vintage svt question.

[anonymous02282011]

I understand your reasoning, but i've put in hours of use. no damage. nada.

jerrold states that if "the tubes also get the voltage spike, and if they were not properly processed, they may arc over inside, with very bad results."

maybe the tubes in my amp were properly processed?

then jerrold says:

"(Dave, you may recall the big hassle with amps years ago where we had to "zap" a bunch of tubes to prevent the internal arcing.... A step was left out by the mfgr. That failure was faster if the speaker was a higher impedance, and didn't happen at a lower one, IIRC)"

If that instance of failure was only possible in amps with the mfgr.'s error, maybe my amp doesn't have the mfgr.'s error?

he concludes by saying that running a lower impedence can wear out tubes.

i'm going to make some calls.

in the meantime. . . .

anybody else using a single 8ohm cab with an old SVT?

 

[bobyoung53]

I am in the same boat. I was thinking the fliptops 1x15 in 4ohm would be a perfect solution...and it will match!

I bought two B-15T's about ten years ago which were a B-15 with 100 watt heads in them, they sounded good but SS is not my cup of tea, so I pulled the heads out and use the cabs with SVT heads and they sound great. They're 4 ohm cabs BTW and match perfectly and the sound is still there and they are nice for smaller rooms. I need to EQ the head a little differently though, they don't have as much bottom as the SVT cabs. I like them better than a 4 x 10 as you can still stack them and hear yourself right out of the top speaker rather than having the bass top end blow right by your feet, plus you still have one on the floor so you get more bottom.

 

[bobyoung53]

I understand your reasoning, but i've put in hours of use. no damage. nada.

jerrold states that if "the tubes also get the voltage spike, and if they were not properly processed, they may arc over inside, with very bad results."

maybe the tubes in my amp were properly processed?

then jerrold says:

"(Dave, you may recall the big hassle with amps years ago where we had to "zap" a bunch of tubes to prevent the internal arcing.... A step was left out by the mfgr. That failure was faster if the speaker was a higher impedance, and didn't happen at a lower one, IIRC)"

If that instance of failure was only possible in amps with the mfgr.'s error, maybe my amp doesn't have the mfgr.'s error?

he concludes by saying that running a lower impedence can wear out tubes.

i'm going to make some calls.

in the meantime. . . .

anybody else using a single 8ohm cab with an old SVT?

When I was young and didn't know any better I cut an 8 x 10 in half and made two 8 ohm cabs and plugged them into both outputs which meant I was running a 4 ohm load off of the 2 ohm tap and yes I wore out tubes and lost power but luckily didn't blow the head up. I did this for a long time and couldn't figure out what was wrong with the amp. My guitarist even noticed the lack of power and thought my power tubes were toast. I did really wear that amp out though (did this for a couple of years) and it was a a real early blue line 6550 head, not a good idea no matter what anyone says.

 

[bobyoung53]

EDIT: If the SVT has an extension speaker jack rather than a switch (I know the oldest ones did), and you don't want to modify the SVT, you can still rewire the cab, but use two input jacks instead of one. The cab effectively becomes two separate 4-ohm cabs in one box. Then run a cable from each output on the amp to an input on the cab (two cables, in other words). You can still have a switch on the cab to allow 8-ohm operation as well.


Hope this helps.

Reid

ALL 69-79 SVT's had auto impedance switching, extension jacks in other words, no switch, I believe that was started when they were bought by the Japanese manufacturer.
If you run a tube amp at twice it's rated output impedance you will lose power and wear it out much quicker and possibly blow it up, I wouldn't want to take that chance again now that I know what I did to my first one all because I was too lazy to haul the 8 x 10 around.
Incidentally I blew up an SVT head once by playing with no speaker plugged in for about two minutes (double heads, didn't realize it at first) I don't know if maybe the shorting jack was not working correctly or what but it went quickly AFTER I plugged the cab in, it did not like being shorted (or open whichever the case was). Your damping factor is also going to change a lot with incorrect impedance, making it a lot slower than it is supposed to be, you lose that tight sound SVT's are known for and actually pioneered along with their tuned cabs.

 

[rodl2005]

Is it possible for you to rewire your 8 ohm 4x10" cab to be a 4 or 2 ohm load??????????

 

[68Goldfish]

Yeah, I gotta tell you guys. After owning this thing for a couple of months now and using it through the 8x10 thats really the only way I'm interested in using it. I've not played a gig yet where that combo didn't work perfectly. Especially considering it's the best bass tone ever concieved of.

 

[denton57]

I didnt realize how GREAT my 8x10 was until I played outside a couple weeks ago. It cant be beat IMO. My new fliptops 4x10 is great as well, but that straightback 8x10 is just perfect!!

 

[Psycho Bass Guy]

I understand your reasoning, but i've put in hours of use. no damage. nada.

...no damage that you are AWARE of, not "no" damage, and as I said earlier, your 15" may simply not have a large enough impedance resonance to cause problems. IOW, you've been VERY lucky. A speaker is NOT a static resistance. 8 ohms is just the average. The actual impedance varies with frequency. An 4 ohm speaker may have impedance peaks of 64 ohms at resonance and may drop very low at other frequencies. The reason higher than rated averages are bad is that the 64 ohm resonance of the 4 ohm cab will be double with an 8 (assuming otherwise identical drivers).

jerrold states that if "the tubes also get the voltage spike, and if they were not properly processed, they may arc over inside, with very bad results."

maybe the tubes in my amp were properly processed?

Good tubes are less likely to arc, but no tube is indestructible. "Proper processing" just means "built well."

"(Dave, you may recall the big hassle with amps years ago where we had to "zap" a bunch of tubes to prevent the internal arcing.... A step was left out by the mfgr. That failure was faster if the speaker was a higher impedance, and didn't happen at a lower one, IIRC)"

If that instance of failure was only possible in amps with the mfgr.'s error, maybe my amp doesn't have the mfgr.'s error?

He wasn't talking about bad amps. He was talking about a bad batch of tubes and using that as an illustration that Dave should be familiar with.

he concludes by saying that running a lower impedence can wear out tubes.

That is just ONE example that I could find easily. If you search his comments for 8 ohm loads on an SVT, you'll see that he is as equally opposed to the idea as I am.

i'm going to make some calls.

I hope you're not expecting quality technical assitance from Loud. SLM's customer service was famous for giving bad info and Loud's is worse.

 

[bobyoung53]

I hope you're not expecting quality technical assistance from Loud. SLM's customer service was famous for giving bad info and Loud's is worse.

Yes, they didn't even want to know about pre-SLM Ampeg's, they did have the schematics and other paraphernalia for the 70's heads though which I still have, but I'm taking maybe 20 years ago now.

 

[anonymous02282011]

couple things.

I called this audiophile I know, the guy who turned me on to SVTs and such. He owns 6 old SVTs and he's run TL606 cabs loaded with EV15" 8ohm drivers and he says he's NEVER had a problem. Now, this cab didn't exist when SVTs were being made. With impedence ratings being nominal and all, based on experience, it seems this particular 8ohm cab is OK with my SVT.

That being said, the amp does sound better when it's presented with a 4ohm load!

So what I'd like to do is continue to use my 8ohm box but trick the amp into making it think it's a 4ohm load. Can I use a Y-cable or something along those lines? PSB, any thoughts?

Also, this thread has been helpful:
http://www.talkbass.com/forum/showthread.php?t=391956&highlight=vintage+SVT+ohms

particularly billfitzmaurice's and PSBs posts.

 

[Psycho Bass Guy]

I called this audiophile I know, the guy who turned me on to SVTs and such. He owns 6 old SVTs and he's run TL606 cabs loaded with EV15" 8ohm drivers and he says he's NEVER had a problem. Now, this cab didn't exist when SVTs were being made. With impedence ratings being nominal and all, based on experience, it seems this particular 8ohm cab is OK with my SVT.

That being said, the amp does sound better when it's presented with a 4ohm load!

Again, it's not a case of the TL606 being some sort of special design. Apparently the 15" in that vented enclosure does not produce an impedance peak sufficient to damage the SVT's output section, however your second point speaks to something else: when you present an impedance mismatch to a transformer (in this case the OT of the SVT) the frequency response shifts. Since the 8 ohm cab is higher than 'expected' by the amp, the entire frequency response of the transformer shifts upward and the amp sounds brighter.

So what I'd like to do is continue to use my 8ohm box but trick the amp into making it think it's a 4ohm load. Can I use a Y-cable or something along those lines? PSB, any thoughts?

Can't do it. The simple solution is to sub a 4 ohm driver or get a pair of 8 ohm cabs.

BTW, for more comment check these out:

http://www.talkbass.com/forum/showpost.php?p=5039359&postcount=25

http://www.talkbass.com/forum/showpost.php?p=2807826&postcount=7

http://www.talkbass.com/forum/showpost.php?p=1429179&postcount=13

 

[anonymous02282011]

Since the 8 ohm cab is higher than 'expected' by the amp, the entire frequency response of the transformer shifts upward and the amp sounds brighter.

exactly!

well, I guess I will either be daisy chaining a couple 15s, or . . . .

there's a berg HT2x10 (4ohm, 500watt) on my local CL that might be interesting. PSB, can you recommend a few 4ohm cabs that will get along well with a 66p bass -->74SVT?

 

[ReidK]

Is it possible for you to rewire your 8 ohm 4x10" cab to be a 4 or 2 ohm load??????????

As I already pointed out, it can be rewired to a 2-ohm, or to two separate 4-ohms with separate jacks. The SVT probably doesn't have an output selector switch, so he'll need to do it the latter way. It should work fine.

Reid

 

[ReidK]

PBG,

I think we're really not too far apart on this issue.

I said I thought you were exaggerating the risk. I didn't say there wasn't any. Yes, running a higher Z load stresses the OT and tubes and they will age faster, I wouldn't have disputed that. But your first comment on the subject was that your amp "might blow up" and you didn't say much else. When questioned about that, you replied with links to an earlier thread in which you explained how running an amp with no load can damage an amp, implying (in the context of the OP's question) that the same thing would happen with an 8 ohm load. That's the context in which I posted my comment about exaggeration.

Here's another quote from Jerrold, which pretty much sums up what I'm saying:

1) Don't try to put in my mouth a claim of "instant amp death" from out-of range impedances.
All we claim is that it DOES stress things more, and do you really want to do that with a limit design? I have NO PROBLEM doing that with a lower powered amp, with lower plate voltage, etc.

2) These are "tendencies", not a hard and fast rule. IF you do that AND you push the amp, you MAY or mau not) have a problem that you would NOT have if you didn't do it.

I hope this clears things up.

Reid

 

[Psycho Bass Guy]

PSB, can you recommend a few 4ohm cabs that will get along well with a 66p bass -->74SVT?

If you're looking for compact, check out Bag End. I LOVE their 2x12 sub cabs, but their bass cabs are killer too.

 

[anonymous02282011]

can you order these in 4ohm? where from? the bag end website shows two 4ohm models; a 10 and a 12.

 

[Psycho Bass Guy]

I said I thought you were exaggerating the risk. I didn't say there wasn't any.

Consider that this is a public forum and a BUNCH of young players here are convinced they can crack open any amp and resolder it to become the holy grail of bass tone. Since most are going to go off half-cocked regardless of all the warnings to the contrary, it's important that the fundamentals be stressed, especially since the issue of tube amp output impedance is so commonly misunderstood or flat-out wrong in MOST cases.

Yes, running a higher Z load stresses the OT and tubes and they will age faster, I wouldn't have disputed that.

..and you're still wrong. A higher secondary load does not "age" the output tubes faster. As a matter of fact, their current flow, and hence operational temperature, drops. It's the resulting voltage spike which can cause them to arc short, all while putting out a LOT more power than normal, albeit mostly in class B.

But your first comment on the subject was that your amp "might blow up" and you didn't say much else.

That's because I have a few dozen OTHER posts that go into greater detail, as well as a much more comprehensive explanation in the FAQ that nobody seems to bother to read.

When questioned about that, you replied with links to an earlier thread in which you explained how running an amp with no load can damage an amp, implying (in the context of the OP's question) that the same thing would happen with an 8 ohm load. That's the context in which I posted my comment about exaggeration.

If you don't understand the relationship between a higher load and no load, all you're doing here is splitting hairs to argue semantics. You've offered no technical info to back up your assertions; you've just said that mine are wrong. They're not.

can you order these in 4ohm? where from? the bag end website shows two 4ohm models; a 10 and a 12.

Yes. I have three of the 2x12 subs, and they're all 4 ohms. (I use them with my 400PS.) I got the first from Bass Northwest on clearance a few years ago and the other two came from a store stock liquidation on eBay.

 

[anonymous02282011]

cool.

all of this has been very very informative. PBG, to your credit, i did read your FAQ posts and then some. again, very informative.

I guess the thing is, the SVT pairs well with the classic 8x10, but that cab def won't fit in my car, let alone some of the stages I play. So I imagine people have come up with some creative cab solutions to get the classic SVT tone from portable, powerful little 4ohm cabs. you can bet i'll do a little research on TB about this, but, and this is for anyone, what are some of the 4ohm cabs that you've found to work well with your old SVT?

I guess I'd prefer a 15 or a 4x10.

 

[Psycho Bass Guy]

The reason I recommended the Bag End 2x12 subwoofer cab is that it sounds VERY much like the original 8x10. When I a/b'ed it against my main gigging cab, the only difference was volume. It's sealed and designed for high excursion since it would normally be used with a Bag End Infra/ELF processor in PA functions.

 

[ReidK]

I said I thought you were exaggerating the risk. I didn't say there wasn't any.

Consider that this is a public forum and a BUNCH of young players here are convinced they can crack open any amp and resolder it to become the holy grail of bass tone. Since most are going to go off half-cocked regardless of all the warnings to the contrary, it's important that the fundamentals be stressed, especially since the issue of tube amp output impedance is so commonly misunderstood or flat-out wrong in MOST cases.

I don't see how this refutes my assertion that you were exaggerating the risks. Are you saying that you should exaggerate the risks, to keep newbies out of trouble? Is that what you mean by "stressing the fundamentals"? I don't equate "stressing" with "exaggerating".

Yes, running a higher Z load stresses the OT and tubes and they will age faster, I wouldn't have disputed that.

..and you're still wrong.

Well, here's what you yourself said earlier in this thread:

Also, since it puts your output tubes above their rated limits, you may have had to replace tubes much sooner than you should under normal conditions.

I call that "stressing the tubes".

And here's what Jerrold said in one of the threads you referred to:

What CAN happen if the load is too high?

1) Some amplifiers use feedback around the output transformer. Without proper loading, the transformer may not provide correct phase feedback, and that may cause oscillation, either an audible squeal, or an oscillation at above-audio frequency. Neither is good for the amplifier, output tubes can be "worn out" in seconds to the point where emission and power output is very low.

2) "Spike" voltages from transients may be considerably higher because the "reflected impedance" is higher than designed. That can cause damage of varying severity. In an amplifier that already "pushes" the tube ratings, that can be serious.

The SVT falls into category 1, BTW, since it has a global feedback loop from the OT secondary to the cathode of the first driver in the power amp.

But your first comment on the subject was that your amp "might blow up" and you didn't say much else.

That's because I have a few dozen OTHER posts that go into greater detail, as well as a much more comprehensive explanation in the FAQ that nobody seems to bother to read.

I was replying to what you posted in this thread.

When questioned about that, you replied with links to an earlier thread in which you explained how running an amp with no load can damage an amp, implying (in the context of the OP's question) that the same thing would happen with an 8 ohm load. That's the context in which I posted my comment about exaggeration.

If you don't understand the relationship between a higher load and no load, all you're doing here is splitting hairs to argue semantics. You've offered no technical info to back up your assertions; you've just said that mine are wrong. They're not.

I have not demonstrated any such lack of understanding. What I did was point out that there's a huge difference between a factor of 2 and a factor of several million, a difference which has a very real impact on potential damage to the amp. That is in no way "splitting hairs".

Anyone reading this thread can examine my previous postings, and they will find that I have, in fact, posted technical information, and that I have not, in fact, said anywhere that your technical information was wrong.