Weber 5F4 build - grounding question

[Enzo]

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.

[yaryaryar]

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!

[Enzo]

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.

[überfuzz]

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

[DRH1958]

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

[yaryaryar]

Crazy question: What would it mean if my heater voltage is only 3.3 VAC?

[DRH1958]

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.

[Justin Thomas]

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

[yaryaryar]

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!

[DRH1958]

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.

[yaryaryar]

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.

[yaryaryar]

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.

[epis]

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

[frus]

Instead of 100K I use 56k and 47k in series.

... and connect the following .022 cap between them, I presume

[epis]

yep