ODS #183 Build

[ijedouglas]

(Bus bar isn't touching the FET jack, weird image thing there.)

Very nice work!

[pfarrell]

Yay! It does make me feel better. Haha. Been reading the PI trimmer thread...

[ijedouglas]

Yay! It does make me feel better. Haha. Been reading the PI trimmer thread...

Don't stress the PI. I went way down that rabbit hole. Get the amp sounding good and get a feel for what it can do, then you can start mucking with the PI. The PI trimmer IMO is not a one-size fits all. It can focus or smear the sound and you may prefer either. I like to use the method described by Gil in that thread and then fine tune by ear (I'm still not sure I'm doing it right but I like what I hear ).

[pfarrell]

Been playing around... Anyone want to weigh in on some settings that you all like? Out of my own curiosity and learning....I know this could be a loaded topic...

I have to say—it's glorious. I don't have an amp that sounds/does whatever is going on here... It's completely rich...and super fun, beyond versatile. I know Martin answered this question—but I didn't frame it very well—with no pedal plugged in—and both switches on the back on "pedal" that's effectively "off" as far as OD and PAB are concerned—that seems to be how it's working...

I have "matched" tubes in... it seems that -41VDC on the bias put me right in the ballpark of .033mV on the test points... on each pair one runs a little over and one a little under, so that seems okay?—.031/.035 each pair respectively...

WHAT AN AMP!

(Oh, and FET jack totally works—didn't look at the voltage yet—amp is mounted in the cabinet so I can figure out what I'm doing there—sounds super crunchy with a regular guitar in—I don't have an amped acoustic at this time—totally nice sounds though...edgy or something... like a different voicing.)

Picked up some pine for a 2x12 cab. Good for Trainwrecks too...

PF-183_22-WOW.jpg

[ijedouglas]

with no pedal plugged in—and both switches on the back on "pedal" that's effectively "off" as far as OD and PAB are concerned—that seems to be how it's working...

- Correct

Been playing around... Anyone want to weigh in on some settings that you all like? Out of my own curiosity and learning....I know this could be a loaded topic...

Preamp vol at around 11

For HBs I usually have the bright and mid off with RJ in rock position. SCs I just pop the mid switch on.

TMB to taste... mine are set with T at just past noon, M at just before noon and B between 9 and 12. I usually turn the pot until I hear a change and then back off a smidge.

Presence around 6.

OD Drive at around 11 and OD level at noon

Master at 10:30 to 11 (pretty loud but that is where it starts to sound good)

What speakers are you using?

[pfarrell]

Thanks!!
EV in that cabinet in the pic, only speaker I have that will handle it...making a 2x12 with G12-65s... Damn it sounds really really good though—makes you want to play. We liked the Rocket in a different way (tried it as it was sitting on the bench), not nearly as versatile—but I really think that's about the speakers—EV probably not being the right choice for the Rocket (I have a creamback and a greenback that are likely gonna be good for it—plus the aforementioned 2x12). Guitar at hand has some mini humbuckers in it...my daughter was playing I was messing with the settings... and vice versa... so happy to have this amp!

[ijedouglas]

EVs are by far my favorite with an ODS. I have a 2x12 with a EV SROs (coffee can) and another 2x12 with EV12Ss. I have another 2x12 with G12-65s but I just can't bond with them (I have really tried). I just recently got the 12S's and had been using 12Ls in that cab. I think I prefer the 12Ss. The SROs are really nice but they are hard to find in good shape.

[bepone]

Been playing around... Anyone want to weigh in on some settings that you all like? Out of my own curiosity and learning....

jazz mode can be special, first volume to 3pm, drive vol also, drive volume to half or so, master as high as possible
+slide on it

[martin manning]

... It's completely rich...and super fun, beyond versital.

The ODS is versatile... it can go from singing clean to totally grungy. The key to using the OD is to recognize that the clean and OD stages are cascaded. The clean channel stands on its own, but in OD the clean volume and OD Drive level work together to make up the OD sound. If you are switching between the two, you may have to compromise, depending on which mode is most important. The Volume/Ratio control is just used to balance the OD/Clean volume (some versions have independent masters instead).

I have "matched" tubes in... it seems that -41VDC on the bias put me right in the ballpark of .033mV on the test points... on each pair one runs a little over and one a little under, so that seems okay?—.031/.035 each pair respectively...

Yes, and where you are now is fine. The exact bias voltage doesn't matter, it's more important to focus on the idle current and plate dissipation.

FET jack totally works—didn't look at the voltage yet—amp is mounted in the cabinet so I can figure out what I'm doing there—sounds super crunchy with a regular guitar in—I don't have an amped acoustic at this time—totally nice sounds though...edgy or something... like a different voicing.

Before multiple pickup systems with on-board preamps were common in acoustics, the high input impedance of the FET worked well with a simple piezoelectric bridge pickup. That may have been one reason it was there, but it can also function as a clean boost to drive the input stage harder and fatten up a SC guitar. Several people have added a third relay for a FET boost function, and moved the FET gain pot to the front panel in place of the FET input jack.

What did you do with the PI balance?
I've had my 124-ish amp out of the box for the past couple of days for some tweaking. Hard to believe it is almost 12 years old! The easiest way to get the PI balanced is to feed a clean sine into the power amp input to get 40-50W undistorted output power, measure AC Vrms at the PI plates with a DVM (at the junction of plate load resistors and coupling caps on the preamp board), and adjust the trimmer to match the readings. If you have a scope you can match the peak AC voltages at the PI plates, and if the scope has FFT, you can minimize the second harmonic in the output signal, which should be essentially the same setting as long as the power tubes are well matched.

[pfarrell]

Thanks guys! This is all super good info.
Martin—haven't messed with the PI balance yet. I'll get into it. I'm super pumped about this amp....haven't stopped thinking about it...
This is the first 100W amp I've built—is any part of how amazing this is due to it's raw power potential, say compared to my Vibroverb? Even at low volumes, both of us just want to keep cranking it up.... it's addictive in a certain way—different from the other amps we've built... just hunting for what might be going on here and it's all so subjective.... and my guitar skills are less than a couple years old...

I'm pretty jacked to get the Ceriatone fixer, fixed—to see what's up there by comparison. I think it's #124 based? Somebody totally messed it up very nicely—it was certainly a kit build....

[ijedouglas]

The easiest way to get the PI balanced is to feed a clean sine into the power amp input to get 40-50W undistorted output power, measure AC Vrms at the PI plates with a DVM (at the junction of plate load resistors and coupling caps on the preamp board), and adjust the trimmer to match the readings. If you have a scope you can match the peak AC voltages at the PI plates, and if the scope has FFT, you can minimize the second harmonic in the output signal, which should be essentially the same setting as long as the power tubes are well matched.

I used to use this method but it does not take the power tubes into consideration and only balances the PI output to the plates. I prefer the method described by Gil https://ampgarage.com/forum/viewtopic.p ... 78#p216778 where you balance the cathodes of the push-pull pairs. It's still not totally accurate as it doesn't account for transformer mismatch, but I feel it gets you a little closer than just balancing the PI plates. I think it is just as easy.

[martin manning]

I'm on record liking that method too, which would more correctly be called the Ampeg voltmeter method (from SVT schematic). IMO an additional requirement (whichever method is used) is to drive the power amp directly to avoid any 2nd order harmonic that might be coming out of the preamp (and I've edited my post over in the PI balance thread on pg.2 to say that). I think you can get very close by equalizing the PI outputs. Using cathode current might be better, but there is screen current included which doesn't go through the OT primary. Minimizing the second harmonic in the output is really the best method since it looks at the final product.

Today I matched the cathode current peaks (DC coupled into a scope) with the trimmer, and found the PI outputs (AC) to be matched as well. I checked the DC voltage across the cathodes and measured 3 mV, so the three methods are essentially in agreement. I didn't take the time to measure the power tubes, but one side of the power stage idles a few mA higher than the other due to one of the four tubes running ~5 mA higher than the others. I did put a matched tube in the PI, and the trimmer is nearly centered with 110k and 100k plate resistors, .

[pfarrell]

Whew—been a bit... finally a home for this super special amp. Finishing touches next (aluminum and diamond cloth)...Couldn't have done this without some super peeps on this forum!

PF-183_24.jpg

[martin manning]

Really nice!

[pfarrell]

Thanks Martin! Yours and Ian's guidance made it possible!!....(Visually, the next Dumble build will be that brown suede/silver combo I've seen around...).
Gotta build a pedal now...

PF-183_26.jpg

PF-183_25.jpg

PF-183_27.jpg