Vintage SVT ohms ?

[B-string]

Originally Posted by xoir

I never run the amp "live" without speakers I mean ever, the issue began a couple of weeks ago, I unplug the external speaker "live" only today I'm sure about it, I could not find information that plugging the external speaker output live could harm the 2 ohm tap in the OT, besides I'm aware that is not recommended, I don't think at this point, that the OT is damaged because running it at four ohms or with one cabinet the amp works right.

Did you have to be told to pee when the need arises?
With a live amp unloading (unplugging) a speaker can cause the back EMF to punch through the windings. All that stored energy goes some where. Not saying you have done this yet but it can and will happen at some point.
Do as you wish, but as someone who has been repairing amps for over 40 years I can only warn you. I won't repeat it again.

[xoir]

Thanks for the adviced, I really apreciate, I'm just trying "in my head" to look for an easy and les expensive solution than replacing the OT, the fact that I have cab exclusive for specific amp's and impedance transformer to protect my amps and speakers shold count as a very paranoic person taking care of my amps right?.

[B-string]

No one should ever replace an OT without it being properly tested. That was not my suggestion Your "impedance transformer" can not protect your amp from user error, in fact with added inductance can make things worse with miss-use.
Now you know? The best "protection" is education.

[xoir]

hahaha agree! I hope this lesson does not cost too much !!

[B-string]

Originally Posted by xoir

hahaha agree! I hope this lesson does not cost too much !!

Chances are it won't. Just don't do that powered up speaker cable thing anymore okay?

[beans-on-toast]

Originally Posted by xoir

The issue is fixed with the plug half-way inside jack, so it may be related to what beans-on-toast told, but I suspect from a ground issue.

Since it works with the jack half in, I'd carefully inspect all the cables, 1/4" plugs, and jacks. Look for shorts or dirty switch contacts. That bent plug doesn't look too good.

You have to inspect that jack where it is working when partially plugged in. Look inside the chassis, plug the cable into the jack part way and all the way in and see what the switches on the jack are doing. Do this with the amp off. Make sure that the switch contacts and the grounds are clean.

When you plug into the main speaker out, you are connected to the 4 ohm tap on the transformer. When you plug into the ext spkr jack, the two cabs are daisy chained together (connected in parallel), the 4-ohm transformer tap connection is broken, and you are switched to the 2-ohm tap on the transformer. The switch is on the ext spkr jack. See the image below. The round connector (P3) with pin-1 and pin-4 is the main speaker output, like on a B-15. In later SVT models this was replaced with a 1/4" jack that is isolated from the chassis.

When the jack is only half way in, it is possible that you are connecting the speakers to both the 2 and 4 ohm transformer taps at same time. Not a good thing. Sometimes the tabs on the ext spkr jack get bent and the 2 ohm tap doesn't get switched in. You have to inspect it. No other way to know what is happening.

________________

Thanks for the pics of your impedance matcher. I've never seen one before. Look like a neat little unit.

[xoir]

understood

I'll take the amp to my local tech this week, in case is not the OT should be an easy fix? I mean change a resistor or so?

[B-string]

The switch on the EXT jack is most likely to be where the trouble is. Cleaning and general maintenance.

[xoir]

Originally Posted by beans-on-toast

Since it works with the jack half in, I'd carefully inspect all the cables, 1/4" plugs, and jacks. Look for shorts or dirty switch contacts. That bent plug doesn't look too good.

You have to inspect that jack where it is working when partially plugged in. Look inside the chassis, plug the cable into the jack part way and all the way in and see what the switches on the jack are doing. Do this with the amp off. Make sure that the switch contacts and the grounds are clean.

When you plug into the main speaker out, you are connected to the 4 ohm tap on the transformer. When you plug into the ext spkr jack, the two cabs are daisy chained together (connected in parallel), the 4-ohm transformer tap connection is broken, and you are switched to the 2-ohm tap on the transformer. The switch is on the ext spkr jack. See the image below. The round connector (P3) with pin-1 and pin-4 is the main speaker output, like on a B-15. In later SVT models this was replaced with a 1/4" jack that is isolated from the chassis.

When the jack is only half way in, it is possible that you are connecting the speakers to both the 2 and 4 ohm transformer taps at same time. Not a good thing. Sometimes the tabs on the ext spkr jack get bent and the 2 ohm tap doesn't get switched in. You have to inspect it. No other way to know what is happening.

________________

Thanks for the pics of your impedance matcher. I've never seen one before. Look like a neat little unit.

Perfect! thanks for providing me all this usefull information!! I'll discus this with my tech based on your comments, suggestions and bad troubleshooting I already did

[xoir]

I wonder from which era the Impedance matching transformer is, I have two numbers inside: 89200010 and 682_035

I bought it because from time to time I like to connect my portaflex to lower ohm cabs using their original xlr speaker.

[JimmyM]

Got to be old as hell since it was still made in Linden. I guess late Unimusic era, early 70's.

[xoir]

Greetings,

Finally last week, I took the amp to my tech and we discovered that the problem is in the ext speaker jack, we clean deep the jack, tight screws, we make sure that were no shorts or cracks in the cables, jack, etc unfortunately without any luck...

So far we discovered that this situation occurs mostly with heavy cables (Gauge 8 in my case), we test the external jack using right angle plugs, wich don't lay all the weight of the cable and plug in horizontal position once is plugged and the issue seems to be solve.

So know my question is, wich kind of plugs and gauge is recomendable to avoid this situation in the future, is there any replacement available for these type of switchcraft jacks?

Thanks!

[xoir]

This is what we see using the heavy connector plug

[B-string]

Unless your cabs are more than 50 feet away from the head 16awg or at the heaviest 14awg cable is all you need. Absolutely NO REASON to be using 8awg wire!

[beans-on-toast]

18 gauge power cord from the hardware store is about as heavy as I use. Take a look inside the amp and see how thin the wires that connect to the jack are. Also, look in the speaker cabinet and note the gauge that they use. There isn't an advantage to using an 8 gauge cable.

You might be able to very carefully bend the contact point on the jack up a bit to make a tighter contact with the plug. As you found, downward pressure on the plug can pull the contact apart.

As for a replacement, they called them open frame jacks. Here they are on the Switchcraft site. Here is the technical drawing, C55B. Also, check the 1/4" plug that you are using. Some, such as military and switchboard plugs have a smaller diameter shaft which lets it pull away from the jack's contact. You should ensure that the plug fits the jack properly.

[xoir]

Originally Posted by beans-on-toast

18 gauge power cord from the hardware store is about as heavy as I use. Take a look inside the amp and see how thin the wires that connect to the jack are. Also, look in the speaker cabinet and note the gauge that they use. There isn't an advantage to using an 8 gauge cable.

You might be able to very carefully bend the contact point on the jack up a bit to make a tighter contact with the plug. As you found, downward pressure on the plug can pull the contact apart.

As for a replacement, they called them open frame jacks. Here they are on the Switchcraft site. Here is the technical drawing, C55B. Also, check the 1/4" plug that you are using. Some, such as military and switchboard plugs have a smaller diameter shaft which lets it pull away from the jack's contact. You should ensure that the plug fits the jack properly.

Well, I agree about gauge, but I found that using two different gauge cables at the same time in the amp, 10 and 14 (sorry not 8) at 2 ohms, the cab that uses the higher caliber cable (10) sounds louder, I switched the cables between the cabs and is noticed... (turned off of course)

Well regarding the switchcraft jack bent, as you may know there is less than < 1mm probably between the rings to play with it, so I'll consider the option of getting a new replacement, from the switchcraft page I found the 39 common jack settings, abusing of your confidence, what is the right part I should look for?

Thanks for your advice!

[B-string]

Look into these http://www.ghplugs.com/ . You can find E-bay sellers. The "Mega Foots" are designed for large gauge and are excellent connectors.

[beans-on-toast]

Originally Posted by xoir

I found that using two different gauge cables at the same time in the amp, 10 and 14 (sorry not 8) at 2 ohms, the cab that uses the higher caliber cable (10) sounds louder, I switched the cables between the cabs and is noticed... (turned off of course)

It makes me wonder if something else isn't going on with your cable. Maybe the solder connection is not good. It is important to have a good electrical connection between the plug and the wire. The solder should be there to hold it in place. What you don't want is plug|solder|wire because solder is not as good conductor. It would have what they call a higher contact mating resistance.

Yes as was pointed out by B-string, there are special speaker plugs that have large surface areas where the wire connects.

Check out this wire gauge calculator. As an example a 12 gauge, ten foot copper cable should have a resistance of 0.01 ohms; a 14 gauge 10 foot cable will have a total resistance of 0.025 ohms. The difference, fifteen one thousands of an ohm, between the cables will not be audible. Something else has to be going on.

[B-string]

beans-on-toast has an excellent point here. Connections should be mechanically sound and tight before solder is applied. Solder is neither a good mechanical connection or the best electronic connection. It is sufficient with electronic transmission but as little space as possible should be used.