New build: Magnatone 213 (Tonemaster/Titano/Evil Robot) + mods --- advice wanted!

[klooon]

Thanks a lot for your considerations and guidelines! I will try them out as soon as possible.

Just had a quick look at photos in post 22 - they are old I can see, but indicative maybe. Some thoughts:

I can't see a central spigot in the 9-pin sockets - the spigots can help screen circuit sections.

I am not familiar with the spigots. How to use them? I assume a spigot could be used (just?) as an extra connection point, but what to connect then exactly (in this case)?

Wherever loose wiring around an input valve is taken from one part to another, then try and separate anode wire from grid and cathode - eg. track the yellow wire away from grid terminal and towards chassis asap.

The input jacks are isolated (I think), so the chassis ground connection perhaps can go to the input stage star 0V. AN perhaps separate 0V returns from the input jacks to star 0V.

Keep Vol A 0V and wiper wires close together and away from output terminal near the pot. The Vol A pot may well be a major noise source - perhaps tack in a temporary 10:1 divider load (say 220k/22k) on the pcb on output of stage 1, with short wiring and reroute screened wiring, with screen to 0V star. The divider is like the pot rerouted as a normal pot, but also lower resistance, and lower noise resistance. With such a high gain, the pot is probably turned down a lot, so that there is a large resistance in series to the next stage, which may be partly contributing some noise.

I will play around with this!! This is the kind of direction I need at the moment. I have to find some time though to do it properly.

Is there really only 0.047u bypass on input stage B+ ?

Yes, this is like the original Magnatone schematic.... Can I just replace it with a higher cap value, in particular also the 22uF? Or should I change the dropping resistors as well then?

Is the noise higher up in the audio band, and hence quite dependant on the tone ?

The noise is there all the time (when nothing is plugged in or when jack 1 is plugged in), with the tone knob minimized it's buzzing and dialing up the tone knob, you get additional high freq buzz.
You always get the wobbling of the LFO interfering with the signal (even with the vibrato in off-position - I didn't change it to stop the total LFO running, because I think it helps the debugging process now) EXCEPT when the intensity knob is all the way UP... then there is just the buzz without the throbbing pulse... (or maybe just very slightly in the background...)

[trobbins]

When making the comment about the intensity knob up, is that with vibrato switched in?

Do you have a mobile phone (android or iphone), that you can load a free spectrum analyser on? The aim would be to check the spectrum of the noise from the speaker by using the phone microphone.

Have you tried a temporary filter capacitor (eg. 20uF) across the input stage B+ ?

[klooon]

When making the comment about the intensity knob up, is that with vibrato switched in?

No, the vibrato switch is in off-mode - strange. It's still according to the original Magnatone schematic, in which the vibrato switch is leading the signal to ground after the 47k resistor just before the FM/AM switch. I wonder if something is wrong here (or elsewhere).
Also in the normal channel with the vibrato switch in off-mode with the normal channel volume maxed, I (now) hear the thumping very remotely.... Strange enough the thumping is less when the vibrato intensity knob is dialed just a bit away from max. Only in max position it's less.... I am really really puzzled here...and I wonder if it's a matter of only "bad lead dress"...

Do you have a mobile phone (android or iphone), that you can load a free spectrum analyser on? The aim would be to check the spectrum of the noise from the speaker by using the phone microphone.

Thanks. These are the spectra:

Note: vibrato switched-off... vibrato intensity knob maxed...

Have you tried a temporary filter capacitor (eg. 20uF) across the input stage B+ ?

Not yet. I will try!!

[trobbins]

Hah, well the second spectrum show 60Hz harmonics are dominant. Is that spectrum for the situation which you describe as 'noisy'?

If it is, then you don't have a noise floor issue (ie. related to high gain preamps and residual noise from parts such as triodes and resistors), but rather a power supply related issue.

That also confirms I think that your first stage is where to focus attention on. I can suggest using a 6V battery to power just that first 12AX7 heater, to see if heater related ingress is a dominant contributor. For that test, you need to remove any heater wiring that goes over to that valve holder.

Another simple test is to remove the wire from pin 1 of the input 12AX7 - leave all the wiring in place - and pots on high. Test for noise, and then drop pot to min and test for noise.

[klooon]

Hah, well the second spectrum show 60Hz harmonics are dominant. Is that spectrum for the situation which you describe as 'noisy'?

If it is, then you don't have a noise floor issue (ie. related to high gain preamps and residual noise from parts such as triodes and resistors), but rather a power supply related issue.

OK! That's right, the second spectrum is the one describing the noise!

That also confirms I think that your first stage is where to focus attention on. I can suggest using a 6V battery to power just that first 12AX7 heater, to see if heater related ingress is a dominant contributor. For that test, you need to remove any heater wiring that goes over to that valve holder.

Another simple test is to remove the wire from pin 1 of the input 12AX7 - leave all the wiring in place - and pots on high. Test for noise, and then drop pot to min and test for noise.

OK. I will try the noise test and find a 6V battery. As regards the power supply related issue, what do you exactly mean by heater related ingress. Is something in my wiring not okay or is it more that the filtering (or power supply design related issues??) is inadequate? I could imagine, also with your comment about the 0.047uF cap, I could improve here...

[trobbins]

The aim of a few tests is to clarify what is the dominant path, or paths that the main frequency gets in. There are many paths!
eg. http://dalmura.com.au/projects/Hum%20article.pdf

Heater paths could be as simple as placing the grid wire too close to the heater wiring. But also could be due to the elevated supply, or a poor 12AX7 ....

Wiring to and from pots, and the pot itself are very susceptible to hum pickup.

Poorly decoupled B+ could be an issue for your circuit.

[klooon]

The aim of a few tests is to clarify what is the dominant path, or paths that the main frequency gets in. There are many paths!
eg. http://dalmura.com.au/projects/Hum%20article.pdf

Heater paths could be as simple as placing the grid wire too close to the heater wiring. But also could be due to the elevated supply, or a poor 12AX7 ....

Wiring to and from pots, and the pot itself are very susceptible to hum pickup.

Poorly decoupled B+ could be an issue for your circuit.

Thanks so much for the info AND explanations! I will start reading and experimenting the coming days and report back!

This is maybe also interesting. This weekend I tried an amp from a friend that was just converted with an added cascading stage that experienced the same problem... It's an pp18 (Marschall 18W clone from ampmaker) and tried to follow the same modification as I did with the Magnatone. Now looking at the "successor" of the pp18, the "pp1800" it seems that with the additional cascading input also the B+ was decoupled (differently) and applied extra (?) filtering (see 33uF and 33uF). Could this indicate that he experienced similar problems...?!

ampmaker pp18 schematic
ampmaker pp1800 schematic

[klooon]

I have been experimenting a lot the last few days...

3 situations that I have been looking at:
1. high vs. normal input with vibrato off
2. high input with vibrato on.
3. normal input with vibrato on.

1. High (i.e. cascaded) vs. normal input channel with vibrato off.
- I have tried to rearrange the V1 wires, but no major changes in noise could be observed.
- The wire that connects the hot lug of the second/normal input jack to the volume pot of the hot input is quite susceptible to interference, e.g. when you’re near with your hand it starts to produce a lot of noise, but no (not much) interference when you’re near and outside the chassis…
- I have rearranged the ground wires around the input jacks and the 0.047uF filter cap, but no decrease in noise was observed.
- Then I tried to put an extra filter cap of e.g. 22uF across the original 0.047uF and BINGO! The noise is gone, the normal and high input have almost the same noise level when you plug in a guitar cable (with the guitar volume pots closed). With the guitar volume open there is a bit more noise but also more gain… as expected…

So I am extremely happy with this result! Maybe I can go even further with extra filtering, but the level of hum and noise and the feel of the amp is very good now.

2. high input with vibrato on.
Then… engaging the vibrato for the high input: I have been rearranging the wires in the modulation part of the circuit, mainly around the V3 tube for days and they go from really a lot of ticking, throbbing, squealing to only a bit quieter - unfortunately. (Note that this is a total different phenomenon than the noise observed earlier.) So actually, this is very much unwanted. And I am thinking of removing the cascaded input, because I don’t want a channel in my amp that starts throbbing as soon as you turn on the vibrato.

3. normal input with vibrato on.
With rearranging the wires in the modulation part, I have been able to get it very quiet. Only in case that the (normal) volume and the tone pot are maxed you hear a remote throbbing.


The thing that still bothers me very very much, is that when I put a wire (e.g. the tip of the voltmeter wire; not even connected to the voltmeter) at a few spots in the (modulation) circuit (plate or cathode or at their varistor locations, it’s absolutely quiet REGARDLESS (and this is I think important) how I move around the other wires that before were needed to be taken with so much care…

It seems that maybe I did something wrong in the circuit or with my interpretation of the schematic/layout – maybe not so much the wires but maybe something on the eyelet board that is ‘fundamentally’ wrong???? I haven’t heard of others cloning the Magnatone, that they experienced similar problems…

Any advice/reflections on this???

[klooon]

It seems that I have found a solution.... finally after again much fiddling around....

It appears that when I tuck/push the (brown on the layout) wire that goes from the cathode of V3 (pin 9) to junction "x2" tightly to the junction/eyelet with the two 2M2s and .0022 from the grid wire (from v3 pin 6) it is absolute quiet and no throbbing whatever I fiddle around with other wires... Especially pushing the cathode wire at the junction/eyelet of the cathode wire is really the key - apparently.

[DrGonz78]

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