Dumble Overdrive Special #124 build video series

[Charlie Wilson]

Hello, one minor detail I have wondered about on the #124 layout are the .1 100v caps on the PAB and OD switches. I assume they are there to smooth out any transients or spikes in the relay voltage but why are the ones in the footswitch .01? In other words, why are the caps in the footswitch and on the toggle switches not the same? I assumed that may have been a mistake(very minor one) in the layout.
CW

[martin manning]

The 0.1u caps are presumably placed across the switch contacts to prevent arcing and associated contact wear. Note however that when the switch on the rear panel is set to "PEDAL," the 0.1u cap in the amp chassis is electrically across the remote switch, so there is really no need for another one. The diodes placed across the relay coils are a form of arc suppression too, so there is already some redundancy. Admittedly there is some additional wire inductance in the footswitch cable, but I didn't put any caps in my footswitch housing, and it works just fine.

[pompeiisneaks]

The 0.1u caps are presumably placed across the switch contacts to prevent arcing and associated contact wear. Note however that when the switch on the rear panel is set to "PEDAL," the 0.1u cap in the amp chassis is electrically across the remote switch, so there is really no need for another one. The diodes placed across the relay coils are a form of arc suppression too, so there is already some redundancy. Admittedly there is some additional wire inductance in the footswitch cable, but I didn't put any caps in my footswitch housing, and it works just fine.

The schematic shows .1 the layout shows .01, I think since the ones on the back of the chassis are .1 the ones inside the foot switch should be the same in my mind. Glad to know skipping them is okay, but for this I already have some, so I'll just put them in

Phil

Sent from my phone using Tapatalk

[martin manning]

The diagram in the layout is authentic, I believe, based on a description of an actual ODS foot switch. The 0.1u value is commonly used for this purpose, but it could be that HAD just used what he had on hand. The LED's and their current limiting resistors are across the relay coils too, so there is yet another arc suppression device.

[pompeiisneaks]

The diagram in the layout is authentic, I believe, based on a description of an actual ODS foot switch. The 0.1u value is commonly used for this purpose, but it could be that HAD just used what he had on hand. The LED's and their current limiting resistors are across the relay coils too, so there is yet another arc suppression device.

Cool thanks for the info guys!

~Phil

[pompeiisneaks]

Here's video 9 of the Dumble Series, I've finished the footswitch up, and have a livestream Saturday, come along for the ride:




Livestream:




Saturday 3/31 11am PST, 2pm EST

~Phil

[pompeiisneaks]

I'm going live here shortly, join in the stream! 7pm PST, 10pm EST:




~Phil

[pompeiisneaks]

I'll be livestreaming the dumble first power up tomorrow 11am PST, 2pm EST (Saturday 4/21) come join in!




For those that haven't yet seen any of it because you build and it's probably not so interesting to see someone do it 'yet again' this may still be interesting to watch
it pop some caps or something

~Phil

[Matt J]

I'll be livestreaming the dumble first power up tomorrow 11am PST, 2pm EST (Saturday 4/21) come join in!



For those that haven't yet seen any of it because you build and it's probably not so interesting to see someone do it 'yet again' this may still be interesting to watch
it pop some caps or something

~Phil

Sweet! I'll be home at that time and should be able to tune in! I'll have to get some popcorn ready, hahah.
I hope it goes well!
- Matt J.

[pompeiisneaks]

I do too, but by the same token, if something doesn't work, its' a learning experience for me, and for anyone else that watches

~Phil

[pompeiisneaks]

Okay so if you haven't seen, the build is complete, and working pretty well, so far, with a few issues I still need to sort out:

1. fet input has no sound, input after the 150k resistor is at 8VDC, but I've not had time to even look at it yet. I'll play with that later.
2. output is pretty soft. I've got it at max volume and max master volume and with PAB engaged I hear a volume spike, and with OD I get good driven tone, but it seems to oscillate.

I'm wondering if part 2 is due to the PI Outputs being out of phase with NFB? I may try swapping those, but would that also make the output quieter? I know the output from the PI is nice and strong, I put in a signal at the input at the millivolt range and I think I've got about 50V at the PI output on each leg, I've balanced it as well and get a nice well balanced looking signal there also.

Any ideas what could be causing the power side to be weak and have the oscillations? IS that the NFB being PFB? I thought it would screech horribly AND be loud, not quieter?

I'm at work and will have to take some pictures later, but thought I'd ask if anyone had ideas.

~Phil

[Charlie Wilson]

Hello Phil, first thing I would do is measure the output wattage in clean mode. It should put out about the same wattage as a Twin and that will give you a baseline to go from. Low volume is kind of a vague description. I would also measure voltages all the way across. The voltages on the power tube screens, grids, and plates should be very close. I don't think the NFB is your problem. I have repaired hundreds of amplifiers and one of the mistakes I still sometimes make is fixating on what I think the problem is and not being more open to other and sometimes more obvious issues. Your problem could be a contact issue, a component issue, etc. I would think if you had a really bad parasitic oscillation issue the signal on your scope would not look so good. Good luck!
CW

[pompeiisneaks]

Thanks, I need to get photos for people to view, but I'm also going to do another livestream tomorrow night at 7pm PST, 10pm EST if people want to join in

~Phil

[pompeiisneaks]

So I've got the FET working, did a video on that, was a 'doh' moment, I was assuming (made an ass outta you and me) that the pinout was top down, but it's bottom up. Once I realized that, I had to pull the FET out, rearrange things, and rebuild the board. Works a charm now. It does seem to have one side of the input squashed, is that part of the 'bias'? I have seen the bias procedure and was trying to figure that out, but the schematic shown in that page is nothing like the #124 FET board. Any tips would be helpful, what resistor do I need to 'swap' to adjust the bias? Is it the 10k in line? If so, can I swap that out with a 10k variable pot and make it adjustable?

Next question, if you listen at the end of the video, I'm still trying to figure out why output is so horribly low and why I seem to get a bad oscillation. Tapping around hasn't found it, I've not yet had time to sift through it with a scope to find the source, but I did accidentally notice something when I was tapping the power tubes to see if they could be microphonic. When my hand got close to the OT the noise changed significantly. When I tap it or squeeze it, the noise changes, it doesn't go away, but it changes. You can hear the sound in the end of the video below.

Any ideas if this means the OT is dead? or is that common with parasitic oscillations?

Additional data I've gotten. The signal at the output of the PI is around 70 to 100V AC when measured with a scope at like 400mV input. Seems pretty well amplified to me. It's distorted obviously at max volume, but not horribly so. The OD switch seems to make it look right as well, so the noise seems to come in at the Output, I've measured all the voltages across the board and they're almost dead on identical on each tube. I've biased it and got it very close to 70% diss. It's about 77 watts dissipation at 0 volume. The signal seems to be around 7VAC at the speaker jack.

Not sure what's common in an amp like that.

Did I get a dud OT, or maybe fry it some when I had the speaker jacks miswired? (If you didn't see, I had the first switching jack wired right, but had the second one backwards so the input went straight to chassis ground).




Any suggestions and/or input welcome.

~Phil

[sluckey]

Power = voltage squared divided by resistance, so if you have 7V across a 8Ω speaker/load then power = 7*7/8 = 6.125W.

OT could certainly be the problem, but you already know what the number 1 cause for a new build not working is. I suggest you eliminate everything else before ordering another OT. Substitution is a valid troubleshooting tool if you have a suitable OT.