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

[klooon]

Finally ... I found the time to continue building! The self-made chassis took way more time and the printing shop for the lettering messed things up.

I have fired it up and without the vibrato it's all fine! But then with the vibrato engaged, there is a lot of ticking and noise unfortunately. I have have played for a month to see what could be the cause. I have replaced most wires from under the board to the top (doesn't look tidy anymore, but now I could play around). It seems that a few wires are extremely sensible to each other and I traced everything back to the modulation part around the 12AU7 that is causing the more and less noise when moving wires a bit around. It seems that wires that go to the pins 6, 7 and 8 are extremely sensitive. Now please help me with this. If I put the probe of my DMM (without attaching it to the DMM, so hanging loose!) to each of the pins (or those points on the board) it's absolutely quiet and the amp is wonderful! What can be the source/solution here???

Here are some pics and a quick sound bite with the probe attached and later I have detached it and you can hear the clicking and noise, and afterwards I put the probe back again...

and with the DMM probe attached:

Klooon Magnatone 213 ticking example 001.m4a

[überfuzz]

Your routing of the wires might be an issue... The wires in your build are very, lets call it, loose. Here are some basic rule of thumb that might lower the noise floor.
#Keep input and out put of each stage apart. Check 1,2 and 6,7 (if you have a bunch of CE-stages).
#Wires should cross each other, literally, i.e. no 45 deg angles and such only 90 deg is possible.
#Use chassis as shield, i.e. route wires close to chassis.

A brief look at your build might suggest that you haven't takes this to heart. I can't open the files so I'll pass on that issue.

[klooon]

Your routing of the wires might be an issue... The wires in your build are very, lets call it, loose. Here are some basic rule of thumb that might lower the noise floor.
#Keep input and out put of each stage apart. Check 1,2 and 6,7 (if you have a bunch of CE-stages).
#Wires should cross each other, literally, i.e. no 45 deg angles and such only 90 deg is possible.
#Use chassis as shield, i.e. route wires close to chassis.

A brief look at your build might suggest that you haven't takes this to heart. I can't open the files so I'll pass on that issue.

Thanks. This is how I usually route my builds. Unfortunately, for this build apparently it was not enough. In order to trace a 'main issue', I have taken all the wires that were nicely tucked away under the board and were 90 deg from each other and put them on top of the board just to play around and debug. What appeared is that if you put the metal pointer of the multimeter probe (a loose wire...) on those 3 specific spots (second halve of the 12AU7 modulation triode), it is all very quiet, so it seems that there is a bigger problem that the lead dress at this point. What could this be?? Do I miss a connection or should I shield something specific?? So strange (to me, that is).....

[überfuzz]

Can you post the schematics, or maybe just the tremolo circuit?

[klooon]

Can you post the schematics, or maybe just the tremolo circuit?

Please have a look in post#27 or click here: http://music-electronics-forum.com/t38394/#post372665

[EDIT] And this is the latest layout that I used (amongst others: reversed the wiring of the tone and speed pot):

[klooon]

And here is the schematic:

[trobbins]

Hi Mark,

Can you describe any problems when the trem/mod switch is set to mod, and the depth pot is set to min? Was that your 'without the vibrato everything is fine!' setup?

With that setup do you get any noises when chopsticking parts and wires around the 12AX7 vibrato modulation circuitry, including back to the depth pot and footswitch, and including wiggling the 12AX7 in its socket? Chopsticking is helpful to identify poor joints, or parts. That testing can be done with no input jack/signal, or a shorting jack inserted for your channel B input, and volume pot heading towards max, and a test speaker connected.

If you don't get any noises then faultfinding would move to looking at DC voltage levels in the circuitry, and looking at test signals passing through the circuitry. For DC voltage levels, a marked up schematic with DC levels on it for the above 'no signal' test condition would be a good start. I'm not sure what test equipment you have for looking at signals. I uploaded an article recently on the maggie vibrato - it derives a few nominal operating levels to be expected.

http://dalmura.com.au/projects/Magna...o%20design.pdf

Ciao, Tim

[klooon]

Hi Tim,

Thanks for your comments!

Can you describe any problems when the trem/mod switch is set to mod, and the depth pot is set to min? Was that your 'without the vibrato everything is fine!' setup?

When the depth pot is set to minimum I also get the noise (ticking and worse). I always get the noise. With the trem/mod disengaged there is also noise (different though). When I chopstick around it gets different and sometimes a bit less. With or without guitar input, pots turned min to max. Always noise.

BUT when I take the metal pin of my multimeter (not attached to the multimeter) and point it at one of the following three points in the circuit then it's absolutely noise-free and the amps sounds fine under all conditions (potmeters min to max, etc)
point 1: point "x2" in my schematic (top varistor)
point 2: pin 7 of the 12AU7
point 3: pin 8 of the 12AU7

but strangely enough not pin 6 of the 12AU7... strange because this should give the same effect as when touching "x2"... or not?

[klooon]

I think I found it.... The only wire that I could not 'chopstick' was the 470k grid stopper resistor, because I hardwired it to the board. I removed this and just replaced it by a wire. Now it seems absolutely noise-free... at all settings. Seemed that the resistor was picking up the noise... ?! OK, tomorrow a bit more investigations...

[klooon]

It seems that especially the grid wires are very susceptible to interference. I have placed them further away from other wires and it seems okay now.


After the vibrato, one thing left was the tremolo. I have tried all kinds combinations of the "buffer" resistor, but it seems that the signal is not strong enough to push through.

I have tried to insert the LFO signal into the junction of the 470k just before power tube grids with an additional 1M to ground (also tried a 470k). Comparable with the 220k/220k and 330k of for instance the Gibson GA5T. (this link) But when dailing in the tremolo depth, the tremolo was strong but also the signal was overdriven (not clean anymore). What could I do/try here??

[tubeswell]

I have tried to insert the LFO signal into the junction of the 470k just before power tube grids with an additional 1M to ground (also tried a 470k). Comparable with the 220k/220k and 330k of for instance the Gibson GA5T. (this link) But when dailing in the tremolo depth, the tremolo was strong but also the signal was overdriven (not clean anymore). What could I do/try here??

Difficult to say without a schematic. In the Gibson GA5T, the 330k acts like a depth pot (set to 330k). If you swapped that for a 250k pot wired as a variable resistor, you would have a depth control there.
Going by your description of 'overdriven' - could be the grid leak resistance for the output tubes is a bit too high. Could be the bias running hot in the output tubes (Having said that - I'm still not sure from your description how the LFO is wired into the rest of the circuit)

[klooon]

Difficult to say without a schematic. In the Gibson GA5T, the 330k acts like a depth pot (set to 330k). If you swapped that for a 250k pot wired as a variable resistor, you would have a depth control there.
Going by your description of 'overdriven' - could be the grid leak resistance for the output tubes is a bit too high. Could be the bias running hot in the output tubes (Having said that - I'm still not sure from your description how the LFO is wired into the rest of the circuit)

I realise that my description is a bit too cryptic. Here is the schematic that I used:

I think you're right with the bias running too hot now.

What to do now? Would it then be possible to insert the tremolo signal into the junction of the two 470k's (so without the 1M, but with the wire to ground). Like this:


Or without the wire to ground like a Princeton reverb? Not sure what this all does to the bias??

[klooon]

Thanks Tubeswell for pointing that out. Of course 470k plus 1M is way too much - I realise now. I have for the moment extra 470k across the 470k's and instead of the 1M now a 220k (so 230k-230k-220k) and it is a nice and deep tremolo!

[klooon]

The tremolo is still giving a bit too much "distortion" at the maximum depth, whereas the vibrato setting is totally fine, also with the trem/vibrato off it's all fine. If you dial back a little bit also the tremolo is fine. What would be the best idea to back off a little bit. I am not sure I want to touch the resistor values the 220k/220k/220k junction. Could an extra resistor (e.g. 100k?) in line with the 47k after the footswitch be an idea? Like this:


Further I notice that the depth pot (audio taper) is not ideal. The tremolo/vibrato kicks in only after half way and then it's hard to dial in differences in depth. So I am planning to put in a linear pot instead.


Then some numbers:
B+1 = 347 VDC (1st node); B+2 = 332 VDC; B+3 = 325 VDC

V_plate = 339 VDC (voltage of pin 3 to ground) and V_cathodeResistor = 21.6 VDC. The current for both 6V6s is (21.6V/270R=) 0.080 A, so 40mA per tube. The total dissipation is (339-21.6)*0.08= 25.4 W. Per tube 12.7 W. The JJ 6V6 are rated a bit higher at 14 W (instead of 12W for "regular" 6V6s ), so they run at 91%. I am thinking of installing a 330R cathode resistor to bring it a bit down. The B+ voltages will go up a bit closer to values that are reported as 'common' for this amp, i.e. 375 VDC. And if I would run another brand of 6V6 this is safer as well... Interesting is that the Hammond transformer is giving relatively lower B+ values than I expected. So no zenering is necessary.

[Tom Phillips]

The tremolo is still giving a bit too much "distortion" at the maximum depth...

Given that, I suspect that the tremolo signal is shifting the output tube bias into a range that causes distortion. Possible solutions are to use a different initial bis setting or limit the range of the depth control. Those are trade offs that are a necessary consideration for this type of tremolo circuit.

From your posts it seems that you have a DVM but do you have a scope, signal generator, dummy load or other test equipment. If so then you can measure the max bias excursions and then set equivalent static bias values for the min & max to clearly see what your depth settings are doing to the power amp and how they affect the signal.

Cheers,
Tom