Sound with pot at 0 + hum.

[tchevy]

Recently finished upgrading my stock crack with the speedball.  After a couple foolish oversights and some advice from Paul I have it up and running. That said after a day it developed a hum in the left channel. I went ahead and messed around re-flowing various connections, however still had similar issues.

Tonight I went through and re-soldered every connection in the stock crack (as I'd done the speedball ones yesterday). I paid particular attention to the black ground wire running to the headphone jack, as well as the RCA jacks, Pot, and power supply caps. I also tightened down all the screws (a bit too much in the terminal ones). All voltages are in spec based on speedball manual.

As it stands I still have a constant low hum in the left channel:
- Hum is constant.
- Does not change when volume pot is adjusted.
- Occurs whether or not there is an input source connected. 

What doesn't affect it:
- Wiggling either tube.
- Tapping anywhere on the chassis.
- Adjusting the RCA connectors or headphone jack.

Here's the kicker and why I'm posting this, I get volume (actual audio output) out of the left channel when I have the pot set to 0 volume (alongside the hum).

I have some new tubes ordered for rolling but they're a ways out. Any suggestions on where to look?

[Paul Birkeland]

How much sound are you getting out of the left channel with the pot all the way down?  Is the jumper between the inner two lugs of the pot well connected? 

[tchevy]

The volume on the left channel is very low, almost like what you'd get with the standard channel imbalance without having the pot padded. I do have the resistors in place to pad the pot. While the jumpers look fine I may just re-wire the pot entirely tonight.

Im wondering if thats the most likely area for this kind of issue. The only other thing I didnt reflow was the ~A3-A6 connectors as I was attempting not to remove the speedball boards.

[Paul Birkeland]

The ground buss enters the bottom of the pot and travels up to the top level, then out.  If the two levels aren't connected well, you'll get weird left channel issues and your DC voltages will move around as you move the pot.

[tchevy]

Hmmm so I remove all the connections on the pot and resoldered them. I made sure the brack ground was well connected mechanically before soldering. I also went ahead and redid the RCA jacks and ground lugs, and reflowed 3L.

- I still have volume in the left channel when the pot is at 0, tho the level decreased from what it was before, barely audible.
- The hum does not seem to be present.
- However there is still a crackling noise (very has flickering sound not really like a crack) from the left channel. I believe this was there before and the hum just masked its presence.

The volume in the left channel really perplexes me. It wasnt there before I reflowed all the base crack connections. As for the flickering sound in the left channel is it possible its related to a speedball board, or likely base crack?

[Paul Birkeland]

If the level changed, then that's a good sign that solder joints may still be a potential issue. 

Sometimes with a pot the 0 volume setting won't be perfectly muted, but there will be nearly no signal coming through, so you may be OK there.  For the remaining noise, you likely still have an iffy solder joint and I would consider adding this mod:
https://forum.bottlehead.com/index.php?topic=11676.0

[tchevy]

I was think as much about the iffy joint considering it improved slightly.

Likewise I was just testing it out and I'm quite sure the cracking noise (more accurately described as static) is interference. It occurs most obviously when i hold the frame, chassis or move near the system, even without input connected. I'll check out that mod and test sources around my apartment. Thanks for that suggestion.

What did happen when testing this morning is the hum was back in the left channel. Same as before, constant, no change when pot is adjusted. So I'm assuming this issue is upstream of the pot.

As I'm obviously a complete novice here I'm not sure if the hum could be related to the small or large speedball boards. I've been avoiding this but I think I'll remove the small speedball boards and reflow all the connection on them and the 9 pin socket.

[Paul Birkeland]

I am just rereading your original post and I'm wondering how the amp worked without the Speedball?

[tchevy]

Amp was fine without the speedball.

I added the pot padding while completing the speedball upgrade, I believe I read that this could increase the amps sensitivity to interference which may explain that.

But the left channel hum started the morning after the speedball installation.

[Paul Birkeland]

Well, you can take the Speedball out and put the resistors back in instead. 

[tchevy]

Ya I was thinking about that. If it were the speedball is it more likely to be te small boards or the large board? Or 50/50?

Thanks again for the assitance btw.

[Paul Birkeland]

The Speedball is very unlikely to be the cause of your issue, but the circuit is much easier to debug without it in there.

[tchevy]

Update:

I pulled the speedball boards, went through the entire crack circuit and resoldered everything, specifically the terminals I could not properly reach beneath the speedball boards. I do not have the resistors on hand to reassemble the stock circuit as I clipped them when building the speedball.

- Hum was gone for ~ 2 days and has since returned. Same as before, left channel, no change when pot adjusted.
- Still have the interference and the very low volume from the left channel when pot is at 0. These aren't immediate concerns.

I added the resistor padding to the pot when installing the speedball initially. I'm going to go ahead and remove this to see if I get any change.


I'm going to order a few resistors so I can reassemble to stock circuit if required. I also want to have replacements for the 22.1K resistors on the speedball board and the 270 resistor in the transformer on hand. I believe these should suffice however wanted to double check that they would work.


22.1k resistors (Both for standard crack and the pair of twisted on the large speedball boards):

    - MBE04140C2202FC100
    - https://www.digikey.ca/en/products/detail/vishay-bc-components/MBE04140C2202FC100/7351959

3k 10W Wirewound

    - SQP10AJB-3K0
    - https://www.digikey.ca/en/products/detail/yageo/SQP10AJB-3K0/18793

270Ω 5W Wirewound

    - PSP500JB-270R   
    - https://www.digikey.ca/en/products/detail/yageo/PSP500JB-270R/2169787?s=N4IgTCBcDaIAoGU4FYAMqBSAhAtGA7KgEogC6AvkA


I apologize for all the questions but really appreciate the help.

[Deke609]

Hum was gone for ~ 2 days and has since returned. Same as before ...

it sounds like something is still loose and that having the amp upside down while you worked on it temporarily fixed the loose connection, which has since become iffy again.

If you haven't already done so, you might want to tighten down all the mounting nuts/bolts that secure ground tabs and terminal strips to the chassis.

Posting some detailed pics of the build might also help.

cheers, Derek

[tchevy]

Ya that seems like a reasonable explanation. Maybe the joint is failing after it warms and cools or something of the like.

Added some photos.