Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: O-Fuzz on December 15, 2023, 12:56:29 pm
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Hi All,
been a while but have my next somewhat 'quirky' amp on the bench, and once again need some help with the tremolo.
I will remind everyone that I am NOT an experienced amp tech, but a fully self taught VERY novice amp lover and tinkerer. Everyones and any advice is super appreciated and very welcomed.
OK, so the Maggie MP5 (i'm sure some of you may not be super familiar) is a bit of a 'quirky' amp, yet sounds INCREDIBLE.
The tremolo has two controls with the 'intensity' control also is the switch to initiate the effect in general - the effect can then also be turned 'on/off' via a footswitch. The overall effect is also wired to a small indicator lamp that pulses along to the speed setting.
OK - so my issue - the tremolo circuit seems to be oscillating as I can here a faint 'whoosh' in the signal as i turn up the intensity control...the speed is definitely working as the faint 'whoosh' as well as the flickering indicator light both increase in speed fine when i turn the 'speed' up...the problem is that it's a 'faint whoosh' - not a full throbbing 'whomp whomp whomp' like it should be.
Please see photos and schematic attached (apologies on the poor quality of the schem but it's all I could find online).
I suspect the issue must be in the 'tremolo circuit' capacitors - however, I'm having trouble accurately identifying those as per my lack of proficiency in fully being able to trace the schematic. I also wonder if the LDR(roach) could be the issue.
The first thing I should note is that the 'turret' board that those elements are mounted onto is rather corroded with that 'thin white chalky dusty stuff' that I have seen in several amps. My first inclination before I start swapping out parts would be to re-flow the solder at the turret points to see if that kicks the tremolo on - but beyond that, I guess I'm looking for any sound avdice, particularly from anyone thgat has actually worked on these specific Maggie models.
Thanks in advance!!
O.
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1. It is not clear that there is an issue. The noise you hear may be "normal"???
2. Always suspect tubes first - including clean tube pins & sockets. This is especially true, because this trem circuit is virtually devoid of components.
3. Verify the actual value of the few components that are there.
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The LDR should be high on the suspect list.
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Thanks Jjasilli - Tube sockets and tube pins have been cleaned and no luck - although I did order some new tubes to try out...should have those in a few days. And maybe the sound I'm hearing is 'normal' - but there's no doubt the amp is not creating the strong 'whoomp' that it should be...
Hey Sluckey,
In terms of swapping out that LDR - the Fender Style replacement on Antique Electronics Supply, will that do? Are there other options?
THX!
O.
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Note that this uses a neon lamp trem oscillator. Rarely see that. One thing about Neons is they don't age well. To get them to start better they dosed a little
Tritium(?) radioactive gas in the Neon. Half-life 12 Years?? Krypton-85, half-life of 10.756 years. So today the ionization is under 4% 2% of new. My old neon pilot power strip switches all flicker bad. Have to think on what that could do to an LFO. I'd think erratic timing then fail to run but.......
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Hey Sluckey,
In terms of swapping out that LDR - the Fender Style replacement on Antique Electronics Supply, will that do? Are there other options?
This roach uses an incandescent bulb to flash the LDR. Fender uses a neon bulb and will not work in this circuit.
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Hey PRR,
I guess I'm a bit confused...the amp has an indicator lamp that flashes along to the speed of the tremolo effect - that lamp is what I believe is neon...as Sluckey mentions, according to the note on the schematic the LDR is incandescent.
Hey Sluckey, understanding that a Fender style LDR wont work, will this work? THX!
https://reverb.com/item/30983627-reproduction-lamp-ldr-tremolo-optocoupler-for-vintage-gibson-tube-amps-ga20-rvt-and-others
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Hey Sluckey, understanding that a Fender style LDR wont work, will this work? THX!
Maybe. I would troubleshoot the LDR side of your roach before buying that.
BTW, that front panel flashy indicator is actually the neon relaxation oscillator which is the heart of your trem circuit. You may find this short video interesting (or not).
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Amazing Sluckey! thanks for sharing - super informative...!
So that 'indicator' is actually part of the trem circuit!?! that is super cool!
So, if when I turn the tremolo effect on, and the indicator lamp/relaxation oscillator starts flashing - why does the effect not work?!? I suspect it has something to do with your advise...
'I would troubleshoot the LDR side of your roach before buying that.'
How exactly do i troubleshoot the LDR side of my roach??
Ths is probably a good time to again apologize for my lack of basic electronics knowledge. I am a lover of vintage guitars and amps, I am excited about the notion of maintaining and repairing tube amplifiers, and I am fortunate to have a source of great old tube amps that keeps feeding me amps in need of help repair. I come to this forum, because I don't know of a better way to continue to educate myself. Everyones knowledge is sincerely appreciated..
OM
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So that 'indicator' is actually part of the trem circuit!?! that is super cool!
So, if when I turn the tremolo effect on, and the indicator lamp/relaxation oscillator starts flashing - why does the effect not work?!?
How exactly do i troubleshoot the LDR side of my roach??
Seeing the neon flash only means a very small part of the circuit is working. The problem is further along the circuit. Change V3. You can simply swap V3 and V6 since they are both 12DW7s. (Look closely at the tube lineup because V3 is not in the position you might expect!)
If no joy, move on to the roach. Strum a chord on the guitar and note the volume. Now connect a temporary jumper (gator clip lead) across the LDR leads of the roach. The volume should increase. Remove the jumper and the volume should decrease. Repeat several times to be sure the volume changes when you connect/disconnect the jumper. Alternately, connect an ohm meter across the LDR. If the LDR is good, you should see the resistance changing at the same rate the neon is flashing. Note, because of the slow display sample rate of the meter, the readout will likely just appear erratic. If your meter has a min/max mode use it to record the highest/lowest resistance reading. If the resistance is a constant value or only changes slightly, the LDR is likely dead.
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Hey Sluckey,
Thanks for the hand-holding!
OK, so some interesting finds:
- Swapping V3 for V6 = No Love...
- jumper across LDR leads of roach = LOUDER (quite a bit LOUDER)
- however, connected my meter across the LDR leads, and NO CHANGE in resistance (see pics) a solid 9.5ohms no matter where I set the speed??? (and i checked with multiple meters,
?now I'm seriously scratching my head? Are we thinking dead roach??
Thanks again Sluckey and all!
O.
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sorry for my intrusion if the ldr is bad you can use a replacement ldr and the same light bulb as the input to it of course then housed in some heat shrink tubing of proper diameter
this might work I use them with white leds in fenders.. https://www.tubesandmore.com/products/photocell-morley-effects-pedals
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Your Fluke is reading 9.49K. Gotta pay attention to the details.
Can you see a pulsing light inside the roach? If so, recheck that resistance reading with all lights in the room turned off.
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Hey Sluckey,
Aha, ,yes...9.49K ohm, correct...
'Can you see a pulsing light inside the roach? If so, recheck that resistance reading with all lights in the room turned off.'
No pulsating light at all inside the roach...i turned off all the roomlight and no light in the roach and still shooting a 9.5K on the meter...
and Mresistor, no worries at all on intruding! your suggestion might be a good one, but certainly above my grasp of amp electronics.
Sluckey is very graciously hand-holding me down a path of step by step troubleshooting.
Next step Sluckey? is it time to kill this roach?
THX!
O.
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With power off, measure the resistance between V3 pin 3 and chassis. What say?
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Hey Sluckey, take a look...
.804K
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Roach bulb is good.
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wow! whats the science/logic behind the conclusion that roach bulb is alive??
And, if its not the tube, and its not the neon indicator, and not the roach - then whats next...
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wow! whats the science/logic behind the conclusion that roach bulb is alive??
And, if its not the tube, and its not the neon indicator, and not the roach - then whats next...
You have to look at the schematic and understand what you see. You checked the resistance from V3 pin 3 to ground and read 804Ω. The schematic shows the roach bulb in series with a 680Ω resistor between pin 3 and ground. So, your meter was reading the combined resistance of the roach bulb plus the 680Ω. This means the bulb is not open.
Next step... With the amp on and the neon indicator flashing, measure the DC voltage between V3 pin 3 and chassis. Schematic says 15V. What have you?
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Thanks for the help and education Sluckey,
Check out the voltage on V3pin3...
with trem on/indicator flashing: voltage fluctuated continuously from about 4vdc to 16vdc over and over, constant fluctuation
with trem off: 9.29vdc (see attached)
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OK. It's time to replace the roach.
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wow! whats the science/logic behind the conclusion that roach bulb is alive??
What is the resistance of a burned-out incandescent lamp?
Since I don't have ANY burnt incandescents around anymore, I'll give the answer: infinite Ohms. (*)
OTOH, a good inky lamp is from a few to a few hundred Ohms depending on rating.
Since you read a few hundred Ohms more than the a few hundred Ohm resistor you know is in there, it is not burned up.
(*)A few unlikely exceptions. Series-string lights for holiday lights, street lights, airport runways, might have a "cutout" to pass current after the filament fails, so you don't lose the whole string/street/runway, just a "dark spot".
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OK Sluckey,
Copy that - time for new roach- what tipped you off to that?
ok - so will this one work?
https://reverb.com/item/30983627-reproduction-lamp-ldr-tremolo-optocoupler-for-vintage-gibson-tube-amps-ga20-rvt-and-others
Incandescent roaches don't seem to just 'grow on trees'...lots of neon ones out there - but this is the only incandescent one I could find...
I notice these folks also mention in the listing that they can evaluate a schematic to other amps for custom roaches so I might reach out to them.
O.
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OK, so after a little Holiday break here's the latest...She's alive! Got the tremolo running on the old MP5.
Ended up going with this 'roach' from the good folks at Music Gear MD - these fellas were super helpful and a real pleasure to deal with:
https://reverb.com/item/30983627-reproduction-lamp-ldr-tremolo-optocoupler-for-vintage-gibson-tube-amps-ga20-rvt-and-others
The only small adjustment was up'ing the value of that 680ohm to a 1K to deal with the fact that their rock bulb looks for 10vdc not the 15(or is that 13) that the original needed....worked like a charm!
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OK, so after a little Holiday break here's the latest...She's alive! Got the tremolo running on the old MP5.
Ended up going with this 'roach' from the good folks at Music Gear MD - these fellas were super helpful and a real pleasure to deal with:
https://reverb.com/item/30983627-reproduction-lamp-ldr-tremolo-optocoupler-for-vintage-gibson-tube-amps-ga20-rvt-and-others
The only small adjustment was up'ing the value of that 680ohm to a 1K to deal with the fact that their rock bulb looks for 10vdc not the 15(or is that 13) that the original needed....worked like a charm!
Where is that 1K resistor that you added located in the images that you posted?