Amp Tech Thread / Ask a tech Q

[justinhedrick]

Anyone feel up to critiquing a dumb Sunday night half asleep thought.

If I have a 120V to 12V transformer, that is a 10:1 turns ratio.

If I connect the primary of a second identical transformer to the secondary of the first, I should get 1.2V which represents 100:1 turns ratio.

If I do it again, with a third transformer, thats 120mV, 1000:1 turns ratio.

So if i hook an 8 ohm speaker to the secondary of the third, I should see 8000 ohms on the primary of the first, so I've just cobbled an output transformer out of three old wall warts, don't imagine it has much of a power rating though. Wonder if I could drive a 6V6 with it  

Now I'll use another 120 to 12 and a 120 to 9 (reversed) for the power

this sounds interesting to me. surely it would be about a watt or two of output, right?

[Hemisaurus]

I don't know. I'm thinking if I have a transformer that does 12V@1A then the primary can handle 120V@100mA, with that as a limitation, that should knock on, so

120V @ 100 mA > 12V@ 1A > BOOM

Working backwards, I can never put anymore than 100 mA into the primary, so I can have on the third transformer secondary 1A, so the primary has 100 mA

The second transformer has 100mA on it's secondary, so that's 10mA on it's primary

The first transformer has 10mA on it's secondary, so 1mA on it's primary, doesn't allow for much power output with a plate current of 1mA tops.

But I think all of the above must be hokum, or at least a lot of it. It's been way too long since I did all this stuff at school.

[inductorguitars]

A question for Hemi:

Is there any reason why you couldn't put switches in the tone stack of a tube amp to switch the caps for different eq/voicing? I could draw a schemetic if that would help.

[Hemisaurus]

No reason atall, that's what the Orange FAC control is. Keep the caps close to, or on the switch to save the signal traveling too far.

[inductorguitars]

Ah cool Thanks. I knew you would know.
I also didn't know what the FAC switch did, I've only played an Orange once long ago but they rebooted the company.

[Hemisaurus]

The FAC on Orange, Depth on Matamp and I think Deep on Electric are just a bunch of caps that can be switched in, either between the input and first gain stage, or first gain stage and the tone stack. I think Dumble and Mesa have different caps that can be switched in or out of the tone stack itself, with various push/pull knobs.

Schematic Heaven is your friend

[inductorguitars]

Aha I've been using other sites with schematics, I like Heaven.

[Metal and Beer]

Hemi-Sunn0))), I got an easy one for ya. The preamp out (solid state Randall, old'n) is rated at "500 M.V.". I'd like to run that signal through my Acoustic 370 but it doesn't have a power amp in jack. How far off are the values of the preamp signal and what the instrument input can take on the Acoustic? I know on some amps the signal is much too hot for the receiving amp but it worked okay when I tried it befo'. Is it possible that it would sound fine but be hurting something? Ol' Nessie has been in duty since 1974 w/ one service in '96 and I don't wanna fuckle 'er up but if she can take this "500 M.V." like a champ, then 'ey. I got me these two women, see, and they ain't up to snuff alone, put 'em together and the domestic bliss is the order of the day
   Wait, it gets more stupiderer. Instrument cable or speaker cable for pre-out power-in? Inst. I reckon...

[Metal and Beer]

Edit: Fucked this one all up by accident, original post quoted below

[Hemisaurus]

Hemi-Sunn0))), I got an easy one for ya. The preamp out (solid state Randall, old'n) is rated at "500 M.V.". I'd like to run that signal through my Acoustic 370 but it doesn't have a power amp in jack. How far off are the values of the preamp signal and what the instrument input can take on the Acoustic? I know on some amps the signal is much too hot for the receiving amp but it worked okay when I tried it befo'. Is it possible that it would sound fine but be hurting something? Ol' Nessie has been in duty since 1974 w/ one service in '96 and I don't wanna fuckle 'er up but if she can take this "500 M.V." like a champ, then 'ey. I got me these two women, see, and they ain't up to snuff alone, put 'em together and the domestic bliss is the order of the day
   Wait, it gets more stupiderer. Instrument cable or speaker cable for pre-out power-in? Inst. I reckon...

500 millivolts is less than what an active bass puts out, and about 5 - 10 times what a passive bass puts out, you should be fine running it, if you keep the input gain low-ish.

And yes, instrument cable

[Hemisaurus]

But wait, there's more !! I saw the ohms breakdown but there's still fog in my brain. I have two speaker outs, 4 ohm it says, right between the jacks. Here be confusion betwixt my beloved bandmates/brothers, we still have this bone of contention to the point we don't even discuss it anymore. Examples of our disconnect:

a) Amp has two speaker outs, there's one label saying 4 ohms exactly between the two. My buddy says each out goes to its own 8 ohm cab, reason being that only one out at a time is 4, and they're bridged to 8 if both used.. I run one out to one 8, then chain out to another 8. I explained daisy-chaining as best I could, that two strung together cuts the number in half. He says that occurs at the spk outs on the amp. If that's true (and maybe it is?) why mislead a dummy like me with "speaker outs 4 ohms".  instead of "Each spkr out is 4 ohms individually, in tandem it is 8 ohm".
  Wouldn't they specify "each output 4 ohms alone. If used together each output is 8 ohms"? At any rate I told ol' boy if his way were true, wouldn't it be indicated at the output? By his reasoning, if I chained two 8 cabs in to one out, that'd match one output = 4 ohms. If I do that again, then cab 3 and 4, both 8 ohms, go into the other 4 ohm out
....My brain all hurty, and now there's a shiny thing on the ground I can't quit looking at.
Once on the old forum, there was a rumor that someone was spotted who not only knew Ohm's Law of Resistance, but could explain it so musicians could understand. Bill Curtis is still looking for that guy !
   Another one is that across equal wattage amps, four ohms is twice as loud/powerful as eight which is twice as 16. Buncha bunk, right?

OK, your friend is wrong, and your are almost right.

99% of solid-state amplifiers have a specified minimum impedance, and you can connect as many speakers as you want down to that minimum impedance, so for a 4 ohm, you can have four 16 ohm cabs (16/4 = 4) or two 8 ohm cabs (8/2 = 4) or one 4 ohm cab (4/1 =4). Most speaker outs are normalled or parallel connected, which means it doesn't matter whether you daisy-chain from one speaker out, or connect one speaker to each speaker out.

A few solid-state amplifiers have a series connected speaker out, usually you'd see this on a combo amp, and the jack would be marked external (though the word external on a jack is no guarantee it's in series), in this case you can connect as many cabs as you like to that jack, as long as the internal speaker is connected, because in series every cab you connect is added in series, ie you have an 8 ohm internal and you connect an 8 ohm external you've got 16 ohms.

Tube amps are, hang on I think I covered all this in the first post in the thread, go read it.

The 4 is louder than 8 is louder than 16 is true, but only for solid-state amps, and it's not twice as loud, and it's not twice the power. If you have a solid-state amp that delivers 100W into 4 ohms, it'll deliver about 75W into 8 ohms and 50W into 16 ohms, this is not including losses, which would mean the amount of loss an amp has to deliver 100W into 4 ohms, it may deliver nearer 80-90W into 8 ohms and 60-70W into 16 ohms.

Human hearing can't hear changes much less than 3dB, and double the power is 3dB, so basically you can barely hear the difference between a 50W amp and a 100W amp. To get twice as loud you need to have 10 times the power, so 100W amp is half as loud as 1000W amp (assuming the speakers have the same efficiency, which they won't )

[Metal and Beer]

Yeah, I was in the ballpark then about the ohms/power thing. So my two 4 ohm spkr outs send a 4 ohm load each, no matter if one, the other, or both are being used, jah ?
Thanks man, reading my coffee-fueled paragraphs had to be a chore

[Hemisaurus]

No, you have to have 4 ohms total, one 4 ohm cab to one jack, or one 8 ohm cab each on two jacks, or two 8 ohm cabs on one jack.

[johnny problem]

A friend of mine is using a Boss Equalizer GE-7.  It's got some age to it and when you try to switch it on, doesn't really work and are required to spazz out and stomp it a few times in order for the pedal to stay on.  I was able to get into the battery compartment, messed around with the switch (turning it on/off), this seemed to have helped a tiny bit, but still required to spazz out on it.  I believe the switch might just be dirty, but still uncertain. 

Any ideas/solutions?

[Hemisaurus]

Switches you can use the non-lubricating contact cleaner from the hardware store OR the lubricating contact cleaner from radioshack on, just spray it on the little switch, try and get it to drizzle down the shaft and enter the body of the switch, and work the switch a few more times, if you don't have a hardware store, try an auto store for contact cleaner.

If this doesn't work, maybe it's time to replace the switch. In Boss gear it's a momentary microswitch that triggers a buffer, you can replace it with another similarly sized momentary microswitch, or follow one of the many online guides and put in a regular 'true bypass' switch.

If the guy is like my buddy, and leaves the EQ on all the time, it might be easier to hardwire that, let me know. WCS you can do it with a penknife

[Metal and Beer]

No, you have to have 4 ohms total, one 4 ohm cab to one jack, or one 8 ohm cab each on two jacks, or two 8 ohm cabs on one jack.

Man, I'm still trippin', thanks for being patient with me. Lemme restate my confusion for ya, it'll be humorous if nothin else ! The two outs are simply labeled once: "4 Ohms". I totally get how I could chain two 8 ohm cabs together to make one 4 ohm load, and plug into one jack on the amp. Can I not do the same thing with the other jack? Can I not plug one 4 ohm load into one jack, and another 4 load into the other? If I do use only one spkr out w/ a 4 ohm cab, how does that affect (if any) the other jack?

[Hemisaurus]

I'm really struggling for a good analogy here, back in school we were taught the plumbing analogy, it can sort of be applied here. Imagine your amp is a hosepipe, the end of the hose is the speaker jack, you can screw a Y adaptor onto the end (or cut a hole in the pipe) it's not going to make the water come out any faster.

Your speaker outs are a Y adaptor, they both connect to the same hose. Your amp can handle 4 ohms total, that's it. If you put 4 ohms on one jack, and 4 ohms on the other jack, that's the same as chaining 4 cabs together, you get 2 ohms, boom, your amp is fried.

That's where the hosepipe analogy breaks down, you split too many hoses off of one hose, things don't blow up, the water pressure just drops. If more things blew up on people, more people might understand ohms law

PS If anyone has a good analogy let me know, I seem to be explaining ohms law to someone every other week.

[johnny problem]

Thanks Hemi!

My friend doesn't ride the pedal all the time, so hardwiring it will be out of the question.

[Hemisaurus]

OK, well if cleaning and/or replacing the switch doesn't work, I can explain to you how to fit a regular stomp switch to it, it's just a matter of Q4 and Q5 coming out, and wiring the switch in there.

[VOLVO)))]

Deoxit is the easily obtainble cleaner/lubricant. I use this canadian 13 dollar a can cork sniffer contsct cleaner, so im not much help haha.

[Metal and Beer]

Thanks man, the Y split clarification was what I was looking for; I see now that it's gotta be 4 ohms, no matter what's plugged into either/or/both jacks. The orig. owners manual probably covered that, but it's not clearly labeled for the average idiot. (me, most people). Some amp mfg's specify what to plug/what not to plug on the back...

Anyway, thanks dude, and yes if more shit was damaged by incorrect use then more people would learn. "I always do it like _____and it's never been a problem!!" is tough to argue with. I've mismatched every which-a way with this particular amp and it still works, wrong as I may have been...

[johnny problem]

If someone is running a lot of pedals but want to cut the signal loss, would an A/B or A/B/Y pedal do the trick? (e.g fuzz, distortion, overdrive, octave on one side and tremolo, phaser, chorus etc on the other side)

[Hemisaurus]

Most ABY pedals are passive, and do not buffer/boost the signal, so they won't help you. I think some Radial ones are buffered. http://www.tonebone.com/tb-loopbone.htm

Boss has a pedal as well http://www.bossus.com/gear/productdetails.php?ProductId=150&ParentId=262 for buffering effects loops.

Or you can add a booster pedal in at the beginning of the chain (at the end works but will be more noisy)
http://www.pedalspluseffectswarehouse.com/Clean_Boost_s/44.htm

[johnny problem]

Thanks for the info Hemi!

[The Shocker]

Master volume question:

I understand the concept of Master volume amps with a separate gain volume and Non-Master volume amps, but what confuses me are Master volume amps with a gain knob, volume knob AND a master volume.  Can you explain that one?