New TMB w/reverb Build

18watt-specific Tech Talk - Building, Fixing, Parts, Mods...

Moderators: CurtissRobin, colossal, zaphod_phil, Daviedawg, Graydon

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 3554
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: New TMB w/reverb Build

Post by JMPGuitars »

foreverstrung wrote:
Wed 12/21/22 1:57 pm
https://youtu.be/50sEk-FK4l4
You can hear the reverb channel sounds like crap. Noisey. Little to no actual "reverb".
Like I said, I haven't spent a lot of time chasing this down. I've swapped out the RCA cables. And then back. Changed out the 12AX7. I did flip it over and stare at it for a while. LOL. Looking for the obvious.
Besides the reverb, this amp is quiet quiet.
It sounds like you have a wiring issue. You also need to double check and make sure the 221K resistor to ground on the red reverb jack is good. With the amp powered off, you can try measuring the resistance of V3 pin 7 to ground to test that.

Also make sure the jacks are tight...without breaking that 221K resistor.

Then break out the highlighter and do just the reverb section again.
Side note: instead of assuming the wiring is fine because you checked 3 times already, assume something IS wrong, and be pleasantly surprised if it isn't. Either way, search carefully for whatever is sneaking past you.
1 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Amplifier Tools & Parts Info


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

User avatar
foreverstrung
Occasional poster
Occasional poster
Posts: 158
Joined: Sun 10/10/21 5:53 pm
Location: Seabrook Texas
Contact:

Re: New TMB w/reverb Build

Post by foreverstrung »

So I swapped out my Siglent scope for a 1104x-e. I'm getting ready to fire it up for the first time and trying to get motivated. I've watched tons of videos on scopes but still really don't have a line on what I'm doing. I'd sure appreciate some very basic 1-2-3 direction for setting the unit up correctly and then chasing down the circuits. Like your explaining to a child, because I feel about as smart as one right now with this thing. I know the black wire with the 3 prongs plugs into the wall, but that's about it. lol
Josh mentioned to use a guitar in place of a signal generator a while back? Do I need that sound ever present?
TIA
0 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 3554
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: New TMB w/reverb Build

Post by JMPGuitars »

foreverstrung wrote:
Thu 12/22/22 4:05 pm
So I swapped out my Siglent scope for a 1104x-e. I'm getting ready to fire it up for the first time and trying to get motivated. I've watched tons of videos on scopes but still really don't have a line on what I'm doing. I'd sure appreciate some very basic 1-2-3 direction for setting the unit up correctly and then chasing down the circuits. Like your explaining to a child, because I feel about as smart as one right now with this thing. I know the black wire with the 3 prongs plugs into the wall, but that's about it. lol
Josh mentioned to use a guitar in place of a signal generator a while back? Do I need that sound ever present?
TIA
A signal generator is required. To really test, you need a 1kHz sine wave being fed into the amp. This is the cheapest/decent signal generator I've used: https://www.newark.com/tenma/72-505/han ... dp/66F3575 (though I have a couple nicer ones I don't need anymore if you feel like wasting money 😉).

You can also get a signal generator app on your phone, but that's trickier to calibrate sometimes. Look for the "function generator" app.

Follow the directions to calibrate your scope probes. You'll be connecting the probe(s) to the calibration terminal on the scope. Press the "auto" button, and it will show you a square waveform. Adjust the screw on the probe as per the directions.

After you've calibrated your probes, leave them set to 10x. Then connect them directly to your signal generator, and set the generator for a 1kHz sine wave. Adjust the output of the signal generator to somewhere between 150 to 300mV (millivolts). That's a relative range for guitar, low mV being lower output pickups. You'll need to hit that "auto" button again to acquire your signal (at least until you get good at messing with the knobs).

Once the signal generator is calibrated to the correct range, you can then plug it into your amp's input jack.

Doing this test with the scope is much easier (and safer for your ears) with a "dummy load." Personally, I use an attenuator on my workbench as my dummy load (it has a speaker cone in it). Alternatively, you can google making a dummy load yourself with a high-wattage 16ohm or 8ohm resistor. NEVER turn your amp on without a speaker or dummy load connected.

This page has some great info on using an oscilloscope for an audio amp: http://www.r-type.org/articles/art-125.htm

You can start by connecting the scope probe to your speaker jack. Chances are, that may look normal if you're just troubleshooting your reverb. So you will need to look at the schematic and what points bring audio signal into, through, and out of the reverb circuit.

Thanks,
Josh
1 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Amplifier Tools & Parts Info


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

User avatar
foreverstrung
Occasional poster
Occasional poster
Posts: 158
Joined: Sun 10/10/21 5:53 pm
Location: Seabrook Texas
Contact:

Re: New TMB w/reverb Build

Post by foreverstrung »

A signal generator is required. To really test, you need a 1kHz sine wave being fed into the amp. This is the cheapest/decent signal generator I've used: https://www.newark.com/tenma/72-505/han ... dp/66F3575 (though I have a couple nicer ones I don't need anymore if you feel like wasting money 😉).
This link is a dead end. I'm interested in what you've got laying around and how much. I'd like something reliable, but cost effective too.
I'd also like to see the cheap one you recommended if you can find a diff link for it.
Follow the directions to calibrate your scope probes. You'll be connecting the probe(s) to the calibration terminal on the scope. Press the "auto" button, and it will show you a square waveform. Adjust the screw on the probe as per the directions.

After you've calibrated your probes, leave them set to 10x. Then connect them directly to your signal generator, and set the generator for a 1kHz sine wave. Adjust the output of the signal generator to somewhere between 150 to 300mV (millivolts). That's a relative range for guitar, low mV being lower output pickups. You'll need to hit that "auto" button again to acquire your signal (at least until you get good at messing with the knobs).

Once the signal generator is calibrated to the correct range, you can then plug it into your amp's input jack.

Doing this test with the scope is much easier (and safer for your ears) with a "dummy load." Personally, I use an attenuator on my workbench as my dummy load (it has a speaker cone in it). Alternatively, you can google making a dummy load yourself with a high-wattage 16ohm or 8ohm resistor. NEVER turn your amp on without a speaker or dummy load connected.

This page has some great info on using an oscilloscope for an audio amp: http://www.r-type.org/articles/art-125.htm



You can start by connecting the scope probe to your speaker jack. Chances are, that may look normal if you're just troubleshooting your reverb. So you will need to look at the schematic and what points bring audio signal into, through, and out of the reverb circuit.
Thx for all of that.
I've watched some good videos using a scope on a vacuum tube amp. It's interesting, but slow. I am realizing that I do need to let all of it soak in.
It sounds like you have a wiring issue. You also need to double check and make sure the 221K resistor to ground on the red reverb jack is good. With the amp powered off, you can try measuring the resistance of V3 pin 7 to ground to test that.

Also make sure the jacks are tight...without breaking that 221K resistor.
Pin 7 on V3 reads 221k, so that resistor appears good. I'm going to pull the PCB board again and check that everything looks good on the underside of the RCA connectors. Measuring off the PCB board, all the caps and resistors, inline with the reverb channel, are reading as there suppose to.
I haven't found anything grounding out. I'll keep looking.

This is what I picked up for a dummy load
https://www.amazon.com/gp/product/B0002 ... UTF8&psc=1

In Gerald Webars book, Tube amp talk..., he's got a chapter on building a dummy load with a speaker. What you suggested made me think of it. This resistor I listed above looks like it'll work too.
0 x

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 3554
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: New TMB w/reverb Build

Post by JMPGuitars »

I clicked the link above, and it appears to work, but try clicking here instead.

That dummy load should be fine, make sure to set the amp to 8 ohms.

Be very careful disassembling your amp over and over again.

The cheap signal generator linked above is fine. The biggest benefit of the others I have is that you can enter the exact amplitude without having to guess or test every time. They can also do lots of other things, but for this test all you need is a 1kHz sine wave. Email me if you want more details on the fancier signal generators.

Thanks,
Josh
0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Amplifier Tools & Parts Info


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

User avatar
foreverstrung
Occasional poster
Occasional poster
Posts: 158
Joined: Sun 10/10/21 5:53 pm
Location: Seabrook Texas
Contact:

Re: New TMB w/reverb Build

Post by foreverstrung »

Attached is a pic of the underside of the Reverb RCA jacks. Appreciate you guys telling me if your seeing something I'm missing
You do not have the required permissions to view the files attached to this post.
0 x

User avatar
Bieworm
Verbose Wizard of Oz
Verbose Wizard of Oz
Posts: 1941
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: New TMB w/reverb Build

Post by Bieworm »

foreverstrung wrote:
Fri 12/23/22 10:49 am
Attached is a pic of the underside of the Reverb RCA jacks. Appreciate you guys telling me if your seeing something I'm missing
I think you should put the black wire to ground and the green wire to the tip. They are the wrong way around
2 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
foreverstrung
Occasional poster
Occasional poster
Posts: 158
Joined: Sun 10/10/21 5:53 pm
Location: Seabrook Texas
Contact:

Re: New TMB w/reverb Build

Post by foreverstrung »

That would be an easy enough thing to do. I'll let you know how it works out.
The layout does show it the way that I've wired it.
Thx
You do not have the required permissions to view the files attached to this post.
0 x

User avatar
Bieworm
Verbose Wizard of Oz
Verbose Wizard of Oz
Posts: 1941
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: New TMB w/reverb Build

Post by Bieworm »

Could be…it’s just 2 ends of a coil. But I never noticed this. All other reverbs anywhere are with black to ground… I see Josh has it wired like you did in the building docs. I wonder if it switches polarity and if that is an issue
9A3A2166-89B7-422E-B3E8-A9CD7C9661E7.jpeg
40D88368-0137-4B00-A1DD-EE6FADD9C094.jpeg
5250188E-5AD3-4D27-8216-AD13DAF58D7F.jpeg
You do not have the required permissions to view the files attached to this post.
1 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 3554
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: New TMB w/reverb Build

Post by JMPGuitars »

That's arbitrary, and it depends which manufacturer whether green or black is the start of the winding. I don't remember why anymore, but that was intentional in the docs.

MM is reverse of that one you posted: https://www.mercurymagnetics.com/wp-con ... /FBFRT.pdf

Hammond is also reverse of the ClassicTone:
p-t1750a.pdf
You do not have the required permissions to view the files attached to this post.
0 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Amplifier Tools & Parts Info


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

User avatar
Bieworm
Verbose Wizard of Oz
Verbose Wizard of Oz
Posts: 1941
Joined: Mon 02/10/20 8:24 am
Location: Belgium

Re: New TMB w/reverb Build

Post by Bieworm »

Oddly enough the hammond 1750A has the black wire as start of winding too. So if Fender puts that to ground it must not be a big deal if it’s switched around?

https://www.tube-town.net/ttstore/hammo ... everb.html
0 x
"THIS should be played at high volume..preferably in a residential area"

User avatar
JMPGuitars
Super Duper Admin
Super Duper Admin
Posts: 3554
Joined: Tue 09/18/12 8:00 pm
Location: South Central, MA
Contact:

Re: New TMB w/reverb Build

Post by JMPGuitars »

Bieworm wrote:
Fri 12/23/22 5:28 pm
Oddly enough the hammond 1750A has the black wire as start of winding too. So if Fender puts that to ground it must not be a big deal if it’s switched around?
These are simple transformers, and transformers use AC. AC voltage is electromagnetically induced from one winding (primary) to the next (secondary), how much does it matter which end you're connecting of the secondary winding? Do you worry about polarity when wiring your HT?

Here's some light reading for you:
https://www.electronics-tutorials.ws/tr ... asics.html
https://www.electronics-tutorials.ws/tr ... ormer.html

Thanks,
Josh
1 x
'I installed a skylight in my apartment yesterday... The people who live above me are furious.' - Steven Wright

Modern Ground Schemes
Soldering Technique
B+ Voltage Reduction
Amplifier Tools & Parts Info


Web Design: DolceVittoria.com
Guitars / Amps / Effects: JMPGuitars.com
(anti)Social: Facebook · Instagram
Items for Sale

User avatar
foreverstrung
Occasional poster
Occasional poster
Posts: 158
Joined: Sun 10/10/21 5:53 pm
Location: Seabrook Texas
Contact:

Re: New TMB w/reverb Build

Post by foreverstrung »

https://modulusamplification.com/
Any one familiar with these guys?
Reviews?
0 x

Daviedawg
Superior Amp Tech
Superior Amp Tech
Posts: 719
Joined: Fri 01/08/10 2:00 am
Location: Scotland

Re: New TMB w/reverb Build

Post by Daviedawg »

Yes I am. I have bought stuff from Mike. He is very reliable. Also lives 40 miles or so from me.

Dd

PS I think he may have been active on here when I first joined up about 12 years ago.
2 x

User avatar
foreverstrung
Occasional poster
Occasional poster
Posts: 158
Joined: Sun 10/10/21 5:53 pm
Location: Seabrook Texas
Contact:

Re: New TMB w/reverb Build

Post by foreverstrung »

So my UNI-T audio generator arrived today. Model UTG932E. Fired up the oscilloscope (Siglent 1104X-E) and generator. Followed the basics with the probe n AG. Calibration.
Had to use the speaker for load as my dummy resistor isn’t arriving until tomorrow.
Anyways, honestly, idk what I’m looking for.
When I plugged the AG into the scope n hit auto, I got the sine wave that I'd expected. Then when I plugged AG into amp and applied the probe to input jack of the amp, it’s not the same sine wave as straight to the scope. Idk if I just had it hooked up incorrectly or what. The sine wave that did appear at the input jack was the same at the reverb pot. I got mixed readings on the PCB board @ resistors n caps online with V3 and Reverb pot. I realize this is not the right way to locate the issue I'm having with the reverb, but I was just trying to understand how the tools work. The sine wave observation that I just described is just that. Observation. Not meant to imply anything as that’s about how much I currently know about these tools. Lol
I'm just getting familiar with the equipment as I'm sure this is most important for me to get this all dialed in.
I’m watching lots of YouTube videos on all this stuff. Daunting but I push thru.
Lastly, How do I change the volts per division from 1x to 10x on the scope?.The probe has a switch, so that's a no brainer, but the first time I ran through everything I got the scope set up to 10x after calibrating the probe and getting everything set up. But now I've got no idea how I did it. I'm watching more vids and reading online manual. Appreciate any direction here.
Thx
0 x

Post Reply