Some more 300B questions...

Okay, Im nearly there just one or two more questions...

In the original instructions for the 300B amp (WAD btw) it states the earth lift goes between chassis earth point and main input, I have a follow up from Neal (I think) that states this is no longer deemed acceptable practice and thus the earth lift goes between the signal earth and chassis point. Does this mean ONLY the signal earth is subject to the earth lift??

Secondly are the two 20uF polyprop caps bypassed by taking another wire from the output tx to the cap input points on the pcb via suitable caps??

Many thanks

Stu

[NealG]

Hi Stu, the chassis should be solidly earthed to the incoming earth at the IEC. The rest, signal, returns, psu returns etc etc are all lifted via the 10R

I don't follow your second Q, sorry.

[Richard]

Hi Stu

Yes, as Neal says, with the later arrangement the mains earth wire is connected solidly to chassis. All the amp circuitry (inc power supply and signal in/out) is isolated from the case and its ground connection is made to the chassis earth point via 10R and cap. (Note that you'll need insulated phonos as the original had them connected to chassis.)

If you wish to try bypass caps the best place is nearest the load and with shortest wiring to minimise inductance. A practical fitting for the ones you mention would be from OPTX CTs to black speaker posts.

Rich

[simon2a3]

Originally Posted by Richard

All the amp circuitry (inc power supply and signal in/out) is isolated from the case and its ground connection is made to the chassis earth point via 10R and cap.

Hi Richard,

Can you explain a little what the cap parallel to the resistor does, I don't think I've ever seen an explanation of why this is used in addition to the earth lift.

Thanks,
Simon.

[NealG]

It shunts any HF noise/interference from the circuit to ground.

[Richard]

Morning Neal!

Yes (again!). Inductive impedance rises with f. One of the reasons for small, low inductance, bypass caps is to shunt RF to ground. They are used as the inductance of a large cap appears as an open circuit to RF. That RF must then have a path off the circuit ground to earth. Due to inductance again, a wirewound resistor may effectively be an open circuit so the .1uF cap provides the path.

Rich

[simon2a3]

Ah, now I see. Thanks guys.

Simon.

[Greg.]

I concur with that written above. However, I read somewhere something about such earth networks where linking the earth from the mains Tx to the network can have a negative effect on sound. Sorry, I can't remember the detail now, but in consequence, I wired all my return ground leads to the network apart from the Mains Tx lead that went straight to chassis earth which in my case is directly linked to IEC socket earth. Maybe Neal or Richard would like to comment on this?

Best wishes,

Greg

Doh! You mentioned the tx to chassis point in an email you sent, I've gone and soldered directly to the earthing point, oh well I'll change if needs be.

All is finished by the way (except the above change) all I need is the toobs and away we go. Got a price off China-arts on fleabay of £157 for 4 EH Gold valves including postage, seems okay to me. Might have to wait a few weeks though, other hobbies are draining the piggy bank at the moment...boo....

[Greg.]

http://www.partsconnexion.com/ will currently supply a matched quad of EH gold grids for £126-26 but that doesn't include packaging. They tell you that once you submit your proposed order. You'll also be more likely to get caught by import duty etc. Chinaarts tends, on his own initiative (no coercion from the customer), to put information on the package that results in you not paying import charges.

Best wishes,

Greg