ODS #183 Build

[pfarrell]

Couple of questions... I inadvertently followed the #124 layout for inputs vs. the #183—seems the difference is really grounding—#124 assumes a good contact with the chassis? and has the jack grounds linked—#183 has a connection to GND 1... I'm not sure if the differences matter...I've never the double TRS style jack before...

The other question is pedal related—is DIN the move? I have a hole that sized in the chassis and female DIN jacks—could change it to something else with a new/rev back plate. I see on one schematic 1/4" jacks referenced for the pedal—seems not enough connections there though.... I don't object to DIN. I didn't buy any plugs yet—but might have one around here for tonearm stuff (vinyl). Will have to sort out a custom pedal...

I have the PI wired... and the other connections Martin referenced... Need to do heaters next. Got sucked into the front switches... they were fun.

[martin manning]

The net result of the inputs should be the same. I would add the redundant short wire to the ground lug.

DIN jack is the best option iMO. 5-pin with 180 degree included angle for the pins is MIDI standard, meaning a cable is easy to get or replace. 5-pin can handle three functions using the Dumble technique of running 12V to the foot switch for the LED's. The TRS audio jack system puts the relays and their LED's in series, so only three conductors are needed- two functions plus ground.

[pfarrell]

Thanks Martin! Redundancy was my instinct.. Roger that, on the DIN.

[pfarrell]

Front shot... Obviously kept the design ethos, but drew it from scratch and fonts I'm assuming are different. File available...

PF-183_19.jpg

[pfarrell]

Well..... SHE WORKS!!!! Haha... pretty happy about that. I don't know an amp like this at all...and have to take the kiddo to song writing camp in the am—so I've only spent 10 minutes with it to just make sure it wasn't going to blow, play some chords, twiddle all the knobs—Check the voltages and flip the switches—knobs were facing away from me on the bench... so that's where it stands for "evaluation" in this moment. Now, I'm not sure EVERYthing is okay—mostly since I don't really know what exactly to look for with the relays etc. All knobs have an effect—all front switches do something—Rock/Jazz is pretty subtle—Mid has a satisfying effect, big difference there—the bright switch is also effective. BUT, the most awesome thing on this first power up is—this is the second quietest amp I've built to date—tied with my 5E3 build. Like, dead quiet as far as noise floor into the EV 12". I was pretty shocked about that actually. And sounds super good to me on a quick play—couldn't find the break up point yet—nice clean tones (and LOUD). Here's the voltages:

B+ 486VDC
B+2 443VDC
B+3 419VDC
B+4 306.5VDC
B+5 300VDC

Bias: -41VDC (I dialed this in based on one of the schematics please advise—test jacks?)
Trimmers on the amp board? B+3 and OD settings? Dial in to voltages on the schematic?

(Oh, and NFB was right on the getgo—I'm actually 75/25 for good guessing with all the amps to date!)

Pretty full up with gratitude to have access to an amp like this—and huge thanks to all the fine contributions from you guys.

PF-183_20.jpg

[ijedouglas]

Well..... SHE WORKS!!!!

Great looking build! Congrats!

Check your V2a grid wire. Not sure from the pic if it crosses the plate wire. If so, you may want to separate them by as far as possible. You may also want to run your V2B plate wire alongside your V2A cathode wire (same as V1). Play around with that but I've had better results running them together for 1-2 inches. One other thing you may want to check is your tone stack wires. Try and keep them away from your input jack. I ended up moving my OD relay further left so I could keep them clear of the input.

Screenshot 2022-07-31 200317.png

[martin manning]

Well..... SHE WORKS!!!! Haha... pretty happy about that.

Fantastic, and it looks great!

Here's the voltages:
B+ 486VDC
B+2 443VDC
B+3 419VDC
B+4 306.5VDC
B+5 300VDC

Those look great.

Bias: -41VDC (I dialed this in based on one of the schematics please advise—test jacks?)

Check the voltage at each of the power tube cathode test jacks. I would start by setting bias for 65% of the EL34's 25W max dissipation: 0.65 * 25 / 486 = 0.033V, and see how that sounds. Swap the power tubes around to get the best balance (the highest and lowest reading on one side and the two in the middle on the other, e.g.).

Trimmers on the amp board? B+3 and OD settings? Dial in to voltages on the schematic?

Center the PI trimmer (at B+3) to start. There's a long thread on setting it stickied in the discussion section: https://ampgarage.com/forum/viewtopic.p ... 91#p207691. Use a scope if you have one, or you can use the DMM technique. Either way it's best to have a signal source to inject at the power amp input.

The OD entrance trimmer sets the OD distortion floor. You can set that to taste, but start with the wiper at ~25%.

How about the FET? Is it working? Did you get it biased before installing the board or is that yet to be done? Measure the voltages at the top and bottom of the 10k drain resistor and report back.

[pfarrell]

All good, thanks a ton guys!

I'll follow all the guidance—and report back. Re: FET, I didn't try it last night—but did go through the setup process you outline, Martin, in one of the PDFs with the 9V batts after it was built. All checked out—is that the bias procedure? I didn't look at actual B+ to the FET board yet.

Ian—of course I appreciate your lead dress notes—and will obviously take them seriously (not sure I will jump on moving the OD relay board immediately, not hard to do really—just have to get over my OCD about vacant chassis holes!—the pic I put up makes everything look tighter than it is in actuality—yours does have more room there by the inputs I do believe.)—but I'm curious....If the noise floor of the amp is seriously wow quiet, and it is—assuming of course I don't have the bias way low (don't think I do)—does the noise floor quietness indicate good lead dress etc? I think I put some pretty quiet tubes in there too. Or are there certain intonations parasitic playing harmonics etc. that would/could be compromised by the slightest lead dress detail? That would be real hard for me to eval with my current understanding and might venture into woowoo (IMO) and I'd think would be obfuscated by guitar/player/speaker/room...but of course you have built a bazillion of these amps and I value your opinions....!! (Not trying to start anything here—online communication can be faulty to say the least—and I'm open to all ideas—and there is no "tone" here with my question!!! Just my own experience...)

One tech question related to the pedal functions. The schematic isn't helping me understand this... When I switch PAB and OD to "manual" vs "pedal" what should happen there? The pedal would have an "ON" and "OFF" state with the pedal switches. And.. on the layouts there's pin "1" on the DIN that connects back to the relay supply board—Martin's iteration of that board doesn't have the 1K resistors there...? These go in the pedal box in this case?

[martin manning]

Re: FET, I didn't try it last night—but did go through the setup process you outline, Martin, in one of the PDFs with the 9V batts after it was built. All checked out—is that the bias procedure? I didn't look at actual B+ to the FET board yet.

Yes that procedure sets the bias. Measure Vdd and Vd (top and bottom of the 10k) and see where it is now (running off the amp's B+5).

does the noise floor quietness indicate good lead dress etc? I think I put some pretty quiet tubes in there too. Or are there certain intonations parasitic playing harmonics etc. that would/could be compromised by the slightest lead dress detail?

Low noise flow indicates good lead dress and good grounding. Once you start sending signal through there are opportunities for feedback and/or signal loss to occur. That can be minimized by having wires (especially in-phase wires) cross at 90 degrees. Running the plate and cathode leads in parallel for an inch or so adds a small (1-ish pF) capacitance that could cut high frequency gain, potentially tuning out some very high frequency noise or oscillation. Some people have reported sensitivity in the V2a grid area, but if the grid and plate wire cross at 90 degrees it's probably fine. You can push the wires around a bit and see if you hear anything changing.

When I switch PAB and OD to "manual" vs "pedal" what should happen there? The pedal would have an "ON" and "OFF" state with the pedal switches. And.. on the layouts there's pin "1" on the DIN that connects back to the relay supply board—Martin's iteration of that board doesn't have the 1K resistors there...? These go in the pedal box in this case?

When a relay switch is set to manual it energizes the relay coil (by grounding it), and then the pedal doesn't have any control over it anymore. If the switch is set to pedal, the pedal has engage/disengage functionality. Yes, you need current limiting resistors in the foot switch for the LED's.

[ijedouglas]

does the noise floor quietness indicate good lead dress etc? I think I put some pretty quiet tubes in there too. Or are there certain intonations parasitic playing harmonics etc. that would/could be compromised by the slightest lead dress detail?

Yes. In my experience it is not just about noise floor but (as Martin stated) phase relationships.
I noticed an increase in sustain when moving the tonestack wires away from the input jack. You may be good with leaving the relay where it is though and just routing them further away.

[pfarrell]

Yep—got it. What about under the OD board? There's space under there—and I could add more easily, shift the whole bundle to the right of the PAB board—sonic conflicts?. Also usually I twist all the stack wires... not sure why I didn't here. I'll definitely do some chopsticking. I wasn't expecting it to be as quiet as it is...again—you guys have been invaluable!

[ijedouglas]

Yep—got it. What about under the OD board? There's space under there—and I could add more easily, shift the whole bundle to the right of the PAB board—sonic conflicts?. Also usually I twist all the stack wires... not sure why I didn't here. I'll definitely do some chopsticking. I wasn't expecting it to be as quiet as it is...again—you guys have been invaluable!

Yup, that may work. I haven't seen where HAD did this but I think as long as you can get them away from the input, you should be good. Most of the pics I have seen has them running next to the OD relay standoff and them between the 2 relays

[pfarrell]

When a relay switch is set to manual it energizes the relay coil (by grounding it), and then the pedal doesn't have any control over it anymore. If the switch is set to pedal, the pedal has engage/disengage functionality. Yes, you need current limiting resistors in the foot switch for the LED's.

What's causing "engage/disengage" when in manual? Or am I complicating this and "manual" IS engaged (period), which would mean one can't hear "disengaged" without the pedal? I have footswitches...I even think I have a box, was planning something out of wood... but...

Pin 1 on the DIN is left not connected to anything... I'd imagine the dropping LED resistors would just be utilized with the 12V feed (in the pedal)?

[martin manning]

What's causing "engage/disengage" when in manual? Or am I complicating this and "manual" IS engaged (period), which would mean one can't hear "disengaged" without the pedal? I have footswitches...I even think I have a box, was planning something out of wood... but...

"Manual" grounds the relay coil and engages the function, even if the pedal isn't plugged in. If the pedal is plugged in, then that function is engaged regardless of what position the pedal switch is in. The chassis switch must be set to "Pedal" for the foot switch to work.

Pin 1 on the DIN is left not connected to anything... I'd imagine the dropping LED resistors would just be utilized with the 12V feed (in the pedal)?

The jack has a 12V pin, a ground pin, and one pin for each function. That means there's an extra pin, which you can use for a third function if you want.
The 12V pin goes to all LED/resistor pairs in the pedal, and they are grounded by the pedal switches along with the relay for each function.

[pfarrell]

PF-183_21.jpg

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