Hot Rod Deluxe Hum

[tubeswell]

Yep sometimes hum can come from a bad triode. I had this recently with a second hand mullard. I put in in V1 of one of my tweeds and the 'normal' channel hummed when the tone was dimed. I thought at first that the tone pot was bad, but when I replaced that tube with another, the problem went away just like that.

[Enzo]

And you will also find that the humminess of a tube makes a difference which socket it is in. The high gain stage on the end is a lot more sensitive to it than the phase splitter for example.

All those New Sensor tube brands - Tung Sol, Mullard, etc in that EH family all come from the Sovtek factories.

Don;t obsess over the tubes, the current brands all work just fine, and the differences are subtle. All 12AX7s sound like 12AX7s.

JJs are fine tubes, I mentioned that Bob only sells them in case you are set on some other brand for whatever reason. I would recommend a chat with Bob, I think JJs are a more mellow tube than some others. And Bob will discuss YOUR tone before selecting tubes for you

[tmlesko]

I have read that 12AT7's sound much cleaner and more mellow than the 12AX7's and are a great substitute, any experience with these? This is getting off the topic of hum so I won't continue this much longer. thanks.
Tom L.

[tubeswell]

12AT7s are higher-current lower-gain tubes than 12AX7s (Whereas a 12AX7 has a mu of 100, a 12AT7 has a mu of 60). So in a PI slot, 12AT7s provide more headroom (fuller, less-clipped 'sine' wave) for the output tube grid signal. They can be okay in a V1 slot if your B+ voltage is high enough to get them to sing. In my view, NOS ones are much nicer than current production ones. I have some GE triple-spacer blackplates made in '57 and they are very nice in V1 with a higher B+

[mr.jetski]

plastic phono jacks

hi

from experience, check the plastic phono jacks for broken solder joints #1 cause of problems with a hotrod

[oc disorder]

Although Mr Jetski is correct in saying that the input jacks cause problems I think this is not the case as you say its independent of volume which means the problem is after the volume control which is way down the line from the jacks.However it may happen one day.What to watch out for is the nut
getting loose on the top of the chassis where it holds the jack..
The socket is soldered directly to the circuit board thus with normal wear and tear (Pete Townsend leaps with a short lead) the socket moves and the solder joints crack as they are mechanically holding the socket in place.
I'll attach a picture of the main offender which is a switching jack.
This type has a little chrome insert which often goes west after a few years.
The recent ones have a different style socket which hopefully doesn't cause the same problems.
Back to the hum... do the tone controls affect the hum ? I suppose they would as you said the hum was reduced by changing the first tube which lowers the gain.
From the schematic the volume control is after the first half of tube 1 and before the second half which is followed by the tone controls.
As Enzo stated it could be a number of things they do suffer from cracked solder joints although it could be something else.
With your previous experience I guess you are aware that an electric guitar (especially with single coil pickups) can pick up hum radiated from the field of the power transformer but again that is pre volume control.
I noticed from your picture that there are some "White" Fender/Groove tubes in the amp.
These are usually fine (at the risk of being shot down by the tube squad)and what is usually found fitted in Fenders.
Here is a link to their tube page
http://www.fender.com/products//sear...tno=0994001102
If you look really closely at yours you will probably see that it is a Sovtek which Groove Tubes buy in bulk and individually test and grade them.
Harmonica players often change the first tube for a lower gain one ie a 12AU7 so that the harp mic is not so likely to feed back.
I would check the 500v power supply capacitors a 47uF and 3 x 22uF to see if that is the source of the hum.
Also the 2 100 ohm 1/2 watt resistors that go to ground from the heater supply.

Cheers