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

[Raoul Duke]

Hey all,
Been lurking, reading, researching, planning, and trying to figure this out for the better part of a year without bugging you all too much, lol. I’ve been using others’ build threads as my guide as well as cross-referencing the many layouts here. I try to use schems as well, but I have no formal background in electronics - so that doesn’t always work out - but sometimes it does help me.

Quick build summary:
- 50 watt 102 build using black panel Bassman iron all around
- added external bias points
- eliminated half-power switch
- made all my own boards using Martin’s excellent templates
- decided to use the Hoffman relay boards with 12v NTE relays and Martin’s suggested Hammond 166F12C trans
- got my chassis from Erwin, resistors from Jelle, and a lot of knowledge from everyone here - so thank you all!

I have a few subjective build questions that I’d like some informed opinions on if possible:

1. Does the switchable LNFB off V1 option provide a noticeable variation in tone?

2. Is putting the OD trim outside the chassis truly worthwhile?
2a. Is an OD trimmer with more range a worthwhile change as well? I’ve read here that it can be helpful when dialing in the OD.

3. When building 102 as a 50w, would I need to adjust any power supply values (resistors on the PS board etc)?

Got some pics to post and will start a build thread soon - just a little bashful about my abilities, lol.

Any help, advice, opinions, suggestions are truly appreciated!
Thanks again and regards,

Marc

[ijedouglas]

Hey Marc,

Good luck on the build.

1. Does the switchable LNFB off V1 option provide a noticeable variation in tone?

Yes. I have added it to a Wonderland build I did. Do I use it...no. I would add the LNFB and just leave it without a switch, no need to add extra wire to a sensitive part of the circuit

2. Is putting the OD trim outside the chassis truly worthwhile?
2a. Is an OD trimmer with more range a worthwhile change as well? I’ve read here that it can be helpful when dialing in the OD.

I don't think so. In my experience it's pretty much set and forget. Again, adding needless wire to a sensitive part of the circuit.
2.a HAD used anything from 100K thru 1Meg. I don't believe he did it for more range though. The larger the trimmer, the gainier the amp. If doing a #0102 I would highly recommend going with a 100K.

3. When building 102 as a 50w, would I need to adjust any power supply values (resistors on the PS board etc)?

Not necessarily, you may want to reduce the reservoir caps by half but the dropping string should be the same.

[Raoul Duke]

Thanks Ian!
I’ve read a bunch of your threads and appreciate your feedback here for sure! I tend to think in the same terms of trying to reduce risk by not adding features that are more novel and less practical as well; hence my questions I guess.

Very meaningful advice! Thanks again!

Hoping for more perspectives as well…

[ijedouglas]

Thanks Ian!
I’ve read a bunch of your threads and appreciate your feedback here for sure! I tend to think in the same terms of trying to reduce risk by not adding features that are more novel and less practical as well; hence my questions I guess.

Very meaningful advice! Thanks again!

Hoping for more perspectives as well…

One more word of advice. Build it as close to the original as possible. Get it up and running and sounding good before you start modifying it. Far too many builds have gone awry as the builder had elaborate plans but didn't get the base amp working correctly. Troubleshooting something like this becomes a nightmare

[Raoul Duke]

Thanks again, makes lots of sense!

Quick follow-up to the LNFB question: so would you just build V1 as the original? I’ve seen some builds where they’ve subbed two 22M for the four 10M. I have both and can do either, but was considering the two 22s just to save space. Probably a dumb question in light of “build it as is”, but wanted to ask so that I can hopefully wrap my head around how small changes can make big differences.

[martin manning]

2x 22M is fine, R-C-R. Build it as in the layout, and then you can ground either side of the cap to eliminate the LNFB and see how you like it. Add a switch if you think it's worth it. Personally, I love the clean tone with the FB.

[ijedouglas]

Thanks again, makes lots of sense!

Quick follow-up to the LNFB question: so would you just build V1 as the original? I’ve seen some builds where they’ve subbed two 22M for the four 10M. I have both and can do either, but was considering the two 22s just to save space. Probably a dumb question in light of “build it as is”, but wanted to ask so that I can hopefully wrap my head around how small changes can make big differences.

You can use 2 x 22Ms. HAD used 2 x 22Ms. In my previous build, which is an exact (as I can get it) clone of an actual Dumble, he used 2 x 22M NTE 1/4W flameproof resistors.

[Raoul Duke]

Thank you both!
Martin - I think I’m the proof that your board plans are idiot proof, lol. Really comprehensive and easy to follow!

[Raoul Duke]

Started the chassis work, but built the cabs first since that’s what I’m most familiar with. I build custom cabs for a local amp builder on the side.

[jelle]

Quick build summary:
- 50 watt 102 build using black panel Bassman iron all around
-->Nice, that works well
- added external bias points
-->I am not a fan, for the simple reason that when tubes go bad, its wise to check if all is still OK inside the amp.
- eliminated half-power switch
- made all my own boards using Martin’s excellent templates
- decided to use the Hoffman relay boards with 12v NTE relays and Martin’s suggested Hammond 166F12C trans
- got my chassis from Erwin, resistors from Jelle, and a lot of knowledge from everyone here - so thank you all!

I have a few subjective build questions that I’d like some informed opinions on if possible:

1. Does the switchable LNFB off V1 option provide a noticeable variation in tone?
-->Yes it does, and at the same time the preamp circuitry is adjusted to match this change. If you are unaware of the parameters, just copy the 102 layout

2. Is putting the OD trim outside the chassis truly worthwhile?
2a. Is an OD trimmer with more range a worthwhile change as well? I’ve read here that it can be helpful when dialing in the OD.
-->Outside trimmer: I am not a fan, and I have not seen it on Dumbles except one 150W ODR. I would set and forget, the tone changes radically and often not for the better.
A different value trimmer will radically change the amp. If you'd like to experiment, this is a good spot to play around.

3. When building 102 as a 50w, would I need to adjust any power supply values (resistors on the PS board etc)?
-->You can but I would not. Aim for 200W on V1A/B by adjusting the dropper resistor between the Screen Grids and the Phase Inverter.

Got some pics to post and will start a build thread soon - just a little bashful about my abilities, lol.

Any help, advice, opinions, suggestions are truly appreciated!
Thanks again and regards,

Marc

[jelle]

The one thing I would be worried about is the amount of global negative feedback. If you have my 100W SN#102 resistor set, I'd run it off the 8 Ohm tab to compensate for the reduction of negative feedback from going from 100W to 50W.

Have fun!

[Raoul Duke]

Thanks for the input Jelle!
Not sure if I have the 102 resistors. When I purchased the set - I wasn’t certain which circuit I was using so I just asked you for a 4th gen resistor set. I can list out the values to determine which I have if that helps?

[martin manning]

Not sure if I have the 102 resistors. When I purchased the set - I wasn’t certain which circuit I was using so I just asked you for a 4th gen resistor set. I can list out the values to determine which I have if that helps?

This is a 100W vs 50W thing. Voltages throughout the amp are the same until you get to the speaker, where the 50W's will be lower, (50/100)^0.5 = 0.707 times what a 100W would have. The feedback resistor should be reduced by that factor to achieve the same feedback voltage with respect to the PI input, or the source can be moved to the 8Ω tap which will increase the feedback voltage by that same factor.

[jelle]

Thanks for the input Jelle!
Not sure if I have the 102 resistors. When I purchased the set - I wasn’t certain which circuit I was using so I just asked you for a 4th gen resistor set. I can list out the values to determine which I have if that helps?

if you have 330r and 4.7k....use the 4 Ohms tab

[Raoul Duke]

Yes, I have 330r and 4.7k.
Just as a learning opportunity for me - based on Martin’s post - what would be the correct values for 50w?

As I’m certain you guys know - my OPT has 2, 4, and 8 ohm taps. Would this make the 2 more useful?

My apologies, I know, my questions are probably pretty simple to someone with the actual knowledge. I appreciate any opportunity to learn though.

Thanks again guys! I really appreciate you all taking the time to keep me on track.