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I managed to get my "Powerman" amp working, but it hums a lot. This is a small amp made with two 6021 tubes.

Can anyone point me to the first things I should look at to reduce hum? This is an eyelet board amp, and there are quite a few long wires under the board. The heater wire pairs are pretty long, but I did twist them together tightly. There are no shields on the tubes.

any pics of the guts? could be a few things: tubes to close to the transformer, heaters, grounding, bad tube just to start with.

It looks like there are a few obvious mistakes.

1. The transformers are on the same side of the board as the tubes. Leaning the board away from the transformers doesn't seem to make any difference, though. I am wondering if the transformers should be mounted on top of the amp, outside the chassis. The guy who designed the amp didn't have to do that.

2. The heater supply wires are twisted together up to a point, but because the tubes are a few inches apart, the wires separate and can't be twisted together all the way to the ends. This leaves about an inch and a quarter of untwisted wire in each direction. These are the green wires exiting a Hammond 261C6.

3. The wires feeding the rectifier diodes (red, Hammond 261C6) have a red and yellow wire between them which is supposed to be grounded. I'll be honest; I have no clue what this wire does. In my only other amp build, I was instructed to twist this wire together with the wires to the rectifier, but in the amp I just built, it has to separate from the two red wires before going to the nearest grounding point. I don't have a bus; I'm just connecting grounded points with jumper wires.

4. I grounded the power cord to the base of the power transformer.

I can post some photos, but I may be laughed off the forum. I have a zillion little jumper wires on the back of the board. I have never seen an eyelet board before, let alone designed one, so I pretty much winged it.

I will upload a schematic and post links to the amp designer's page.

http://www.harmonicappliances.com/power ... erman.html

The New Steve H wrote: I can post some photos, but I may be laughed off the forum. I have a zillion little jumper wires on the back of the board. I have never seen an eyelet board before, let alone designed one, so I pretty much winged it.

Steve

I would sincerely hope that you wouldn't get laughed off the forum for posting photos. You will however get some very good advice as there are some very experienced guys on here who are more than willing to help anyone who asks.

A few photos would be very useful to identify any problem areas.

We all had to start somewhere, winging it is all part of learning.

Thanks, Fred. I got distracted, but here are photos which may give an idea what we're dealing with. I probably should have copied the eyelet board the designer used, but I thought I'd leave some blank space in case I wanted to add something later.

Right now, the power cord ground is attached to a green wire that loops around one of the screws attaching the cord receptacle to the chassis.

The sound is a little dark; maybe I shouldn't have bought Weber's cheapest speaker! That speaker is also dark-sounding with my Firefly. I'm wondering if it would help if I stuck a high pass filter on the input jack.

Lost one!

There is only one switch (left of front panel when amp is right side up). There is a 125V power light on the right side. The jacks are on either side of the power switch, with the out jack on the outboard side. There is only one pot.

Sorry to keep replying. To be clear, the amp is upside down in the photos. When it's assembled, the tubes point upward. The preamp tube (or what I think is the preamp tube) is close to the 125C. The output tube is by the main power supply.

Steve

Try to keep the input jacks/wires/components as far away from any mains and heater wiring as possible. You never see a commercial amp with the input jack next to the mains switch, they are always as far apart as possible. Tubes have pretty high input impedances so will tend pick up any stray AC from anywhere.

Transformers are usually mounted on top of the chassis for the above reason, they can radiate quite a bit of magnetic interference, particularly the cheaper ones. (Toroidal transformers have the lowest magnetic leakage but are also the most expensive.)

Edit
I've just read your last post. It sounds like you have the input away from the mains transformer but I would still move both the transformers to the top of the chassis and separate the input jack and mains switch.

Hope this helps

I'll give it a shot, Fred. Thankfully, it won't wreck the chassis, and it will make it look considerably cooler. I guess I can live with 4 extra holes in the back.

I can't figure out why the original Powerman didn't have this problem. It sounds clean as a whistle.

Do any of you experienced tube building guys want to add anything, I am more into the fault finding and repair side rather than building?

It turned out all of the hum was coming in through the input jack. I pulled it and moved it around to see where it wanted to be. In the end, I had to get rid of the 125V power light. It seemed to generate hum no matter where it was. I put the input jack in the light hole. Now I have an extra hole. I'm wondering if there is a place in the circuit where I could bleed off enough DC to run an LED power light. Maybe an LED and a resistor downstream from the rectifiers?

The transformers generate noise, but it appears that they are not close enough to the input jack to matter, so I guess there is no point in moving them.

The amp sounds pretty good, but while I like it better than the Firefly, it's still slightly muddy and dark. I'm wondering if there is a way to fix that. I guess there aren't many people out there trying to get a hot Fender tone out of a sub-one-watt amp.

I don't think the cheapo Weber speaker is the issue. I tried it with my Super Champ XD, and it seems fine.

This amp takes pedals very well, but you have to crank the volume on the pedals. For some reason it really likes a Tube Screamer TS-9.

It will be a great practice/travel amp. I need a tiny cab to go with it. I have some small 4 ohm speakers that came out of my pickup; maybe I should use one.

Thanks for the clues.