102 build advice

[cdale-amp]

All,
I have some questions regarding my recent build and would appreciate any suggestions.

Quick background:
I got into amp building last year and completed a 5e3 kit. For my second build I wanted to try a full build on my own and chose ODS 102. I have no background in electronics but am a math and science teacher so I kind of have the brain. I've watched Uncle Doug videos, scoured Rob Robinette's, Aiken's, and this site, read Merlin's book, etc. On a personal note, I'm totally hooked and if anyone has suggestions to help further my education I'd gladly accept them.

My amp:
My build is a 102 with a few minor exceptions: switchable LNFB on V1B (because 183 did not include this, so I could try both variations), 3 position bright switch like the 183, 183 rectifier/bias board, and 10K variable resistor in series with the 4.7K NFB. I also built my own chassis with an old aluminum street sign. In the photo you'll notice I started to cut the PT hole on the wrong side, first cut, not an auspicious start. Also, the aluminum is starting to tear at the folds, bummer.

The tone is great with plenty of volume. However, I'd like to tweak the OD tone if possible. If I pick a single note it starts great, good attack and pick percussion, however, as the note decays it fizzles a bit (hard to describe sound with words) sizzles, warbles, etc. It lacks the smoothness I read about from other descriptions of these amps. With OD off the amp sounds great: quiet with good sustain.

My two biggest questions during the build were: 1) what types of components to use, 2) how to avoid electromagnetic field interference.

With regard to components, I chose to go with metal foil resistors with larger than necessary wattage (to limit hiss), and polypropylene film capacitors. When selecting, I looked for a balance between $, tolerance, operating temperature, and temperature coefficient. But I don’t know if there are other determining factors I should have considered. I plan on swapping components in the OD circuit to carbon film and orange drops to hear the sound difference. Lacking a deeper understanding, I’d appreciate any suggestions regarding changes, with reasoning if possible to help my education.

My voltages are (100W setting): B1 – 445V, B2 – 443V, B3 – 429V, B4 – 335V, and B5 – 330V. Not sure why B4 and B5 are higher than “normal”. When purchasing resistors for the power supply I got wire wound 10W, overkill. I did not realize how big they are so I needed to change the layout to accommodate, but it’s basically the 102.

Tubes: I read Robben Ford currently has JJ tubes so I got JJ-6L6GC power tubes and JJ-ECC83 S (12AX7) pre-amp tubes. I’ve read about old stock tubes and specific tubes HAD used, but I just wanted to get this thing fired up so I spent no time tube searching. I’m guessing I could refine my tone with some different tubes, particularly with the pre-amp. Again, suggestions? I'm also guessing the tone will change as I burn in these tubes?

With regard to EMF, if anyone is willing to look closely enough, please let me know if I missed something. I’m particularly unsure about the transformers and power tube wiring. I think I did the heaters as recommended. Or, let me know if I totally missed something and created a dangerous situation!

Thanks for any suggestions. This site has been a huge resource for me so also thanks to all of the previous posts I’ve read.

Mark

[drew]

I plan on swapping components in the OD circuit to carbon film and orange drops to hear the sound difference. Lacking a deeper understanding, I’d appreciate any suggestions regarding changes, with reasoning if possible to help my education.

My voltages are (100W setting): B1 – 445V, B2 – 443V, B3 – 429V, B4 – 335V, and B5 – 330V. Not sure why B4 and B5 are higher than “normal”. When purchasing resistors for the power supply I got wire wound 10W, overkill. I did not realize how big they are so I needed to change the layout to accommodate, but it’s basically the 102.

Tubes: I read Robben Ford currently has JJ tubes so I got JJ-6L6GC power tubes and JJ-ECC83 S (12AX7) pre-amp tubes. I’ve read about old stock tubes and specific tubes HAD used, but I just wanted to get this thing fired up so I spent no time tube searching. I’m guessing I could refine my tone with some different tubes, particularly with the pre-amp. Again, suggestions?

You need to get the voltages right before you start swapping components and tubes. Do you have an as-built schematic or layout for the power supply?

This probably should be moved to the Dumble section.

[xtian]

Mark, great job on this. Dumbles are NOT beginner amps, and you've done a good job with the lead dress--you must have studied the photos of previous builds very closely.

You mention EM interference. Are you having trouble with hum or AM radio reception? Something else?

For the OD, fizzy decay is sometimes due to oscillation due to poor lead dress. But your amp is nicely wired. I might complain about the turret tops that are only partly filled with solder--it's hard to tell by looking if these are solid connections. Perhaps someone can offer a suggestion to improve your OD tone.

Don't worry too much about component choice. You'll find swapping capacitors to be very low on the scale of what changes tone for the better. Just choose good components that are rated for the job.

[Colossal]

Your B+4 and B+5 voltages are 330 and 350V but your caps are rated for 250VDC. Those should be 500VDC.

[cdale-amp]

Colossal, thanks for seeing that, I'll change it immediately, glad I posted the photos.

Xtain, thanks for the words of encouragement. In some cases when the turret heated the solder flowed down faster than I could feed it. In those cases there is a small solder void but all components are solidly soldered. Re components, I went in the direction of robustness as opposed to historical copy, glad to hear you don't think this was a mistake.

Drew, re B+, my B+4 is 335V and my B+5 is 330V but the data I found says they should be 320V and 310V respectively. I'm not sure what the tolerance is and if a 6% difference is a concern. The voltage on my plates, however, are exactly as stated on the schematic: V1A - 198V, V1B - 202V, V2A - 206V, and V2B - 208V. I do not have an as-built schematic but it is 99% the 102 I found on this site https://ampgarage.com/forum/download/fi ... &mode=view.

Thanks
Mark