6M18VIB build test voltages

[Billy_Goat]

Anyone have or know where I can find a schematic with voltage test measurements and / or a listing of expected voltages for a vibrato build? I have a copy of schematic and build document, but these unfortunately do not contain any voltage test points. I did my best to search this forum and across the Internet to no avail. Maybe I missed something? Anyway, I did find a voltage test sheet here for a TMB that gets me some numbers ( JMPGuitars.com 18 Watt TMB Voltage Test Sheet ), but I'm not sure how the TMB section differs from a vibrato build beyond V1 & V3. My power section and initial DC measurement seem to be ok, but the measurements across the 3 12AX7's seem to be quite whacked. I don't have the power tubes yet... Clearly I have a wiring / component issue somewhere that I need to identify and resolve. I've build quit a few tube and solid state studio rack devices so far, but this is my first full guitar amp build . Yes, I'm a noob guitar amp builder. Any assistance anyone cares to share would be greatly appreciated. Thanks in advance!

[Bieworm]

You mean tremolo? There is a significant difference between tremolo and vibrato. Lots of folks confuse those to ...
Tremolo:
is volume fluctuating on an adjustable speed and depth
Vibrato:
Is pitch modulation where the notes become bent out of tune in two ways. This is a very complex system and requires I think 5 triodes, meaning 2.5 tubes.
If you mean tremolo, that's easy and the voltage charts are to be found in the download area. If you take the 18 watt classic, and not the TMB. Those are different and not to be compared except for the normal channel on some designs

[JMPGuitars]

Use the Modern Classic docs in the download section instead.

[Bieworm]

Better option indeed, but be aware that V2 and V3 are swapped, so V3 is the PI...

[Billy_Goat]

Thanks for the responses guys.

Yes, I understand the difference between vibrato and tremolo. Weber calls their non TMB build a VIB (Vibrato) so I was just following their nomenclature.

That aside, I think I've figured this out - maybe? The amp powered up nicely after fixing a couple of cold solders and a missing connection to the PI. (Ugh!).

I'm now having some oscillation issues on the normal channel only when the volume is at (near) full (guitar unplugged - no signal) and / or the tone is at (near) zero (guitar plugged in or not) .

I've poked around with a wooden stick to see if massaging the connecting wires throughout would resolve, but no luck. The VIB channel seems to be working fine. So, working on resolving the last two issues.

Otherwise, the amp sounds amazing!

[Bieworm]

I read somewhere that someone with a similar problem solved this with grid stoppers. The weber schematic shows no grid stoppers.

Shielded wire to the grids is always adviseable, grounded on only one end.

[Billy_Goat]

I read somewhere that someone with a similar problem solved this with grid stoppers. The weber schematic shows no grid stoppers.

Shielded wire to the grids is always adviseable, grounded on only one end.

Bieworm,

Hmm... I'll look into grid stoppers.

Yes, I used shielded wire with only a single sided ground connection.

Appreciate the input.

Cheers

[Bieworm]

I read somewhere that someone with a similar problem solved this with grid stoppers. The weber schematic shows no grid stoppers.

Shielded wire to the grids is always adviseable, grounded on only one end.

Bieworm,

Hmm... I'll look into grid stoppers.

Yes, I used shielded wire with only a single sided ground connection.

Appreciate the input.

Cheers

Maybe use 10k. Those are used on the tremolo TMB...

[Billy_Goat]

Maybe use 10k. Those are used on the tremolo TMB...

Yep, 10K will be my first test.

Thanks again!

[Billy_Goat]

I added the 10k grid stoppers to the normal channel and that solved the oscillation issues. Thanks!

Not sure why the Weber schematic is so far off from the basic schematic posted within this site, but it is what it is.

So... not sure if the following should be made into a new thread or still part of this thread?

At this point, the amp is more or less functioning with the following caveats...

1.) Should I add grid stoppers to the tremolo channel and the power tube section even though the amp is not outwardly suffering from the lack there of?

2.) While the amp is fairly loud, sounds nice and has no buzzes, pops or other anomalies pointing to bigger issues, it does seem anemic in over all volume. Something along the line of about 1/2 has loud as I would expect an 18w amp to be in potential volume. I searched the Internet and this forum hoping to find a quick and easy means of calculating and verifying output wattage even if only a ball park number. Using one calculation I found, this amp measured out at about 8w. The calculation I found... (output V x output V) / speaker ohms. Is this a legitimate means of ball parking the output?

My procedure... 1kHz signal input from a signal generator into normal channel. Amp speaker out connected to a 100w dummy load. Oscilloscope connected to dummy load. I cranked the normal channel input up until sine wave displayed on the oscilloscope started to flatten (distort), then backed it down until the sine wave was once again smooth. That measured ~8v AC. 8 x 8 divided by 8 (ohms) = 8watts. Cranking to full volume only raised the output voltage a tad higher above 8v.

Did the same with the tremolo channel. Same results.

3.) With the above in mind, my DC measurements on pins 3 of the EL84's is only about 7V instead of the schematic indicated voltages indicating 12V. The 'JMPGuitars_18_Watt_Modern_Classic_Schematic.pdf' I download from this site has a note (#3) indicating that 180Ω 5W resistor should be altered in size to bring the voltage to 12V. Do I want to do this? Any other things I should check / test to bring this amp up to full output assuming my calculations above here are correct? ( My full measurements are included as an attachment here. )

4.) The above aside, and potentially very serious, for reasons that I do not undertand atm, the solid state rectifier 'tube' that Weber provides ( https://tedweber.com/wca4/ ) measures over 300v DC between chassis (ground) and the outer copper covering. I found this out after attaching the metal tube retaining clip (doing this provided continuity between the outer shell of the rectifier and the chassis) which lit up my current limiter light bulb as a warning - thankfully.

I immediately powered everything down and checked that the mains fuse did not pop. It did not. I then removed the the retaining clip and re-powered up the amp. All seemed well.

Measuring from chassis ground to the outside shell of the SS rectifier tube resulted in over 320v DC. I triple checked the Weber schematic and build doc to ensure I wired all as it should be. Seems to check out. I then tried to see if I could find a continuity issue between the rectifier shell and any of the pins while the rectifier was unplugged thinking maybe I has a defective device. All checked out normal - no continuity to be found.

I then plugged the rectifier back into the amp with the power off and unplugged and tried measuring for continuity again. My best guess atm is the rectifier tube is getting continuity from the CT of the power transformer should the retaining clip be attached or someone touches the rectifier shell and a ground source unknowingly. Leaving the retaining clip off would of course mitigate this - sorta, but the Weber website suggests that hanging this tube upside down as it will be in the build should have the retaining clip attached. More important though, leaving this as it is now could open up a potential life threatening, high voltage shock hazard should anyone accidentally touch the rectifier shell and a ground source down the road. No bueno! In any case, I don't undertand how the rectifier shell is connected to anything that I can find.

The Weber rectifier 'tube' ( representative drawing attached ) is a direct drop in for a EZ81 which I thought I should consider dropping in assuming my build is indeed correct, but looking at the basic build schematic from this site, it appears that the EZ81 requires it's own separate filament power from the rest of the tube compliment or is that just a option with the transformer indicated on the basic schematic? Either way, my build at the moment does not have any filament wiring to the rectifier tube position. I guess could add that, but it would be a bear and a headache at this point.

Anyway, sorry for the long post. I also understand that the Weber build is NOT what folks here normally build here. Lesson learned. Unfortunately the Weber support forum is more-or-less a ghost town - much to my chagrin. I appreciate any input that can be provided here knowing that I created my own world of noise by choosing this kit.

Thanks in advance!

[Bieworm]

I suggest that you disconnect the rectifier and make a SS rectifier. Put them on the 290V HT wires to the standby switch. You'll find an example on the "tremolo TMB" schematic. That will raise the B+, which is kinda low on your voltage chart. It wil bring the voltages on your PI plates more to spec too. For sag you can put a 5 or 10W 100 Ohm tesistor in series with the conjunction of the diodes. You won't hear a difference with the EZ81 that way.

What value cathode resistor are you using? 7V means you'll have to raise that resistor. You need to be in the 12V ballpark. I think that will solve your output problem too., along with the higher B+ voltage. It's all a matter of balance.
Also put a 2200uf cathode cap for augmented bass response.

It's not necessary to add more grid stoppers if there are no audible issues.

[JMPGuitars]

I'm currently in a food coma, so the only advice I can give is to check your dissipation here: https://robrobinette.com/Tube_Bias_Calculator.htm

[Billy_Goat]

I suggest that you disconnect the rectifier and make a SS rectifier. Put them on the 290V HT wires to the standby switch. You'll find an example on the "tremolo TMB" schematic. That will raise the B+, which is kinda low on your voltage chart. It wil bring the voltages on your PI plates more to spec too. For sag you can put a 5 or 10W 100 Ohm tesistor in series with the conjunction of the diodes. You won't hear a difference with the EZ81 that way.

What value cathode resistor are you using? 7V means you'll have to raise that resistor. You need to be in the 12V ballpark. I think that will solve your output problem too., along with the higher B+ voltage. It's all a matter of balance.
Also put a 2200uf cathode cap for augmented bass response.

It's not necessary to add more grid stoppers if there are no audible issues.

Bieworm,

I ordered a EZ81 just in case, but I thankfully have the components on hand to throw together the solid state rectifier as you suggest and that the TMB schematic indicates. I don't have a 10W 100R resistor, but I do have 5W's. Based on your suggestion above, a 5W will suffice at least for testing purposes? And, my current in place cap / resistor filter build will work or do I need to build the TMB cap / resistor network less the 'D' section?

The cathode resistor that was in place when I measured 7.11v was two 250R resistors in parallel which of course nets 125R. Not sure why the Weber build specifies two resistors instead of a single resistor, but I digress... Anyway, I replaced the two cathode resistors with a single 500R and then tested. That netted a 11.04V measurement across the resistor. That also reduced the plate current to 9.1mA from 22-23mA using the 125R Weber build size. The plate voltage jumped from 305v using the 125R cathode resistor to 354v using the 500R so I guess I'll lower the cathode resistor initially after adding the SS rectifier just to be safe.

I'll cobble the SS rectifier circuit together and test that hoping for a little bit higher B+ voltages and then see where I am as you suggested.

I'm currently in a food coma, so the only advice I can give is to check your dissipation here: https://robrobinette.com/Tube_Bias_Calculator.htm

JMPGuitars,

Yeah, I hear you! Turkey overdose.

I have been reading Rob's website hoping to get my head around all of this. After building and testing Bieworm SS suggestion, I'll report back with calculations.

As an aside, I have a 9 pin bias tester from EuroTubes that allows me to view the plate and current reading at the same time without having to probe. It's more-or-less displaying the same probe measurements I'm manually getting using my Fluke DMM.

Assuming I'm using the calculator correctly, based on the 500R cathode resistor stated above, I get the following numbers. See attachment.

Thanks for the replies!!!

[JMPGuitars]

Assuming I'm using the calculator correctly, based on the 500R cathode resistor stated above, I get the following numbers. See attachment.

Thanks for the replies!!!

First, you need to pick your power tubes in that list. Then you need to input the correct voltages. Plate to cathode voltage is plate minus cathode. 354 - 11 = 343.

But yeah, 500R is way too high. Drop that to 180R and see what you get.

[Bieworm]

I'll advise you the same Josh did when i was struggling with my power tube biasing. Get some 180, 200 and 220R resistors to start tweaking. And maybe some 5V 5W zener diodes forvthe B+ tweaking...
Start with the 180R and get your B+ to spec. If the cathode voltage is still too low you raise the resistor to 200R.. and so on..
Your B+ should be ideally at 335 to 345V. Mine is 354V for higher headroom, but with the classic 18W you don't want that. It's going to be pretty loud before it starts to break up with too high B+. I might lower my classic to 335V some day, but that's not my priority right now, and I hardly use the classic because the tremolo TMB is way more my holy grail. In fact I'm building a second one right now .. it's so good I want 2!!!

20201128_204212.jpg

[JMPGuitars]

Your B+ should be ideally at 335 to 345V.

335 is too low. Ideal B+ is 345VDC (+/- 5V is okay).

[Billy_Goat]

I'll advise you the same Josh did when i was struggling with my power tube biasing. Get some 180, 200 and 220R resistors to start tweaking. And maybe some 5V 5W zener diodes forvthe B+ tweaking...
Start with the 180R and get your B+ to spec. If the cathode voltage is still too low you raise the resistor to 200R.. and so on..
Your B+ should be ideally at 335 to 345V. Mine is 354V for higher headroom, but with the classic 18W you don't want that. It's going to be pretty loud before it starts to break up with too high B+. I might lower my classic to 335V some day, but that's not my priority right now, and I hardly use the classic because the tremolo TMB is way more my holy grail. In fact I'm building a second one right now .. it's so good I want 2!!!
20201128_204212.jpg

Bieworm, JMPGuitars,

After building the SS rectifier as suggested, and changing the cathode resistor to 175R ( I don't have any 180R, 200R or 220R 5W resistors, so put a 100R + 75R in series just so I could test while waiting out resistors on order...)

Voltage measurements seem to be more inline with what is expected, but amp is still anemic in loudness. ( See attachments )

I also don't have any 5v/5w zeners or 1N4744's on hand as noted on the TMB schematic. Not sure where you are going with the 5v/5w zener suggestion, but I have also ordered up some of these along with some 1N4744's - just in case. The 1N4744's are 15v devices and are something other than the 5v/5w zener suggestion? Can you provide some clarity here, please?

Beautiful build picture, Bieworm!

@ JMPGuitars, yes, sorry. I see what I did on the first posting of my Tube Bias Calculations. Hopefully, this one is correct.

Thanks again for the assistance. Much appreciated!

[JMPGuitars]

After building the SS rectifier as suggested, and changing the cathode resistor to 175R ( I don't have any 180R, 200R or 220R 5W resistors, so put a 100R + 75R in series just so I could test while waiting out resistors on order...)

Voltage measurements seem to be more inline with what is expected, but amp is still anemic in loudness. ( See attachments )

I also don't have any 5v/5w zeners or 1N4744's on hand as noted on the TMB schematic. Not sure where you are going with the 5v/5w zener suggestion, but I have also ordered up some of these along with some 1N4744's - just in case. The 1N4744's are 15v devices and are something other than the 5v/5w zener suggestion? Can you provide some clarity here, please?

You don't need to drop your B+, so the 5W zener diodes aren't relevant.

1N4744 along with UF4007 are for the "Paul Ruby Mod" which you don't need unless you're having fizzy / swirly distortion issues. Ignore all of that.

All you need to do regarding your voltage now is to correct the power tube bias, everything else looks good. You want the dissipation between 75% to 85%.

If you still have issues after that, you will need to highlighter test the build again.

Thanks,
Josh

[Billy_Goat]

Josh,

I’ll give your lower cathode resistor suggestion a try, but won’t lowering the cathode resistance also lower the voltage drop across the cathode resistor making that voltage farther away from the desired 12v rather than closer?

I originally had 125R and that was getting me just over 7v across the cathode resistor.

Seems like using 120R - 130R will get me back to where I started at the top here not withstanding the higher B+ voltages I now have? That resulted in about 23mA current on the plate.

Lastly, ‘highlighter test’? I’m not familiar with the term or what it means?

Cheers!

[crgfrench]

Review the build in detail with a schematic and a highlighter, coloring each part as you verify it's correctly spec'd and placed.