Noise

[RockNRollDr]

So I just finished my 6th amp build, a little 5 watt single ended EL84 amp. I have a problem though, when I power the amp on (I am using a champ style switch built into the volume pot) there is some hum as the tubes warm up and as I turn the volume knob the noise changes from a hum to a squeal to crackling, it's really a horrible sound. Although through the hum and squeal it still does its job amplifying my guitar, that is as long as i play the strings softly, when I pick them harder the guitar sounds just turn into a muffled buzz sound. I have changed tubes and grounds but it hasn't helped any. Also I am using a Fender Champ 275-0-275 power transformer and there is no center tap from the high voltage taps, does Fender ground that inside the transformer maybe through a mounting bolt? Can anybody give me an idea of what is going on with the noise? Any information is appreciated, thanks.

[Jana]

There is no center tap? Are you using a full wave bridge rectifier? If not, that is your problem right there. If you are using a FWB, then what are your voltages? Stating it is a 275-0-275 is a bit confusing given that you don't have a center tap. If the center tap is missing and this transformer is indeed 275-0-275, don't, I repeat, don't hook this up to a full wave bridge, you will have about 700 to 800 volts DC! I don't think you are prepared for the excitement that will cause.

[RockNRollDr]

There is no center tap and the only indication on this transformer says 275V 6.3V, but it confused me because every PT I have used has a center tap for the high voltage taps. Yes I am using a 4 diode full wave bridge rectifier, and I haven't checked my voltages yet but I would assume that my tubes would have blown along with melting the sheathing on my hook up wire if there were 800VDC going through it. Let me know if you have any more suggestions, I also must say that I didn't use shielded cable on my input or volume pot, could this cause such a big problem?

Thank You.

[RockNRollDr]

I ran my shielded cable from pins 2 and 7 on my 12AX7 to my input and volume pot and that got rid of most of my noise but still if I use any force when picking the strings the notes turn into muffled buzzes. Any ideas?

[KT66]

An idea I like to suggest in this situation, especially when you like to build amps and will probably build more in the future, is to make a signal tracer. Just get an old boom box and hook a probe with a high-voltage bolcking cap in front to the auxillary input and connect the ground from the aux input to the chassis or ground buss or whatever. Run it off of the batteries for less noise. Use the probe to listen to the circuit one stage at a time to isolate the source of the noise. When you find the offending stage, you can then sub tubes/caps resistors or look for a possible short. If you have a ground loop ( which your problem does not sound like) this method might not make it so obviuos. Hope this helps.

[Bob-I]

It sounds like a high frequency oscillation. Use a chopstick to move wires around and see if you can isolate where it's coming from. If you have a scope you can often see it on the scope as a wave at the output of a gain stage.

[David Root]

I am having the same problem. This is on the amp I discuss in an earlier thread "5F6A/8417 80W". I have shielded leads, 470K gridstoppers on the second stage, 3rd stage and PI, and it is still there. I have split plate resistors to cut gain in the first stage, third stage and PI and it is still there. It is driving me nuts. I have switched plate leads too.

[tubeswell]

Maybe the squeal is +ve feedback. (Did you try swapping the OT primaries around?)

I wonder if the crackling is something else, like a voltage leak across the insulating material of a tube socket or pot, or somewhere on the board between eyelets/turrets (maybe where some leftover flux/residue, or having a high voltage eyelet/turret too close to a lower potential). Inspect the spaces between the eyelet/pot lugs/socket pins. Maybe try changing the pot for a new one?

[David Root]

I just figured it out, the reverb circuit dry recovery tube is far too gainy and it was blasting way too much voltage with lots of bass back into the main preamp thru the anode mixer tube.

So I disconnected the reverb completely. Will have to rearrange the reverb circuit, but at least I can put the gain back in the main preamp.