Maggie's Mystery

[zodiac_analog]

Hi all,

I've got an Magnatone M2 on the bench with a peculiar problem. It seems to have extremely weak vibrato (Max depth I'd say about 10-20% of what I'd expect). Also worth noting, I believe I'm about the 3rd or 4th tech in this game of kick the can, and it seems damn near everything has been tried (all caps replaced, including added 20uf filter on B node for vibe triode). I've measured the LFO at the output to be about 27v rms, and judging by the Tim Robbins paper on the maggie vibrato this seems much lower than the expected 60-70vrms described. Voltage on the plate resistor is the expected 278v. The only thing I've tried is temporarily sub a new grid leak 3.3meg resistor (the massive 3w in photo) as the original had drifted to just over 4meg, but this had no effect on the output. Has anyone built Sluckey's M2 clone to confirm the voltage output of this LFO?

[sluckey]

I've measured the LFO at the output to be about 27v rms

Scope shows 2.7V_rms. Forget to tell your scope you're using a 10X probe?

Anyhow, you need much more LFO signal. Trust Tim's numbers.

[zodiac_analog]

Forget to tell your scope you're using a 10X probe?

Anyhow, you need much more LFO signal. Trust Tim's numbers.

Thanks for the confirmation Sluckey!  (and yes the scope and I have an unspoken gentleman's agreement on using 10x on tube amps, last scope was not as fortunate)

Any idea what could be dragging the signal down so low?  Looks like maybe time to gut the LFO circuit and start over, but it seems all the parts check out ok...

[sluckey]

I doubt the signal is being dragged down. More likely it'd just weak. It's a simple circuit but it needs a strong tube. Connect your scope to the hot side of the INT pot just so we both know where you are.

I would... put a strong tube in the socket. Make sure pin 3 has a very good connection to chassis. Replace the three caps between the plate and grid. Check all resistors. Clean up that nasty looking terminal strip to the left of the tube socket.

[sluckey]

I just updated the pdf for my M2. Page 7 contains everything you should need to fix your LFO. Voltages were just verified 10 minutes ago...

     https://sluckeyamps.com/lil_maggie/Magnatone_M2.pdf

[zodiac_analog]

Thank you slucky, this addition helps tremendously! 

Strange thing this seems to indicate though, is that your voltages (112p-p) are also pretty significantly different than the 196vp-p/65vrms in Tim's paper, but I assume yours works and sounds great so maybe my voltages aren't TOO far off...

Yes, I also measured at the .02 output which is wired to the INT in, and also measured at 5hz. I've also checked all grounds including pin 3 measured from the tube side socket to ground. Also replaced the tube with multiple new in box tubes from ehx and jj with all identical results....the hunt continues!

[sluckey]

Strange thing this seems to indicate though, is that your voltages (112p-p) are also pretty significantly different than the 196vp-p/65vrms in Tim's paper, but I assume yours works and sounds great so maybe my voltages aren't TOO far off...

That 112V_pp was measured on the INT pot. I show 252V_pp on the plate. And my B+ is 328VDC. I have no idea where Tim's voltages were measured, so it a meaningless comparison.

[sluckey]

Would you post some hi-rez pics of that original M2? I'm especially interested to see a gut shot of the entire chassis. THX...

[zodiac_analog]

Would you post some hi-rez pics of that original M2? I'm especially interested to see a gut shot of the entire chassis. THX...

will do!  ...it's been bumped off the bench for now, but in just a few days I'll have it back up and get some hi-res pics of the chassis for ya. let me know if you'd like any measurements as well.

[sluckey]

Your weak vibrato is due to the weak LFO signal. But, to increase the strength of the vibrato even more, change this 1M resistor to 100Ω...

[zodiac_analog]

Your weak vibrato is due to the weak LFO signal. But, to increase the strength of the vibrato even more, change this 1M resistor to 100Ω...

Oh yeah I did try that as I had read it in your notes on the clone, also found the previous tech had bypassed that filter network entirely in a vain attempt at more LFO gain...

[zodiac_analog]

chassis photo1

[zodiac_analog]

chassis photo2

[zodiac_analog]

chassis photo3

[zodiac_analog]

well....I've completely rebuilt the LFO section and unfortunately there is zero change in the output voltage. I did notice that the voltage drops 92v p-p to 74v p-p when the intensity knob is turned up, but I assume this normal? again, this measurement was taken at the input of the intensity pot with the frequency set at 5hz. Looks like the mystery continues...

[Carlsoti]

How do the plugs of the umbilical look? If there's been a bunch of people in the amp, I'd assume it's been plugged/unplugged a bunch. If one of the pins isn't making a great connection, it might explain the voltage discrepancies.

[tubeswell]

completely rebuilt the LFO section

How does the rebuilt LFO differ from the last version? Can you post a schematic?
What is your HT voltage? And did you use a fresh oscillator tube (and other components)? Or did you re-use the old parts?

[zodiac_analog]

How do the plugs of the umbilical look? If there's been a bunch of people in the amp, I'd assume it's been plugged/unplugged a bunch. If one of the pins isn't making a great connection, it might explain the voltage discrepancies.

It seems all the B+ voltages are correct coming from power supply. Getting approx 300v on node A, 278 node B, and 250 on node C.

[zodiac_analog]

How does the rebuilt LFO differ from the last version? Can you post a schematic?
What is your HT voltage? And did you use a fresh oscillator tube (and other components)? Or did you re-use the old parts?

I only reused the socket, plate resistor (reading 283k) and the panasonic .02 to ground (also reading good). everything else is new installed by myself. And I installed a new, known good 12ax7 (tried a few just to be sure).

[zodiac_analog]

I'm working from the original M2 schematic provided by sluckey in his clone document.

Oh, also today I had a friend bring his original m10 (which has an identical LFO circuit), and measuring from the intensity input it's putting out approx 60vrms (and sounding glorious), so it seems I'm getting about 50% the output from this M2....

[zodiac_analog]

Ok buds, I think the mystery has been solved! ...after going through my friend's m10 and checking all positions and values, I found a pretty major discrepancy in that the .02 to ground shown in the schematic is wrong. It's actually directly tied to the anode in the m10. I then immediately moved that cap directly to the anode and BOOM voltage jumped to 156pp! (EDIT: the voltage is actually 128vpp at 5hz, didn't realize the speed knob was changed in the process)

sluckey, would you try this and remeasure your output? I suspect your 112pp is also low due to this schematic error...

[zodiac_analog]

ok well the excitement was short lived....the vibrato is only slightly improved, maybe 10-15% stronger but still VERY weak...

[tdvt]

Been following this...

But just to clarify, on the M10 all three .02uF caps connect to the junction of the 270K B+ resistor & pin 1..? (One to the LFO side, one coupling cap & the one you mention goes to ground, correct?)

[zodiac_analog]

Been following this...

But just to clarify, on the M10 all three .02uF caps connect to the junction of the 270K B+ resistor & pin 1..? (One to the LFO side, one coupling cap & the one you mention goes to ground, correct?)

yep, that is correct. on the m10 all (3) .02 caps are connected to the anode (it's pin 6 on the m10). It does seem accurately represented in the m10 schematic, so it's entirely possible the m2 was just done differently, but for whatever reason in this circuit it results in about 30-40v difference in voltage output.

[zodiac_analog]

ok I think I have a final update... I noticed the m10 had an output cap value of .068, so I added an .047 in parallel to the current .02 and then noticed another large jump in output voltage from 128pp to now 178pp! The vibrato was improved, but still sounded a bit weak, so I then investigated downstream and found a previous tech had subbed in a .02 where the schematic calls for an .002 on the output of the second triode. Putting that back to an .002 made a sizable difference in the audible effect of the vibrato and I now think it sounds as it should (still not as nice as the m10, but that is a 4 varistor unit)

[zodiac_analog]

Also, thought it might be helpful for anyone working on an M2 or building a clone to take a photo of the actual schematic inside the unit (honestly I never even bothered to look inside...oops). Here you can see the original ground cap is indeed tied to the anode and is labeled a .01.

[tubeswell]

take a photo of the actual schematic inside the unit

Thanks for posting - better than trying to find schematic variants in old books


@EL34 - note Actual maggie M2 schematic

[trobbins]

Good to see the issues were sorted, and to see Steve's detailed M-2 doc that nicely shows the influence of the two 0.02uF caps on the waveform change from what is generated at the LFO anode, to that presented to the INT pot. 

For clarification, the 213 model LFO's 65Vrms level was measured across the INT pot, as I also had my soundcard probe at that point.  I haven't looked at the low frequency rms measurement accuracy of modern digital scopes, but some analog meters have restricted LF bandwidth that may not be too accurate for lower LFO frequencies.  The 213 LFO circuit is somewhat different from the M2, with only a simple coupling cap of 0.1uF to the pot, so I'd anticipate noticeable signal voltage change.  There certainly were many Maggie models and variations on a theme.

[zodiac_analog]

Hi Tim, just wanted to say thanks for the wonderful write up on this circuit! It really helped me understand how this vibrato works. Oh, speaking on that I wanted to ask if you still thought adding the 100p neg feedback cap was a worthwhile/audible improvement? It absolutely makes sense to filter out all the upper harmonics...

[trobbins]

Good to see it helped - Maggie vibrato certainly interested me.  If the doc can help you identify how the circuitry works, and what may or may not be worth trying, then hopefully you do get some time and incentive to try out changes.  Imho it is all down to your ears and your gear, so enjoy.