The Amp Garage

Ashman wrote: ↑Mon May 27, 2024 10:33 pm Any problem with or benefit to swapping the two anode wires from the OT?

No problem but also no benefit. A 5E3 does not have a NFB loop.

bepone wrote: ↑Mon May 27, 2024 6:45 pm edit, this post on top is for working amp

Well, it was working the first time he built it. But he recently rebuilt it. After the rebuild it no longer works, so treat it as a new build.

Is this jumper missing or is it underboard. It's a very important ground and if missing would explain the loud hum as soon as the rectifier warms up. If it were my amp, I'd want that ground on top of the board in plain sight.

@bepone... 5E3 has no NFB.

Disconnect the NFB wire from the speaker jack. Any better?

I would be very nervous running a 6K6 with 350-400V on the plate.

akabek wrote: ↑Thu May 16, 2024 4:44 pm I will definitely replace the mains cord with a 3 prong. Hopefully one of my irons will have enough heat for the earth ground wire.

No need to replace the cord. Just put a new 3-prong AC plug on the end of the existing cord.

You cannot share cathodes of series gain stages w/o creating a feedback loop.

Read this discussion. Listen to what PRR has to say.

Sorry for the filament string wild goose chase. I missed you saying you had correct voltage between the filament pins.

Remove the lamp limiter and variac. Plug the amp straight into the wall. Plug in the power tubes. Do the filaments light up? Voltage readings any better?

Most likely a bad solder joint, broken wire, or wiring error in the filament string. Easy to locate with your meter. DON'T MEASURE FILAMENT VOLTAGE WITH ONE PROBE CONNECTED TO GROUND! Connect one probe to one of the PT green wires and the other probe to the other green wire. Voltage should be as exp...

I would be nervous. It's very common to use two 350V caps in series for the reservoir cap.

For an 18W amp in an aluminum (not steel!) chassis I'd just bolt to the chassis. Don't forget heatsink grease/paste or one of those fancy silicone pads. For a 50W amp I'd use a real adequate heatsink.

For a larger amp I'd just get an 18W amp.

If power handling is a worry and I'm worried about that MosFET, can I just add a second MosFET in parallel? I can't imagine a tube amp needing parallel MOSFETs for VVR. Thes devices are rated for high power/high current with proper heatsink. For example, the NTE2973 that is commonly seen in many VV...

My electrolytics are rated for 450V, but with a 5U4, my B+ nears 500V while warming up. Is this okay, or should I get higher rated caps? I would be nervous. I like to measure the AC voltage on pin 4 or 6 of the rectifier tube, multiply that number by 1.414, and chose filter caps with a voltage rati...

... and this I what I get when using my new favorites link. This is correct.