Noisy build, any suggestions?

[wjdunham]

..

[Bob-I]

It sounds like a ground loop on that master. I'd move the flying ground from the board and see if that changes it.

[tubedogsmith]

Are the heaters grounded? What's that white wire you have hooked with the trannie bias wire?

[talbany]

Here is what I would do

Disconnect the wire running from .047 coupling cap going to the relay's run that straight into the PI input...While doing this I always disconnect the PS for the relays... If it's still there it's somewhere in the first stage tone stack... It's an easy test and isolates much of the front end and switching etc... Good Luck!!


Tony VVT

[wjdunham]

The long white wire running from the bias supply is indeed the bias voltage from the PT, I stole a trick from the Fender Prosonic and use the bias tap to generate the relay power, with a 6V zener diode to set the voltage. I've used that same trick in my two other builds and it seems fine, I disconnected that wire at the source to make sure it wasn't causing the problem. My concern when I fist saw that circuit was that I would get a pop when I switched the relays and the current draw caused a spike on the bias supply, but never had a problem with that.

Tony, I ran a wire directly from the CL2 coupling cap to the master input, I did not try to run it directly into the PI, I'll try that tonight. My guess is that it's going to be noisy, but that's just a guess.

I do not have the heaters grounded, nor do I have the 120R resistors to ground. I have one amp with the resistors, another without, both seem fine. I have to admit that I don't understand the purpose of those heater resistors, I'll do a search as I'm sure I've seen some threads where the subject was discussed.

[ChrisM]

The long white wire running from the bias supply is indeed the bias voltage from the PT, I stole a trick from the Fender Prosonic and use the bias tap to generate the relay power, with a 6V zener diode to set the voltage. I've used that same trick in my two other builds and it seems fine, I disconnected that wire at the source to make sure it wasn't causing the problem. My concern when I fist saw that circuit was that I would get a pop when I switched the relays and the current draw caused a spike on the bias supply, but never had a problem with that.

Tony, I ran a wire directly from the CL2 coupling cap to the master input, I did not try to run it directly into the PI, I'll try that tonight. My guess is that it's going to be noisy, but that's just a guess.

I do not have the heaters grounded, nor do I have the 120R resistors to ground. I have one amp with the resistors, another without, both seem fine. I have to admit that I don't understand the purpose of those heater resistors, I'll do a search as I'm sure I've seen some threads where the subject was discussed.

You need those resistors or a heater CT, one or the other, not both.

If you don't have one of them you will get some nasty 60Hz hum coming from your heaters.

[talbany]

I do not have the heaters grounded, nor do I have the 120R resistors to ground. I have one amp with the resistors, another without, both seem fine. I have to admit that I don't understand the purpose of those heater resistors, I'll do a search as I'm sure I've seen some threads where the subject was discussed

Bill
Chris is correct.. The Filiments need to see a CT..Either by CT wire if the transformer has one which it looks like yours does not.. So you need to create a false CT using the 120 or 100 ohm. resistors..The 2 also help protect from a heater short which will in theory take out the resistors instead of the winding..Good Luck!!

T

[Guitarman18]

wjdunham wrote:

I have to admit that I don't understand the purpose of those heater resistors

Those resistors create an artificial CT as a substitute for a trannie CT, which helps to balance each end of the winding.

Cheers,

Paul.

[butwhatif]

the resistors 'pull down' the wide open heater winding, and give it a reference point---remember the heaters are inside each and every cathode,
and can induce hum if they are not ref'ed to a clean ground---

[wjdunham]

Guess when I get home I'm gonna put 120R resistors to ground on the heaters That would be why the same PT on my 100W build didn't have any problems, I followed the #124 layout for that one and put them in there. My DLite doesn't have the resistors, and I don't remember the PT having a center tap for the heaters, but I could be wrong. I should add the resistors there if not. As always, thanks for the suggestions! I'll send an update when I get a chance to try it out.

Bill

[tubedogsmith]

That'll probably do ya! When I tore that Boogie SOB apart a while ago and got it modded, it hummed when I fired it up. There must have been ground referencing resistors on the PCB and I just never checked, I just thought it had a ct. As soon as I heard the hum I knew what it was and added them.

[BobW]

the resistors 'pull down' the wide open heater winding, and give it a reference point---remember the heaters are inside each and every cathode,
and can induce hum if they are not ref'ed to a clean ground---

Butwhat is absolutely correct!

The hum balance on older amps was a pot w/ the wiper grounded and the outer lead attached to the 100 Ohm heater resistors. This allowed a fine tuning out of the hum by balancing the circuit.

[Structo]

Something I have wondered about those heater balance pots is how much current is that pot handling?

[wjdunham]

Well, a big thanks guys! Put the resistors on the heaters last night and things are much better. This one is as quite or even better than some of my others now. All the suggestions are much appreciated, I guess this was an easy one

Didn't have a chance to open it up since it was late when I got to work on it, but the clean channel sounds good. One thing I did notice is that the drive channel seems to have a lot less drive with OD entrance trim set somewhere around the middle which is where I have it set on other amps. I have to set the drive control around 7 to get the same bite that I get normally at 3 or 4 with a Les Paul. Need to check my voltages and resistor values first I guess. Don't know if anyone can spot anything obvious in the photo.

Bill

[ChrisM]

Something I have wondered about those heater balance pots is how much current is that pot handling?

I am just guessing here...

But heater resistors are usually 1/2W. Most pots are 1/2W, I believe Alpha's are. So my guess is that most commercial pots could handle the job.