My 50 watt #183 with footswitchable FET build.

[amplifiednation]

Looking good brother!!!

[martin manning]

Matt J, you have a problem lurking here... The positive-to-negative jumper in the main filter stack should be going from the other positive to the other negative. As it is, you are shorting the rectifier output to ground.

[martin manning]

Well.... I just attempted to set the FET bias and things didn't go so well. Not long after hooking up the two 9v batteries the capacitor popped right in front of me on my bench. (Thank God for always wearing safety goggles)

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So, obviously I screwed up something major (and scared the bejeezus out of myself and my cat that sitting in the floor near me)

I think I'm just going to leave things "as is" and keep the stock values listed on most FET layouts. I'm using a NTE 452 FET and really hope what just happened didn't kill it!

- Matt J.

It's very likely that you just hit it with reversed polarity, either in hooking up the battery or by installing the cap backwards. I can't see the traces on the PCB you are using, and it has no silkscreen, so I can't comment further. I highly recommend following the bias procedure posted in the files section. With the source resistor value shown in the layouts it may not work at all.

P.S. I've just updated the FET bias diagram to make it more foolproof. It now shows the battery connected across the e-cap.

[Matt J]

Matt J, you have a problem lurking here... The positive-to-negative jumper in the main filter stack should be going from the other positive to the other negative. As it is, you are shorting the rectifier output to ground.

Thanks for catching that Martin!! I'll swap the wire over to the correct position and double check everything! It's always the little things that are so easy to overlook!!
- Matt J.

[Matt J]

Made a little more progress this afternoon and evening....

Rewired the power supply board to the proper layout per the error Martin noticed. Wired up the board to test the power supply and ran it first with my current limiter. Limiter dimmed properly, so I tested it normally.

Mind you, this is with no tube installed or the preamp wired up. I used a 5 amp fuse and got the following results:

PT secondaries = 352 VAC on each lead
PT heaters = 3.3 VAC on each lead
PT relays 3.9 VAC on each lead, 5 VDC on the output
Bias = 59 VAC from bias tap, -41 VDC feeding into the bias pot
B+ from rectifier board = 488 VDC
B+1 to choke and OT = 488 VDC
B+2 to screens = 488 VDC
B+3 = 488 VDC
B+4 = 485 VDC
B+5 = 484 VDC

Should those values on the dropping string be expected since no current is being drawn from the B+ by the tubes? They just seem a little higher than expected to me. Those caps are rated for 500 volts, but after my recent unpleasant experience with an electrolytic on the FET board has left me a little wary with allowing caps to come that close to their max rating.

- Matt J.

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[pompeiisneaks]

Yes that's very common. Once the tubes start conducting, they drop voltages significantly.
Looks beautiful!

Phil

[Matt J]

Yes that's very common. Once the tubes start conducting, they drop voltages significantly.
Looks beautiful!

Phil

Thanks Phil!!

I've still got some tidying up to do in that area. The power LED on the front is very dim. It's more of a dull orange than the red it should be. I have some more LEDs of different colors, so I might test them with different dropping resistor values. The OT primaries connect to the octal sockets via the terminal strips. That's to keep the leads long so I can easily switch them once the amp is finish to test which polarity is correct.

It's mostly out of frame, but the OT secondaries have been trimmed and soldered in. There are a few more odds and ends I need to take care of before I wire in the preamp board, then I'll get back to working on the FET. I'm probably going to breadboard that first to practice biasing it again.

- Matt J.

[norburybrook]

Yes that's very common. Once the tubes start conducting, they drop voltages significantly.
Looks beautiful!

Phil

Thanks Phil!!

I've still got some tidying up to do in that area. The power LED on the front is very dim. It's more of a dull orange than the red it should be. I have some more LEDs of different colors, so I might test them with different dropping resistor values. The OT primaries connect to the octal sockets via the terminal strips. That's to keep the leads long so I can easily switch them once the amp is finish to test which polarity is correct.

It's mostly out of frame, but the OT secondaries have been trimmed and soldered in. There are a few more odds and ends I need to take care of before I wire in the preamp board, then I'll get back to working on the FET. I'm probably going to breadboard that first to practice biasing it again.

- Matt J.

the 330R as per the layout has worked perfectly with every type/colour LED I've ever used.


Marcus

[martin manning]

330Ω will be safe with a 12V supply since the nominal 2V drop for a red LED will leaves 10V and a 30mA current, the typical limit for small 3-5mm devices. Blue or white LED's have 4-5V drop, and so the current is reduced to ~22mA. That might still be too bright, depending on the LED. The "super-bright" blues still look like a welding arc to me.

[Matt J]

330Ω will be safe with a 12V supply since the nominal 2V drop for a red LED will leaves 10V and a 30mA current, the typical limit for small 3-5mm devices. Blue or white LED's have 4-5V drop, and so the current is reduced to ~22mA. That might still be too bright, depending on the LED. The "super-bright" blues still look like a welding arc to me.

I don't have a 12V supply in the amp, so I'm running them off of the heater wires like in the original layout, so they're seeing ~3.15 vac from either wire going to the dropping resistor and the 1n4003 diode. I have a new red LED from NTE (NTE30041) to try. max Vf is listed at 2.5v, max If is 25mA but I wouldn't want to run it at anything over 20 mA.

- Matt J.

[Matt J]

Update:
Swapped out the LED I was first using. I think there was an issue with it. It came with the LED holder I purchased. Installed the new NTE red LED and it lights up just fine with the 330r dropping resistor. It's in the "Goldilocks Zone" - not too bright, not too dim, just right!

- Matt J.

[Matt J]

Took the opportunity to test fit the chassis again with the speaker installed in the cabinet. Now that all of the transformers are installed, I wanted to make sure that everything fit ok inside the cabinet and none of the trannies are potentially hitting the speaker. Fired it up to test the LED again while I was at it. There's a long way to go, but I love how it looks!

- Matt J.

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[Matt J]

Inside shot of the footswitch. Tested it again yesterday and everything seems to be working properly. LEDs light up fine.

Going to be working on finishing up the power section this afternoon, then it is on to hooking up the preamp. Any good tips on routing the preamp lead wires to prevent oscillations or any similar issues? I'm a little anxious about the B+ wires that feed V1, V2, and V3 interfering with any of the leads that have to completely underneath board.

- Matt J.

[norburybrook]

I've never had any oscillation issues regardless of where they were routed.

M

Sent from my Nexus 6 using Tapatalk

[martin manning]

Installed the new NTE red LED and it lights up just fine with the 330r dropping resistor. It's in the "Goldilocks Zone" - not too bright, not too dim, just right!

As I mentioned above, I find high-brightness LED's far too much at rated current, either for the front panel or for the footswitch. In this case, you have maybe 4-5mA average running through it. I like to power the panel LED from the relay supply, which keeps a small current flowing through the regulator and eliminates the extra junk needed to power it from the filament string. Next time I think I'll integrate the regulator with the relay supply board and clean that area up a bit more.