JM wonderland build using tone geek boards

[Garveyte01]

Hi, I decided to build a JM/wonderland using the Fairfield county PCB boards from Ryan @ “tone geek”. I’ve checked all my grounds and wiring. I did a smoke test without tubes, my voltages on pin 3 of all power tubes is 455, the pins 4 and 6 also show 455 on all power tubes. I’m reading -41 volts on pin5. Heaters have correct voltage. I also just pulled my first two caps tested those and they seem good. I took a video of my start up with a bias probe to show the plate voltage in relation to the current. It seems as when the power tubes come to full temp the power tubes start to draw current. I pulled all preamp tubes to try to isolate the problem. It does seem to be in the power section but I can’t figure this out. Here is a video if anyone could give me some insight on what is going on I’d greatly appreciate it!
-Tom
https://youtu.be/cMd5fHmdFII?si=BBrJau3mRiCEPUWZ

[martin manning]

Video?

I don’t see anything wrong with what you describe above, unless you are seeing excessive current draw on your bias probe.

[Garveyte01]

Video?

I don’t see anything wrong with what you describe above, unless you are seeing excessive current draw on your bias probe.

The video hopefully shows more of what is going on, thanks Martin

[martin manning]

I hear the low frequency oscillation start as soon as the plate current comes up. Maybe you have the OT primaries reversed and you have a positive feedback?

[Garveyte01]

I hear the low frequency oscillation start as soon as the plate current comes up. Maybe you have the OT primaries reversed and you have a positive feedback?

I swapped the primaries still is drawing plate current, and dropping the plate voltage and it is lighting up the light bulb limiter. Also interesting is When the amp is on I have no reverb at all but when I shut it down and I’m still playing the guitar I can hear the reverb as the power fades

[Guy77]

I would confirm all the wiring in the power section and compare it to your layout to make sure everything is connected properly.
I would especially check how the first two large caps in the power supply are wired in series.

This can be easily wired wrong.

Cheers
Guy

[Garveyte01]

Ok so I got the low frequency oscillation out, amp is running fine with correct voltages. But I have no reverb. On my reverb transformer I have 465volt on the red primary and 355 on the blue wire…but I’m not showing any volts on the green wire which would normally drive the reverb tank input. When I move the reverb tank I can hear the “splash” of the springs in the tank but no reverb to play with.

[martin manning]

What was it that fixed the low frequency oscillation?

On the reverb, a 100V drop across the reverb driver transformer primary red-blue is way out. Normally there is 3-4 mA flowing through a DC resistance of 1k or so, and a drop of 3-4 volts. The secondary green-black shouldn’t have any DC voltage on it. The problem is on the primary side; the splash means the recovery amp is working.

[Garveyte01]

What was it that fixed the low frequency oscillation?

On the reverb, a 100V drop across the reverb driver transformer primary red-blue is way out. Normally there is 3-4 mA flowing through a DC resistance of 1k or so, and a drop of 3-4 volts. The secondary green-black shouldn’t have any DC voltage on it. The problem is on the primary side; the splash means the recovery amp is working.

Thank you Martin, the low frequency oscillation subsided after I replaced one of the 220uf capacitors that didn’t test well the OT primaries were left to there original layout.

In regards to the reverb, I feel embarrassed but I found a cold joint on V3 pin2 which is bridged to pin 7. The socket lug was filled with solder but I found the smallest movement in it, reflowed it and I have reverb. Check the voltages again and the reverb trans primaries were within 7 volts of each other. One note is with the reverb maxed, it isn’t that “wet” sounding.

Building these amps have been some of the most satisfying moments in my life it really is magical to hear these things come alive!

[martin manning]

Yes, always exciting to power up for the first time, These are complex builds, requiring patience and careful work. There are a lot of rabbit holes to explore too.

[Guy77]

What was it that fixed the low frequency oscillation?

On the reverb, a 100V drop across the reverb driver transformer primary red-blue is way out. Normally there is 3-4 mA flowing through a DC resistance of 1k or so, and a drop of 3-4 volts. The secondary green-black shouldn’t have any DC voltage on it. The problem is on the primary side; the splash means the recovery amp is working.

Thank you Martin, the low frequency oscillation subsided after I replaced one of the 220uf capacitors that didn’t test well the OT primaries were left to there original layout.

In regards to the reverb, I feel embarrassed but I found a cold joint on V3 pin2 which is bridged to pin 7. The socket lug was filled with solder but I found the smallest movement in it, reflowed it and I have reverb. Check the voltages again and the reverb trans primaries were within 7 volts of each other. One note is with the reverb maxed, it isn’t that “wet” sounding.

Building these amps have been some of the most satisfying moments in my life it really is magical to hear these things come alive!

Glad you got it working! When I heard the sound in your video I had a feeling it had something to do with the caps in your power supply.

In regards to getting the reverb more wet sounding you can replace the .01 uf cap that is connected across the Reverb Return pot to a smaller value like .005uf or .002uf. This will increase the reverb.

Cheers
Guy

[martin manning]

One note is with the reverb maxed, it isn’t that “wet” sounding.

Check out the posts starting here: https://ampgarage.com/forum/viewtopic.p ... 35#p433635