Another 5F2-A hum question!!!

[jimboyogi]

Hi all,

I built myself a 5F2-A a few months back, and it was my first successful tube amp build. I am very happy with the sound, and have been playing it and planning on checking/tweaking in the future.

The future has arrived.

As i was designing the layout i was referring to Randall Aikens "Star Grounding" article on his website, and I thought that I had it right. Since I finished the build, I think that I understand grounding a bit better, and could improve the layout and grounding scheme, but only if I take the whole thing apart and start again!

The amp does hum a bit, the hum is effected by the settings on the volume pot exactly as you would expect, also the tone control alters the tone of the hum as expected. I have tried changing V1 between various 12AX7's and the current choice, a 5751. The hum was louder with the 12ax7's as expected, quieter with the 5751, which makes sense for the lower gain tube.

But with nothing plugged into any input, the amp is almost dead quiet, with only a slight hiss audible with the volume control maxed.

Does this mean that the amp does not have a hum problem, and any hum is being generated by the guitar and cables?

[txstrat]

Does this mean that the amp does not have a hum problem, and any hum is being generated by the guitar and cables?[/B]

...and furthermore -> open input jack. When you plug a guitar cable into your input jack the tip (that is grounded with no cable plugged in) is no more grounded and signal can be amplified by the tubes - so can the hum.
I'd say a slight hum is normal and you've probably done everything right.

[tubeswell]

The amp being almost dead quiet with nothing plugged in indicates everything in the amp is fairly solid. However, when there is nothing plugged in and the volume is dimed, is there still only hiss?

If so, it sounds like your hum problems are coming from a bad cable and/or a poorly shielded guitar. Out of curiosity, are there fluoro lamps, or some other source of EMI, in the room where you are testing it?

[jimboyogi]

Thanks for the replies txstrat and tubeswell

When nothing is plugged into the inputs then V1a grid is grounded (via 68k grid stopper), and no hum is evident. In this condition, with the volume dimed, there is only a slight hiss coming through the speaker.
Am I correct in thinking that this means that I do not have any grounding/hum problems post V1a?

Is it still possible that there is a problem with V1a?

Should I try putting a blank plug into the input jack to un-ground (is that a phrase?) V1a grid, to check it's hum status inside the chassis? Does this blank plug need a ~10k resistor across it to simulate the impedance of the pickups?

Tubeswell, i have tried a few different cables and guitars (some humbucker, some single coil). Humbuckers definitely show less hum. Cable swapping made no difference. Throughout my house I have fitted compact flourescent light globes, can these be the culprits?

Perhaps I expect too much

Also, I am running a false heater CT (2 * 100 ohm resistors) to the 6V6 cathode for heater elevation to minimise heater-cathode noise pickup. I am using s/s rectifier, with big filter caps, 47uF for the OT, then 47uF for the screen supply, then 10uF for the preamp supply.

[tubeswell]

Fluoro lights are the worst kind in terms of EMI with geetar amps. Try the amp in a different location. You could also try shielding you single coil geetar like this:

GuitarNuts.com - Shielding a Strat(tm)

(I did this to my strat and it worked a treat. YMMV)

[jimboyogi]

Thanks for the tips tubeswell.

I suspect that the hum and noise is just normal, but will try the amp somewhere else. Almost everywhere I go has some form of fluoro these days!