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Using a dummy plug on an early 70s blue line Ampeg SVT

#21
Well, we just designed what we needed.

Output transformers are interleaved with many primary and secondaries tied in series / parallel.

The process is called interleaving or sectioning.

We simply brought those windings out. Each of those seconcaries is really I believe four inside the transformer for a total of eight.

That keeps the transformer well behaved out to 80kHz. which makes the feedback loop respectable at 20 kHz.
 
#22
Ok I get it, the 16Ω winding is simply not pulled out.

I figured that by having a 4 Ω 810 at the time, it also probably helped Ampeg sell more cabinets. Especially if you were using two of them. If there was an 8Ω tap on the back of the amp, people might have tried using cabinets made by another company.
 
#25
Yes, leave it alone and do not use the dummy plug.

I also said that if you put those 4 speakers in parallel you would then have a 2 ohm load.

Using a dummy plug would provede 300 watts to your 4 X 10 box, probably blowing up the Eminence speakers.

I would avoid that!
 
#27
I'm under the impression that the speakers in the cab are 12" Eminence [Invalid or Expired Link Removed], rated 8Ω 250W each.

In series-parallel 8 Ω wiring, 500W capacity; wired 2Ω in parallel, 1000W capacity.
 
#29
My error, thought you said 4-10.

I loooked at the eminence web site...the have 2 inch bobbins,
using 250 watts of program material is believable, they would probably work fine in parallel and using the dummy plug.

Either situation will be OK.
 
#32
If you want it optimal, you could ad another input and run two lines to the box and do speaker pairs wired parallel for two four ohm loads, as suggested earlier. Then the amp sees exactly what it likes to.

However, since Bill says you're good to go, I'd tend to trust his opinion! ;)
 
#33
This is awesome!

Is there a performance difference between plug vs no plug? Now I am just trying to understand the advantage or disadvantage of using it.

Glad I will be fine either way! But I am curious to know the difference. So happy to know I haven't been slowly killing my amp and that I can use the cab I like!
 
#34
It was always my feeling that not much bad would happen running an 8 Ohm box off the nominal 4 Ohm output on an SVT as the circuit needs to be tolerant of impedance variations anyway inherent in speakers to be robust (and I doubt anyone would question the robustness of old SVTs). It's good to have that feeling validated by the guy who designed it. :D

Welcome aboard Bill! It's awesome to see you posting here and we definitely appreciate any comments or insights you have into the mighty SVT!
 
#35
No performance differance, the plug is just setting the impedance switch. The rated impedance of a speaker is really the minimum impedance. As an example, the SVT 8-10 that we released in 1969 is 4 Omm minimum at about 250 Hz, rising to around 75 Ohm at 60 Hz, and 5Khz.

If you run with no speaker at all, (open circuit or infinite Ohms) you can destroy the output transformer. Run with a short (zero Ohms) and you will beat up the 6550's
 
#36
whitespikebass said:
This is awesome!

Is there a performance difference between plug vs no plug? Now I am just trying to understand the advantage or disadvantage of using it.

Glad I will be fine either way! But I am curious to know the difference. So happy to know I haven't been slowly killing my amp and that I can use the cab I like!
Click to expand...

The further off the nominal impedance you are, the less power the amp will be able to deliver. There's no advantage of using the plug unless you rewire that cabinet to be 2 Ohms. If you leave the cab at 8 Ohms you'll get better performance out of the amp by not using the dummy plug. By better performance I mean more potential output power. Don't know if there'd be any sonic difference other than very slightly earlier distortion onset.

I ran a SWR Goliath III rewired to 2 ohms off my SVT using a dummy plug for quite a while. Worked great.
 
#38
whitespikebass said:
Is there a performance difference between plug vs no plug? Now I am just trying to understand the advantage or disadvantage of using it.

Glad I will be fine either way! But I am curious to know the difference.
Click to expand...

Just to summarize and to add a couple of comments:

If you run the amp with a 4Ω cab, two 4Ω cabs, or a 2Ω cab with the dummy plug, the performance will be the same. The impedance of the cab(s) will be matched to the 2Ω or 4Ω taps on the output transformer. The amp will perform as designed.

If you run an 8 Ω cab connected to the 4Ω speaker out it won't hurt the amp. There will be an impedance mismatch which will change the performance of the amp. The operating points of the power tubes is going to shift a bit. The power output will be lowered, Bill estimates maybe to around 250W. But at 250W or 300W you aren't going to hear much of a difference in volume, if any at all. You might notice less headroom and more distortion as a result of running the amp with the impedance mismatched.

In general, if an impedance mismatch between the speaker cab and the output transformer taps is too high (towards an open circuit), the output transformer and the tubes can be damaged. To low (towards a short circuit) and there is a strain on the power tubes and output transformer that will shorten their life. The higher currents in the output transformer will generate more heat. If the transformer gets too hot, especially a vintage one that is 40 years old, you could lead to premature breakdown and cracks in the coating on the wires that are wound inside the transformer. This would lead to transformer issues such as shorts.

It is important to ensure that the continuous wattage rating of the cab matches the amp. A general rule of thumb that I like to follow is for a 300W amp, the cab should be rated at twice the capacity or 600W. It is more complicated than just looking at the wattage. As Bill pointed out, thermal and mechanical limits of the drivers and the cabinet design and porting come into play. If you crank the bass, you are going to be putting more strain on the drivers. Use common sense and your ears when cranking the amp to the point that the speakers are distorting.

The performance of the amp will also be affected by the bias setting and the power tube set in the amp. Some matched tube sets have an earlier onset of distortion and less headroom than others. The tube bias and voltages within an amp can change from venue to venue, depending on the wall voltage. If you bias an amp at 125VAC and go to a club that has 115VAC, the amp isn't going to perform in the same way. So if it sounds a little different one day, keep that in mind.
 
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#39
I am only familar with the origional SVT produced in Linden N.J. and later by Magnavox at Jefferson city Tn.

Speaker power ratings are different between manufactures. Driver specs alone don't tell the full story.

There are two limits, thermal and mechanical. Mechanical limits are also linked to cabinet design and vent tuneing.

There was an attempt several years ago by the EIA (Electronic Indistries Association) to come up with a standard rating system. It is called EIA RS426, later abended to EIA RS426B.

Bill
 
#40
Mr. Hughes, if you don't mind me digressing a tad, how involved were you with the original V4?

Lots of folks (myself included) have been dying to know if the original V4's power transformer (and OT, for that matter) can safely cope with the extra filament current of 6550's rather than 7027's. (Many folks have made the switch, but do you consider it acceptable?)
 

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