Who's afraid of the Big, Bad SVT? 'Cause I Need Help!

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9/18/1999 9:45 PM

Rob

Who's afraid of the Big, Bad SVT? 'Cause I Need Help! I'm working on an SVT right now that was blowing fuses because one of the filter caps shorted. I replaced it and it's fine now, but it still pops intermittently, which is what it was doing before it started blowing fuses. Do I need to replace all of the filter caps and tubes before I can be sure of anything? Has someone had similar experiences? I don't think a power tube or socket is arcing, but I can hear a click in the chassis at the same time as the pop in the speaker. It does it without the phase inverter tube in, but it sounds a little different when I do that. When you try to play through it, the popping sucks the signal out for a half second or so. I noticed it will only start popping when it gets hot, so I can't get it to pop when the chassis is out of the cabinet (I thought I might be able to see some arcing.). It pops more when I try to play through it. I checked a whole hell of a lot of other components, and they all seemed fine. What components should I replace just to be on the safe side?

 

9/18/1999 10:25 PM
Ken Gilbert	Replace the tubes. It sounds like one is arcing inside, and upsetting the equilibrium of the amp momentarily.         After all, they're really the easiest thing to replace. You just pop 'em in.         Check those direct coupled 12BH7 cathode followers. Their plate supply is shared by the screens of the outputs.         KG

 

9/18/1999 10:42 PM
Don Symes	I agree with Ken, but think it would be worth a careful check of the power tube sockets and the post-PI driver(s) if any. Use a high-intensity light source and a good magnifier.         Luck man,     Don

 

9/19/1999 1:53 AM
jb	Re: Who's afraid of the Big, Bad SVT? 'Cause I Need Help! Good advise so far. Also check the board real good for cold solder joints. These were very well built amps but old solder does funny things. Keep us posted on what you find. Good luck,     jb

 

9/19/1999 4:47 AM

Marc

Re: Who's afraid of the Big, Bad SVT? 'Cause I Need Help! Rob: Here's advice I got from Bruce along those same lines...         First thing: replace ALL the first filter E caps with what ever will     give you at least 50uF at 900v to 1000v!     More uF is OK to a point.     I use two series 100uF/500v caps for that section. No sweat.     LCRs have a common ground (tricky if it's a multicap)...but they     make singular 500v caps too.     Go over and make up what you need in uF/Vs to replace the other     awfull E-caps they used in there!     Then flip the thing over and remove ALL the 10 ohm and 22 ohm     power resistors on the plates and the screens (current limiting     resistors).     Replace what ever is there with LKQ (like kind & quality)... some     amps were different but they all get majorly stressed when the     power tubes blow up (which is regular when not set up right)...     replace them all with known good ones and don't cheat.     Check the sockets of the power tubes to see if there has ever been     a flash over on any. Put new in if so. Rivets YUK!     You might want to check the 1ohm cathode resistors and make     sure they are still really 1ohm.     The balance and bias adjust relies on them being pretty close to     right on.     Don't pull too much current at idle through the tubes.     The probes measure mv of DC through those 1 ohm resistors so     your DVM should be set to read DCmv.     Milivolts measured here will mean miliamps now.     About 60ma to 70ma for three (one side) is close to right at this     VERY high plate voltage.     Use the other pot to balance the two halves as close to the 60ma to     70ma as you can.     CAREFULLY! Don't load this amp lopsided with too much     current.     Pay attention to this amp and respect it's power because the power     supply is capable of some serious voltage at high current and can     damage or stop your heart if you even get mildly poked by it.     If you are new to this kind of DC power, have somebody nearby     incase you have an accident.     I've NEVER worked on an SVT that didn't pump out at least     280watts (or more) of pure tube horsepower!!     KT88s are very cool in these too.

 

9/19/1999 6:32 PM

Psycho Bass Guy

You are experiencing the joys of its protection circuitry. There is probably one bad tube as Ken pointed out. Pull the tubes, replace all the power tubes and try new drivers as well. Go back through the amp with the new tubes and measure the current draw and make sure that a socket hasn't been shorted due to a faulty tube. Watch for any kind of output distorion on the scope as this will give a clue as to where to look for damage. Test the signal at all the different stages after the preamp, too. The voltages in them boogers do wierd things to them sometimes. The shorted filter cap was a symptom and not a cause. Remember, an SVT will continue to operate with an unbalanced power section but it's very hard on the amp in the long term. Look for burn marks very carefully on the bottoms of all the tubes you pull. The plastic base of most 6550's will scar quicker than the socket itself. This also will give you a clue as to where to look in the power section. Oh and, there is a reason it's called the Big Bad SVT. You don't get to make a mistake, so be very careful. The most dangerous SVT of all is one that works, but has some sort of problem in the output stage.         PBG

 

9/20/1999 12:59 PM

Graywater

Rob,         I suspect that the SVT was made by Magnavox in TN - if so, it probably has lotsa coupling/shunt caps in a right-angles lead axial package with stripes. I spent a few weeks exorcising evil corporate spirits from a V-4 this summer and almost all of the weird symptoms (too numerous to list) turned out to be bad coupling caps. While I'm not linking these components to your current woes. I would consider checking the appropriate ones for a high resistance short.         GW

 

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