60 cycle hum as soon as turned on.

[ServiceBob]

Hi, y'all!
I've pretty much finished my 6th little SE amp build. It is essentially a Silvertone 1471 with Loctal equivalent tubes (i.e. 7C5 for the 6V6 and a 7Y4 for the 6x4). I've actually built a similar version of a 5F1 champ with the 7C5 and a 5AZ4 and it works and sounds great! I bought a large box of NOS tubes at a local auction and the majority were Loctals so I bought some sockets on eBay and I've been using those tubes when I can.

The problem with my current amp is that there is a low but audible 60 Hz hum AS SOON AS the power is switched on. It doesn't get louder when the tubes warm and it doesn't get quieter either. The amp sounds great otherwise. All of the heater wires are and the other AC wires are twisted and the heater wires are done in the Fender style - off the board and moving down for each tube, not laid against the chassis. This is the way I've built all of my amps and they have not hummed. I'm using Hammond 269JX power transformer and the PT-31 (5K/8 ohm @ 8W) transformer sold by AES, which I have used on two other builds without issue. The transformers are mounted with their axis at right angles. It makes no difference what speaker I use. It's not very loud and isn't noticeable when playing, but it's there and all of my other builds have been dead silent until the tubes warm up,

7Amp.jpg

Thanks in advance!
Bob

[T Wilcox]

Does the PT have a center tap for the 6.3v and its grounded?

[Stevem]

Your power transformer is magnetically coupling into the output transformer due to there positioning to each other and or location to each other,

To prove this I will bet that even with all the tubes out of the amp that if you turn it on you will have the same level of hum out of the speaker.

I have two 1966 Univox B 45 amps that do this very same thing!

Even if the heater center tap is not grounded, or if your PT does not have one and you have not made a artificial one yet, then amp will not hum until the tubes have warmed up,

[ServiceBob]

Hello!
Yes, the PT has a center tap and is grounded. Virtual center tap on heaters. I tried floating it on the 7C5 cathode, but it made no difference.

And Stevem, you are correct. No tubes - no difference. It looks like I'll be moving the OT, fortunately, I have the room on the top.

I will follow up after I've had a chance to move the OT, It might be a day or two.

Thanks, guys!

[Stevem]

If your PT has basically a square foot print then it might be easier to just spin that one 90 degrees, and then give er a try!

[ServiceBob]

I had a chance to move the OT (the PT isn't square) and SHAZAAM! no hum!
Thanks, Stevem!

[Stevem]

Great news to hear, and for you to not hear the hum any longer , lol!