First D-style (102) build opinions solicited & build thread:

[martin manning]

The 100W ODS documented here (102, 124, and 183) all use 4k7 and 390 from the 4 ohm tap. A 50W adaptation would reduce the 4k7 to 3k3, or move the FB source the to the 8 ohm tap. Either way you will get the same level of NFB as the 100W. The resistor change results from using the calculation I posted above.

[jelle]

I would run 330r and 4.7k off the 8 Ohms tab. Have fun

[jelle]

I would run 330r and 4.7k off the 8 Ohms tab. Have fun

[jelle]

The 100W ODS documented here (102, 124, and 183) all use 4k7 and 390 from the 4 ohm tap. A 50W adaptation would reduce the 4k7 to 3k3, or move the FB source the to the 8 ohm tap. Either way you will get the same level of NFB as the 100W. The resistor change results from using the calculation I posted above.

yes, if he changes the 4.7K, the presence cap value changes too, if aiming for the same frequency response. better keep it simple and move the tab

[Raoul Duke]

So if I’m following this correctly for the sake of understanding the math; the presence cap would go from 2.2uf to 1.5uf?

Also, what I’m gathering is that using what I already have - all I’d need to do is run the wire from the 4k7 off the board to the 8ohm tab on the impedance switch rather than the 4, correct?

[martin manning]

Jelle's post above (as originally written) listed the configuration for 100W, so I reiterated the options for 50W: Change 4k7 to 3k3 or move the FB lead to the 8 ohm tap, your choice. If you change the 4k7 FB resistor to 3k3, and keep the 2.2uF there is a small effect on the lower frequency pole, but it's only 3%. This is not worth worrying about since the presence capacitor's tolerance is probably 10%.

[jelle]

So if I’m following this correctly for the sake of understanding the math; the presence cap would go from 2.2uf to 1.5uf?

Also, what I’m gathering is that using what I already have - all I’d need to do is run the wire from the 4k7 off the board to the 8ohm tab on the impedance switch rather than the 4, correct?

Yes,

Keep the 100W GNFB values: 330R and 4.7K as provided, and use your 2.2uF cap as per schematic/layout for the original 100W SN#102... __and__ move the GNFB from the 4 Ohms tab to the 8 Ohms for your 50W amplifier.

The 4.7K and 2.2uF work together. Do not change them.

Do this or recalculate the amount of GNFB and the Presence knee...

Jelle

[jelle]

The 100W ODS documented here (102, 124, and 183) all use 4k7 and 390 from the 4 ohm tap. A 50W adaptation would reduce the 4k7 to 3k3, or move the FB source the to the 8 ohm tap. Either way you will get the same level of NFB as the 100W. The resistor change results from using the calculation I posted above.

yes, if he changes the 4.7K, the presence cap value changes too, if aiming for the same frequency response. better keep it simple and move the tab

Exactly!

[Raoul Duke]

I really do appreciate both you guys taking the time time to talk me through this. I completely understand both methods now and will press forward.

Thank you Martin and thank you Jelle!

[Raoul Duke]

FET and bias boards completed. I used 1/16” fiberglass board for these. FET is an NTE452.

[martin manning]

FET and bias boards completed.

Great! Now, if you have a bench supply, or a couple of 9V batteries, you can check the FET bias point. A how-to is in the Dumble files section.

[Raoul Duke]

Yes Sir, I just printed out your FET “how to”. Probably tackle that sometime this weekend if/when the house is more quiet, lol.

Quick question regarding the PAB relay board when you have a second Martin:

Your instructions say “22M from NC to COM on both sets of contacts” for populating that relay. Every layout I’ve seen shows the 22M only from Com2 to NC2. Are there jumpers under the board that connect Com1 and NC1 to the aforementioned #2 contacts?

Thanks in advance!

Marc

[Raoul Duke]

Relay PS and relays:
Mapping out the Hoffman relays to translate to Martin’s instructions were a bit of a thought exercise for me, but I figger’d it out by drawing it out, lol. My first time working with relays.

[ijedouglas]

Your instructions say “22M from NC to COM on both sets of contacts” for populating that relay. Every layout I’ve seen shows the 22M only from Com2 to NC2. Are there jumpers under the board that connect Com1 and NC1 to the aforementioned #2 contacts?

There should be 2 x 22M on the PAB relay board. On #0124 he ran the second one to the treble pot but it's electrically identical. If you look at the #0183 layout you will see that both are on the relay board between Com1 and NC1 AND Com2 and NC2.

[Raoul Duke]

This 22M off the J/R switch to the treble pot Ian? I put it there because I was following Tony’s layout; but if it works better on the relay I can certainly move it.