ODS 124 PCB Build

[professormudd]

If you're speaking about F temperature I'd recommend MUCH hotter temps for good joints. 650-700 easy. I use a Eutectic solder (Sn, Pb, Cu, Ag) and an iron at 680 F.

Yes, Fahrenheit. That is the hottest temperature my iron reaches.

I use a fan to blow fumes out of my face.

[pompeiisneaks]

yeah but also realize that unleaded solder basically always looks 'frosty' unlike the nice shiny solder blobs you get with leaded solder.

and yes the flux is nasty, when I first started I didn't have an exhaust fan and I started having flu like symptoms, found out it was the toxins from the flux and I use a fan now always. Have a hakko fan myself, and it works well enough for the light soldering I do.

~Phil

[Strider0007]

I built one with the same PCB. Came out great!

I see you opted to use a separate JFET board. Mind if I ask why?

Where did you get your chassis and head cabinet?

I made that JFET boost pcb and wanted to test it out on this amp. It's bypassable, so I can just use one jack. It's also adjustable through the jack, so I don't have to open up the amp to tweak it.

The chassis I ordered from Thailand I believe, found it kind of by accident on a google search.

The cab I bought off eBay and tolexed myself because I wanted to learn how to do it. This was my first attempt, so it's not perfect but good enough for me.

[professormudd]

I installed the new relays and they work just fine. When I initially switched to OD, the amp output drop significantly. I replaced the shielded wire from the drive pot along with the 180k and 68k resistors that connect to V2. This resolved the volume issue.

So, to make an inventory of remaining issues:

1. 60hz hum in the preamp section.
a. Increases/decreases with the master volume.
b. Still present while all pots are set to 0.
c. Plugging directly into the FX return also drops the hum to the lowest volume (though still present).
d. Has not responded to chopstick manipulation of wires.
e. Grounding points all seem to be good.
f. Resistors and capacitors that connect to V1 and V2 have been placed with as much distance from the filament wiring as possible.
g. Increases/decreases with tone switch changes.

2. Loud scratchy-screech when the bright switch is engaged if the input volume is passed 12 o'clock.
3. Drive or ratio turned to 10 o'clock or higher led to loud squealing (oscillation?) that dropped with the master volume.

[norburybrook]

I installed the new relays and they work just fine. When I initially switched to OD, the amp output drop significantly. I replaced the shielded wire from the drive pot along with the 180k and 68k resistors that connect to V2. This resolved the volume issue.

So, to make an inventory of remaining issues:

1. 60hz hum in the preamp section.
a. Increases/decreases with the master volume.
b. Still present while all pots are set to 0.
c. Plugging directly into the FX return also drops the hum to the lowest volume (though still present).
d. Has not responded to chopstick manipulation of wires.
e. Grounding points all seem to be good.
f. Resistors and capacitors that connect to V1 and V2 have been placed with as much distance from the filament wiring as possible.
g. Increases/decreases with tone switch changes.

2. Loud scratchy-screech when the bright switch is engaged if the input volume is passed 12 o'clock.
3. Drive or ratio turned to 10 o'clock or higher led to loud squealing (oscillation?) that dropped with the master volume.

Swap your output primaries


M

[professormudd]

I was missing the ground connection from the 2 20uf 500v caps that feed the B+ to the other side of the board.

Correcting that fixed 90% of my hum issues. There is some noise remaining but it's much lower.

Overdrive channel is still touchy as hell. It wants to squeal/feedback uncontrollably pretty early in the sweep.

[professormudd]

I installed the new relays and they work just fine. When I initially switched to OD, the amp output drop significantly. I replaced the shielded wire from the drive pot along with the 180k and 68k resistors that connect to V2. This resolved the volume issue.

So, to make an inventory of remaining issues:

1. 60hz hum in the preamp section.
a. Increases/decreases with the master volume.
b. Still present while all pots are set to 0.
c. Plugging directly into the FX return also drops the hum to the lowest volume (though still present).
d. Has not responded to chopstick manipulation of wires.
e. Grounding points all seem to be good.
f. Resistors and capacitors that connect to V1 and V2 have been placed with as much distance from the filament wiring as possible.
g. Increases/decreases with tone switch changes.

2. Loud scratchy-screech when the bright switch is engaged if the input volume is passed 12 o'clock.
3. Drive or ratio turned to 10 o'clock or higher led to loud squealing (oscillation?) that dropped with the master volume.

Swap your output primaries


M

I think my OT primaries are correct.

After fixing a missing ground contact, the clean channel is working well now (excepting the low hum).

[professormudd]

Got the squealing worked out with chopsticks. Wire connecting to V2-1 was entangled with other wires from V1. Moved the wire to a more careful path and we're good.

The only issue remaining is the persistent low-volume hum. It somewhat changes with volume knobs, but it never goes silent.

[xtian]

The only issue remaining is the persistent low-volume hum. It somewhat changes with volume knobs, but it never goes silent.

How does this hum change if you remove the PI tube?

[professormudd]

The only issue remaining is the persistent low-volume hum. It somewhat changes with volume knobs, but it never goes silent.

How does this hum change if you remove the PI tube?

I've tried removing each of the 3 12ax7 tubes and the hum did not change.

[xtian]

I've tried removing each of the 3 12ax7 tubes and the hum did not change.

If you pull the PI tube and still have hum, that limits the issue to the power tubes (mismatched?). If you ALSO pull the power tubes out, all that remains is the PT radiating EM into the OT--pull the power tubes and let me know what you hear out of the speaker.

[professormudd]

I've tried removing each of the 3 12ax7 tubes and the hum did not change.

If you pull the PI tube and still have hum, that limits the issue to the power tubes (mismatched?). If you ALSO pull the power tubes out, all that remains is the PT radiating EM into the OT--pull the power tubes and let me know what you hear out of the speaker.

Dead silent with the power tubes removed. The set was purchased as a matched quartet. I do not have a scope, is there a way I might be able to verify they're matched?

[xtian]

Bite the bullet and install those 1R bias sense resistors (goes between power tubes' cathode pins and chassis ground--anywhere nearby, no worries about introducing hum). Then you'll be able to see exactly how much current each tube is drawing.

Another simple solution is to pull out one pair of power tubes, leaving the others in far L and R sockets and see how they're balanced. Repeat with other set, then mix and match--should be able to see if one is way out.

[professormudd]

Bite the bullet and install those 1R bias sense resistors...

I actually ordered some of those resistors when I ordered the replacement relays, but mouser sent me an empty bag. The reship should be here Monday, hopefully.

[martin manning]

Dead silent with the power tubes removed. The set was purchased as a matched quartet. I do not have a scope, is there a way I might be able to verify they're matched?

You can do a quick check measuring the voltage drop across each side of the OT primary, and use the resistances to calculate current.