Amp Tech Thread / Ask a tech Q

[jibberish]

The first transformer was the original transformer from the factory.  I sounds like it was originally built with the incorrect transformer.

can your [cough] techs get the original test points from the mfr of your amp?
by test points I mean a diagram with places to put the DMM leads throughout the circuit and a list of values you should be reading.

what I am getting at is, what if certain other electrical parameters have changed over time to be pulling too much juice vs original.

I can't see amp mfrs making sketchy products like that that would need constant service
if you beef up the power supply, later you might find something else that finished failing and when fixed only draws <500mA now

just something to consider in the back of your mind. I don't think we have the whole story yet

[RacerX]

The first transformer was the original transformer from the factory.  I sounds like it was originally built with the incorrect transformer.

That's what Foxen & I are getting at. And if they knew it was the wrong tranny before replacing it with a clone of the same, I'd call it at least partly their fault, too.

[jibberish]

I have one more question about that SMF head. 

at this point in time, we know that the state of the circuit is such that it has a 6 month field life between service. if you use it again as is, it looks like it will be back in 6 months again.


here is the key question: how old is this amp?

IF it was defective initial design, then I have to assume this amp is about 1 year old.  6mos from factory new=1st tranny meltdown, 6mos later=#2 meltdown.
you would be best served by upgrading the power tranny to 1 amp model.

If the amp is several, or quite a few years old, then logic dictates, that for all those years, a 500mA supply was fine. the 500mA tranny is NOT THE PROBLEM. something changed and your less than impressive techs need to find it.

[beerrhino]

Sound City made the SMF series somewhere around the late 70's-early 80's I believe.  I've had the amp for about 7 years although it hadn't seen much use for the first 6. 
The techs tell me that everything else in the circuit is running exactly as it should.  I have to take them at their word, they've done excellent work for me in the past.

[jibberish]

Sound City made the SMF series somewhere around the late 70's-early 80's I believe.  I've had the amp for about 7 years although it hadn't seen much use for the first 6. 
The techs tell me that everything else in the circuit is running exactly as it should.  I have to take them at their word, they've done excellent work for me in the past.

ok, good luck. hope you figure it out.

[mortlock]

stupid question from me number 216..
I want to add another cab to my bass rig. I want a 2x15. I currently have a peavey mark vi head and an ampeg 4x10.

the cab is 200 w at 8 ohms and the head min 2 ohm load. rated at 400 w at 2 ohms and 250 w at 4 ohm. what do I need to make it work in the way of a 2x15. watts?? ohms?? please educate my dumb ass..thanks..

[jibberish]

mortay, i'm writing you a book because I think you are worth it heh.   this trickery frees you if I can explain it effectively.

stupid question from me number 216..
I want to add another cab to my bass rig. I want a 2x15. I currently have a peavey mark vi head and an ampeg 4x10.

the cab is 200 w at 8 ohms and the head min 2 ohm load. rated at 400 w at 2 ohms and 250 w at 4 ohm. what do I need to make it work in the way of a 2x15. watts?? ohms?? please educate my dumb ass..thanks..

hey chief, I want to give you a couple tools.

1)reduce the confusion factor by using a concept called a "black box".  powerful analysis trick universally used.  here's how it works: you literally only look at the inputs and outputs to some chunk of circuitry.  an op amp is a great example of black box in action and no one even knows it. do you need to know that there are whatever 20 transistors and 50 capacitors and 100 resistors and whatever all in that little can? no. we only need to know it needs power and some input and you get some output. we black box the guts.
really any chip like a 555 timer or w/e is a black box.

ok wtf does this have to do with bass and speakers?  ignore the contents of the box. just go with the black box parameters of the entire cabinet. you have one black box that is an 8 ohm load that can dissipate 200watts of power. you want to add another black box with some resistance and some power dissipation rating(this is max watts)
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

2)
now ask yourself questions: how do I want my power distributed?  do I want half to go to each black box?

[side story. been talking with my nephew about LED's for his super van. he is a wiz at IT shit. I bow before him, but he is pure nub at electrical theory. I showed him how to set a voltage by using a pair of resistors as a voltage divider. like how tube bias or pedal transistor bias is set with 2 resistors. it was awesome seeing the light go on and he was happy amazed how that worked and went right home and sawed a 12v signal down to 4v or w/e he needed]

electricity likes path of least resistance more, but some goes through the higher resistance too if another path is added in parallel. like one faucet on full(not much resistance to current flow)and one on a trickle(big resistance). the total water flowing is the total of both faucets, but you can see the higher resistance flows less water.   feel that inverse ratio. feel that if both faucets are opened the same, they will flow the same amount of water....feel that if both black boxes have the same resistance, they will both dissipate the same amount of energy. they will split whatever the amp is dishing out fiddy-fiddy.

so , to split power in half, both resistances must be equal.  if you want 1/3 and 2/3, then resistance 1 is 2/3 and resistance 2 is 1/3. see that inverse shit.  a 4ohm and an 8 ohm resistor in parallel would make that ratio and send 2/3 power one way and 1/3 the other way respectively.  so that's how you set the actual power distribution, by juggling how each unit resists the flow.
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
number 3

power handling adds no matter how you run juice through . that black box is burning off power. if you have that 200w dissipation(ie handling) cab and a second 200w cab, total power handling is 400w. CAVEAT:make sure you distributed power correctly(see #2) so you dont put too much power through either cabinet. the 50/50 thing is safe math haha.

-------------------------------------------------------------------------------------------------------------------------------------------------------

number 4

what total resistance does the amp see so we know what kind of power it will be putting out.

first off, i dig stuff that is stable at 2 ohms. that is a fucking beefy amp that can flow some current and not burn up. i love it.
the lower the resistance, the more current it will flow until you get to zero, which then asks the supply to flow every last electron it has , voltage drops to zero and you have shorted out your supply. this melts the outputs. so 2 ohms stable is badass.

2 ohms is going to sound sloppier than 4 or 8 because the amp really has lost tight control over the situation, but tha could be a good thing for your tone, or a bad thing. have to play with it. here again, the 50/50 thing makes for easiest math. you have an 8 ohm cab. IF your new cab is also 8 ohms, then everything is split evenly.  in a parallel hookup,the amp sees both flows so the 2 smaller 8 ohm flows become a double sized 4 ohm flow when they are combined..feel the inverse symmetry.

your amp does 250 watts max power output at 4 ohms resistance(load)

------------------------------------------------------------------------------------------------------------------------------------------------------------------------

put it all together:  i want to make loud but no smoke.

we dont overpower a cab, or short out our amp. that's what we have to watch. solid state likes high resistance and hates low resistance, basically opposite of a tube amp. (open output circuit destroys tube amps = infinite resistance load. a short would cause less havoc)

so of we do the 50/50 thing just to set a relative point AND skate with EZ math. we get ourselves a second 8 ohm cab rated at 200 watts

1) i have 2 of these 8ohm, 200 watt black boxes so i go halvsies the whole way. symmetry
2)power, split in half due to equal paths
3)cab total power handling is 400 watts, split in half
4)amp sees 2 8 ohm loads in parallel for a grand total of 4 ohms
5)amp can put out 125 watts max into each cab (250 total split down the middle)

------------------------------------------------------------------------------------------------------------------------------------------
epilog.  if you had a 4 ohm and 8 ohm cab in parallel. logic that the total flow will be 4 ohms big plus even more since we are adding some smaller 8 ohm flow

fuck the math, just interpolate this: 4 and 4 would be 2ohms, but this is 4 and 8, so not as much flow as 2 ohms, but more than 8 and 8 which is 4. HEY i go with 3 heh
3 is close enough for government work. and you can estimate amp output somewhere between 2 and 4 ohms to get the sense maybe it would be doing 325 into 3 ohms..

watch this mismatched ohm thing becasue that double current throws bigger wattage down lower resistance path and you could just be mostl;y playing one cab and then cook it when you turn it up too loud. obviously a 200 w amp isnt going to brutally rape 200 watt cabs, but you could still cook it if 200+w goes to one cab and 100 to the other.

read this over a few times. once you get this feel. these issues get a lot easier to sort out quickly. and symmetry always keeps it the simplest.

i think i am going to have a rare cigarette. i earned it. go rest my ash after all that typing.

[mortlock]

dude you rule.. thank you for taking the time to try and get this shit to sink into my head.
basically what I took from that is I want a cab that matches my ampeg and ill be in good shape splitting things 50/50. 200 watt/8 ohm 2x15. ill never understand this crap, I have tried but electricity fucks me up.

[jibberish]

i'm glad you checked that out.

ya, if you stay 8 ohm, the power gets split even. this is about volume. keeps cabs about same volume.

you could get a cab that can take 400watts max or something. if it is 8 ohm, it still is getting 1/2 the amp's power

as you audition speakers, you might want to keep relative efficiency in mind also. especially big wattage stuff, like w/e a 1000w system. sometimes those take a lot to get going and it would be way quieter than a more efficient lower power handling one when they get the even split. idk. maybe they have very efficient hi-power bass setups. then it wouldn't matter as long as it played the same volume as the 200w 4x10

this is where the bass players can recommend shit to try. the good thing is if you stay in bounds, you can try stuff without fear of damage.

happy hunting

[mortlock]

is there a correct way to stack cabs?? I usually see the 4x10s on top of the 15s. does it really matter??

[RacerX]

Whichever your hernia repair prefers.

[khoomeizhi]

bigger (wider) goes on the bottom. smaller (lighter) goes higher up. nothing to do with sound, just physics and laziness.

[jibberish]

this is the speaker that goes UNDER the stage.

we are not worthy.......

http://www.crutchfield.com/p_13692116/JL-Audio-13W7AE-D1-5.html?tp=111&awkw=75621282025&awat=pla&awnw=g&awcr=47439192385&awdv=c


edit:  32mm cone excursion. hello

[mortlock]

any suggestions on a good 15" speaker for bass guitar application?? 

[Pissy]

Eminence Beta and Deltas are pretty tried and true.  Easy to get re-coning parts if they need it too.

[fallen]

How much maintenance pain am I getting into if I buy a '70s Orange OR80 or OR120?

I've heard of the Matamp versions eating tubes and generally falling apart but maybe it's just because they sound so sweet when they are cranked?

How about the Oranges?

[VOLVO)))]

No worry. Good amps.

[Danny G]

JCM800 2203 Pre-Amp pot question:

Since I run my 800 loud and clean, I use the pre-amp set between 1.2 and 1.5. And there is a HUGE diff in that 0.3 range.

Can I swap the out the pot with a lower value to get a more useable sweep? 

I'd like some wiggle room between LOUD and TOO FUCKING LOUD \m/,

[RAGER]

How much maintenance pain am I getting into if I buy a '70s Orange OR80 or OR120?

I've heard of the Matamp versions eating tubes and generally falling apart but maybe it's just because they sound so sweet when they are cranked?

How about the Oranges?

Love my 72 Or120 great dependable amp.

JCM800 2203 Pre-Amp pot question:

Since I run my 800 loud and clean, I use the pre-amp set between 1.2 and 1.5. And there is a HUGE diff in that 0.3 range.

Can I swap the out the pot with a lower value to get a more useable sweep? 

I'd like some wiggle room between LOUD and TOO FUCKING LOUD \m/,

Try running your master lower and the pre amp higher.  Is that too dirty?

[Danny G]

I like to have the master up as close to 7 (or higher if need be) as possible to drive the E-34Ls and get the clean cutting power

Below 6 and it's just not "loud" enough. Turning up the preamp for more volume when the master isn't where it needs to be doesn't really help it cut through better

I run a 12AT7 for V3 to further pull the gain back a bit. Get all my dirt through a pedal

[Pissy]

Try a pot with a different taper Danny. If its linear in there now, go audio or vice versa. Maybe it'll do what you're after. Maybe it'll be worse. I'd almost guarantee one or the other.

[Danny G]

50/50 chance. My inner samurai likes those odds heh \m/,

[spookstrickland]

JCM800 2203 Pre-Amp pot question:

Since I run my 800 loud and clean, I use the pre-amp set between 1.2 and 1.5. And there is a HUGE diff in that 0.3 range.

Can I swap the out the pot with a lower value to get a more useable sweep? 

I'd like some wiggle room between LOUD and TOO FUCKING LOUD \m/,

I think you might just want to pull the clipping diodes, would be easy and give you a much cleaner sound.

[SoupKitchen]

Marshall model 2203/2204 doesn't have clipping diodes. That's the 2205/2210.

[VOLVO)))]

And jcm900s