Tonemaster Troubadour 214 (Magnatone 213) - Help diagnosing crackling

[GoKart Mozart]

This is kind of a "part 2" to a previous thread I started last month. I recently got a Tonemaster 214 in a trade. I put in a 3 prong cord, removed the death cap, and replaced the filter cap can with a new one from CE Manufacturing. When I got the amp, it had some static/crackling and I was hoping that replacing the electrolytics would help but unfortunately it has not.

When I got the amp, it had the original Telefunken 12AX7s, GE 12AU7, Sylvania 6V6s, and GE 5Y3. I thought I had some of the static/crackling isolated to the 12AU7 so I replaced it with a NOS GE tube but it really didn't help any. I've swapped around all of the tubes with ones from other amps and the crackling is still there.

Here's a video.


I've cleaned the tube sockets but that hasn't helped any. Is there a "right" way to troubleshoot an annoyance like this?

 

[GoKart Mozart]

Forgot to post this; this is the closest schematic I've been able to find. This was a later evolution (Magnatone model 413) that replaced the 12AX7 oscillator with a 6AV6 and added some tone switches in the preamp. Other than those changes, I believe it's identical to mine:
http://www.magnatoneamps.com/schematics/magnatone_413.png

 

[Nickfl]

At this point I'd suspect a leaky coupling cap. See if you have some DC voltage on the pots or somewhere else you shouldn't.

 

[GoKart Mozart]

At this point I'd suspect a leaky coupling cap. See if you have some DC voltage on the pots or somewhere else you shouldn't.

I checked them and the only one showing DC voltage is the coupling cap going from the V1 plate to the channel A volume pot (measuring at the lead going to the pot). I had to set my meter to the lowest DC setting; it’s jumping around between -20mV and 20mV.

Seems small but none of the other coupling caps appear to be letting even that small amount of voltage through. Could this be it?

Thanks!

 

[peteb]

Crackling is a bad connection.

 

[corliss1]

The "right" way is to use a signal tracer to check various points of the amp, sending the signal out to your test amp. When you find where the crackle noise starts you've located the component or area of the issue.

I would bet on plate resistors causing this noise as that is the classic "bacon frying" sound of old plate resistors.

 

[LarryBudMelman]

Do what the gurus say first, (Im far from that ) But I have found a few bad parts/connections with the chopstick test. Its not very scientific.

 

[GoKart Mozart]

I would bet on plate resistors causing this noise as that is the classic "bacon frying" sound of old plate resistors.

Thanks for the tip! So even if they’re old and causing the crackling, will they still measure the proper resistance?

 

[corliss1]

Yup - resistance will still measure fine and everything will "work" - just you have that noise.

 

[wanderin kind]

does it sound like bacon cooking or microwave popcorn? (dang, now i'm hungry, bacon and popcorn, never tried that combo,)

if bacon, then change out the plate resistors and you should be good. they absorb moisture and will get noisier over time.

 

[GoKart Mozart]

I ordered some resistors and replaced the plate resistors on the first two preamp tubes but unfortunately that didn’t fix it. I was hoping I wouldn’t have to dive into the other two preamp tubes since they control the vibrato circuit and it’s a big mess o’ stuff all around those pins. Haha
I’ve got a few more parts that I’m going to have to order for those, so more to follow.

All of the resistors on the input jacks are carbon comp and I’m REALLY hoping I won’t have to resort to replacing those.

 

[GoKart Mozart]

Also, I replaced the carbon film screen grid resistors on the power tubes. The cathode resistor gets REALLY hot (one of those old blocky ceramic ones) and I thought that maybe it being right on top of the screen grids could’ve cooked them but replacing them didn’t help any.
It got me thinking though, might the cathode resistor be the cause of the crackle?

Cathode resistor is in the top left corner:

 

[corliss1]

<cough>signal tracer<cough>

But really, that's the right way to go about this. Without doing that you're just sorta guessing at parts, swapping them out, and hoping that's the fix. You might get lucky, or you might replace every part in the amp.

 

[GoKart Mozart]

<cough>signal tracer<cough>

But really, that's the right way to go about this. Without doing that you're just sorta guessing at parts, swapping them out, and hoping that's the fix. You might get lucky, or you might replace every part in the amp.

Crap...you’re probably right. Do you have any low budget signal tracer recommendations?

 

[corliss1]

One capacitor and a cut off guitar cord. Google how to make one.

I should just write an article on my own site since I link this a lot but I don't like it...

http://musicfromouterspace.com/analogsynth_new/HOT_TIPS/ampsignaltracer.html

Do that, skip the resistors. You can even skip the alligator clips and use the one end of the cap as the probe. Get in the habit of touching it to the chassis after each test to discharge it - the cap is there to keep DC from going out into your test amp.

 

[GoKart Mozart]

One capacitor and a cut off guitar cord. Google how to make one.

I should just write an article on my own site since I link this a lot but I don't like it...

http://musicfromouterspace.com/analogsynth_new/HOT_TIPS/ampsignaltracer.html

Do that, skip the resistors. You can even skip the alligator clips and use the one end of the cap as the probe. Get in the habit of touching it to the chassis after each test to discharge it - the cap is there to keep DC from going out into your test amp.

Ah, interesting. So this doohickey goes into a separate amp than the one I’m actually working on, correct? Then I just run some kind of audio signal into the input of the amp being worked on?

 

[corliss1]

This takes the signal from your amp under test and sends out the specific point you probe to another amp so you can listen.

Since you're listening for noise you wouldn't need to have a signal in the amp under test - you just want to find the noise and don't care about the "good" audio over it.

It may also be helpful to plug the amp under test into a dummy load so you don't hear the noise in two places.

 

[GoKart Mozart]

This takes the signal from your amp under test and sends out the specific point you probe to another amp so you can listen.

Since you're listening for noise you wouldn't need to have a signal in the amp under test - you just want to find the noise and don't care about the "good" audio over it.

It may also be helpful to plug the amp under test into a dummy load so you don't hear the noise in two places.

Sweet, thanks for the tip at the end! I’ve got a UA Ox that I can use as a load and mute the output.

 

[GoKart Mozart]

I built one of the signal tracers out of an instrument cable (thanks again for that tip!) and while it helped figure out where the crackle is and where it's not, I'm still not sure where it's originating. In the images below, I highlighted where the crackling is in red. In a nutshell, no static on V1 (12AX7) but V2 (12AX7) & V3 (12AU7) have crackling though a few components only have crackle on one side. There's another 12AX7 not depicted below, but there's no crackling anywhere on it and I believe it's mainly just controlling the vibrato speed/intensity.

*You'll notice that I've temp soldered a few resistors in series while I was doing some earlier troubleshooting. I'm going to clean that up later.

First, there's no crackling at all on anything connected to V1 (12AX7):

On V2 (12AX7), basically anything attached to the triode that has pins 6, 7, and 8 has crackle. The carbon comp resistor on pin 8/cathode only has crackle on the tube side; the lead attached to ground does not crackle. The two ceramic discs have crackle on both sides. The string of 3 ceramic film resistors going from the plate of V2 has crackle until the point that it attaches to the high voltage red wire + plate resistor from pin 1 of V2.

Another angle of V2:

Here's V3 (12AU7), which contains the varistors that contribute to the vibrato mojo and also one of those funky rectangular mica capacitors. All of those components are connected to a tag board, which is connected to the ceramic disc from pin7/grid of V2. The crackle is only on that side of those components; the opposite ends actually connecting to the V3 are quiet.

Here's another angle of V3. As previously mentioned, all leads connected to the tube are quiet.

Any ideas?

 

[corliss1]

First idea - who ever did the previous work on that needs to practice some soldering.

So - we eliminated V1 from the equation. Good If you pull V2 does the crackling stop?

We would never expect to find noise/crackle on an audio path to ground. This would be the other side of the resistor connected to cathode you mentioned, but would also be the other side of the plate resistors on pins 6 and 3 of the 12ax7 tubes.

If pulling V2 stops the crackle, or at least changes the crackle noise, I'd start by swapping the remaining carbon comps there - specifically the one between pins 7 and 8 and the 2 that go up and twist into each other.