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

[Raoul Duke]

Thanks as always Ian! That’s a great organizational guide for sure. I’ll definitely use that on my next build (which I’m already planning, lol).

Just got given a bag of old Q-line caps, an NOS F313X, and Stackpole slide switches - so the next will be a 2nd gen inspired by universal providence

For now, back to this one…

[Raoul Duke]

Ok,
Just repeated the FET bias process 3 times and came up with a value of 1791+/-2 each time starting from scratch (parts unassembled, follow Martin’s FET setup instructions for clean boost to the letter). Sounds like 1.7 or 1.8k resistor for Rs to me.

My guess (literally) is either would work given tolerances? Also, just to confirm - is 1/2 watt acceptable in this position?

Thanks in advance!

[martin manning]

1.8 is a common value, and much closer to 1791. A 1/2 or 1/4 W will be fine.

[Raoul Duke]

Awesome, thanks Martin!

I’ll be going back to Marcus’ first build thread and re-reading your advice to him regarding the testing of the power section and relays before installing the main board. I’ll probably have questions for you; I’m nearly all wired-up for that. Hopefully ready in the next day or so.

Thanks again

[martin manning]

I do like the test as you go approach as opposed to wiring it all up and then hoping you got it all right. That does mean adding the weight of the transformers to the amp earlier in the wiring process, but that's personal preference, I think. I don't see why people like to put the preamp tubes in first, either. It's the power supplies and the power section that are likely to smoke, so getting that squared away first makes more sense to me.

You can wire everything on the left side of the layout up to the power board/preamp board gap (PT, choke, and OT, plus the relay PT, relays, and footswitch jack), and get all of that working. You will want to run all of the leads that go under the preamp board done before dropping it in, too. Other than checking supply voltages, you can't bias and test the power amp until you have the PI and heaters up to that point wired, but that is certainly possible.

[Raoul Duke]

I agree Martin; especially this being my first “complicated” build. Incremental progress checks are far more preferable than deconstructing layers to find problems. In my previous professional life - that was part of the culture I lived for just under 28 years and is a “comfortable” routine, lol.

Haven’t made a lot of progress in the past few days due to being whacked by a monstrous head cold - but it’s giving me more time to read and do more research. Found some on-line guides for determining fuse value and my conclusion given 120 VAC (US) into a 50 watt amp of the type I’m building is that my mains fuse should be 3 amps. Hopefully I’ve come close - but welcome any wisdom/guidance as always.

Thanks again! MTF…

[Raoul Duke]

Ok, I think I have everything in place to test everything to the left of the power board/pre-amp board gap (looking from the back). Got a couple questions:

1. Was my calculation for the 3 amp mains fuse correct? I’ve seen some charts showing a 2 amp was the fuse for old Fenders with similar specs.

2. How exactly would I go about accomplishing the testing? I’ve built the bulb current limiter from the files section and have 25, 40, and 60 watt bulbs on hand and ready to go.

3. Do I need the FET board in place? I see that it runs from the B+5 tap - so my understanding is that it isn’t needed but wanted to confirm just in case.

[ijedouglas]

Ok, I think I have everything in place to test everything to the left of the power board/pre-amp board gap (looking from the back). Got a couple questions:

1. Was my calculation for the 3 amp mains fuse correct? I’ve seen some charts showing a 2 amp was the fuse for old Fenders with similar specs.

2. How exactly would I go about accomplishing the testing? I’ve built the bulb current limiter from the files section and have 25, 40, and 60 watt bulbs on hand and ready to go.

3. Do I need the FET board in place? I see that it runs from the B+5 tap - so my understanding is that it isn’t needed but wanted to confirm just in case.

I would use a 2 amp fuse as Fender did.

I'm not sure how you are going to test the power section without the preamp tubes and FET. All of these will pull down the voltage. If you are just trying to see if you have your wiring correct, you could keep the variac (Im assuming you have one... if not, good investment to make) real low ( 5 or 10 volts) and see what happens with the current limiter.
I prefer to wire everything up and then test with a DMM for grounds and connectivity before slowly applying power from the variac.

[martin manning]

1. Was my calculation for the 3 amp mains fuse correct? I’ve seen some charts showing a 2 amp was the fuse for old Fenders with similar specs.

You want the smallest fuse that can reliably stand the initial startup surge. If a blackface Bassman worked with a 2A, yours probably will too, but you won't know until you have the whole circuit working.

2. How exactly would I go about accomplishing the testing? I’ve built the bulb current limiter from the files section and have 25, 40, and 60 watt bulbs on hand and ready to go.

Start with a low wattage bulb in the limiter, 40W should be good since that will allow a maximum of ~330 mA. At power on you should see a brief flash at near full brightness, and then a steady glow. You can then check for heater voltage and DC voltages on all of the power supplies: B+1 through B+5, the bias supply, and the relay supply at its reservoir capacitor. The regulated 12V won't appear until you have sufficient voltage at the regulator's input. All voltages will be low due to the limiter, in proportion to the AC voltage you see at the power inlet (after the limiter) divided by 120. With no tubes installed, all of the B+ nodes will be similar at this point. From there you could change to the 60W, and then finally to full line voltage. The unregulated voltages will be high by 5-10% since there is almost no load, and you should have something very close to 12VDC at the relay supply's regulator output.

3. Do I need the FET board in place? I see that it runs from the B+5 tap - so my understanding is that it isn’t needed but wanted to confirm just in case.

No, it's not needed, and you can't get a good reading on the FET voltage and bias until the amp is complete and the power tubes are biased.

[Raoul Duke]

Thanks very much guys! Always appreciate both of your feedback for sure

Martin, as usual your direction is clear, concise, and even a NEMF like me can follow the logic. Let me run through this mentally a few times and I’ll report back with what I discover once I do the actual testing.

Thanks very much again!

[Raoul Duke]

quick clarification:

Do I need tubes in place for this test? If so - which ones? My hunch is that I don’t, but honestly I’m just not sure.

Thanks!

[martin manning]

Don’t put any tubes in yet. This testing is just about the power supply voltages. You don’t have the bias voltage connected to the power tube grids, since that arrives through the two 220k’s on the preamp board. If the tubes are installed, and there is power going to the heaters, the tubes would be destroyed in short order. Also, if the OT is connected, you must have a load connected to the output whenever power is applied with power tubes installed.

Exactly what is wired up at this point? Maybe post a gut shot of the power section?

[Raoul Duke]

Here we go: power section,

IMG_0157.jpeg

pre-amp area,

IMG_0160.jpeg

and view of the back panel from inside.

IMG_0161.jpeg

The bias pot has the 27k 1watt resistor, but my gut tells me that might need to change. I also need to neaten up the wires coming over the PT. Trying to compensate for my lack of technical knowledge with neatness and careful forethought, lol.

[martin manning]

Looks pretty decent! Are you ditching the 1R current sense resistors on the power tube cathodes? I'd recommend keeping those, even if you don't fit test jacks.
Have you applied any power yet?

[Raoul Duke]

Thank you Martin, I appreciate the feedback!

So I should keep the 1R resistors from pins 1 to ground on both power tubes that were part of the bias test point “network” then? Sure thing, easy enough to put back. I removed them and the test points after considering Jelle’s advice about reducing add-ons because:

1. I’m not a tube roller. I tend to put a set in, bias, and leave ‘em be until they wear out.
2. I’ve always been a “keep it as simple as possible” person in other parts of life - this doesn’t seem to be the project to change that philosophy with. The test points were in the box o’ parts I traded for - so I installed them without much consideration thinking I ought to use everything in the box.
3. Jelle’s advice made sense all things above considered; and he’s built way more of these than I ever will, lol.

No Sir, I haven’t applied power yet. Been mentally preparing and doing some more research to ensure I’m doing it safely and correctly as well as taking the readings properly so that whatever I post here makes sense. Also, really haven’t had a quiet house and the free time yet this week - which I find key when trying new things like this. Possibly tomorrow if everything looks good to you in my pictures.

One last question: what function will the 1Rs serve when re-installed, act as a grounding “safety valve” for the chassis?