8R load for an SVT

[bob p]

It's funny how people will say something on the internet and then it becomes sticky to the point that it becomes prevailing wisdom. I'm running across an example of this right now, where people are claiming that an SVT *must* be run into a 4R or 2R load, and *only* a 4R or 2R load, and that an 8R load will kill the amp.

Of course, around here we're the type of people who look at vacuum tube operating characteristics and we understand what happens when you displace the operating point on the tube's impedance curve.

What strikes me as odd is that there are people who vehemently insist that you can't run an SVT into an 8R load, or that you'll kill it if you try to do that. I can't say that I agree. The first thing that we need to ask ourselves is WHY did Ampeg spec the SVT for 4R or 2R cabinets. I think that the reason for this is because back in 1969 there weren't any good drivers that could handle 300W of output, so Ampeg was forced to use an 8x array of low power drivers to have enough power handling capacity. Using an 8-pack of 32R speakers in parallel gave them a 4R load. They didn't design the OT for an 8R tap for obvious reasons.

Things are different now. There are 8R cabs that can handle SVT power. But the SVT has never changed, it's still made with a 4R tap and a 2R tap.

The question often comes up whether or not you can use an 8R cabinet with an SVT. Of course, my answer to this is yes, because I do it all the time. But try to tell the 4R zealots that it's OK to do this and you've got a fight on your hands.

Has anyone else here used an SVT with an 8R load at high output levels? I do it all the time with no problems whatsoever. The math tells me that it's OK to do this, but I thought I'd ask if anyone has an opinion that the math is wrong.

[Steve Conner]

I dunno, but I've cranked one all the way when it was hooked up to a 4 ohm cabinet with the impedance switch set to 2. Nothing bad happened.

[J M Fahey]

Agree.
2:1 mismatch in tubes means still within the working area, although obviously not optimum.
You will lose some power but not burn the OT, because you still have a reasonably reactive load to absorb possible flyback peaks.

[52 Bill]

I will agree with the lack of drivers that could handle the load theory. As I recall it Ampeg would not guarantee the speakers in the 8-10 cabinets if you did not run the amp with 2 cabinets.

[bob p]

Well, thanks, Steve. You've proven the point about an 8R load on a 4R tap.
Bill, I had no idea that they wouldn't warrant the drivers unless you used the 16x10 configuration. Interesting.

[g1]

As I recall it Ampeg would not guarantee the speakers in the 8-10 cabinets if you did not run the amp with 2 cabinets.

And I believe the lifetime warranty existed at that time. Further to that, here's something that was posted on another forum by methodofcontrol:
"if i remember correctly....ampeg wired the SVT speaker jacks in such a way that you are unable to access all of the amps' power unless two cabs are plugged in.
before that trick, ampeg had a lifetime guarantee on their speakers and it was "highly reccomended" you purchase TWO 8x10's when buying an SVT amp. many cheap musicians (read: broke) would buy only one cab to save some dough and then take advantage of ampeg's generous lifetime guarantee whenever something blew. to combat this, ampeg rewired the speaker jacks on the SVT to limit the output power unless BOTH speaker jacks were in use.
furthermore...there's a trick to get around ampeg's clever wiring which basically amounts to putting a dummy plug into the second speaker jack fooling the amp into giving you the full 300+ watts. (although, i don't own an SVT...so try to confirm all of this info before trying anything that might damage your vintage gear.)"

[bob p]

It'd be interesting to see a schematic that shows how the switching worked. I wonder if they did something sneaky like interrupting the cathodes.

[g1]

Maybe it was just a misunderstanding of the impedance tap switching. The only switching I know of is the ext.spkr. jack switching in the 2 ohm tap. And the early 4 pin connector that had the jumper wire so the power supply was disabled if the cab was not connected.

[Enzo]

Foo. The SVT had two speaker out jacks. Speaker and Ext speaker. The speaker jack has a shorting contact, so you must use it first. Plugging only into it gets you the 4 ohm tap. ANything plugged into the ext. speaker jack transfers over to the 2 ohm tap. SO a pair of 4 ohm cabs plugged intop the two gets the 2 ohm tap. If you shove an unwired plug into the ext, then your main jack is still transferred to 2 ohms.

That's it. There are no other circuits involving the OT secondary, other than the NFB feed. The amp has no idea what you have connected.


I think this story is just some wishful thinking. The full power of the amp is available on the 4 ohm tap or the 2 ohm tap, like on any tube amp. Someone may have decided that only 2 ohms made the full power and by shoving a blank plug into the EXT, you could get the 2 ohm output onto a single 4 ohm cab or something. But that is just wrong, it confuses solid state performance and tube amp performance.


Yes, the very early ones had the XLR4 with power enable instead of a main jack. That jack had no shorting contact, but without the XLT jumper the amp wouldn;t power up fully, so you still had to use it first. Otherwise the EXT jack worked the same as later versions.

[bob p]

Fender made one of the big PS amps that used cathode switching. Each speaker jack connected to it's own OT, and IIRC, they did cathode switching to turn on/off power tube pairs. To get full power you had to use a cabinet on every output.

[Leo_Gnardo]

I worked for 8 years as guitar/amp tech with T Bone Wolk, an SVT fan if there ever was one. On many occasions had to run just a single 8 ohm cab with an SVT and nothing went up in smoke. (On maybe 3 occasions an SVT output tube did fail, but it was on first power-up before sound check, and likely due to being rattled around in transport.) When Ampeg started to offer a 16 ohm version of the SVT 10-inch speaker @ 2004, I loaded one of his 4x10 cabs with them and after that never had a worry about impedance matching for those small-stage gigs.

There WAS a caution issued by (aherrrmmm.... cough, cough) Gerald Weber claiming that Ampeg, in ALL the original SVT's, purposely reversed the 2 and 4 ohm taps so that only half power would reach speakers. Look there, on the deli shelf between the salami and the cheese, it's BALONEY. Now I have found in rare cases, the 2 and 4 were swapped but it was not a regular thing, just something else we have to look out for when servicing old SVT's.

There IS an auto-switching jack on every "old" SVT aux speaker output I've seen. The idea being the 2 ohm tap would be selected when running a second 4-ohm cab. At this late date, 30-45 years on, I find these fancy jacks corroding and failing so I put in a heavy duty toggle switch in one of the back panel holes occupied by a Line Out and hope the player or tech will select the appropriate setting.

[Leo_Gnardo]

Fender made one of the big PS amps that used cathode switching. Each speaker jack connected to it's own OT, and IIRC, they did cathode switching to turn on/off power tube pairs. To get full power you had to use a cabinet on every output.

That they did, the 300-PS and 400-PS. Total beasts! Rare too. Maybe that's a good thing... Also a 160-PS PA head but just one pair of 6550's. Read all about the 400-PS by Time Electronics' Rich Koerner here:

The Fender 400 PS Amplifier Web Pages, by Rich Koerner

[g1]

I think this story is just some wishful thinking. The full power of the amp is available on the 4 ohm tap or the 2 ohm tap, like on any tube amp. Someone may have decided that only 2 ohms made the full power and by shoving a blank plug into the EXT, you could get the 2 ohm output onto a single 4 ohm cab or something. But that is just wrong, it confuses solid state performance and tube amp performance.

Agreed, and I think it's the same kind of ill-logic that leads to the original issue Bob mentioned, people claiming that they must be run into 2 or 4 ohm load and 8 ohms will kill them.

[bob p]

I had never heard of some of those Weberisms. I guess I'm lucky that way.

Lots of Fenders only came with a 4-ohm tap and people use them with other loads all the time without worry. I just don't understand why people think that the SVT is going to be any different in this regard than your garden variety Fender. Tube are tubes. The same laws apply to both Fenders and Ampegs.

[Leo_Gnardo]

I had never heard of some of those Weberisms. I guess I'm lucky that way.

For entertainment purposes only: GW's article "Getting Rid of Doze SVT BLues" appeared in Vintage Guitar magazine, Jan 1995, Vol 9 #4. Sorry no link. This is where I got his words of whiz-dumb. Quoth Gee Dubya:

"Another tidbit of information about SVT'ts: the impedance of the output transformer is deliberately wired backwards. This was done because when the amps were first sold, some players did not want to use two speaker cabinets and would blow speakers that were under warranty. To stop this, Ampeg wired the impedance switch on the external speaker jack so that when only using one speaker cabinet, the transformenr was actually set at 2 ohms. This reduced the power somewhat and kept the speakers from blowing. You could actually increase the power of an SVT by plugging a dummy phone plug (not connected to anything) into the external speaker jack. This will trip the impedance switch to 4 ohms and increase power when using a 4 ohm speaker load."

Also "there is a 99.999% chance that the hum balance control of any SVT is fried." OK maybe some but no need to change until proven guilty.

Enough GW baloney for one day. Yes, as you wrote in your first sentence, funny how "it becomes sticky to the point that it becomes prevailing wisdom." I'm sure our fellow MEFer's are wise enough to regard GW's advice in many cases to be taken as somewhat questionable.

As for SVT's, there's a lot of energy to be disposed, as compared to the usual 15 to 80 watt amp. Of course it's always best to have a matching load but we've seen in practice, you can get by with an 8 ohm load, no damage.