noise problem in 5F4 build

[Bluefinger]

I have a bit of a problem in my 5F4 build I can't solve. The build is done more or less and everything is checked more than once against the schematic and layout. Voltage readings are ok as well. Still I have two noise problems:

There is a low hum coming from one of the trannies. It is still there if I connect a dummy load to the output instead of the speakers and it's there as soon as I turn on the amp, before the retifier has warmed up. It's coming through the speakers a bit as well when they are connected. It's not loud or very annoying but I wonder if it's normal.

Here's the real problem: there is a high frequency noise all the time that sounds like you have a tinitus in your ear. At some positions of the treble and presence pot it gets louder and depending on how fast I turn these pots the amp starts to feedback with a low frequency hum sometimes. If I disconnect pin 2 of the PI (V3) which is the red wire coming from the tone stack it's gone but if I disconnect the two wires feeding the tone stack it's still there. I checked the tone caps and the readings of the tone pots but everything is normal. I even changed the lead dress in a way that I shortened all wires to a minimum lenght and ran them in a way that no two wires are running directly alongside. still there is no change. Any Ideas? The link below shows some pics of the lead dress ... maybe this helps to track down the problem. Thanks in advance!

http://www.king-bees.com/5F4.htm

[TD_Madden]

BF,

can't address the second problem, but I have a similar hum on my Ceriatone Plexi 1987 build....a slight hum through the speaker from when first turned-on (even in standby).....

I have a Mission Super build that's similar to yours...I'll take a look at the routing when I get home.

[Bluefinger]

Thanks Madden ... I'd love to see some pics of the lead dress of a working 5F4 cab. Maybe I put some wires whrere they shouldn't be.

[what_wires]

Here's the real problem: there is a high frequency noise all the time that sounds like you have a tinitus in your ear.

If you pull the NFB wire from the speaker jack does it go away? If so, swap the wires connected to pin 3 of the output tubes.

[Bluefinger]

Thanks, that was it. The amp is running great now!

[TD_Madden]

hate to hijack...but has anyone figured-out the reason for the hum thru the speakers evern with the standby engaged?

[Bluefinger]

Mine was gone as soon as I reversed the wires ... it also hummed while the amp was on standby. If you don't experience the other problems I described as well the the reason for your's will probably be a different one tho ...

[TD_Madden]

dang!

[Jag]

Depending on the orientation and proximity of the transformers, with respect to each other, the PT might be coupling magnetically with the OT and inducing the hum. I have a build that does this even with the output tubes pulled. Confused the hell out of me until I did some research. Unfortunately, nothing I've tried has resolved the issue. It's not really enough hum to worry about, it's an SE amp and there's only so much that can be done anyway.

[Bluefinger]

Depending on the orientation and proximity of the transformers, with respect to each other, the PT might be coupling magnetically with the OT and inducing the hum. I have a build that does this even with the output tubes pulled. Confused the hell out of me until I did some research. Unfortunately, nothing I've tried has resolved the issue. It's not really enough hum to worry about, it's an SE amp and there's only so much that can be done anyway.

Have you tried relocatint the OT or changing it's orientation?

[Jag]

Yeah did that for sure. Problem is the chassis is too small. I can move the transformers around and pretty much eliminate it, but the transformers would no longer be sitting completely on the chassis (should have used a bigger chassis). It's not bad enough to be a real bother, I've even recorded with it and had no problem in the final mix.

[Bluefinger]

In which order are your trannies bolted to the chassis? The original Super had the OT sitting next to the PT and the choke was located towards the inputs but I arranged mine the "traditional way" (PT, choke, OT) ... maybe that'S causing the hum ...