Post by thetragichero on Apr 8, 2019 15:25:33 GMT -5
around Christmas time I made a round of all the pawn shops in the area seeing what musical gear caught my eye. one of the shops had this old solid state pa mixer with a "blowout pricing $99.99" tag on it. made a note and went on my way this past Friday I thought maybe I would want to pick it up so I ended hoop buying it for $70 out the door I am starting a doom metal band with a friend of mine, and that crowd already loves old sunn solid state heads so I wondered... what if I cascaded all of the channels together?
what a simpler time, when amp makers provided information to help techs repair them instead of pretending like they came up with some brilliant technological breakthrough
inside is a lot simpler and tidier than I expected
contact cleaner on all the jacks (input 1 originally wasn't working but contact cleaner cleared that all up), tested by running fx loop out into channel 2 input and indeed was getting some crunchiness
first order of business was drilling holes for switching jacks that will send the output of each of the first 5 channels either top the fx send side of the switch or to a cable plugged into the jack. also replaced power cord that had a fraying end with a receptacle for a standard power cord
since each channel has a James tone stack, I ran some simulations in the Duncan tone stack calculator with values I've found from around the net. orange amps, ampeg amps, use the James stack so I will throw several different in there
will clean all the pots as I go (pretty much all were scratchy) has several notch filters to avoid feedback. will attempt to voice with more guitar appropriate frequencies as long as I can find an lcr filter calculator I can make sense out of (already figure a simple rc low pass filter around 60hz would be quite helpful)
one quirky thing I found is that this does not like to be dimed, as I kept popping the panel-mounted circuit breaker with everything at 10 (but man it sounded good for the 5 seconds). plenty loud on 3 so I guess I should just not be such a butthead
time to prepare for a bike ride with the better half. more to follow
That's a nice old unit, easy to understand and to work on. I guess '74 was just before such units became standard using opamps instead of seperate transistors.
Does the reverb work? Its a nice bonus if it sounds good
I have an old english mixer from about 1980 It was a nice project and served as our band PA for a few years. The one thing it needed most was new power-supply filter caps, which had bulged/oozed goo.
If you plan to feed guitar into yours, I reckon it might benefit from an input buffer with high impedance to avoid dulling the guitar tone. This could be as simple as just using any buffered pedal, switched off, or a jfet stage.
Also, the circuit is well within range of Spice modeling, maybe in stages. This could give good insight into how the tone stack or notch filters interact.
Post by thetragichero on Apr 10, 2019 17:23:47 GMT -5
reverb works. full-sized accutronics tank. since it's footswitchable, I'm thinking maybe I'll add a switch so it's either reverb or a boost (from the recovery stages). will be easy enough to test since the tank is connected by rca jacks
my understanding of amps is limited, but my assumption was that the 1M5 resistors to ground mean the input impedance is 1M5, but I welcome correction either way, there is PLENTY of low and high frequency content available. one neat aspect about the James tone stack is that the bass and treble is either boost or cut, allowing a mid boost as they're both turned down. I don't think any of the six channels have either a bass or treble knob at even the center position; they're all in cut mode
you're 100% right about needing to model out in spice; I wish there were a smart phone app as I loathe sitting in front of the pc I have modeled the various tone stack changes in the Duncan tone stack calculator which was a HUGE help. the only thing it didn't tell me was that using a very large (1M) resistor for r4 in the first stage would cause crossover distortion/that splatty octave fuzz sound, but fortunately I tested each stage as I went
on the schematic, the 5A circuit breaker is in series with the output, I guess to protect the speakers? I've noticed a couple other places where some repairs were done, and this unit had it rewired *somehow* on the mains/power switch, so that after awhile playing it would trip. since there's already a mains fuse, I bridged the two contacts and played for quite awhile without it shutting down. I may later use the chassis hole for the reverb/boost switch
I don't have any meters that measure yup to 4700uF, but as long as it works and there's no visual issues with the filter caps should I just leave them for the time being?
Post by thetragichero on Apr 10, 2019 20:36:41 GMT -5
looking at schematic earlier (okay, i was skinning fish to cook for dinner after looking at the schematic...), and i noticed a pattern with the resistors in the notch filter section
seems that the resistors going horizontal (390R 680R 1.5k etc) and the ones on the actual filters (390RR 1k 2.7k) are related... ie 800hz filter r29 is 2.7k while the horizontal resistors in that section leading up to it are 390R + 680R + 1.5k = 2570... almost like we've got a resistive voltage divider in play! i may be too lazy to enter things into spice and figure out how to make it work, but i can certainly recognize patterns
so i thought, what if i used c15 (80uF) and the appropriate resistor to ground (online calculator gave me 33R) to get my low pass filter on the first switch pole?
i think i can lift a resistor leg, solder in a 33R resistor, and see if what i think will happen will happen
i also found an LCR filter calculator that is dead on for the 100hz switch and close enough with the others that they could've been rounded
it appears that the inductor and capacitor set the frequency, while the resistor affects the damping. does that matter or not? i'm not sure. but it appears some well-chosen capacitor values can give me the ability to cut out mid frequencies that may be troublesome ('muddy,' etc)
or i'm off on some tangent and i'll have to revert back to what i know works. either way i am open to suggestions
Id expect that loading the circuit at C15 with that 33 ohm resistor will squash all sound.
If you want to specifically cut 60hz, id suggest to try to increase the cap value of the 100hz filter to lower its notch frequency.Nominal value would be so 1/(2Pi.(LC)^0.5)=60hz. In the case, the 5uF cap goes up to 14uF. Could try adding 10 in parallel with the current value of 5, to get close.
Post by thetragichero on Apr 11, 2019 9:28:57 GMT -5
had a feeling i'd be getting off too easily with a simple RC filter
i'm actually looking for a high pass filter at 60Hz (guitar doesn't belong there and on the bass forum i'm seeing guys use hpf around that frequency to not compete with the kick drum... psychoacoustics is fascinating), which apparently in an LC circuit is a T filter link
so Fc = 1/(4pi sqrt(LC)). if i want Fc to be 60Hz, and i'm using the stock inductors of 500mH (taking one out to measure would be prudent), then we just solve for C. i got 3.5181uF since i would need to double the capacitance for the 2C with inductor to ground, i'm looking right around 7uF. i know i have some 6.8uF caps, which would give me right around 61Hz. close enough for punk rock, as my father would say
i was a math major in college, but that's the most advanced i've gotten in a long while. mainly just length and area and figuring out complementary angles for installing trim - it's nice to know some of this stuff is still in there (my brain) somewhere
only other thought is that the two caps in series with the signal... will they effect the other filters?
spent some time with the better half last night instead of firing up the soldering iron - glad i did
the 350hz notch filter i'm planning on putting a .875uF capacitor (well, two capacitors in series that give me that value) to get a notch at 240Hz. trying to give options to reduce possible low mid/mid bass mud
edit: since i would need to use electros for the 6.8uF caps, my gut tells me that the anodes would have to "face each other" (both be in the direction of this filter, since it would be the "most negative" part?
Post by thetragichero on Apr 11, 2019 21:42:17 GMT -5
didn't get a lot of testing time in but the high pass filter works, done get that satisfying THUMP that sounds so good playing by myself but awful in a band mix changed the middle three notch filters to 240hz, 400hz, 700hz (left the 5k filter in tact, I like it). couldn't tell much difference between the three or if I like them, but I also was unable to crank it due to a difference of opinion between "it's barely breaking up!" and "it's way too loud!" so we will revisit later
confirmed that connecting the reverb send to return (using the recovery circuit as a boost) works, gives some KILLER saturated tones (with its own tone and volume controls... it's almost like having a two knob rat internally!)
tried a couple footswitches in the reverb footswitch jack and was unable to get anything to switch so I will have to investigate the switching, besides that it's clean up the remaining pots and putting 'er back together
edit: realized I was testing it wrong. works as it should. salvaging a dpdt switch for reverb/boost from a partscaster I'm parting out
... and you can get bipolar capacitors to avoid the problem with the polarity question (I used one to fix my solid state Marshall Amp years ago)
I think the term is actually 'non-polarized'. Bipolar can be used to describe certain transistors and well ... me.
Of course - not something I refer to often but I believe you are right about the conventional name for these things (however with a Google search just now I saw both terms used here and there but I still think you are correct).
Whatever they are called, my solid state Marshall combo works well to this day since I used one in place of a reverse-biased tantalum capacitor that was in the original build. It was odd, since the board was assembled correctly according to the silk screen designations but there were a few volts of DC reverse-biasing this coupling capacitor. It created occasional 'pop' sounds but otherwise worked properly, and several repair shops were unable to find the problem so I bought the amplifier very cheap.
I also picked up YVM-6 .... couple of issues ... pots really scratchy of course so need to clean and lubricate ... how did you remove chassis from cab? Removed 2 bolts .... chassis loose but wont slide out?
Also, did a qwik test and it cut out .... power cycled and seemed to work again ... might be the pots so will start there?
to be honest i don't remember how i removed, sometimes i just keep taking out screws until i can get the thing out did it cut out when you had it loud? doubt it's the pots, could be output section issues but it is likely the circuit breaker with the long red button, which is in series with the output. i had an 80s carbon 4x12 that had a fuse in the cabinet... I'm guessing folks were concerned with blowing speakers or something in the 70s/80s? i just jumpered the two lugs of mine. I'm much more concerned with getting the most output and distortion from mine, like every guitarist 😁 if the pots are scratchy but working, the deoxit for faders (has a lubricant in it) should fix em right up it's a fun little project and i ended up with surprisingly nice results... i think of it as a poor man's sunn concert lead (does the sound the doom guys are all into). have fun with it, learn some stuff about solid state amps, etc