Troubleshooting Speedball... [solved]

[StivVid]

I've just finished upgrading my Crack with the Speedball kit.  Before the upgrade, the Crack was singing beautifully.  Now not so much.  When I did the voltage checks, everything checked out except for 15 and 21.  15 reads 230 volts and 21 reads 255 volts.  I think that's a little more than spec.  Also...  when I check the voltage at the tip ring of the headphone out, voltage climbs to more than 30 volts then rapidly drops back down to zero.  I plugged in some sacrificial headphones and hear a hum.  If I touch the 12au7, I get a loud cracking noise.  Same if I jiggle the headphone plug in its jack.  Any advice on where to start troubleshooting?

[Paul Birkeland]

If all the voltages are in line except 15 and 21, then you have a working Speedball.

If you have hum and noise when you jiggle the 12AU7, then you have flaky solder joints around the 9 pin socket.

-PB

[StivVid]

Thanks for the quick reply.  I'm relieved to hear that the voltages check out ok.  I just tracked down and fixed the problem.  Silly mistake.  I missed a solder joint on the ground for the headphone jack.

Question...  I've read other posts that describe a significant bump in gain after the speedball upgrade.  I'm running HD600's with mine, and don't really notice significantly more gain.  Is this gain increase headphone dependent?

I do wish, however, that more of the potentiometer was in a usable range, if that makes sense.  It gets very loud very quickly.  I'd never listen anywhere past 10 o'clock on the dial.  That's much less than half a turn of the knob.

[Paul Birkeland]

I just tracked down and fixed the problem.  Silly mistake.  I missed a solder joint on the ground for the headphone jack.

I'm glad you found that. 

Is this gain increase headphone dependent?

Nah, the 12AU7 doesn't know what's plugged into the amp. 

I do wish, however, that more of the potentiometer was in a usable range, if that makes sense.

See the Crack FAQ on this board,  I put up some instructions to modify the level control to have more usable range.

-PB

[StivVid]

Thank you very much, PB.  You're a gentleman.

[Nathan]

The suggested mod to pad down the volume using 75k ohm resistors and 33k ohm resistors works very well and with HD600's are necessary

[Nathan]

"Question...  I've read other posts that describe a significant bump in gain after the speedball upgrade.  I'm running HD600's with mine, and don't really notice significantly more gain.  Is this gain increase headphone dependent?"

It's minimal

[StivVid]

Thanks, Nathan.  I'll definitely give it a shot.

[StivVid]

Nathan--

Have you done the mod on the volume pot on your own amp?  If so, could you send a pic?  I just want to make sure I don't screw it up somehow.  As I understand it...  I disconnect the red and white leads from the potentiometer.  I attach one end of a 75K resistor to each of the wires, then reattach the wires with resistors installed to their original positions.  That part seems clear to me.  The next part is what I'm unclear about.  Where exactly do I attach the 33K resistors?  It sounds like I place one between the left most lug of the top row and the left most lug of the bottom row, then do the same with the right lugs.  I leave the center lugs alone.  That sound right?

[Paul Birkeland]

I disconnect the red and white leads from the potentiometer.  I attach one end of a 75K resistor to each of the wires, then reattach the wires with resistors installed to their original positions. 

Yes, the red/white wires coming from the RCA jacks, not the ones leaving the pot and going to the tube socket.

Where exactly do I attach the 33K resistors?  It sounds like I place one between the left most lug of the top row and the left most lug of the bottom row, then do the same with the right lugs.  I leave the center lugs alone.   

One goes between the left most lug of the top row and the right most lug of the top row.  The other one goes between the left most lug on the bottom row and the right most lug on the bottom row.

[StivVid]

Ah!  Gotcha now.  Thanks so much, PB.

[StivVid]

I'm about to order parts to do the volume mod.  I understand that power rating of the resistors doesn't really matter.  I assume, though, that tolerance ratings do matter.  Is that right?  The easiest parts for me to source are 1/2 watt metal oxide film resistors with 5% tolerance.  Would they do the trick?

[Paul Joppa]

Use 1%

Honestly, the gain control is not all that precise so 5% is probably not going to be a problem. But it's better to use metal film if you can. Metal film is quieter than oxide, and usually 1%. If you get carbon film (OK) at 5%, they are cheap -  why not get 5 or 10 of each, and sort them with your meter to find matched pairs?

[StivVid]

Okay.  Found some Xicon 1/2 watt 1% metal film resistors.  Should be good to go.  Thanks so much for the advice!

[hjc]

I do wish, however, that more of the potentiometer was in a usable range, if that makes sense.  It gets very loud very quickly.  I'd never listen anywhere past 10 o'clock on the dial.  That's much less than half a turn of the knob.

My Crack kit hasn't arrived yet but to reassure you and as a point of comparison, that's a similar range to my WA6. With stock tubes in it (came with 6DR7's) I wouldn't turn it up much over 10 o'clock. I found there's quite a bit of leeway when changing tubes - e.g. 6SN7's let me get up to about 12 o'clock on quieter recordings. So the Crack volume pot range doesn't seem all that unreasonable.

cheers Howard