Weber 5F4 build - grounding question

[yaryaryar]

Hello all,

I'm new to the forum, but I've been reading for quite a while. (Or rather, my google searches for amp questions always lead me here.) So, thank you so much for all your advice!!

I've been tinkering with amps for a number of years, I've built one kit (a 5F1 from Marsh that had lots of instructions), and I can do simple repairs. The project I'm working on now is a "rescued" kit build of a Weber 5F4. I bought it from a guy on craigslist, who completed about half and then gave up and put it in a closet for 5 years. I know Weber doesn't actually make these kits anymore, so googling info on this thing has been tough.

My first question has to do with grounding. The previous builder had already put in the ground bus soldered to the pots, so I stuck with that. But, I'm not sure where to ground the power tubes? They're 6L6s, so I'm looking at pin 8 of both tubes. Can I just ground that straight to the chassis? That's how I have it now, but I'm getting some pretty crazy screeching sounds coming from the amp - which could be unrelated, I know. This is just something I'm unsure of so wanted to ask about it first.

Also re: grounding, I'm following a grounding scheme for the input jacks that looks like this: https://robrobinette.com/How_Fender_...Jacks_Work.htm . Does that look applicable to a 5F4?

Thanks so much in advance. I'm sure I'll have more questions, but thought this is a good place to start.

[DRH1958]

Have you got the amp to a point where it works, as in plugging in a guitar will pass a signal to the speaker? Sometimes, this screeching sound is when the negative feedback loop is connected incorrectly and it actually turns into positive feedback. Try reversing the leads from pin 3 (plates) of the two 6L6s that go to the output transformer.

Yes, you can connect pin 8 directly to the chassis next to the tube socket. You might want to insert a precision(1-5% tolerance) 1 ohm, 1/2 to 1 watt resistor between pin 8 and ground. This way, you can use it for sensing current for biasing purposes. You use your meter's millivolt range to measure the voltage across the resistor and it translates directly to amps since it's 1 ohm. So if you read 40mV, it's cathode current is 40mA. This is the safest way to set bias because you are measuring less than a volt. If you are going to change it to adjustable bias in the future, then they are already in place. The 1 ohm value will not change the performance of the amp. Even if you don't change it to adjustable bias, you can still use the resistors to monitor bias current to see if your tubes are closely matched.

From what I can tell, an original 5F4 has a bright cap across one of the volume pots, so the amp should have the high/low, bright/normal scheme. It's the top channel that has the .0001 cap across the volume.

Fender_super_5f4_schem.pdf

[yaryaryar]

Hey, thanks very much. That's exactly the kind of info I'm looking for. I'm still pretty foggy on grounding stuff - as I'm sure you know, most layouts just say "this thing goes to ground", but doesn't say where. Googling for info on grounding leads to some pretty serious info on exactly what types of grounding schemes are best and why, but I can never find something that just says "ground this thing here".

So, screeching. I did actually already did have to reverse the wires on pins 3 of the output tubes. What's weird is - that screeching sound goes away when I have it wired opposite from Weber's layout. I'm using the same transformer that the layout calls for, so I'm not sure what the deal is there.

But after I did that, there's a different screeching/squealing. Parasitic oscillation, I think, from what other searches tell me. I thought it might be related to heater wiring - the first guy had the heaters wired really messy, not twisted pairs, etc. So I totally removed and redid it the standard Fender way. It looks nice and neat now, but I'm still getting the squealing. After messing with some of my ground connections, I did get it to a point where I can pass guitar signal through, but it was pretty low volume and distorted quickly, and the oscillations/squealing would come and go.

I know I'm being pretty vague here - let me know what info I can provide to be more helpful. I'll do some more tinkering tomorrow, and post with any updates.

[DRH1958]

Make sure all wiring that handles any signal is either shielded or twisted and as far away from heater and high voltage wires as possible. Here is a great primer on grounding. Randall Aiken really knows his stuff. You might find something in here that helps: Grounding

Even if your wiring is opposite of the layout, whatever way gets rid of positive FB is the correct way. You have a 50/50 chance of getting it right the first time.

BTW, welcome to the forum.

[lowell]

Please tell us what the bias current of your power tubes is. Incorrect biasing can lead to low volume distortion issues.

Another test, once bias is confirmed to be within normal range, is to use a signal probe. Poke around the signal chain starting from the power amp and working your way back to the input until the squeal isn't there. If you dont have a probe one can be built in minutes with some basic parts. This is cool as he built it into a pen. Troubleshooting your amp with audio signal tracing |. My probe is a cap taped to a chopstick, and wired to a 1/4" plug. I plug it into my cheapo bench top solid state amp. You want to narrow down the source of the squeal.

Since you received the amp half built there are some things to confirm before troubleshooting. If it were me I would check all plate, grid, cathode voltages and confirm they are within range. This simple process can reveal lots of problems, or eliminate variables if you know they are correct.

[Enzo]

I don't know what Weber has on their web site, but have you contacted them directly to ask if the original assembly instructions could be sent to you? That might answer some questions.

[frus]

Their layout just shows the ground connections to the grounding plate/chassis, like usual fender

Weber 5F4

[yaryaryar]

Hi all. Thanks again for all the advice here.

I didn't have much time to poke around this evening, but I did take a bunch of measurements at the tubes. Some of this stuff already looks way wrong to me (the biases don't match! And are way high!), but also, it's entirely possible that I'm just measuring wrong. Please feel free to tell me so. I haven't had time to dig in and figure out what's what. But maybe there's something obvious here you can spot right away?

Also, I am using the Weber layout posted above. For bias, I used a Bias Tester from amp-head.com - it's your standard tube-socket-looking-thing that you plug in to a multimeter. This list follows the signal path, as in "tube 1" is over by the inputs, and the power tubes are last.


preamp 1
1 435
2 29
3 -
4 heater
5 heater
6 443
7 11
8 -
9 heater

preamp 2
1 434
2 340
3 -
4 heater
5 heater
6 360
7 439
8 -
9 heater

preamp 3
1 448
2 48
3 -
4 heater
5 heater
6 452
7 -
8 93
9 heater

power tube 1
bias: 76
1 -
2 heater
3 448
4 453
5 -39
6 -39
7 heater
8 -

power tube 2
bias: 56
1 -
2 heater
3 454
4 454
5 -39
6 -39
7 heater
8 -

[lowell]

If this is correct...there is positve DC on your preamp grids. ?? Is this really true? Also, what are the cathode (pin3/8) voltages? 0v? "-" Doesn't tell us anything.

Preamp grids V1a/b, V2b, and V3a should all be 0V on the grids. Cathodes of those tubes should be apprx +1-1.5v.

Swap your power tubes with eachother. Do the bias readings stay with the tubes or the sockets? 20ma difference could cause some hum for sure. Swapping them can reveal if it's the tubes or the circuit.

[frus]

there has to be at least 1-2 volts on cathodes (except V2b and V3b where there should be like 200 and 65 volts respectively)
I'd say your preamp cathodes are not grounded at all, and preamp tubes are not drawing any current, except maybe V3b (?)

[yaryaryar]

Thanks for the input. I knew something goofy was going on here.... I'll double check these measurements when I have time tonight or tomorrow. As for pins 3/8 on the preamps, the multimeter was jumping around a bit very close to zero, so I just assumed that 0v was the reading.

So, I'll re-measure, and I'll also try swapping the tubes around and re-checking the bias, and I'll update tonight.

Maybe it's time to double check and redo every single connection in here. The previous guy had the board all populated and some connections to the tube sockets and pots. Maybe he knew something I don't... and that's why he gave up on the project. Oh well. I'm learning!

[DRH1958]

there has to be at least 1-2 volts on cathodes (except V2b and V3b where there should be like 200 and 65 volts respectively)
I'd say your preamp cathodes are not grounded at all, and preamp tubes are not drawing any current, except maybe V3b (?)

Agree, preamp tubes aren't conducting. The plate voltage proves this. Pins 1 and 6 shouldn't be anywhere near 435 and 443 on V1. Somewhere in the low to mid 200 volt range or so seems better. I think you are correct, look over every connection. I would treat the amp as if nothing had been done. I seriously doubt he knew something you don't.

The bias current for the 6L6s should be around 40-45mA for 60-70% bias at those plate voltages.

Also, the plate and screens are taken from the same supply node so the screen will be a higher voltage than the plate. Evidence of this is power tube 1 readings, screen is 5v higher than the plate. Generally, you want the screens lower than the plate. I would solder in a 470 ohm 1-2 watt resistor in series with pin 4 of the 6L6s to protect the screens.

[Enzo]

None of your small tubes are conducting, and they all have B+ voltage, so I must ask, are those tubes even lighting up inside?

[yaryaryar]

Yup. Preamp tubes are lighting up. I thought that was just the filaments' job?

Like I said, I didn't have time last night to dig in to what these voltages should be, or even what pin corresponds with what inside the tube (I did look that up today). And I'm (obviously) not at a point yet where I can look at a measurement and say "well that ain't right". So I thought just throwing out these measurements would reveal something obvious to you all. And it did: something is really, really wrong with my wiring (or possibly my measurements). I get now that there shouldn't be full plate voltages on the preamps (right? And where's that coming from?), and also that there shouldn't be DC at all on the grids (that should be AC only from the signal, right?)

DRH: Can you clarify what you mean by: "solder in a 470 ohm 1-2 watt resistor in series with pin 4 of the 6L6s". Does that mean putting a 470ohm resistor from pin 4 of tube 1 to pin 4 of tube 2?

One more question: can this wackiness be the product of bad solder joints? As I mentioned, the board was already populated when I received this. If so, what's the best way to correct that? Heat it up and reflow/add solder? Or is it better to totally remove and redo?

[loudthud]

One more question: can this wackiness be the product of bad solder joints? As I mentioned, the board was already populated when I received this. If so, what's the best way to correct that? Heat it up and reflow/add solder? Or is it better to totally remove and redo?

It could be a bad solder joint, but I would look for a missing ground wire from the board to ground. These usually go from an eyelet on the board with one or two components leads in it to the brass plate under the pots. Since the amp has a ground buss soldered to the backs of the pots, ground the wire to the ground buss.