New Crack Build - Good Tests but Loud Hum, Low Volume

[bharris]

Hello everyone,

I finished my first Crack (no speedball) build yesterday.

All tests hit targets, tubes glow brightly and LEDs light but there is a loud (ground) hum and very low volume with pot all the way up (with HD650's).

I visually checked my connections and reflowed one or two that weren't perfectly shiny with no improvement.
I switched out both tubes, one at a time and then both together, with no improvement (except for a slight volume bump when trying a 12BH7 in place of 12AU7).
I tried a different power cable, power strip and outlet but still no change.

I'm using an Alps volume pot, Kimber wire and AEC RCA's but everything else is stock from kit.

Any ideas?
I can add pics but I thought I'd wait to see if there were specific points of which photos would be most useful.

Thanks,
Ben

[JamieMcC]

Sounds possibly like you have forgotten to bridge the left and right ground channels on the Alps pot together. 

Edit I am sure the Bottlehead team will be able to resolve the problem quickly for you.

[Doc B.]

I'm using an Alps volume pot, Kimber wire and AEC RCA's but everything else is stock from kit.

That could be the reason for the problem. I think Jamie's hunch is a good one, worth investigating. If the voltages are right it stands to reason that the problem is most likely in a part of the circuit that does not affect the voltages. That would be the signal input part of the amp - the RCA jacks and volume pot. And hum and low volume tend to indicate a lifted ground. We could possibly be more helpful if you had used the stock components, since we know those parts and we know how they are connected into the circuit. When parts are changed during the initial build it becomes much more difficult for us to trouble shoot because it is not the exact layout and wiring we describe in the manual. That's why we suggest - wait for it guys - build the kit stock first, then modify it.

[bharris]

Well, I've used the Kimber wire and these RCA's a fair bit so I didn't think I was stepping out too far but when the wisdom comes from on high you just have to nod your head and accept it. Haha.

I'll switch out the volume pot first and see if that doesn't bring me back to life.

[bharris]

Back to stock pot with no change...

Probably won't be able to get the old RCA's back in until tonight.

thanks again,
Ben

[Paul Birkeland]

Are you 100% sure that your voltages are all within specifications?

Hum and noise with the volume pot all the way up aren't significant.  Do you get this issue with the volume all the way down?

-PB

[bharris]

The hum doesn't seem to be affected at all by the position of the volume pot.

It also isn't increased or diminished by either having a source attached to the RCA's or not.

I'll replace my RCA's with the stock part tonight but it's hard for me to comprehend that being the problem. That said, I'm not above finding out that I'm wrong.

[ALL212]

Have another set of tubes around? 

[fullheadofnothing]

Post pictures.

[Paul Birkeland]

You likely have either a loose grounding wire or a loose connection on one of your 220uF 250V capacitors.  Having a disconnected capacitor doesn't change the DC voltages, but the power supply hum will be very apparent.

-PB