A #102 Attempt - First amp

[Vertigo]

The rest of the preamp tubes are in...no sound.

[martin manning]

Get voltages on all of the tubes, pins 1, 2, 3, and 6, 7, 8 on the 12AX7's, and 3, 4, 5, and 6 on the power tubes. Are the heaters all glowing?

[Vertigo]

scratch that...it was a bad speaker cable. I tested it, replaced it and then thought I had no sound again. I cranked the volume controls and got the slightest noise out of my guitar. I happened to start flipping switches when my wife walked into our office just as I got to the OD switch and holy hell! We both just about loaded our pants.

So I have the clean channel but no OD.

[Vertigo]

V1
1: 172
2: floats around 0mV
3: 1.92
6: 161.1
7: floats around 0mV
8: 2.12

V2
1: 194
2: 0
3: 1.701
6: 196
7: 0
8: 1.702

V3
1: 290
2: 32.8
3: 53.9
6: 287
7: 30.6 (it's very loud when the probe hits this pin, regardless of amp volume knob)
8: 54.1

[Vertigo]

I turned up the trimmer on the OD and now it's good. I'll recheck the voltages after I goof off for a while.

After that, I need to sort out the FET. It doesn't seem to be doing anything at all.

[martin manning]

Ok great! Voltages look good, maybe a little low on V1. Check the voltage on both ends of the 10k drain resistor on the FET board and post them. Remember there is a trimmer on the FET board too. Make sure it’s up about half way.

[Vertigo]

Ok great! Voltages look good, maybe a little low on V1. Check the voltage on both ends of the 10k drain resistor on the FET board and post them. Remember there is a trimmer on the FET board too. Make sure it’s up about half way.

B+ on the FET is 307
14.12 on top and 22.6 on the bottom of the 10K resistor. I biased the FET using your trick but with a benchtop supply set to 18V rather than two 9v batteries as the voltage was dropping rather quickly while I was fiddling with it when I used the batteries. I landed on a 3k3 resistor which gave me 9.4V.

I've got a push/pull 10K pot in the FET hole rather than the trimmer and adjusting it doesn't seem to to anything at all. In/out, min/max, it's all the same. I wonder if I borked up the relay wiring

On the layout there are notes for V1 and V2 that say "200V" and "205V" respectively. Is that what the plate voltages are supposed to be? The location of the notes is a bit ambiguous to me

[martin manning]

Yes I see you have the FET relay set up. The voltages look like the bias cold be a little hotter (you’re only dropping 8.5 of 22.5V, but it should function where it is. What value did you wind up using for the source resistor? I think the problem is most likely some wiring error.

[Vertigo]

Yes I see you have the FET relay set up. The voltages look like the bias cold be a little hotter (you’re only dropping 8.5 of 22.5V, but it should function where it is. What value did you wind up using for the source resistor? I think the problem is most likely some wiring error.

I ended up with a 3K3 resistor. With 18V applied it was reading 9.4V at the drain. The 3K gave me 8.7V

I'll check the relay wiring again.

I think we were posting at the same time, so I'll ask this again...On the layout there are notes for V1 and V2 that say "200V" and "205V" respectively. Is that what the plate voltages are supposed to be? The location of the notes is a bit ambiguous to me

[Vertigo]

I think the problem is most likely some wiring error.

I think I just found it...forgot the ground wire on the push/pull

[martin manning]

Ok, good. Sorry, missed the 3k3 in the post above.
Yes those are plate voltages on the layout They can vary quite a bit due to The variation in tubes. Swap V1 and V2 and you can see what that does. I would select a quiet tube for V1.

[Vertigo]

Ok, good. Sorry, missed the 3k3 in the post above.
Yes those are plate voltages on the layout They can vary quite a bit due to The variation in tubes. Swap V1 and V2 and you can see what that does. I would select a quiet tube for V1.

Thanks, I'll try that. I've got a 5751 from Tube Amp Doctor in there now. I have another and a bunch of 7025's that are set aside for the 002.

I just put the ground wire on the FET switch and that was it. Sounds pretty darn good.

Thank you for all your help, Martin. I'm sure I'll ask a bunch more simple questions but I really appreciate all your advice. This has been a great learning experience. I'm sure the 002 that I'll get working on soon will greatly expand on this. I'd better make a headcase for this first though.

[Vertigo]

I was able to spend about an hour playing and fooling around with the amp today and naturally I have some questions.

Does the OD trimmer do some magic that I need to figure out? I started out with it turned up about 25% from fully off and the OD volume knob was set to about noon for it to be about the same volume as with the OD off. When I turn up the trimmer, I need to make a corresponding decrease of the OD volume but it doesn't seem to get any dirtier. I have not turned the trimmer full up, mostly because I'd have to barely crack the OD Vol open to match the volume of the non-OD signal. Is this the expected behavior?

I've read in several places that these amps sound great with humbuckers but sometimes not so great with single coils. That's definitely the case with this one. My PRS sounds amazing and my Strat and the tele copy I made sound pretty good, but they're a bit muddy in the mid range even with the mid-boost off, the middle knob at 8:00 and the bright switch on. When the OD kicks in, chords lose their note separation in an unexpected way. Just playing a few power chords on the strat, I checked several times that it was in tune because it sounded so bad. Turning the Drive level down helped significantly, but it wasn't very overdriven to start. It's not quite the same sound as the flubby bass heavy nastiness that affects some amplifiers (I have a Mesa MKV that I have to run the bass knobs nearly off) but more like a rolled back wah that still lets the high end through.

Last question for now...is there any reason that I shouldn't run the 12V power LED off the relay power supply rather than off the heaters?

The clean tones are pretty great. With my preamp vol set about noon, it's chimey and incredibly responsive to pick and finger pressure. As I turn up the preamp volume, it looses some composure and gets murky but I think I've found it's happy place for now. Is this another situation where swapping in different tubes could be the answer?

edit: I totally forgot to ask the most important question for now. When it starts up, there's a random scratchy sound that sounds a lot like a dirty pot or bad instrument cable connection, but swapping cables doesn't change anything. I poked every single wire, component and tube socket pin with a chopstick and while there were a few things that are microphonic, nothing creates the scratchy noise. After about 5min, the noise stops and doesn't come back but it's loud and doesn't seem to be affected by the volume level. Any ideas about that one would be much appreciated.

[norburybrook]

no reason at all not to run the power LED from the relay board I've done that on a build and it's a lot easier/nicer regarding cable runs.

the #102 doesn't have a separate master for the OD and clean. The master is the overall master and the ratio control is a blend/level control between the OD and clean. if you look at pics of #102 amps live the you'll always see the ratio set below 12 O'Clock for this reason to balance the levels.

the #102 file show that the OD trimmer is set at about 23K IIRC so that's about 1/4 of the range, again IIRC.

#102 isn't a 'gainy' amp by modern standards but the OD should be clear and you should have good note separation on chords. Just think early Robben ford tones and that will give you a good idea of what to expect. His classic OD tone is humbuckers on the bridge pickup but he plays a tele as well.


as an aside, the Dumble ODS isn't for everybody. I've quite a few pro guitar friends who don't like the Robben Ford OD tone at all so bear in mind just because they're incredibly expensive and legendary amps doesn't mean they're for everyone and everyone should like them.


M

[martin manning]

Does the OD trimmer do some magic that I need to figure out? I started out with it turned up about 25% from fully off and the OD volume knob was set to about noon for it to be about the same volume as with the OD off. When I turn up the trimmer, I need to make a corresponding decrease of the OD volume but it doesn't seem to get any dirtier. I have not turned the trimmer full up, mostly because I'd have to barely crack the OD Vol open to match the volume of the non-OD signal. Is this the expected behavior?

about 25% is the place to put the OD trimmer. I look at it as setting the minimum level of OD distortion. Since the clean and OD stages are in series, the clean volume affects the OD. It sounds like you need to use more clean volume. There is a lot of flexibility in the settings, and it may take some time to get used to them.

I've read in several places that these amps sound great with humbuckers but sometimes not so great with single coils.

Have you tried the FET boost with your SC guitars?

is there any reason that I shouldn't run the 12V power LED off the relay power supply rather than off the heaters?

That's the best way, IMO.

The clean tones are pretty great. With my preamp vol set about noon, it's chimey and incredibly responsive to pick and finger pressure. As I turn up the preamp volume, it looses some composure and gets murky but I think I've found it's happy place for now. Is this another situation where swapping in different tubes could be the answer?

V1 is the most critical tube, IMO, since it is in the lowest S-N position in both clean and OD. Select one that is both quiet and bright.

When it starts up, there's a random scratchy sound that sounds a lot like a dirty pot or bad instrument cable connection, but swapping cables doesn't change anything. I poked every single wire, component and tube socket pin with a chopstick and while there were a few things that are microphonic, nothing creates the scratchy noise. After about 5min, the noise stops and doesn't come back but it's loud and doesn't seem to be affected by the volume level.

This is likely to be caused by a bad component or solder joint. Its disappearance with time suggests that it is temperature dependent, and just poking at the offending part may not expose it. Can you isolate it to preamp vs. power amp, and clean vs. OD using the channel switching and volume controls? For example, does it appear in both clean and OD, and if you turn the master all the way down does it stop? If so, it's not in the power amp.

Now that you have everything working, can you get a set of voltage readings for both the tube pins and the power supply nodes? Measure your line voltage too. Have you done anything with the PI trimmer? For now, you can set it at mid range.