ODS #183 Build

[pfarrell]

Fired up the PSU....NO drama!!!

Here's the voltages @20VAC on the variac—with limiter—it was about 20VAC on the IEC //
B+, B+2 79.7VDC, B+3 78.9VDC, B+4 75.4VDC, B+5 75.2VDC

LED didn't light up—probably not enough voltage there yet.

I didn't take it up to full...dinner time...

[martin manning]

Fired up the PSU....NO drama!!!

Here's the voltages @20VAC on the variac—with limiter—it was about 20VAC on the IEC //
B+, B+2 79.7VDC, B+3 78.9VDC, B+4 75.4VDC, B+5 75.2VDC

LED didn't light up—probably not enough voltage there yet.

Great! So if we multiply by 6, 480V B+. How about the negative bias and the relay supply? The regulator won't work at that low voltage, so no LED, but you should see ~16VDC at its input, pin 1.

[pfarrell]

Bias 1 shows about -9VDC.
Pin 1 on the regulator is 2.1VDC, but the AC is only about 1.2VAC from the 12V trafo with 20VAC on the vairac and bulb limiter...relay supply is similarly low...not hooked up yet. Should I take everything to full voltage?

[martin manning]

Seems a little low on the 12VAC transformer. Is that 20VAC at the IEC inlet? I think you can take it on up, though.

[pfarrell]

Yes, that was 20VAC at IEC inlet—w/ bulb limiter.

Here's 120VAC, still with limiter:
B+, B+2: 412.3VDC
B+3: 408.5VDC
B+4: 389.7VDC
B+5: 388.7VDC

Bias: -45VDC

Relays: 12VDC
Pin 1 on reg: 14.67VDC

And a nice red LED.

[martin manning]

You have 120VAC after the bulb limiter? What is drawing current through the B+ rail? FET board?
Seems like you're safe removing the limiter now.

[pfarrell]

FET board isn't connected... amp B+ legs are and bias... being super careful with flying leads off the amp board....

[martin manning]

FET board isn't connected... amp B+ legs are and bias... being super careful with flying leads off the amp board....

Ok, it's just the balance resistors on the 2 thru 5 filters drawing current :^)

[pfarrell]

Martin... after that successful PSU test run... I'm looking at the relays now...

I'm feeling clear on the PAB relay...

OD relay: Is NO2 on the OD relay board the in from the amp board .05u? NO1 is the out to the pot (coax)? Coax on Com1 to the MV? Where does NC1 go? This seems different from the #124 layout—which is more clear than the #183 layout. I can't make a lot of sense from the 2 schematics I have—I'm sure it's me.

And DC- eyelets off the diodes run to their respective manual switches at the back of the amp..?

Screen Shot 2022-07-28 at 10.55.26 PM.png

[ijedouglas]

OD relay: Is NO2 on the OD relay board the in from the amp board .05u?

Yes - also jumper to NC1

NO1 is the out to the pot (coax)? Coax on Com1 to the MV? Where does NC1 go?

- see above

And DC- eyelets off the diodes run to their respective manual switches at the back of the amp..?

- yes

[martin manning]

Thanks for covering for me, Ian. Re 50n vs. 0.05u, I prefer to use units that don't require a decimal point.

pharrell, once you get the relays wired you can do some testing there, switching them in and out and making some continuity checks as shown below.

If you like the test-as-you-go method, you can wire up everything over to the PI input cap, including the PI ground, and test the power amp. Put a shorted plug into the loop return (or clip the PI input to ground), connect a dummy load, install power tubes, set bias, and confirm that your NFB is NFB.

[pfarrell]

Thanks guys! I think I'm understanding... I do like the test-as-you-go method—really just following your direction/suggestions/flow Martin, but I like it—and I feel like I'm learning new things or at least solidifying concepts since this is hardly my vocation—it's not a hobby either—but I digress.

So, make all connections to the PI socket—including the ground off the tantalum (there was earlier discussion about where this goes? I'd have to reread) and the signal in .02u (shorted at input jack for testing) from signal access jack(s). Do I need to wire the pots, etc. off the relays? I think the answer is yes—but just confirming (I tacked in a bus bar for the pots late last night). Also would have to run the heater wires for the power tubes, assuming.

How does one test/confirm NFB if I make it that far!?

[ijedouglas]

How does one test/confirm NFB if I make it that far!?

You WILL know The positive feedback is pretty startling. It lets you know...

If you wire it the same as the wiring scheme I sent, you should be fine

[martin manning]

I do like the test-as-you-go method...

Me too.

So, make all connections to the PI socket—including the ground off the tantalum (there was earlier discussion about where this goes? I'd have to reread) and the signal in .02u (shorted at input jack for testing) from signal access jack(s).

Correct, and to test the power amp you need the presence pot wired in.

Do I need to wire the pots, etc. off the relays? I think the answer is yes—but just confirming (I tacked in a bus bar for the pots late last night).

It would be enough to have all the leads that are soldered to the relay boards connected to do the relay checks. I tack-solder things in place to test, and then when any remaining leads are added, I fill the joints completely. The power amp test doesn't need any of that, as there is a clean break at the loop jacks.

Also would have to run the heater wires for the power tubes, assuming.

For testing the power amp, yes, and the PI tube too. Since the power section wiring will be complete, building the elevated heater string over to the PI socket will not get in your way as you are finishing up the preamp wiring.

How does one test/confirm NFB if I make it that far!?

If your OT primary leads are reversed you will have positive feedback, and a 100W oscillator.

[pfarrell]

How does one test/confirm NFB if I make it that far!?

You WILL know The positive feedback is pretty startling. It lets you know...

If you wire it the same as the wiring scheme I sent, you should be fine

Ah! Yes, I have experienced this... didn't connect the dots—DID get in the habit of making the OT leads equal for potential switching.