5f2-a building. a few more questions

[pietro_moog]

hi guys.
i'm slowly building my amp, because i'm still waiting for some parts.
i have a few questions:

1) i'm not sure about the power transformer. i have an hammond aex290
black are primaries. i need to connect red and yellow secondaries to the
rectifier tube as shown in the schematics ( is it correct? ) and green sec
go to lamp. i'm not sure here,i have to connect them to the lamp and stop?
there's a red/yellow wire, is this supposed to go to ground? (to the brass
plate ground or to the orange screw near the power trafo holes?)

2) do i have to connect a .05µ cap to the fuse-holder? i read to connect this to
ground wire coming from the wall.. but i'm not sure about it..

3) my speaker has a +/- sign on it, does it matter the way i connect it to
the secondaries of the output trasformer?

thanks guys

[JHow]

Do you mean hammond "290AEX"? The two reds are high voltage and go to pins 6 and 4 onthe rectifier tube. Red/yellow is the high voltage center tap and it goes to ground/chassis. Yellows are the 5v for the rectifier pins 8 and 2, greens are for your heaters - they typically go to the lamp and then on to heaters for the 6V tubes in your circuit.

[RWood]

For your other questions -

Since you'll be using a 3-prong power cord, you can avoid that .05 cap to ground.

No, it really doesn't matter which way you hook up your speaker since it doesn't have a second speaker to be in or out of phase with. Nevertheless, I typically connect the + to the positive OT tap and the - to the black/ground.

Buona fortuna!

[pietro_moog]

thanks to everyone guys!
let me see if i got it right:

-i don't need the power grounding capacitor, i just need to connect the ground wire (from the power wire) to the amp ground
- the power supply green wires (both) go to the lamp, and from here a wire goes to pin 7 of the power tube and pin 4 and 5 of the pre tube. just a wire, no return. everything is grounded by pin 1&2 on the power tube and pin 9 on the pre tube. is it correct?


i don't understand the output transformer wiring, i'll do some search

[RWood]

-i don't need the power grounding capacitor, i just need to connect the ground wire (from the power wire) to the amp ground

Yes to the capacitor.
The ground wire from your power cord needs to terminate in a crimped ring and then bolted to the chassis alone. with nothing else connected along with it.

- the power supply green wires (both) go to the lamp,

Yes

and from here a wire goes to pin 7 of the power tube and pin 4 and 5 of the pre tube. just a wire, no return. everything is grounded by pin 1&2 on the power tube and pin 9 on the pre tube. is it correct?

No. From the lamp it remains a parallel pair, twisted and going to the power tube (pins 2 and 7, one wire to each pin) and then the preamp tube (pins 9 and 4/5, one wire to each).

Now, the filament circuit still needs a reference to ground. To easily accomplish this, connect one 100Ω from pin 2 of your power tube to pin 8, the cathode. Then take the other 100Ω and connect it from pin 7 of the power tube to pin 8.

This will not only ground your filament string, but it will elevate it about 14 - 20 volts above ground, and thereby lower the amount of hum a little bit more than connecting those resistors straight to ground.


RWood

[pietro_moog]

thanks RWood!
everything is more clear now.
i don't wanna abuse of your kindness, but if you culd help me the last time this would be great!

i'm not really into the output transformer thing (at least i'm not sure)
i need to connect one of the primaries to pin 3 of the 6V6, and the other to the
16µ capacitor before the 10k resistor (i'm not sure. that's in the schematics but it's not in the picture). about the secondaries: both go the speaker jack, one is in parallel with ground and the other with the signal coming from the 22k resistor.

is this correct?

many thanks

[JHow]

Re, the output transformer, it sounds like you have it right. If you have the typical color sheme:

primaries:
Red to high voltage (junction of cap and resistor)
Blue to 6V6 pin3

secondaries
Green to speaker jack "tip" and lead to nfb resistor
Black to speaker jack "barrel" (ground, chassis)

[pietro_moog]

thanks guys

i just have some 100Ω, 1/4 W resistor now, do you think it's enough?
or i need a major wattage res?

[RWood]

I would either wait until you have 1 watt resistors or use a different value. If you have 150Ω or 180Ω or 220Ω they'll work fine.

[pietro_moog]

thanks guys.
my amp is ready but i didn't powered yet, i'll check it tomorrow.

everything seems ok to me.

- i grounded the earth wire alone.
- i grounded the hi volt center tap alone
- i grounded the circuit to the brass plate
- i grounded the heater to pin 8 of the 6V6 with 2w 100Ω resistors as it was
explained to me
- i grounded one of the secondaries of the output transformer just connecting
the sleeve pin on the jack socket (so it should be connected to the chassis)

is everything right?



my output transformer is an hammond 1760c.
i have a 4Ω speaker.
i connected the black wire to signal and the yellow to ground.
now i have seen the paper for the trafo, and it says Black=Com.
does it mean ground? should i reverse me wiring?

[RWood]

it says Black=Com.
does it mean ground? should i reverse me wiring?

Yes, it does, and yes I would, just as a matter of habit. Your amp will work fine as-is but if you ever decide to use an 8 or 16Ω speaker, then you'd only have to swap the hot lead.

Are you using negative feedback? If so that is another reason to use the speaker wiring as intended. Black = common = ground.

Good luck on your fire-up; let us know.

RWood

[pietro_moog]

thanks

my amp has a problem, i'm going to open a thread in the debugging & troubleshooting section