document clearly shows all the caps get grounded to a lug on the chassis
Ground them where ever you like
Hopefully your amp will not hum too badly doing it some other way

I also have this grounding document on the library page
http://el34world.com/charts/grounds.htm

[Rp3703]

Yeah, I have an 800 clone I've built that is humming like crazy that I used a start grounding scheme in. I'll try wiring it up using your method and see if that fixes things. The internets make it pretty confusing with Valve Wizard, Aiken and Kevin O'connor telling you to only use star grounding. In reality though, I have seen a lot more Marshalls in the world than any amps those guys built and they all sounded pretty quiet to me. No offense meant by my use of the word "vague". Thanks a lot for the help.

[Willabe]

The internets make it pretty confusing with Valve Wizard, Aiken and Kevin O'connor telling you to only use star grounding.

Their not telling you to use star grounding. (I don't know about what Aiken says and Merlin does have a short/small section/blurb in his grounding page but he says in it you'd have to arrange the chassis/PT, OT, choke/ tubes very differently then is the norm.)

KOC and Merlin do have ground stars (KOC calls it galactic grounding, these stars make up the galaxy) BUT each star is arranged around a single B+ filter cap that's feeding the tube(s) and their components. The B+ filter cap and the components it feeds/supplies, get grounded together with the filter cap being as close as possible to the circuitry it feeds/supplies. This way the ground/B+ wires are short and the current flow in them will not disturb other circuits which can/would cause hum. Each ground star is then daisy chained to the next ground star.

True star grounding has ground stars BUT each stars ground runs a ground wire back to 1 chassis ground point. So the sensitive preamp grounds are tired to the dirty high current power amp grounds. 

Their similar but they are different.

Doug's grounding separates the power section from the preamp section, power section gets grounded at a PT bolt and the preamp gets grounded at the input jacks. Doug also keeps the B+ filter caps close to the circuitry they feed. This works great.

There's 4 main things;

1. The power section has large and dirty ground current that if connected to the preamp ground can/will modulate the preamps ground and cause hum, separate them. (Either like Doug or like KOC and Merlin.)

2. Keep the B+ filter caps close to their circuitry.

3. Ground the B+ filter cap and only the circuitry it feeds together.   

4. Try to use as few chassis ground ground connections (random grounding) as possible.

[Rp3703]

I have a couple of more questions for you guys. First off, I'm using a Hammond 272jx PT which has center taps on the main secondary as well as the HT secondary. The 100 watt power supply wiring diagram you drew shows the center tap on the main secondary going between the first two 100 uf filter caps and the negative side of the bridge rectifier going to the negative tab of the second filter cap. It was my understanding that the negative half of the bridge rectifier would only be necessary if there was no center tap on the PT. I'm only asking because I have only seen power supplies wired one way or the other, not both. My other question is what to do with the HT center tap. Your diagram shows it grounded and another drawing I saw showed it unused but if there is a center tap, should the center tap not just run through a 100 ohm resistor to ground and eliminate the two hum balance resistors on the HT taps.

[sluckey]

The 272JX is suitable for a 50 watt plexi so follow the 50 watt schematic. It is not suitable for the Hoffman 100 watt plexi. The 100W uses a different PT. If you use that 272JX and follow the 100W schematic your B+ will be about 848 volts! If you are building a 100W plexi you will need a different PT, one that has a much lower high voltage winding. So, which Plexi are you planning to build?

It was my understanding that the negative half of the bridge rectifier would only be necessary if there was no center tap on the PT. I'm only asking because I have only seen power supplies wired one way or the other, not both.

You would NEVER connect the negative side of the bridge to ground AND also connect the center tap to ground, but it's perfectly OK to connect the negative side of the bridge to ground and connect the centertap to the junction of those two caps. The rule is that you cannot connect the negative side of the bridge and the center tap together, regardless of where they connect in the circuit.

My other question is what to do with the HT center tap. Your diagram shows it grounded...

No it doesn't.

...if there is a center tap, should the center tap not just run through a 100 ohm resistor to ground and eliminate the two hum balance resistors on the HT taps.

Don't do that. The center tap would connect DIRECTLY to ground. Or, you can follow Hoffman's drawing which shows the center tap unused, in which case, you would use the two 100Ω resistors on the board.

[Rp3703]

I’m actually building a 2203 not a Plexi. I bought the 272JX years ago after Kevin O’Connor showed it being used in a bunch of his designs from his second book, including his version of a Plexi and an 800. His version is wired the same way as a 50 watt, using a full wave instead of a bridge rectifier. He did run each main secondary tap through a 10 ohm 10W resistor first and then through the double diode full wave rectifier and then the combined B+ goes through another 10ohm 10W resistor. Other than that(and his complicated bias system) his design is pretty much the same. According to my math, the resistors should only drop the voltage by 6.5V per resistor and with since my measured voltages were on the low side already, I just assumed the resistors were just more of Kevin’s over engineering and I removed them. Now If what I have just written is correct, Then I should just run my B+ center tap to the first ground lug. Correct? And there should be no ill effects if I just eliminate the 100 ohm HT center taps and wire the center tap from the PT to the same ground lug. Correct?

[sluckey]

Yes, both centertaps would connect to the same ground lug.

[Rp3703]

Thanks, one more question. Is it better to connect the output jack grounds to the power supply lug, the preamp lug or just straight to the chassis?  I've read that chassis is bad.