Svt 2 pro ohms question

[oceanoasis]

So, just for fun I decided to check the actual ohm measurement on both of the cabs that I own. To do this I used a standard 1/4 inch speaker cable plugged into the cab. Then I measured the resistance, using a multi meter, at the other end of the cable.

Cab 1 - Mesa powerhouse 2x15 4 ohm. Actual ohm reading 3.2 ohms.
Cab2 - Eden 2x10 Xlt 4 ohm. Actual ohm reading 2.4 ohms.

As you can see each cab is not actually running a 4 ohm load.

So by my calculations both cabs running in parallel will equate to a 1.371 ohm load that my 2 pro will see. That's not even close to the 2 ohm load the amp is designed to handle.

So, question. What is the standard deviation or error rate when manufacturing cabs at a certain resistance? Will the deviation in ohm's damage the amp? Or is this well with the margin for error and cause no problems with the amp?

I'm asking because I just blew a power tube in the amp and I'm not sure if it was just tube age or if the cabs were outside the total error rate in ohms for what the head was designed to provide. was wondering if the cab not running at exactly 4 ohms each will cause problems like this with the amp.

When running each cab separately I run the amp on a 4 ohm load. When running the cabs together, in parallel, then I switch the amp to a 2 ohm load.

Any thoughts or recommendations are appreciated.
Matt

[Grooveman1961]

all you're measuring with your ohm meter is the dc resistance of the voice coil. The total impedance of the speaker is that resistance plus the inductive reactance created by the coil moving in the magnetic feild. speakers are rated "nominaly", basically meaning that its an average value over it's frequecy response. There's a little more to it than that, but that's a general overview.

as to the standard deviation, that's dependant on the specific manufacturing process used to produce the voice coil.

the values your seeing are correct, because you're just seeing the voice coil resistance component. Measure again, and this time move the speaker in and out and see what happens. The value will be different.

Actually, what a lot of folks don't realize is that the impedance varies all over the place, from tens of ohms near resonance, to less than rated nominal in the middle of the frequncy range, climbing again on the other end of the frequency spectrum. It all depends on frequency and construction of the driver and the box its in.

I think you're A OK with respect to speaker impedance. Amps are designed to take this variance into consideration. Your problems lie elswhere

[RickenBoogie]

+1 to that^

[oceanoasis]

Grooveman and Ricken,

Awesome, thanks for the advice guys. I figured I wasn't seeing the whole picture, hence my post. I think the tube was due for failure soon anyways. They are the original tubes and the amp was made in July of 2006, so it seems like it was about time for a re tube anyways.

$200 just for the power tubes. Ouch!

Cheers

[JimmyM]

you should only need to replace 3 power tubes at the most if one blew. svt tubes are grouped in triplets, so you can either try to find a tube with similar measurements to the ones in that group of 3, or replace 3. you don't have to do all 6.

[oceanoasis]

Thanks Jimmy.

I'm not sure about which tubes act together in the push pull configuration and I haven't found any schematics for the 2 pro yet, so I figure I can't go too wrong by replacing the whole power tube section.

I was toying with the idea of replacing the preamplifier tubes as well, but they are working just fine, I think, and it will add another $120.00 to the bill for tubes.

It sucks, cause I just got my Mesa cab and haven't been able to hear what it sounds like at all due to the amp not working.

[James Judson]

Not sure where Jimmy gets his information but heres my take on tube and class B (commonly refered to as push/pull) amplifier basics. All output tubes need to be a matched set. Half the tubes pull (positive signal swing) the other half push (negative signal swing). If any of the tubes mismatch, those tubes do not push/pull the same as the others creating a distortion in your amp (not a calculated musical kind of distortion). Preamp tubes do not work with each other in that sort of way so mix and match those to get the best sound possible. I frequently buy 8 matching tubes at a time. Then I can mix those matching tube without consequence. You don't half to believe me, take it to any amp tech. Or Jimmy might be able to explain how Ampeg has some magic to mix and match groups of 3. Jimmy doesn't make sense to this amp tech of 40 yrs.

[Jerrold Tiers]

Originally Posted by James Judson

Not sure where Jimmy gets his information but heres my take on tube and class B (commonly refered to as push/pull) amplifier basics. All output tubes need to be a matched set. Half the tubes pull (positive signal swing) the other half push (negative signal swing). If any of the tubes mismatch, those tubes do not push/pull the same as the others creating a distortion in your amp (not a calculated musical kind of distortion). Preamp tubes do not work with each other in that sort of way so mix and match those to get the best sound possible. I frequently buy 8 matching tubes at a time. Then I can mix those matching tube without consequence. You don't half to believe me, take it to any amp tech. Or Jimmy might be able to explain how Ampeg has some magic to mix and match groups of 3. Jimmy doesn't make sense to this amp tech of 40 yrs.

Yo.... Apparently you never worked on an SVT.....

There are two groups of tubes, 3 per group. EACH HAS ITS OWN BIAS CONTROL.

So you CAN replace 3 at a timeand they will share nicely..... The only issue is if the gain is way different in the groups, the SVT versions which do not have a "balance" control may not be totally symmetrical in that case.

Vintage SVT, MTI, SVT-HD, and SVT-VR all have balance controls The II, 2-PRO, CL & AV do not.

Jimmy is no dummy..........

[Grooveman1961]

Originally Posted by oceanoasis

Thanks Jimmy.

I'm not sure about which tubes act together in the push pull configuration and I haven't found any schematics for the 2 pro yet, so I figure I can't go too wrong by replacing the whole power tube section.

I was toying with the idea of replacing the preamplifier tubes as well, but they are working just fine, I think, and it will add another $120.00 to the bill for tubes.

It sucks, cause I just got my Mesa cab and haven't been able to hear what it sounds like at all due to the amp not working.

I've got a 2 pro down stairs that I recently retubed, I'll go look, but if I remember, the power tubes are in triangles...

[Grooveman1961]

Originally Posted by oceanoasis

Thanks Jimmy.

I'm not sure about which tubes act together in the push pull configuration .

Yes, it's as I remembered. ( a small miracle)

so the tubes are like this:

O O O
- O O O

1 2 4
-3 5 6 - 123 are one triad, 456 are the other.

FWIW, you'll get a lot of opinions on power tubes. My own experience has been good with "Lord Valve" at nbs electronics. He recomended Sovteks, about 170 for a sextet, tested,matched and they come with a warranty. the svt I have now delivered about 330 clean watts, rms.

[Grooveman1961]

Originally Posted by Jerrold Tiers

Yo.... Apparently you never worked on an SVT.....

There are two groups of tubes, 3 per group. EACH HAS ITS OWN BIAS CONTROL.

So you CAN replace 3 at a timeand they will share nicely..... The only issue is if the gain is way different in the groups, the SVT versions which do not have a "balance" control may not be totally symmetrical in that case.

Vintage SVT, MTI, SVT-HD, and SVT-VR all have balance controls The II, 2-PRO, CL & AV do not.

Jimmy is no dummy..........

Mr. Tiers, I believe that the II is configured like the early SVT's, with balance and individual bias for each group. If my memory serves me, the pro cl and av were different. If the II is the first of the rack mounted svt's that is.

[coreyfyfe]

Originally Posted by Grooveman1961

Mr. Tiers, I believe that the II is configured like the early SVT's, with balance and individual bias for each group. If my memory serves me, the pro cl and av were different. If the II is the first of the rack mounted svt's that is.

Looks like it. SVT-II



SVT-II Pro

[oceanoasis]

Good news!! I pulled out the tube that was malfunctioning, sprayed tips with electronic cleaner and then stck it bak in it's spot. Powered up the amp, and what do ya know, it's firing on all six tubes again. Wicked!!

According to what I've read about the push pull thing and ampegs design of the two pro, there are two groups of three tubes, so minimum replacement of the power section will be three tubes, as I understand it. They make biasing easy too with those nifty lights above the bias screws! There are two bias trim pots so that further cements the two groups of three theory for me.

Thanks for all the help and info everyone. I learned more about my amp this weekend than I have in the last five years of ownership.

Kind regards,
Matt

[Grooveman1961]

Mmmmmm...look at all that iron. Those trannies, 6550's, and 700v on the plates are the svt tone and power. (IMHO)

[oceanoasis]

Originally Posted by coreyfyfe

Looks like it.

SVT-II Pro

Mmmmm, tasty. The chassis in mine is grey and the banana plugs on yours have been replaced with a speak on connector on mine. I think mine was one of the last to come out of st. Louis, and yours was probably among the earlier runs.

[Jerrold Tiers]

yep... you bees correct......

I was remembering wrong, I went and looked and the II DOES have it, but 2 PRO etc do not, as shown in pics in thread.

In any case, the OP has a 2PRO per the thread title, so HE does not have one.