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Q & A / 1968 Sunn Sceptre reverb troubleshoot
« Last post by joshjames86 on October 06, 2017, 04:45:05 am »
Hello all. Long time viewer, first time posting.

I recently picked up a 1968 Sunn Sceptre for a good price being it has two issues. One the treble control acts in reverse. When treble is turned all the way down the amp plays and sounds great; when turned up the amp loses volume. For this I'm going to check the pots and wiring and go from there, unless someone has a different theory?

But more importantly; the reverb. I didn't get a chance to pull the tank out yet to inspect but how difficult are the two diagnose or replace (probably hard to find)? Reverb doesn't seem to work at all and if you turn the reverb knob up then amp starts to sound trebly. I'm going to look at it, check the wiring and test connections then go from there. But i'm curious if this is a common/well known issue and something treatable.

Otherwise the amp sounds good but I'd like to get it right all around before dropping money at an amp tech. Any help or feedback is appreciated. Thank you!
Q & A / Re: 1979 Sunn Concert Bass Schematic
« Last post by cosnerguitars on October 05, 2017, 02:19:16 pm »
Hey Loudthud,

Thanks so much for the schematic!  You saved the day!
Q & A / Re: Blowing fuses after re-wiring for a three conductor power cord.
« Last post by Stratmanx on October 05, 2017, 07:33:34 am »
Thanks Greg,

I'll go with that approach.

I did have all tubes removed when doing this, and did find that the Red / black and Red / yellow seemed to be the culprit, as with both of these removed and isolated, no fuse blown, but like you were saying both the solid reds were still connected, but with no tube rectifier in place.

I do appreciate all your help and the time you've taken to assist, very thankful for your knowledge. 
Q & A / Re: Blowing fuses after re-wiring for a three conductor power cord.
« Last post by Soundmasterg on October 05, 2017, 06:10:35 am »
Don't disconnect different wires willy nilly on the secondary and try to'll blow fuses. There is one primary on the transformer and many secondaries, some with center taps. If disconnecting wires, you should always disconnect or reconnect the whole secondary winding and center tap at a time. The two yellow wires on the secondary are the 5V filament wires for the rectifier tube. The schematic doesn't show the 6.3V filament wire color, but I am assuming they are green. There should be two of them, one for each end of the winding, and also a center tap, which is often green-yellow. There are also two red wires which are the wires that go to the rectifier tube. The center tap for that winding is the red-yellow, which should go to ground. You should leave that connected to ground if you have the two reds connected or the tube rectifier plugged in. If you disconnect it while those are plugged in the fuse will blow. The red-black is the bias tap and that should always be connected if the tubes are installed.

If you are testing to verify power transformer function, then 1)Make sure your primary wiring is correct as everything starts from there.  2)Unplug all tubes, and disconnect all secondary wires and leave them floating and not touching each other. Power the amp and if the line fuse does not blow, then the PT is ok. If the fuse does blow with all secondary wires disconnected, then the PT is likely bad. If the transformer works, then 3)Connect each secondary up correctly, one at a time, and test function. This will allow you to see if one secondary alone is bad. If fuse doesn't blow with all secondaries connected then PT is fine. 4) Add rectifier tube only and check function. If fuse blows then rectifier tube or wiring past that point are bad and need to be corrected. If ok, then add power tubes and repeat.

You will get continuity if some of the wires are connected to ground. Transformer windings have very low resistances and if a center tap is connected to ground then you will get continuity on that winding. The negative bias circuit is also very low resistance and you will get continuity if the center tap is connencted to ground too.

Tube amps have many little localized circuits that all work together but when you have a problem, you need to try to isolate where the problem is. By taking all of the tubes out, including the rectifier tube, then you are able to localize any potential problems to the PT and the power supply wiring. By disconnecting all of the secondary windings on the PT, then you are able to isolate to the primary winding and the PT itself. Once you verify that function, then if you connect one secondary winding at a time, you can verify function of each one, or isolate the problem winding. You need to take a systematic approach to it like I described instead of playing around with this wire or that wire to see what combinations affect things. You end up chasing your tail with that approach.

Q & A / Re: Blowing fuses after re-wiring for a three conductor power cord.
« Last post by Stratmanx on October 02, 2017, 05:23:21 pm »
Thanks for the help Greg !

As you said I doubled checked my wiring on the primary side, and did correct it from my first post so I'm good on the primary side.

Black to Fuse to switch connection 1

Switch connection 2 to one side of the primary.

The other side of the primary to the white wire.

What I did find is that there is an issue on the secondary. If I disconnect both the red / yellow that goes to ground and the red / black that goes to the diodes the fuse stays intact.

However if I lift the Red / yellow secondary wire that goes to ground, I hear the transformer hum for a bit and then it blows the fuse.

Also the 50uf 150 cap after the diode gets hot.

If I lift the red / black wire that goes to the diodes and keep the red / yellow that goes to ground connected, it immediately blows.

I did replace that capacitor with a brand new Sprague atom with the proper rating and voltage, and double checked the polarity with the original, so Id didn't get that reversed.

Old Cap

New Cap

I also replaced the multi cap can with an updated higher rated can, but checked and verified correct wiring for that.

I've also noted that if I check continuity from either of the red / yellow or  red / black wires to ground individualy  I get continuity.  That doesn't seem right.

Q & A / Re: Power transformer availibility for a 1968 Sentura I ?
« Last post by Soundmasterg on October 02, 2017, 08:40:22 am »
Triode electronics sells exact replacements, but before you assume the transformer is bad, fix your wiring as I mentioned in your other post. You've got it all wrong at the moment and it will blow fuses all day when wired like that.

Q & A / Re: Blowing fuses after re-wiring for a three conductor power cord.
« Last post by Soundmasterg on October 02, 2017, 08:39:10 am »
It is hard to see what is going on completely from the pics you posted. If you have it wired like on the drawing you posted, that would be incorrect. You only show essentially a single winding like a choke for your transformer. You also have the white wire going to the switch, but that won't work because the switch is a single pole switch and it will only switch one thing at a time. The white and black are separate and have to stay separate. White needs to attach directly to the other side of the transformer directly and not to the switch unless everything is wired like the Sunn was originally with fuse and switch on separate sides of the line. Take a look at the original Sunn schematic and you'll see that the primary side of the power transformer has one end of it going to one side of the AC line from the wall, and the other end of the transformer goes to the other side of the AC line from the wall. They put the switch on one side and the fuse on the other. You need to look closely at the wiring from an electrical perspective. Imagine each wire's electrical connections as if you are looking into the amp from the cord. The modern technically correct way is to have hot (black in US) go from the cord to the fuse (through the fuse) then to the switch, then to the transformer. Neutral (white in the US) is supposed to go from the cord direct to the other end of transformer primary directly. With the Sunn and trying to light the light on the power switch, it isn't that straightforward.

Where does the black wire from your cord go at the moment? That should go to the front connection on the fuse. Then there should be a wire going from the back end of the fuse to the switch. Then there should be a wire from there to one side of the transformer primary winding. You can just connect that side of the transformer winding to the same point on the switch as your black from the fuse. For the white wire from your power cord, it can't just go to the has to connect to the other side of the transformer winding somehow. Some people will use the center connection on the ground switch as a tie point to connect the white wire from the power cord and the transformer wire. Wiring it all that way won't light the light on the power switch however, but the amp will function.

The usual way to test the power transformer is to take the tubes out, disconnect all secondaries and make sure they are safely isolated from each other. Replace your fuse and then flip the power switch. If the fuse blows, you either have the wiring incorrect or the power transformer has failed. If it is fine in that situation, then you can reconnect one secondary at a time to see if one winding has an issue. However in order for that to work, your AC wiring has to be completely correct. Be careful with the AC wiring as if you get it incorrect, it can kill.

One thing you can try is to wire it up the way it was originally but minus the death cap and with green to chassis ground on its own bolt. It isn't as technically correct as having hot go to fuse then switch then transformer, and neutral to transformer, but you know it functions and the light will light. If it works that way it is ok to run it that way. When I first was getting started with tube amps I couldn't figure out how to get the light to light up and still having the wiring technically correct for modern sensibilities, so I just wired it up the original way, but with the addition of the green chassis ground. The amp is still working fine 20 years later and is perfectly safe.

I think your problem is that it is wired incorrectly. The power transformer is probably fine. Redraw the correct way to wire your amp before you un-solder and reconnect the wires. Draw it up in a proper schematic fashion for that section with the way the electrical connections should be, then from there, draw up a layout that has the same connections, then physically re-wire it.



Q & A / Re: Power transformer availibility for a 1968 Sentura I ?
« Last post by Stratmanx on October 02, 2017, 07:59:26 am »
Good point !   

Thanks for bringing that to my attention.

Q & A / Re: Power transformer availibility for a 1968 Sentura I ?
« Last post by Isaac on October 02, 2017, 07:22:26 am »
Don't confuse the power transformer with the output transformer. Only the output transformer is ultralinear.
Q & A / Power transformer availibility for a 1968 Sentura I ?
« Last post by Stratmanx on October 02, 2017, 06:30:02 am »
If in the chance my power transformer is blown, is there a Mercury Magnetics, or other transformer supplier that has a substitute for this ?

I know that Sunn stuff is the ultra linear, so there may not be an exact replacement ?

Thanks !
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