Amp Tech Thread / Ask a tech Q

[VOLVO)))]

Pretty much, what you said. Wire it like the volume knob in your guitar, it'll be passive, but it'll do what you want it to do.

[Jake]

Are there any do's and don'ts to consider? Is any type of switch better than another? What resistance pot should I use? How many licks to get to the center of a Tootsie Pop?

[Hemisaurus]

You'll want a 3PDT if you want to add an LED indicator, and then you'll also need to add a battery and a resistor to limit the LED current.

http://www.generalguitargadgets.com/faq/40-technical-questions/180-many-bypass-switching-options

have all the links you need.

Also be aware you are putting this in parallel with your guitar volume control,. it will change the tone, it will move the sweet spot. If you don't want that to happen get some of those no-load pots for your guitar, which are out of circuit when they are turned up full.

[SoupKitchen]

Others who know better should prob pipe in, but if this is a foot-controlled passive volume pedal, you should be able to use the same pot as in your guitar. So 500K for humbuckers, 250K for singles is common wisdom. As far as switches, a DPDT switch will give you true bypass. If you really want to get fancy, use a DP3T switch for an LED, but that'll require a battery. Your average mono non-switching jacks should work too. Stereo input jack with the battery negative connected to the ring terminal will disconnect the battery when the pedal is unplugged, if you go the LED/3PDT/battery route. I'd wire the pot exactly like it is wired in your guitar, since you're cloning your volume knob and just making it switchable. If you need parts numbers, I could get those for you.

One...two-who...thrrree...(crunch)...three.

So basically, what Hemi said.

[Hemisaurus]

Others who know better should prob pipe in, but if this is a foot-controlled passive volume pedal, you should be able to use the same pot as in your guitar. So 500K for humbuckers, 250K for singles is common wisdom. As far as switches, a DPDT switch will give you true bypass. If you really want to get fancy, use a DP3T switch for an LED, but that'll require a battery. Your average mono non-switching jacks should work too. Stereo input jack with the battery negative connected to the ring terminal will disconnect the battery when the pedal is unplugged, if you go the LED/3PDT/battery route. I'd wire the pot exactly like it is wired in your guitar, since you're cloning your volume knob and just making it switchable. If you need parts numbers, I could get those for you.

One...two-who...thrrree...(crunch)...three.

So basically, what Hemi said.

That is only true if the pots in the guitar are taken out of circuit. Otheriwse his 500K in his guitar is now in parallel with 500K in the pedal, yielding 250K. Hence either:

1. Deal with it, find the new sweet spot
2. Fit no-load pots to the guitar
3. Fit push/pull pots and wire it so they can be switched out of circuit
4. Fit double the value you need in the pedal and guitar so, 1M in the guitar and 1M in the pedal gives you 500K
5. Just wire past the pots in the guitar, do all the work with the pedal.

There may be other options too.

[Jake]

I'll take Door #1.

[Hemisaurus]

If that's the case, the Ernie Ball passive is 250K I think. There used to be a Behringer one for about $25 too.

[Hemisaurus]

Simpler still, if there's space on your guitar, or if you fit a switched pot. Add a couple of fixed resistors, or a trimpot under the scratchpate, that you can switch to, to get the sweetspot, no enclosure, jacks, or extra cable necessary. No need to use your foot.

[sheikyerbouti]

i just bought a sunn beta lead off ebay and it has a few issues.
the master volume doesn´t react after 3 and the amp is very quiet.
if i plug into both channels it basically beeps uncontrolably and the input jacks seem too tight and they scratch.
some of the pots seem too loose and some too hard to turn.
is there an easy fix for these problems or will i have to consult a technician? (i have little to zero experience with repairing amps.)

[Hemisaurus]

Beeping sounds bad, sounds like the channel switching logic is messing up.

If you are using monster cable, that may explain your jack problem.

You probably will need to take it to be fixed, if your tech needs it, I have the full service manual, it's probably online somewhere too.

Yep it's here, http://sunn.ampage.org/site/svcmans/beta/ you may want to check the troubleshooting flowchart.

Err, nevermind the flowchart, incomprehensible

[sheikyerbouti]

thanks for the quick answer.
i opened it up to look for anything that was burned and found out that one of the input jacks is a bit corroded. i can´t see any bad solderjoints or any other visibly broken parts though.
it wasn´t cheap anyway and now i have to pump more money into it if i can even find someone in my area to fix it.

[VOLVO)))]

I baby my Betas. They are far from quiet. Both max out volumewise at 4ish. I never even use that much.

[Hemisaurus]

Well any tech worth their salt, equipped with the service manual especially should be able to fix it. It is however an awkward beast, there's not many amps use switching logic, even less of that vintage. The logic control circuit is explained, clearly, at the start of the manual, if the IC's on your's are socketed it will make it easier, but I suspect they won't be.

I've often thought it might be easier to pull out all the complicated channel switching and fit a channel switcher module to one, if channel switching is still desired.  

Looking at the schematic to remind myself, I think it might be possible to jack channel 1 by lifting the end of R130 and connecting it to pin 6 of IC107, and removing Q104, that way you just removed the channel switch from the circuit. Likewise on channel 2 the end of C124, could be connected to pin 2 of IC108 and Q106 removed.

That way at least Channel A input and Channel B input should work, and you could control channel switching with an A/B pedal instead.

I only mention this in case you have a tech friend, you could save the parts and put it back together again, if / when you decided to / got the cash, etc.

Hey Nick, want to check my work

[The Shocker]

What do you guys think of these?

http://www.ebay.com/itm/1950s-Military-Tube-Amplifier-15-Watts-Fantastic-Guitar-Harp-Amp-Tone-/150810837881?pt=LH_DefaultDomain_0&hash=item231d06c379

I can't imagine coming across 150 of them.  He probably bought all of them for $150...

[Hemisaurus]

Not a technical question, it's soliciting an opinion, go directly to jail, Sunn will delete your post, then your account, then your life

Also remember with military equipment, the military didn't care who they killed, including their own operators

[The Shocker]

Not a technical question, it's soliciting an opinion, go directly to jail, Sunn will delete your post, then your account, then your life

Also remember with military equipment, the military didn't care who they killed, including their own operators

Dang, I debated posting a new topic vs here.  Guess I chose wrong.

My apologies.

[Hemisaurus]

Technically speaking, I'd be asking, was it designed to be mains powered. That tube compliment doesn't suggest the old suicide boxes with heaters in series BUT what does the army need a mains powered tiny amp for?

12AX7 is common enough 6AQ5 you might be best getting someone to change the tube socket from a 7pin Noval to an 8pin octal and allowing you to fit a 6V6.

For $250 I'd shy away, for $30 in a rummage sale or mil surplus deal, I'd go for it. Just think to yourself what tube amp can I buy for $250, designed for guitar already, brand new, warrantied, safety passed etc.

http://www.musiciansfriend.com/tube-amplifiers#fT=2012:100.00-200.00|200.00-300.00&gP=1&pS=20&v=g&sB=pLH&lP=c&catId=site1HAH
http://www.musiciansfriend.com/tube-amplifier-heads#fT=2012:100.00-200.00|200.00-300.00&gP=1&pS=20&v=g&sB=pLH&lP=c&catId=site1HAMD

or send me $250, a friend of mine is selling a weird tube amp and a tester, you can have the amp Zenith amplifier and the tube compliment is: 12ax7, 6BQ5, 6BQ5, and a 5Y3 rectifier

[Jake]

I've got a ABY/phase question for you fellers. After reading this article, I thought that it might be wise to potentially add a mini toggle to be able to flip the phase if needed when running amps in stereo.

http://www.premierguitar.com/Magazine/Issue/2010/Apr/Getting_Back_In_Phase.aspx

My question is this: Could I simply add a mini toggle that that lives right at one of the outputs that can change the whether the +/– goes to the ring or tip of the jack? Does that make sense? Would it work? Is something like that going to introduce any unwanted "gremlins" into the signal? Also, Conan, what is best in life?

[Hemisaurus]

You would have to change all the jacks to insulated type, the problem with just flipping one of the inputs or outputs would be you might inadvertently ground something you didn't mean to.

I love those articles written by idiots that don't know the difference between phase and polarity, having all your speakers moving in the same direction is good, but that's not phase.

Oddly the safest place to flip speaker polarity is at the speaker, make a little box that reverses the polarity at the speaker, or just make a reversed speaker cable and mark it accordingly.

After you've done all this, go back and remember high school physics on wave theory and realise the difference between the speakers is just as critical. If you're speakers are a half wavelength apart, then they are out of phase, and will cancel each other out, then realise your wavelength varies with the frequency ie. note you are playing, so there is always going to be points where your speakers are out of phase, and then just before your head explodes, realise that means that at some points your out of polarity amps are in phase.

BOOM!

[dunwichamps]

i wouldnt recommend doing a phase flipping that way since ur not doing an active phase flip. What you can do instead is use a unity inverting gain stage, unity means gain of 1 or no gain but make the freq response flat so you get 180 phase flip on the one output

And I would not use the word polarity when referring to AC, phase is a more useful term. Polarity has more context when talking about electric fields or magnetic dipole alignment

[Hemisaurus]

i wouldnt recommend doing a phase flipping that way since ur not doing an active phase flip. What you can do instead is use a unity inverting gain stage, unity means gain of 1 or no gain but make the freq response flat so you get 180 phase flip on the one output

And I would not use the word polarity when referring to AC, phase is a more useful term. Polarity has more context when talking about electric fields or magnetic dipole alignment

Then you should read http://community.calrec.com/?p=1325

[dunwichamps]

when we discuss things in the lab, 180 degrees phase difference is not called polarity. Polarity is usually referred when we discuss DC measurements but if its an AC waveform its phase. Phase is frequency dependent and is useful when discussing the electrical phase of the waveform hitting the speakers and the resultant sound waveform's relative phase is a different subject due to environmental influences of position/reflections, ect ect

[Jake]

Excellent. I think I'm hearing you gentlemen loud and clear and you're telling me to put down the soldering iron and buy one of these.

[dunwichamps]

yea that will work. Hemi and I have this discussion each time this topic is brought up. Engineering death match

The article surmises that since a time shift is not evident then it should be called polarity to avoid confusion with the term phase which can mean time delay correlating to a phase shift

[Jake]

No worries. I saw a preview of it in the comment section of the article.