Built a 5f4 Tweed Super but have low distorted volume!

[Enzo]

Setting bias in an amp is like setting idle speed in a car. You cannot set idle speed while the car is driving, and you can't set amp bias while you are playing through it. When you measure grid voltage or tube current while playing, you are not measuring bias, you are simply measuring tube current.

[hamilcaster]

We have glorious guitar signal at full volume!! We also have a lot of hum. Hum that gets louder and quieter when I adjust the unused channel's volume too. This can be tracked down I bet. I'm just so thrilled to have guitar coming out of the speakers!! You build with unknown transformers and by the end of the build you're not sure if they even work or if you've damaged them in the process.

Since last I posted I:

1) unsoldered the output transformer secondaries from the jacks because I wasn't liking the short I thought I was measuring. Turns out something was shorted. I decided to remove the choke and OT, use headphones to locate a quiet position for the OT and reattached both again. I put the choke in between the OT and PT this time and moved the OT a little farther away. Hopefully this loud hum is not due to it's placement. So I resoldered all connections, twisted the choke wires, OT primary and secondary wires (kept seperate from each other). Plugged it in and had almost zero hum in standby now, which assured me that I made the right choice putting in the effort to move the OT!

As far as the hum: guitar volume off and all amp tone knobs at zero, I'm getting 60hz present at zero volume that gets louder as I raise the volume, and gets even loud when I also raise the unused 2nd channel volume. I turn up the tone and the hum turns into an insect buzzing that gets very loud. When I turn up the bass, a deeper and louder 60hz is present.

I will start the hum elimination process when I get back from work. Thank you everyone for taking the time to make suggestions that ultimately lead to the amp working!

[hamilcaster]

Setting bias in an amp is like setting idle speed in a car. You cannot set idle speed while the car is driving, and you can't set amp bias while you are playing through it. When you measure grid voltage or tube current while playing, you are not measuring bias, you are simply measuring tube current.

That makes perfect sense.

[eschertron]

The heater wiring is not shown in the Fender schematic. However, the heaters need a ground reference or it WILL be very hummy! If you have a center tap on the heater wiring at the PT, ground it. If you don't have a heater winding center tap, then Fenders of that vintage simply ground one side of the heaters. This is frowned upon today, but a quick test with a short piece of jumper wire will determine if the heaters are the source of the hum here.

edit: ..and how are you getting the 6 volts to the heaters at the 6L6 tubes? Wired in series?

[hamilcaster]

The heater wiring is not shown in the Fender schematic. However, the heaters need a ground reference or it WILL be very hummy! If you have a center tap on the heater wiring at the PT, ground it. If you don't have a heater winding center tap, then Fenders of that vintage simply ground one side of the heaters. This is frowned upon today, but a quick test with a short piece of jumper wire will determine if the heaters are the source of the hum here.

edit: ..and how are you getting the 6 volts to the heaters at the 6L6 tubes? Wired in series?

The transformer has a 6.1v tap for the power tube heaters and a 12.6v tap for the preamp tubes. I had a pair of two 100ohm resistors off the 6.1 heaters for the artificial center tap but disconnected them earlier when I read something that said shouldn't use them but I'm thinking I should hook them up again. I'll do that later today. Good catch!

[Enzo]

They did just that here. The schematic shows one side as grounded. The usual method was to run one hot AC wire down the row of sockets and then ground the other side of each heater right at each socket.

Here is the layout for that schematic:
http://bmamps.com/Schematics/fender/super_5f4_schem.pdf


I agree, that is better than nothing, but also the least desirable of all the methods.

[loudthud]

Since there are two heater windings being used, there needs to be two center taps. If the 12.6VAC doesn't have a center tap, use two 220 ohm 1/2W resistors.

[Alan0354]

1)I put the choke in between the OT and PT this time and moved the OT a little farther away. Hopefully this loud hum is not due to it's placement. So I resoldered all connections, twisted the choke wires, OT primary and secondary wires (kept seperate from each other). Plugged it in and had almost zero hum in standby now, which assured me that I made the right choice putting in the effort to move the OT!

Don't worry about the placement of the transformers just yet. You don't know what is the problem until you fix the volume issue. Fix that first before you do anything. You are going to long route, the problem of the volume can just as well causing the hum.

AFTER fixing the volume and if the hum is still there, then

1) let people here look at your grounding location in the chassis. People can see the potential problem. Again, eye ball!!!
2) Filament can cause hum if there's a mistake.
3) check wires of sensitive signals draping over power wires.

If all that fail, then look into the placement of the transformer. Don't speculate, step by step starting with the easiest one first......eye ball check.

Did you eye ball the wiring according to the schematics yet? That' only takes you a few minutes. I bet you 40% chance you'll find the problem.

[hamilcaster]

Loudthud: You are correct, I failed to put a center tap on the preamp heater. I added two 220 resistors as well as reattached the 100ohm resistors on the output tube heaters and 95% of the hum went away! So thanks for that idea.

Alan0354: I realize I was very hasty with my decision to unbolt and move the transformers. Solving one issue at a time is definitely the right way to go. I just took a chance and once I fixed the shorted output jacks, I decided to go one step farther and eliminate the hum while in standby mode by moving the transformers. This afternoon, I went through and checked with the schematic as best I could. The layout I was using compared to the Fender schematic takes some time to reconcile in my head as I'm tracing the paths! Good news is most the hum is gone and the amp is behaving pretty much as I'd hope save for a little hum but that might also be because I'm standing right there with my guitar in sight of the open chassis. The hum does go away mostly if I move away from everything. Still, I hear people build completely silent amps and I want that to my goal for this one so I'm gonna keep poking around to see if I can quiet the signal by carefully moving some wires around. I'm afraid to post a picture of the inside wiring because I know it's not so pretty... but here you go!





Is the the way I grounded all these to the star scary or ok?


Here's the cabinet I built


I guess let me know if something is blaringly dangerous and needs to be addressed please! I isolated all jacks from the chassis. All grounds are star-grounded at the transformer lug so there's only one place where ground touches the chassis. This is the first time I've gone this route with isolating the jacks.

[Alan0354]

See, that was the second item on my list in the hum issue in post #23. The filament supply. Transformer position is not critical. That's the reason I told you not to move the transformer.

Need picture on the power amp section.

I don't like the way you ground the cathode of the preamp tubes to the star ground with one red wire in a string, then you have a separate red wire ground for the potentiometers to the star ground in another string. Someone like doing star ground can help you more. I personally do not do star ground like this. Basically the ground of the first volume should be the same as the ground of the first preamp tube. But this is about hum issue, nothing to do with your original problem.

BUT I suspect your lack of volume is in the power amp, you need to post pictures of the power amp, don't need to be so close up. Just like the one you have for the preamp.

[hamilcaster]

See, that was the second item on my list in the hum issue in post #23. The filament supply. Transformer position is not critical. That's the reason I told you not to move the transformer.

Need picture on the power amp section.

I don't like the way you ground the cathode of the preamp tubes to the star ground with one red wire in a string, then you have a separate red wire ground for the potentiometers to the star ground in another string. Someone like doing star ground can help you more. I personally do not do star ground like this. Basically the ground of the first volume should be the same as the ground of the first preamp tube. But this is about hum issue, nothing to do with your original problem.

BUT I suspect your lack of volume is in the power amp, you need to post pictures of the power amp, don't need to be so close up. Just like the one you have for the preamp.

Here's the layout I was also following, which uses the star-ground scheme. I don't have an opinion on the matter of grounding yet.


Also, just to be clear, I no longer have any volume issues. Only slight hum, which is much better now that I fixed the center tap of the heater supplies. And I also wanted point out that I fixed the shorted output jack and went ahead and moved the transformers hours before you posted advising against it so please don't feel like I intentionally ignored your wisdom because that wasn't my intent In the future, I'll likely not take an unfounded action like that before ruling out other options.

I am in the process of hanging the amp in the cabinet to see how it sounds in that form. I'll post back!

[Alan0354]

Here's the layout I was also following, which uses the star-ground scheme. I don't have an opinion on the matter of grounding yet.


Also, just to be clear, I no longer have any volume issues. Only slight hum, which is much better now that I fixed the center tap of the heater supplies. And I also wanted point out that I fixed the shorted output jack and went ahead and moved the transformers hours before you posted advising against it so please don't feel like I intentionally ignored your wisdom because that wasn't my intent In the future, I'll likely not take an unfounded action like that before ruling out other options.

I am in the process of hanging the amp in the cabinet to see how it sounds in that form. I'll post back!

Glad you hear you fixed the problem. The diagram does not show where the star is. But also there are other opinions around. I do it differently and my amps are dead quiet. Other people did the star ground and their amps are dead quiet also. We had big debates before, it's no point going through that. I am sure other's can make it dead quiet for you too.

One thing, to test hum, do not plug the guitar in. just turn on the amp and crank up the volume. That's the hum from the amp. If it is humming, you have a problem, if it is not, then you are good to go. You can talk to your face turn blue on grounding, but the bottom line is if it is not humming, you are good!!!!

[hamilcaster]

The hum does not seem to be too big of a problem when I disconnect and play with the volume. pretty happy about that! During sound check, the tone sounds decent at low volume but when I crank the amp (which isn't as loud as I thought it'd be...maybe a symptom) the distortion is very rough and the best word to describe it is "blatty"and fizzy. I will swap out some preamp tubes to see if those are the culprit. Thought I'd share the file for you all to hear if interested: https://soundcloud.com/guitylerham/5...storted-signal (if your page looks like mine, just click the play button in the lower right-hand corner).

The tone controls on this amp are very interesting and not what I'm used to! Should be fun getting to know the amp once it's up and running smoothly.

[Alan0354]

Sounds like you have some "motor boating" problem!!!! ( sounds like tremolo or vibrating sound. It is lower frequency that sound like you talk with mucus in your throat!!!) That usually is not enough filter caps.

You see in the schematic where the two 100K plate resistors of the first tube? Follow the connection from the two resistor in the schematic. You'll see a 8uF 450V filter cap. That's way too small. I would put another 10uF in parallel with that one.

You have a filter cap that is over 8uF and 450V cap? Just try putting it in parallel with the 8uF cap and see whether it improve the tremolo/vibrating sound.

[hamilcaster]

Sounds like you have some "motor boating" problem!!!! ( sounds like tremolo or vibrating sound. It is lower frequency that sound like you talk with mucus in your throat!!!) That usually is not enough filter caps.

You see in the schematic where the two 100K plate resistors of the first tube? Follow the connection from the two resistor in the schematic. You'll see a 8uF 450V filter cap. That's way too small. I would put another 10uF in parallel with that one.

You have a filter cap that is over 8uF and 450V cap? Just try putting it in parallel with the 8uF cap and see whether it improve the tremolo/vibrating sound.

I'll source an adequate cap for that tomorrow when I can get to the store. So that's what motorboating is! I think it can also be a constant motorboat sound too which means your caps are suspect to fail. This only comes when it's cranked and I strum a heavy chord and the motorboating comes in as the note decays and then fizzles out after a second. Thanks for that tip off!