SOLVED Any Amp Experts??? Mojotone 5F2A Issues...

[Fingolfen]

Any amp experts... I've gone back through this circuit about 4 times and still can't figure out what's wrong.

I've been trying to finish up this Mojotone 5F2-A... Passed the dim bulb testing, etc. but when I put in the tubes I get a high pitched squeal that scales with the volume knob, and any tones are harsh, robotic, and clearly into breakup. Definitely isn't sounding like it should... to say the least...

As currently built (I've taken the tubes and speaker back out)



Wiring diagram (for 4 Ohm speaker - which is what I have)



Schematic (note, the schematic shows the green 8 ohm lead from the output transformer wired, but per the diagram above and the impedance of my speaker I'm using the yellow)



Thoughts??? Help???

 

[Fingolfen]

I haven't tried swapping the red and blue yet - will try it this evening with the clean-up mentioned and cross my fingers...

 

[RetiredUnit1]

I need to find some of that! We always called it “spaghetti tubing”, but both with amps and pedals, I always seem to need to insulate a lead or 3 here or there

Here ya go!

AWG L/T/S Type 150/300/600V Thin Wall PTFE Tube Sleeve 0.3mm-7.46mm Id | eBay

Excellent electrical performance, PTFE is a highly non-polar material, has good dielectric properties, great resistance, dielectric constant of about 2.0, is the smallest of all electrical insulating materials, is an ideal C-class insulation material;.

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[Fingolfen]

THANK YOU to everyone for the input!!!!!

Swapping the blue and red wires fixed it right up. I'm doing the 60 minute burn in at this point, but the dang thing even outside the cabinet sounds better than the little Champion 20 solid state I'd been using upstairs...

 

[RetiredUnit1]

THANK YOU to everyone for the input!!!!!

Swapping the blue and red wires fixed it right up. I'm doing the 60 minute burn in at this point, but the dang thing even outside the cabinet sounds better than the little Champion 20 solid state I'd been using upstairs...

Ok! I thought it would probably be with the output transformer, but I was so brain dead today. Ugh.....

Izzy is 23, that's 161 years old in Human years. He's almost completely deaf, can barely see, but boy is he loud when he wants out which is just about every morning before sunrise. I love the ol' guy so what ever Izzy wants, Izzy gets. Nickname for Isaiah. whoa, can't believe I spelled that right....

ROCK ON!

 

[Fingolfen]

Ok! I thought it would probably be with the output transformer, but I was so brain dead today. Ugh.....

Izzy is 23, that's 161 years old in Human years. He's almost completely deaf, can barely see, but boy is he loud when he wants out which is just about every morning before sunrise. I love the ol' guy so what ever Izzy wants, Izzy gets. Nickname for Isaiah. whoa, can't believe I spelled that right....

ROCK ON!

You nailed it! Thanks again, I just got it into the cabinet and the thing sounds like a dream at this point...

 

[HamishR]

NFB is scary if the OT primaries are the wrong way around. Simple fix though. I sometimes put the NFB on a switch. In the 5F11 tweed Vibroluxes I have built the sound is so much better without the NFB I just leave it out. I recently built a 6G3 brown Deluxe and put a switch in the extension speaker socket hole. At lower volumes it sounds soooo much better with no NFB. But as soon as you wind the amp up to where it's breaking up by itself the NFB really helps tame the fizzies. It's an interesting area for experimentation.

Another amp I built recently is a JTM45. One of the key differences between a JTM45 and the 5F6-A Bassman it is based on is the amount of NFB - The Marshall has way more. I've been experimenting with various different values of NFB resistor and using less NFB has really opened up the sound at the volumes I typically play at.

 

[Giorfida]

THANK YOU to everyone for the input!!!!!

Swapping the blue and red wires fixed it right up. I'm doing the 60 minute burn in at this point, but the dang thing even outside the cabinet sounds better than the little Champion 20 solid state I'd been using upstairs...

It’s definitely more common on push pull outputs, never knew it could happen on SE, but it makes sense. FWIW, in case it was the OT, is why I recommended a quick snippy snip of the NFB wire as it would have told the same thing as swapping, but would be easier/quicker to undo if it WASN’T the problem. But in this case, your taking a gamble and simply swapping the wires paid off and you didn’t have to undo any “temporary” measures!

Definitely a learning experience! Just wait until you have it happen on a 100w Marshall head sometime. The ringing in your ears will stop about an hour or so after you clean your pants out! Lol

 

[hamerfan]

NFB is scary if the OT primaries are the wrong way around. Simple fix though. I sometimes put the NFB on a switch. In the 5F11 tweed Vibroluxes I have built the sound is so much better without the NFB I just leave it out. I recently built a 6G3 brown Deluxe and put a switch in the extension speaker socket hole. At lower volumes it sounds soooo much better with no NFB. But as soon as you wind the amp up to where it's breaking up by itself the NFB really helps tame the fizzies. It's an interesting area for experimentation.

Another amp I built recently is a JTM45. One of the key differences between a JTM45 and the 5F6-A Bassman it is based on is the amount of NFB - The Marshall has way more. I've been experimenting with various different values of NFB resistor and using less NFB has really opened up the sound at the volumes I typically play at.

I like my 5F1 clone at lower volume in the raw mode. I do this with switching the standard NFB resistor to a 1meg resistor via a pull push pot. The 1meg resistor gives me a very tiny amount of feedback. This helps to dampen peaks a tiny bit. Since the 22k resistor is a kind of middle way damping, I don't need that anymore. I changed it to 12k, which eliminates the farting out of the 8 inch speaker on a higher breakup level.