2nd Gen Build Started (that didn’t take long…)

[Raoul Duke]

Ok,
Got my 9vac OPT tester working and came up with 91.6 on the brown and 76.3 on the blue.

Oddly, the brown plate lead is a good 5” shorter than the blue or the red CT. I might end up having to add a little bit on to the end. Would grafting solid core onto stranded be a bad thing in the event I’m forced to lengthen that lead?

Came this way straight from Hammond far as I can tell (sealed factory packaging).

[martin manning]

Odd, I've never seen a short lead on a Hammond transformer. I'd use stranded wire to extend it if needed. What model number is it?

[Raoul Duke]

It’s a 1760L.
Just did a test fit and it comes up about an inch too short. So close…

If I moved the tube socket over right next to V4 vs the “skip a hole” 50w Dumble pattern, it’d be fine; but then I’d have to move other things. Also - that hole is right under the fuse and switches, which isn’t easy to work around.

I’ll just lengthen it. Just found 5” of 18AWG stranded brown that I trimmed from the Heyboer in my 102. Couple of J hooks, solder, and shrink tube and it should be ok, right?

[martin manning]

So close, but not enough. I would strip ~5/8" from each wire, twist them together in line, solder, and cover with heat shrink. The splice will be barely larger than the wires. Your phase test agrees with the 1760L data sheet ;^)

[Raoul Duke]

I will splice as you’ve advised

Good to hear I did the test correctly. I’m always thrilled when things like that go right.

Small bites, lol…

Thanks Martin!

[Raoul Duke]

Still deciding on whether to include an “in” for the loop. I’m leaning toward doing it if it’s as simple as it looks in my 102 notes; I.e. one wire from the 5n eyelet shared with the “Level W” and sharing the black wire on the “out” jack? Questions:

1. Is it actually that simple?
2. Any downside to this?
3. In Tony’s layout, where does the black wire come from? Is it the “control bus ground”?

Thanks for any opinions and advice!

[martin manning]

Still deciding on whether to include an “in” for the loop. I’m leaning toward doing it if it’s as simple as it looks in my 102 notes; I.e. one wire from the 5n eyelet shared with the “Level W” and sharing the black wire on the “out” jack? Questions:

1. Is it actually that simple?

I would wire it exactly like 102 if you want the power amp in:
Level (master) wiper to the preamp out tip via the single eyelet on the preamp board, and preamp out ip to the power amp in switch lug.
Power amp in tip to 0.005u PI input cap on the preamp board.
Accent switch ground lead to the bottom of the 100R PI tail on the preamp board, and ground the 100R at the power amp in sleeve lug.

2. Any downside to this?
3. In Tony’s layout, where does the black wire come from? Is it the “control bus ground”?

No downside to having the additional functionality. Do you have holes for two jacks on the rear panel? How are they labeled?
The black wire is the shield, I believe. It kind of disappears, but I would think it would carry through to the Level pot ground.

[Dr d]

The phase test for your OPT is an interesting one. I followed the colour codes on my 1760L with the schematic, ie.brown/brown and blue /blue only to find the OPT was out of phase. I disconnected the NFB and the screaming stopped! Swapped the the wires and all was good. Cant rely on the phase by the colour of the wires.

[martin manning]

The phase test for your OPT is an interesting one. I followed the colour codes on my 1760L with the schematic, ie.brown/brown and blue /blue only to find the OPT was out of phase. I disconnected the NFB and the screaming stopped! Swapped the the wires and all was good. Cant rely on the phase by the colour of the wires.

Some OT's leave the factory with the lead colors swapped, that's why I always check. The hookup I described above can be done with the transformer insalled, too. I had one case where the secondary lead colors didn't match the data sheet WRT to the impedances, so I check that as well.

[Raoul Duke]

“No downside to having the additional functionality. Do you have holes for two jacks on the rear panel? How are they labeled?
The black wire is the shield, I believe. It kind of disappears, but I would think it would carry through to the Level pot ground.”

Yes, I have both the in and out holes - labeled as “Interupt In Out”.

Can the shield run to any other ground or must it be to the Level pot ground? I guess it doesn’t matter if I’m using the Accent switch ground lead?

On your 2nd Gen board layout, the Control Bus Ground would now go to the power amp in sleeve lug?

Level W to preamp out via which single eyelet? The top of the .005uf cap? If so, would it share the eyelet with the Level W wire?

How would one go about verifying OPT impedences? Is it as simple as measuring resistance from common to each tap and verifying highest to lowest corresponds with the order given in the factory schematic?

Sorry for the barrage of new guy questions. These are things that wouldn’t have occurred to me in the first build but now are sort of bubbling up as my understanding (hopefully) grows.

Thanks!!!

[martin manning]

Yes, I have both the in and out holes - labeled as “Interupt In Out”.

Sounds good.

Can the shield run to any other ground or must it be to the Level pot ground? I guess it doesn’t matter if I’m using the Accent switch ground lead?

On your 2nd Gen board layout, the Control Bus Ground would now go to the power amp in sleeve lug?

Level W to preamp out via which single eyelet? The top of the .005uf cap? If so, would it share the eyelet with the Level W wire?

Follow the 4th/5th gen board layout for that bit. You can put the 1u electrolytic cap on the board, and have the accent switch play the part of the presence pot, in an all or nothing way.

How would one go about verifying OPT impedences? Is it as simple as measuring resistance from common to each tap and verifying highest to lowest corresponds with the order given in the factory schematic?

You can put your AC on the primary plate leads, ignoring the CT, then measure the voltage across there and across the secondary, say common and 16 ohm tap. The primary impedance is (Vpri/Vsec)^2 x 16. You could check all the taps, but I'd just make sure that the voltage on the secondary taps goes with the impedance of the taps, (8 ohm = 0.71 times the 16, and 4 ohm = 0.5 times the 16.

[Raoul Duke]

Ok, thanks - I think I’m figuring it out now.

The single eyelet is the one in the 102 layout where the Master ties into the “In” (the brown wire turns yellow) over by the 390r that goes to the presence cap.

Can I leave the cap on the switch or is it better to move it?

If I move it to the board, does one end go to the 100R and the other to the single eyelet? Then one wire to ground at the power amp in/out jacks and one back to the accent switch jumping the center terminals on the switch?

[martin manning]

Yes on the single eyelet, and you can leave the accent cap on the switch. Note that all the switch does is slap the cap across the 100R.

PS I’d still run the ground back to the loop jacks.

[Raoul Duke]

Thank you Martin, it must be sinking in, lol…

The control bus ground as identified on your 2nd Gen board to the jacks and not the ground bus? Just the jacks or both ways?

[martin manning]

Ground the Level (maser) to the control bus, and do the board like this.