Troubleshooting 2204 clone oscillation

[2L man]

Thanks guys. Appreciate it.

I've verified that the coupling caps connect to the correct output tubes but I've never been fully sure that the OT primaries were correct and that's just because the Hammond documentation for the 1750N is not really helpful. The wire colors definitely don't match so I just looked at the transformer schem and wired it as the 2204 schematic shows. I'm fully aware what swapping them does as the amp was setup that way when I built it and all it did was howl the first time I powered it up. But if someone wants to confirm the output wiring of the 1750N I'm all ears. Currently Red goes to V4 and Blue goes to V5.

I do have a scope. It will take some time to get it working though.

Ken-

Usually Red is primary CT where HV is fed. Brown and Blue are often anode outputs. Datasheet show White for CT wire color but sometimes transformer builders make mistakes.

Measure that tbere is about 80 ohms between anodes (red and blue) and 40 from anodes to CT/white.

 

[kenrinc]

After pulling the board and doing a little bit of wire re-routing it is back up. Thankfully it still sounds the same but the oscillation is still there. This time I can get to about 6 on the MV before it goes crazy so it did change something. I've looked at the wiring and I'm beginning to see this may be more of an issue with a mcrophonic tube. When I first built the amp I was having an issue with oscillation way low on the gain and MV and after swapping the tube everything worked fine. I'm going to go though some tubes and see if I can get V1 to settle down.

EDIT: Confirmed about 41ohms between legs and CT on OT.

Ken-

 

[FourT6and2]

All of these problems start with bad lead dress. If you take the time to wire it up properly and neatly, you'll also most likely do everything else better too, like catching a bad solder joint or a missing ground or a short. It's like making your bed in the morning. If you do that, it sets the tone for the rest of the day. So... start by cleaning up all the wiring. Once you do that, you'll probably find the culprit.

Is the input feeding V1a or V1b? I see a grid stopper on one input grid, but not the other.

And yes, those green wires going to the power tube sockets from the PI need to be twisted and kept away from everything else. The resistors on the sockets should be heat shrinked TO THE SOCKET PINS, not just left hanging there. Any movement of those parts will cause noise. And personally, I wouldn't use metal oxide resistors there but that's another conversation.

Do you have a schematic, including values for whatever mod you used? That will help figure some things out.