Cash in hand

[Andy Le Blanc]

a couple more pict's

the other end of the box, star ground point... hum dinger, I had to get creative and mount it to the pilot light.

the stand off and resistors is the only mod to the circuit
bleeder and series resistor before the screen grid resistors.

It allows to screens voltage to be set fairly precisely at the operating point you choose

In this case the screens are 7.4 v neg. @ 100ma bias

Its adjustable with the series resistor.

The bias is set to 50% of the PT ma. rating... not any arbitrary power tube plate dissipation assumption.

If the setting is maintained irregardless of tube type, the amp sounds very nearly the same despite the tubes.

as long as the tubes are within there max ratings it don't matter.

this thing can run nearly everything accept 6ca7/el34 without mod.
the socket wiring for "kt" types some times come with one of the pins connected to a metal tube base, better safe than sorry

[Andy Le Blanc]

And out the door it went. Client loves it, already gigging it

There's always lessons.

never let your client talk you into using a unknown material to cover the amp.
I got stuck using mother of boat cushion... I made it work but what a pain in the arse.

Stand by pop... I was surprised at just how apparent it was, sticking to the old scheme.

The solution I gathered off an old repair guy 20 years back, put a high voltage
.1 cap across the stand by switch, worked great... the client expected the pop
associated with the old amps, was very surprised that it didn't pop...


The only other whine for me was having to scrounge a 250r hum pot.
I'm having a time sourcing an affordable 3-5w part for whatever reason

I still have to fit a cover for it, hopefully I'll have a few more pict's

[Structo]

Congrats Andy!

Another amp out the door!

[M Fowler]

Great project Andy

[surfsup]

I like the 1/2inch EMT connector and ground block. I'm getting close to finishing my new home so can't wait to start building amps again!

[Phil_S]

I like the 1/2inch EMT connector and ground block. I'm getting close to finishing my new home so can't wait to start building amps again!

I noticed this and hesitated to say something, especially knowing that Andy is one of those people who seems to be at the top of this game. Maybe I don't see or understand it correctly? I'm not looking to criticize. I want to learn something.

I think I see the green mains ground going to one end of the block and the buss ground going to the other end of the block. It's my understanding that connecting the circuit ground to the mains ground is a decidedly bad idea. In addition to possibly allowing the introduction of hash from the a/c mains, in the event of some catastrophic problem, the mains is connected the ground side of the circuit with no protection or isolation at all.

What gives? Is the use of separate terminal at opposite ends of the block more than good enough? Is this no different than using an isolated bolt at one spot for the mains, and a different bolt at another spot for the circuit, with chassis metal in between?

Having said all that...Andy, this looks like a great build with a great result! I really like it.

[pdf64]

Alpha make a 100 ohm 5 watt ww pot http://www.tube-town.net/ttstore/Resist ... :3476.html

[Andy Le Blanc]

the current IEEE recommendation is to ground the case of mobile equipment.

the star ground is the simplest way to accomplish this.

the only parts of the circuit that are grounded to the case are the opt secondary
and the bleeder of the screens mod.

The input and audio circuit are not grounded to the case, they are grounded thru the higher conductivity copper bus, and each filter node of the power supply has its individual return to the star point.

the ground is differentiated from the AC mains "neutral" or the older two wire system ground.

[Firestorm]

Where did you find a 250R hum pot? Flip tops has the 100Rs, but 250?

[Phil_S]

Andy:
Thank you for the explanation. I don't see the star point!
Phil

[Andy Le Blanc]

the ground block constitutes the star point, all the ground points are essentially in the same spot.

the aluminium and the copper conductors have a higher conductivity than tha steel chassis.
which is why I try to keep the grounds off of the chassis as much as possible

the trend is to separate the Ac neutral and ground into separate buses.
The variations in code still result in separate wires for the neutral and ground
with the newer codes going to 4 wire 220, differentiating between neutral and ground.

I fond the 250r pot on an old tube hifi pre... and finally found a 250r pot thru mouser CTS... any thing from 100r to 500r is usable