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

[Raoul Duke]

After bringing it up to temperature - 30 minutes or so running - the bias has drifted down to 36 and 35 with the 2k7.

Brought it up to 40 and 41, but V1 still sits at 208 and 211.

Temped in a 4k7, readjusted bias, and I’m sitting at V1A: 200.7 and V2A: 206.3
FET is 10.2 top and 20.5 bottom

It’s pretty eye-opening the change in sound. Less constricted and more “open” or “deeper” in all modes. Subtle, but significant.

Do you think trying a 5k1 is worthwhile?

[martin manning]

That has brought it together pretty nicely. I’d say try the 5k1 just to see if there is anything left on the table. I believe there is some "burn-in" that happens too. It's probably time to make friends with it for a few days before considering any more changes.

[Raoul Duke]

Well, I was skeptical that such a small change would matter much - but I tried a 1w 5k1 Ohmite carbon film just to be sure (as Martin suggested). Warmed it up and played and it sounds even better. Thanks again Martin!

V1A: 199.1
V2A: 205.3
FET top 10.4, bottom 19.7
and oddly, both power tubes are sitting steady at 40 vs off by 1mv

My only (uneducated) concern is going from a wire wound 5w to a CF 1w; but I’ll make this 5k1 permanent if you say it’s good Martin

[martin manning]

Measure the voltage across that resistor and calculate the dissipation. V^2/R. You'll find it's pretty low, but I would get a larger WW when you can. The power tubes will usually take some hours to settle in, so no surprise there, and they may drift a bit more.

[Raoul Duke]

Will do.

Many thanks again!

[Raoul Duke]

Ok folks, finally got my Mouser box with the correct bias resistor and dropping resistor (finally) and did the installs.

I’m about to fire it up and re-check the bias again in light of the changes and to confirm settings and voltages. Got one question.

Reading through other build threads, I see reference to “burning in” - which in my previous life meant we lost an aircraft to a catastrophic failure(bad) but in amp building I’m seeing it as a part of the finalization of the build process(good).

My question is: “how long/how much?” I realize a fair amount of this will be me just wanting to play it constantly, but I’ve read reference to what seems like an underlying framework, i.e. “run it for x hours continuously, check the voltages periodically, be vigilant of noises, smells etc.”

What’s the “best practice” for this part of the build?

[martin manning]

You will get various opinions on that. It seems to me that a brand new amp sounds a little harsh/bright, and that seems to go away after a few hours. No scientific data, just my potentially flawed human psycho-acoustic assessment. I've read about boutique builders following a regimen of burn-in for all new amps, where they allow them to idle for some hours. Power tubes will often drift a bit as you have observed, so rechecking the bias once or twice is a good practice. You could look at your PI balance again if you are curious.

[Raoul Duke]

As always, thanks Martin!
I appreciate the wisdom!

Lighting it up now…

[martin manning]

...burn-in... ...we lost an aircraft to a catastrophic failure...

That would be auger in, with the result being a smoking hole.

[Raoul Duke]

And no days off for awhile…
Yikes!

As long as the seat worked and it didn’t happen in a populated area - everything else is replaceable, right?

[Raoul Duke]

I think it’s ready for the cabinet. Here’s the voltages after tinkering with the dropping resistor and running it for about 8 hours over the past two days. I ended up at 5k6.

V5 - 464 plate, biased at 41.1
V4 - 463 plate, biased at 40
B+1 - 471
B+2 - 468
B+3 - 429
B+4 - 327
B+5 - 319
V1A - 197.8
V2A - 202.4

I also now see how important it is to let the amp warm up for at least 20-30 minutes. Seems like everything settles down at that point and readings become much more stable.

Still learning stuff, lol…

[martin manning]

Looking good. I note that B+1 has drifted down a bit, but some of that that could be line voltage variation. it's a good idea to make a note of that when you take voltage readings, or even better adjust it to a standard 120V with a Variac. Did you happen to take a second look at the PI balance? It'd be interesting to see if it has moved in any significant way.

You've come up against many of the typical first time ODS builder questions along the way here, so I imagine this thread will be useful to others. It's been quite popular, almost 55k views at this point! What are your thoughts on the test-as-you-go approach to building? Would you use it on the next one?

[Raoul Duke]

I haven’t re-visited the PI yet; but will likely do it today before the cabinet install. I imagine now that everything is in place and running I can check it with a loop pedal into the front of the amp vs signal? Or should I use the 400hz into the front of the amp?

The Variac is something I have my eye out for. Seems like a handy tool even if you’re not a serious builder (although I plan on a couple more at least).

There’s a lot of lessons I’ve picked up along the way and made note of. Testing as I go is the route I’ll take every time until I’m positive I know what I’m doing with every step - which means I’ll probably always use that approach, lol.

I’m going to try to separate the technical “doing” notes from the conceptual “planning” notes and create a post for first-time builders like me. Looking back, if I’d had a better understanding of what I was looking at project-wise and understood key points about the lineage and attributes that separate the many circuits - I think I could have shortened the planning phase and been better prepared AFA procuring parts (schedule and cost over time) once I made a choice on direction and started “doing”.Hell, just learning how to navigate the Mouser web site was an education!

Thinking back to my first inkling that I might try building this from scratch, I’m very happy (and somewhat surprised) with/by how it turned out thanks to this forum!

[martin manning]

I haven’t re-visited the PI yet; but will likely do it today before the cabinet install. I imagine now that everything is in place and running I can check it with a loop pedal into the front of the amp vs signal? Or should I use the 400hz into the front of the amp?

Definitely feed generator signal into the "Power Amp In" jack. For accurate balancing you need a clean undistorted sine wave.

The Variac is something I have my eye out for. Seems like a handy tool even if you’re not a serious builder (although I plan on a couple more at least).

Very useful if you are fooling around with electronics or doing an occasional repair.

Testing as I go is the route I’ll take every time until I’m positive I know what I’m doing with every step - which means I’ll probably always use that approach, lol.

For hobby builders every amp is unique, so I'm a big fan of this approach. Completing and testing sections of the project helps to insure success as problems are discovered early and corrected, likely saving time in the end.

[Raoul Duke]

Just re-checked the PI balance and needed to adjust down about 1 mv, which wasn’t much - but definitely changed the sound.
Definitely worth the effort to re-check after final assembly; it sounds even better