first time poster/builder with a 5f2a question

[charriman]

Hi everybody,
I just completed a Mojotone 5f2a kit. The amp powered up fine and my voltages all check out, but there is absolutely no sound coming out of the speaker. I've tested the speaker out and it is fine.
My voltages are as follows:
6v6:
1:
2: 3.3
3: 341
4: 344
5: 0
6:
7: 3.3
8: 22
12ax7:
1: 175
2: 0
3: 1
4: 3.3 vdc
5: 3.3 vdc
6: 171
7: 0
8: 1
9: 3.3 vdc
Rectifier pin 8 - 402
1st filter cap - 402
2nd filter cap -402 (no dropping resistor) OT connection blue wire.
3rd filter cap - 343
4th filter cap - 289

I have also used an audio probe to check the signal throughout the signal flow. I'm getting signal at every point from the input jack to pin 5 of the 6v6 where the signal enters. When I probe pin3, there is a giant hum.
Should I be able to hear the signal on pin 3 of the 6v6 like I do when probing the in/out of each triode in the 12ax7? Or is it the fact that there is the high DC B+ voltage also on that pin.

I've been going back and forth with MojoTone support, but i'm thinking it's either the tube or the OT. How can I further troubleshoot this?
Thanks for your time.

[charriman]

More pics:

[martin manning]

I see you have the red wire from the OT going to the output tube, and blue to the power supply. It's shown that way in the layout, but opposite in the schematic. Usually Blue goes to the tube (pin 3). If those are reversed the feedback will be positive. I'd try swapping them.

[charriman]

I saw that, but I was under the impression that if those OT wires were reversed there would be a squeal. As it stands, there is no sound at all. No hiss, no pops when checking voltages etc.

[martin manning]

I would try swapping them. You could have an ultrasonic oscillation, and it's very likely that those leads are connected backwards anyway.

[charriman]

swapping the red and blue fixed the signal issue. Now there is guitar coming out of the speakers. There is a slight whistle that changes when the volume is turned up and when playing the guitar, there is a nasty distortion happening.
Here's a couple of videos.

[martin manning]

Looks like you have some oscillation going on at the front end, suggested by the volume knob changing the pitch. Try moving the control leads around with a chopstick to see if there is any effect.

PS the wiring should look more like this picture from Mojo's site.

[charriman]

It looks more like that now. I've rerouted some stuff @ the 12ax7 and replaced the neg feedback wire as that was a little too short.

[charriman]

Ok. This is all sorted out. Pulled the negative feedback and the whistle disappeared. Swapped the IT wires so red is in pin 3 and I'm good to go. I had moved blue to pin 3 since those pix were taken.
The is for your help!

[martin manning]

Glad you got it working Your lack of output seems to have been due to some problem that was corrected when the wiring was cleaned up(?) In the first post you said you had traced signal all the way to the output tube's grid.

[Rant] Mojo's documentation has come up short. Again. The layout conflicts with the schematic, the detailed contruction manual, and the photo of the finished chassis wrt the OT primary connections. Looking a bit further, I checked the docs for the two transformers called out on the schematic, and find that the "NOS 771" (which is supplied with the Champ kit, according to the description), has its primary lead colors reversed wrt the original (which was, of course, the standard). There is a large print warning to that effect on the data sheet, but really, how hard would it be to change the lead colors so that they are consistent with the standard, and avoid the confusion? [/Rant]

[charriman]

The schematic says blue to pin 3. They layout shows red to pin 3. The finished chassis pic shows blue to pin 3 as well. Maybe Heyboer is inconsistent in their wiring? I definitely had a wiring problem I inadvertently fixed, then I had to put red to pin 3.
But hey it was my first time and I learned a lot.

[martin manning]

For sure, always something to learn. How's it sounding now? According to Mojo's data sheet, the primary lead colors on that transformer are non-standard. See: https://www.mojotone.com/transformers/F ... NOS771.pdf