My 2nd Bass Amp Project

[Scott Anderson]

Well I finally got around in building my 2nd bass amp. I do have some noise issues with the preamp. See the link.
http://home.triad.rr.com/scottvicki/150BassAmpProject.htm

Scott Yellow_Light_Colorz_PDT_12

[TomMcNally]

Beautiful work Scott ! I see you used chassis ground near each tube in the preamp section, that could be contributing some hum and noise. I was looking at the bottom pics trying to figure out where the ground wire from the IEC connector is going ? I usually ground them with a 1" piece of bus wire to a solder lug right on the socket with a star washer.

That monster must weigh about 40 pounds ! Very cool.

... tom

[Scott Anderson]

Thanks Tom. Right now it weights 30 pounds. I'm going to be getting a Hammond 1451-30BK cage and a pair of handles that will sure to add some extra weight. What's funny is that one of my speaker cabinets is an Epifani 110UL that weighs only 22 pounds.

All my grounds go back to a single point. The ground wire is grounded to the left of the SDS capacitor filter board.

Scott

[TomMcNally]

Typically you don't want to mix that power cord ground into your audio ground system, you want that to go to the chassis right at the entry point.

Maybe it's just me - but looking at your brown phenolic terminal strips, it looks like you are using the ground lug which bolts to the chassis ??? That would create numerous ground loops.

This 60 pound monster I built has 4 handles on the corners of the chassis ... make it a little easier to move. It's uses the same Hammond 278CX power transformer as yours, but two full sized chokes, a pair of outputs, and 3 more filament transformers ...

[dhuebert]

I'm going to talk just off the top of my head here and share some observations.

Power amp:I don't like the hammer that the standby switch gives the tubes. I think that between the possibility of cathode stripping and hammering them with the high frequency high voltage transient when the switch turns on, I'll take the possible cathode stripping.

Your schematic shows the on/off switch on the other side of the transformer primary from the fuse. This is a big no-no. On the hot side: first the fuse, then the switch. The way you have it, if the fuse blows the switch is still hot. Very Dangerous! Ask Me How I Know! Also wire the fuse so the hot side is deep inside the fuse holder. If you do it the other way you can zap yourself on the fuse end. Ask Me How I Know!

You might want to double up on the 1N5408s particularly if you want to keep the standby switch. If you turn the switch off with the power on there will be a reverse EMF spike out of the transformer secondary that will blow those 5408s to kingdom come.

I would have moved the choke to the other side of the second set of caps and put the center tap from the output transformer before the choke. The push pull output will cancel the power supply hum and with the much lower current through the choke you can use a single much larger choke to give smoother power to all the single ended stuff in the pre amp.

Ditch the bias adjust pot and use a Bourns H83P trimpot holder. Having the bias pot accessible like it is is asking for trouble.


Preamp: I like to use an insulated input jack and ground the whole input section to the jack. Grounding the jack to the chassis and the preamp elsewhere seems to make a buzzy front end. Apart from the power supply and the output tubes, I don't ground anything to the chassis. Looking at yours, I notice the input jack is bolted to the chassis with a lock washer then the ground is brought to a terminal strip and grounded there. A classic example of a ground loop.



More later.
Don

[Scott Anderson]

Thanks Don for your comments.

I will make the changes you suggest. Especially when it comes to the power switch and fuse. I've seen it done the way I have and wasn't thinking of the safety issue.

The power supply filter capacitor assembly is the SDS board the Triode Electronics sells and I just wired it up per their instructions.

I will isolate the preamp section from ground as much as I can. I do notice that when I bypass the preamp with my DI preamp there is no noise.

One interesting thing. When I plug in an 8 ohm speaker into the 8 ohm jack, the preamp circuit works but noisey. When I connect 2 - 8 ohms cabinets into the 4 ohm jacks the preamp cuts out. I can still use my DI and by-pass the preamp. Strange.

Scott

[dhuebert]

One interesting thing. When I plug in an 8 ohm speaker into the 8 ohm jack, the preamp circuit works but noisey. When I connect 2 - 8 ohms cabinets into the 4 ohm jacks the preamp cuts out. I can still use my DI and by-pass the preamp. Strange.

It might have something to do with your feedback. Try disconnection the feedback altogether and see how you like the sound. I don't use feedback in any of my bass amps.

I forgot to say Good Job! Once we have it all sorted out I think you'll be very happy with it.

Don