Amp Tech Thread / Ask a tech Q

[Slow]

Most 16 ohm cabs ive metered run 12-15 ohms. Sound city says 16 ohm tap. Herb says?

[Hemisaurus]

I'd have personally said the 8 ohm tap, tube amp go low, solid state go high, but if Sound City say the 16, use the 16.

You could stick two 8 ohm drivers in the cab, and make it 8 ohm u'know

[justinhedrick]

I'd have personally said the 8 ohm tap, tube amp go low, solid state go high, but if Sound City say the 16, use the 16.

You could stick two 8 ohm drivers in the cab, and make it 8 ohm u'know

i bet you could fit two 18s in there, right?!

[VOLVO)))]

http://www.soundcitysite.com/sc_webpages/6x10_cab_wiring.pdf

Should I trust it?

[Hemisaurus]

Are you pushing it with a Sound City amp?

Justin, I should have been clearer, if he replaces two of the 16 ohm drivers with 8 ohm drivers, wires the four 16's series-parallel like a 4x12, for 16 ohm. Then wire the two 8 ohm drivers in series for another 16 ohms, wire those two sets in parallel for 8 ohm.

Or get another cab that can be rewired for 32 ohm, and add it on for an 8 ohm load

[Slow]

Nope... v4.

[LogicalFrank]

You would want to run the amp at sixteen ohms. It is better to have the speakers lower impedance than what the amp is looking for.

From my understanding, V4s are bitchy about impedance mismatches though so be careful. I know my buddy destroyed his V4 w/ a mismatch, like cheaper to buy another amp destroyed... but is was a really bad mismatch due to some dead speakers in his cab.

[Hemisaurus]

Yeah, I'd fake it, disconnect one pair of speakers and make it 16 for now, it's not worth the heartache of a blown transformer.

[VOLVO)))]

Should I recap when I retube? Where can I dig up the caps?

Do I have to measure each tube when I bias with that bastard probe, or should I just measure off the pins and do the math?

How long do they have to burn in before I play 'em?

[Hemisaurus]

If you are buying a bias probe from eurotubes, you might as well follow his instructions too. I solemnly believe there is more than one way to skin a cat, or bias an amp in this case.

http://eurotubes.com/euro-Generic-Bias.htm

He just happens to use a JCM 800 as his generic amp

It is a trimmer or at least it's a non-front panel adjustment, depends on your model where it is, you got one of those whacky PCB's so I don't know where it will be for you. It would be fairer to call it more of a bias balance as it affects all the tubes at once, you have to check each tube, and use the adjustment to get a happy medium.

[Hemisaurus]

and no you don't need to recap when you retube, how old is the amp and when was it last retubed?

You don't have to let new tubes burn in, but they may change tone after you play them a while. Some people advise bringing a recapped amp up on a variac, I don't know if thats strictly necessary with todays capacitors.

[justinhedrick]

and no you don't need to recap when you retube, how old is the amp and when was it last retubed?

You don't have to let new tubes burn in, but they may change tone after you play them a while. Some people advise bringing a recapped amp up on a variac, I don't know if thats strictly necessary with todays capacitors.

i always thought you did that in case the new caps threw something else out of spec (can't remember what) so you don't blow your tranny? (unless you are in to that sort of thing!!)

[Slow]

86, and im assuming never. Its only seen heavy use this passed year...

[Hemisaurus]

Yeah sorry, I meant how long since it was recapp'd not retubed 

It might be an idea to give it a cap job, if it hasn't ever had one. Even with no use, old caps can dry out, some of it is just old age, heavy use or no.

Recapping can bring up the voltage, so it is necessary to rebias after a recap, but not necessary to retube, unless those tubes are old, and the new voltage makes them fail. That's not too likely a scenario, as most amps don't run tubes half as hard as they are spec'd.

So what I'm saying is.

• You don't have to recap when you retube.
• You don't have to retube when you recap
• You do have to rebias, when you recap or retube

[beerrhino]

I have a little 10 watt tube practice amp that just suddenly started making a horrendously loud buzzing sound.  The amp still passes signal but the buzz is always louder than what I'm playing.  Tried different cables and guitars (nothing else in the signal path) and there is no difference.

[Hemisaurus]

Take it to a tech.

Not knowing the amp, or how it is built, or how old it is, you could be looking at broken tubes, dried out caps, lifted ground, shorted wires, shorted caps, or whatever.

OR

http://www.diyguitarist.com/Misc/J_Darr.htm

[Slow]

Chopstick poke test...

[Hemisaurus]

Good for dodgy solder joints not much else. A fault that bad shouldn't take muchbench time.

[core9]

I have a question for the people with the knowledge!

I am running 2 full stacks.  One stack is Mesa Boogie Rectifier loaded with V30's.  They are 8 ohm cabs.
The other stack is Matamp loaded with Celestion G12T-100's.  These are 16 ohm cabs.

When I run my GT120 through the Mesa it is fuck-all loud, real nice and in your face.
When I run the same GT through the Matamp cabs, it is loud, but not nearly as loud as the Mesa.

Any ideas about what is going on here?  I'm not sure how the cabs are wired inside.  I don't want to sell my Matamp cabs, but I need them to be as loud as the Mesa's.

HELP!!

[Hemisaurus]

Assuming your remember to change the impedance on your amp correctly for each set of cabs. It would be the sensitivity of the speakers 100dB on the V30's vs. 97dB on the G12T-100's

If you aren't remembering to change the impedance it's because your about to melt the output transformer

[core9]

No I remembered to change the impedance.  Do you really think that 3 db would be THAT noticeable?
So it wouldn't really matter how it is wired inside?

I don't know shit about this stuff man, thanks for your help.

[Hemisaurus]

No I remembered to change the impedance.  Do you really think that 3 db would be THAT noticeable?
So it wouldn't really matter how it is wired inside?

I don't know shit about this stuff man, thanks for your help.

The 3dB is for 1W at 1meter usually, not that it's a linear thing, but across 8 speakers it's going to be significant, anyways the frequency responses are going to be different too, some are more bassy, some more middy, some just plain sound louder.

[core9]

Thanks bro!

[Hemisaurus]

Dumb Fact A tube amp is fractionally louder on it's highest impedance setting, because the higher the cab impedance, the less significant the losses in the cables are.

Example If you have a 4 ohm cab, and your cable has a 1 ohm loss, 20% of your signal is lost in the cable. If you have a 16 ohm cab and a 1 ohm cable loss, 5.9% (1/17th) of your signal is lost in the cable.

Marshall originally adopted 16 ohm (or 15 ohm) as it's standard setting because people were using long runs of 18 gauge wire for speaker cables.

So if your cab is 4 / 16 ohm switchable, and your amp has 4 and 16 ohm taps (switch settings or whatever) go with the higher value.

NB always match the tube amp with the cab impedance, running a 16 ohm cab on an 8 ohm tap is NOT a good idea!

[justinhedrick]

hemi,
I think i'm going to re-wire all my cabs to 16 ohms now!

also, i have a question about cutting a new baffle:

i have an old peavey PA cab that had 2x12 in a tuned enclosure. no speakers in it. i'm going to cut a new baffle to put my two 15s in there (it will fit) but i don't know how to put the baffle in. do i just screw it in from the sides and then caulk around the inside to make it air tight? do i cut a port? i am the lost . . .