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Thread: Weber 5F4 build - grounding question

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    Weber 5F4 build - grounding question

    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.

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    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

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    Last edited by DRH1958; 10-05-2015 at 03:52 AM.
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    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.

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    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.

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    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.

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    Lifetime Member Enzo's Avatar
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    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.

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

    Weber 5F4

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    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 -

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    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.

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    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 (?)

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    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!

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    Quote Originally Posted by frus View Post
    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.

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

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    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?

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    Quote Originally Posted by yaryaryar View Post
    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.

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    Without working heaters, a tube will not conduct. That is just ONE possible reason for all the tubes not working. As LT points out, a missing ground connection they all share is another.

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    Aha! So we're back to grounding issues (possibly). Like I said in the first post, grounding is still a foggy concept for me. Although, that Aiken article mentioned above is hugely helpful. I'll hopefully have time to check all this in the next couple days and report back. Thank you!

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    Lifetime Member Enzo's Avatar
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    There is grounding and there is grounding. We can debate the intricacies of star grounds versus grounding all over the place, but your thing flat out doesn't work, this isn't a matter of 10% more hum than we'd like. Ground is ground, anything that is to be grounded, just connect it to chassis somehow. worry about nuance later.

    If you have a cathode resistor of 1500 ohms going to pin 8 of a tube, then measure resistance from pin 8 to chassis. You either get about 1500 ohms or you get an open indication.

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    Maybe you could post a gut-pictures of your amplifier? Some MEF members have very sharp eyes and will probably spot wonky wiring.

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    Quote Originally Posted by yaryaryar View Post



    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?
    They would be soldered between where the screen grids get their voltage from the power supply(point A on the power supply) and pin 4 which is the screen grid pin number on the tube. Each tube will get its own resistor. This won't be the cause of your problems, just something to help your 6L6s last longer. Many people see this as a design flaw as almost all amps have screen grid resistors. I have attached a schematic of an amp that has them. You have bigger problems, address them first. This is just something to add later on as long as you are working on the amp. Make sure you have 6.3 volts AC on all your heater pins to be sure you have heater voltage. Nothing can work without this. Maybe you can get a list started on all the things to do.
    1. heater voltage, 6.3 VAC
    2. check all wiring against the schematic/layout
    3. check for cold solder joints
    4. check for cathode resistors/bypass caps connected to ground
    5. check for bias voltage on preamp tubes, 1-2 VDC
    6. check voltages against the schematic I linked you to in post #2 as the Weber schematic doesn't have any. You have roughly 10% higher voltages than that amp does, so use it only as a guide. All pertinent voltages are on that schematic. You'll have to cross reference the two.
    7. correct the bias on power tubes
    8. screen grid resistors
    Fender_bandmaster_6g7a_schem.pdf

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    Last edited by DRH1958; 10-06-2015 at 10:45 PM.
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    Crazy question: What would it mean if my heater voltage is only 3.3 VAC?

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    Quote Originally Posted by yaryaryar View Post
    Crazy question: What would it mean if my heater voltage is only 3.3 VAC?
    Did you check the heater wiring? The two greens are the supply and the green/yellow should be grounded. If this is the voltage, then you are only getting half the heater voltage on the tubes. Check the voltage on the 6L6s, pins 2,7. The 12AX7s should have 6.3 VAC between pins 4,5 and 9. This would explain why the tubes are not conducting if you are sure of your measurements. Also, you could check at the pilot light to see if it has 6.3 VAC.

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    My first thought is that you have one probe on the chassis and the other on a heater pin. You check heater voltage between heater pins, not with one probe to ground. As in, probes on pin (4 or 5) and pin 9.

    Justin

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    Gah! You're right - I was measuring it to the chassis. Jeez. There's so much I have yet to learn.

    But, now that I now how to measure AC, I'm now certain that the heaters are at 6.6 VAC. (I know 6.3 is ideal, but 6.6 is acceptable, right?) In the coming days, I'll go down that checklist above and verify some of the other stuff. Sorry to do this so piecemeal, but spare time is tough to come by on week nights.

    Thank you again!

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    It's fine that you only have small amounts of time as most of us have day jobs too. And it will give you a chance for things to sink in slowly instead of trying to absorb all of it at once.
    The 6.6 v heater value is one of those borderline things. Some will say it's OK and some will say it might shorten your heater's lifespan. For now, I wouldn't worry abut it. On your checklist, write down 6.6v next to "check heaters" and come back to it and later on you can decide whether or not you want to do anything about it. There is a solution but don't worry about it for now. See if you can find other things wrong. The important thing is that you are getting enough voltage for the tubes to operate. Also, don't forget about the advice on grounding from the others' posts just before my list. I should have included them as well.

    Notice I put add screen resistors last on the list. Get the amp working first.

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    Hey! It works!

    I spent some time with the amp last night going over every connection with a fine-toothed multimeter. I'll list what I found here, in case anyone's curious.

    1. Missing jumper under the circuit board, going from the 10k resistor to the 56k/100k pair, near the middle of the board (sorry - I can't name the components by their function yet). Or maybe it was just a bad solder joint between the two. No continuity there anyway.

    2. Strange reading on the 56k resistor on the top left of the board (bias resistor?). When measuring the resistor, the meter just jumped around a bunch. I removed it to replace it, and found that when it was out of the board, it measured fine. I soldered it back in place, and it measured correctly. Could a bad solder joint cause a jumpy meter reading like that?

    3. One of the capacitors had shorted. Something new I learned: when you can measure continuity across a capacitor, that's a bad thing. This was the .01 uF cap going to pin 6 of the third (middle of the chassis) preamp tube.

    4. I ended up just removing the bright channel inputs. I couldn't get a correct reading on that 1M resistor, and didn't have a replacement, so out it went. Maybe another bad solder joint? I just removed it (for now) to simplify the troubleshooting.


    Now the amp produces good clean guitar sound, with just a little hum (I'll dive in to that later - I liked Enzo's advice to "just ground it" and worry about minutia later). The voltages look good compared to the Fender layout, and the power tubes are biased at about 40 mA. BUT, there's still a couple issues: The treble and Presence pots don't seem to be doing anything at all. I'll have to look more closely at those issues this weekend. So there's still work to be done, but: Progress!

    Two things I've learned on this project: 1.) Patience! I was too anxious to just get the thing finished and turned on that I didn't bother to check the first guy's work outside of visually confirming that the right components were in the right places. Also, 2.) Double check your work! If it's somebody else's work, quadruple check it.

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    Hi all,

    I'm getting close! The amp is functioning on both channels now, all the controls work as they should, I'm just getting down to some minor stuff now. I have two mysteries remaining, and could use your expertise.

    1. My voltages mostly check out - compared to the old Fender layout, I'm at about 10% above those notations, as recommended above. BUT, the last reading (all the way to the right, going to pin 1 of the first preamp tube), is a little high. In fact, pin 1 of that preamp tube is at 214v, and pin 6 is at 193v. Shouldn't those be the same??? The voltage is coming out of the pair of 100k ohm resistors. Before those resistors, I'm measuring 317 volts. After those resistors, it's 193v and 214v. Both resistors check out fine (both measure 100k ohms). What's going on there? I've attached a picture below with my readings, if that helps. If it makes any difference, I'm using a 12AX7 for the first preamp tube - I know Fender has it as a 12AY7, but Weber supplies three 12AX7s in their kit. From what I understand, that should just be a difference in gain, but maybe that's causing this strange voltage thing?

    2. I'm getting a little noise when I turn the treble up. It's not awful - it's not ridiculously loud - but it's also not ideal. Is that just the nature of this circuit? Or is that a grounding issue? It's only on the treble - this does not happen on bass or presence or the volume knobs. I'm using the ground bus soldered to the back of the pots, if that makes a difference. Although I'm wondering if the noise could be related to my weird voltage issue?

    Again, thank you so much for the help. I'm really learning a lot on this project.



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    Supporting Member epis's Avatar
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    Hi yaryaryar,

    when you increase treble, it's normal to get more hiss, that's thermal noise, you can't really eliminate it. It's not caused by different anode voltages at first preamp tube.
    Simply if you had a perfectly balanced triodes in 12ax7, these voltages would be the same.
    If you put a tube with lower gain as 12AY7 hiss would be less noticeable.
    I like to use 12ax7 with split load anode resistors, it makes it close to 12ay7. Instead of 100K I use 56k and 47k in series.
    Cheers, Damir

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    Quote Originally Posted by epis View Post
    Instead of 100K I use 56k and 47k in series.
    ... and connect the following .022 cap between them, I presume

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    yep

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