New 5F1 Champ Build

[HumanJHawkins]

Hi all.


Thanks for the great forum. I was able to solve a squeal issue by searching here (I think). But now I'm hoping someone can tell me what lead up to this issue. The scoop is... After completing the build and going through some (not enough) preliminary checks, I turned the amp on with tubes in it. As soon as the rectifier warmed up, I started getting a loud squeal even with volume down. Pulling the pre-amp tube brought it down to a nice very low hum.


Searches here lead me to suspect the OT primaries were reversed, causing the negative feedback to become a positive feedback and resulting in squeal. I disconnected the NFB from the output jack to test this. Bingo. I can play and it sounds surprisingly good even with no NFB. I haven't swapped the OT primaries yet, but I suspect that (and reattaching the NFB) should put it right.


But here's the thing... The OT Primaries do not seem to be wrong. I checked the specs for the ClassicTone 40-18031 output transformer, read a bunch here, etc. Everything says I had them right. So what I am wondering is, what could I have done prior to the OT primaries to get something else backwards? Or is there anything else that would explain the symptoms I just described?


Photos follow:






Thanks in advance for any tips or help.

[TIMBO]

Just try it and see, it won't do any harm. 

[stratele52]

All you signal wires ( not heater ) must be close to the chassis and short as possible .


As I see , far from the chassis they may act like antenna . Check the wire from the volume wiper to the 12A7 pin 7


But first disconnect NFB and reverse your OT wires .

[kagliostro]

On SE amp (also if there isn't an NFB circuit) some people pay attention to connect B+ at the wire of the primary which is connected to the inner winding of the primary of the OT as to avoid the more stray capacitance present on the other side of the winding (I hope you can understand what I mean)


K

[Jack_Hester]

Hello, and welcome to the forum.

Are your input jacks wired as in the original schematic?  With no inputs connected, the signal input wires will be tied to signal ground.  An open input can cause squealing, as well. 

Jack

[stratele52]

Hello, and welcome to the forum.

Are your input jacks wired as in the original schematic?  With no inputs connected, the signal input wires will be tied to signal ground.  An open input can cause squealing, as well. 

Jack

+1 , shorting jack contact

[sluckey]

I disconnected the NFB from the output jack to test this. Bingo. I can play and it sounds surprisingly good even with no NFB.

This is all the proof you need that your problem is incorrect phase for the NFB. The problem ain't heater wires and it ain't input jacks.

You must reverse the phase. There are only two ways to do that in the 5F1... Reverse the primary leads, or reverse the secondary leads. But since you have multiple taps on the secondary you'll have to just reverse the primary.

[stratele52]

I disconnected the NFB from the output jack to test this. Bingo. I can play and it sounds surprisingly good even with no NFB.

This is all the proof you need that your problem is incorrect phase for the NFB. The problem ain't heater wires and it ain't input jacks.


+1

You must reverse the phase. There are only two ways to do that in the 5F1... Reverse the primary leads, or reverse the secondary leads. But since you have multiple taps on the secondary you'll have to just reverse the primary.

[HotBluePlates]

... I'm hoping someone can tell me what lead up to this issue. ...

But here's the thing... The OT Primaries do not seem to be wrong. I checked the specs for the ClassicTone 40-18031 output transformer, read a bunch here, etc. Everything says I had them right.

You mean correct color-coded wire soldered to the correct place. I trust you soldered the wires in the right place. Who said the manufacturer got it right? That is, the end of the primary that was blue on the Fender OT got a Red wire on the Classictone OT...

This happens all the time. You have a 50/50 shot of getting it right; leave OT wires long for first connection until you've confirmed correct hookup. If the amp doesn't have negative feedback, you don't have to worry about this potential problem.

[HumanJHawkins]

Wow... Active forum.    I will reverse the OT primaries tonight, re-attach NFB,  and give it a listen.


On the long lead from vol-pot to V1 pin 7, I wondered about that too. There are two holes in the board for routing this wire, and they seem to make it longer than necessary. I think I'll just replace this with a shorter shielded wire at some point and skip the routing guides. (I wanted to build it stock to start for some reason.)



Thanks all.

[stratele52]

Wow... Active forum.    I will reverse the OT primaries tonight, re-attach NFB,  and give it a listen.


  (I wanted to build it stock to start for some reason.)

Thanks all.

Building it to stock is not only put same parts together .........


If you ear some hiss , replace with a shielded one or better wire dress , like original Fender amp .

[HumanJHawkins]

Thanks again all. Switched the OT primaries and... Oh my. She's a beauty. I don't even have a proper speaker in it yet and some of the sounds I am getting are unreal. Now that the NFB is back on, there is essentially no hum. I'm tempted to tinker, and probably will after the honeymoon phase. But for now, it's all good.


Loves me some old-school tube tone.
Cheers.



 

[stratele52]

Thanks again all. Switched the OT primaries and... Oh my. She's a beauty. I don't even have a proper speaker in it yet and some of the sounds I am getting are unreal. Now that the NFB is back on, there is essentially no hum. I'm tempted to tinker, and probably will after the honeymoon phase. But for now, it's all good.


Loves me some old-school tube tone.
Cheers.

Great , enjoy