First D-style (102) build opinions solicited & build thread:

[Raoul Duke]

This is great, thanks Martin!
I’m going to digest this and hopefully get it wired-up tomorrow. I appreciate you taking the time to break this down and explain like you do. I really am learning quite a lot

You must have some background in instructional system design and presentation? I’d bet on it, lol.

[martin manning]

I've had a side hustle teaching for a few decades ;^)

Move this 100Ω over to the other socket so it is grounded to the chassis directly.

[Raoul Duke]

Meant to ask about that and forgot after my smoke test.
Just tidying up some leads and preparing to wire up the PI.

Thanks Martin, thanks Tim!

[talbany]

It's hard to tell from this shot but you want to try and get that grid resistor away from your heater wires (AC) on the socket (pin 1) nearest to your power supply.
The one to the right looks OK.


Tony

[martin manning]

Tony has a good suggestion there. Run the grid resistors from pin 1 to pin [edit] 5 straight across the sockets, just at the level of the solder lugs, and bring the green heater leads up so they approach the lugs on the axis of the pins. Do the same with each segment of the heater chain.

[Raoul Duke]

Understood, sort of…

So I should take the 5.1k going from pin 1 to pin 5 and move it to pins 1 to 8 and shorten the leads?
Here’s a closer view:

[martin manning]

No sorry, keep it going from pin 1 to pin 5, just lower it to the level of the solder lugs.

[Raoul Duke]

Understood, will do

Very much appreciated Martin and Tony! Flattered that two fellers of such renown in the Dumble amp world are helping this complete novice figure it out. Truly grateful and humbled

[martin manning]

Well it's always fun to see a new amp come alive, and I appreciate that you have the patience to give the test-as-you-go approach a try. I do believe it offers more learning opportunity by focusing on the operation of one section of the amp at a time. You won't learn all the subtleties of amplifier design and construction (most of us are still working on that), but I feel you can get much more out of the experience than you would just sticking all the parts together. BTW those 5k1 "stoppers" on the output tube control grid pins are there for two reasons: One is preventing oscillation by making a low pass filter with the tube's internal capacitance, and that is why they are best mounted directly on the tube socket. The other is that they slow down charging of the coupling caps by grid current, which causes bias excursion and blocking distortion.

PS Since you're nearing the point for testing with tubes installed, do you have a load (resistor or speaker) ready to plug in? Does your DMM just have probes, or do you also have "grabber" test leads that can be clipped on? Do you have any clip leads (wire with alligator clips on both ends)?

[Raoul Duke]

I imagine I’ll be learning the basics for a long while yet, but I’m enjoying the process.
For instance: I just learned more about grid stoppers. I’d read about how they act as an LPF during my on-going research; but didn’t know about them slowing down the charging of the coupling caps - so “thanks” for that nugget

No sorry, keep it going from pin 1 to pin 5, just lower it to the level of the solder lugs.

Quick follow up: does it matter which end (pin 1 or 5) I connect the leads from the bias section of the main board? I suspect either way works, just wanted to double check.
Additionally - is it better to run B+ lines over or under signal lines? I want to get this correct the first time so that I don’t have to do it twice, lol.

Also; yes, I’ve constructed a simple load simulator using a 75w/8R wire-wound resistor and my DMM has probes currently. Been searching for a set of grabbers here and there when I think of it - but haven’t picked any up yet. I have a set of clip leads, but they’re only 6” long. Thought about lengthening them but wasn’t sure if that compromises the readings in any way.

Certainly open to advice on these points as well.

Thanks again!

[martin manning]

does it matter which end (pin 1 or 5) I connect the leads from the bias section of the main board?

Definitely. Pin 5 is the control grid, the input in this case, so the signal leads from the PI coupling caps on the main board go to pin 1's. Pin 1's are not connected to anything inside the tube, so here they are just tie points. Check the layout, and mind which lead goes to which tube.

[martin manning]

Sounds good on the load and clip leads. You can use the clip leads to clip a probe tip to the chassis or whatever. I would route the grid and bias leads like this, keeping them close to the chassis surface. Note where the brown lead crosses the pilot LED lead it should be at a right angle or nearly so to minimize any pickup of 60 Hz. Over or under doesn't matter as much as the right angle crossing, and don't run parallel if at all possible.

[Raoul Duke]

Understood, thanks, and will do.

Turns out my V5 lead was about 2” too short, so I replaced the whole line from the cap rather than graft an extension on. Only “problem” is the wire I used is a different brand (but the same spec re: gauge, voltage, and heat tolerance - even color). Gives me a moment of “OCD panic” even though I know it’s the correct fix and the specs are consistent (and once it’s in the cab and running; I’ll forget about it).

Anybody else ever hit that wall, lol?

[Raoul Duke]

Here’s the PT sockets with recommended fixes along with the PI tube socket (not yet soldered) to include the NFB to the 8R tab on the impedance switch. Connected the Presence pot, loop jacks, and B+3 as well.

IMG_0344.jpeg

IMG_0345.jpeg

IMG_0346.jpeg

IMG_0347.jpeg

Anything look like it needs re-addressing? Tried to go slow and deliberate and untangle anything being touted for the next phase so that I only need to lift the board to connect the mid and bass caps. Hopefully I’ve made some progress today - but either way I’m hoping to learn more.

Thanks!

[talbany]

When I did the layout for 124 here I used the same lead dress layout that was in the amp at the time ( including wire colors), Dumble generally followed this same lead dress procedure in most of his ODS model amps throughout the years (although there may be a few variations depending on direction of sockets/Tube type and power supply's used), Anyway it's a good template for HAD approved lead dress.

BTW? It's also good to have a photo of the inside as a reference as well if one is available

Tony