Amp Issues

[Isaac]

I've been playing with my Sunns for years, and never had a problem with any of them until recently.

Playing bass for Reefer Madness, my Model T developed a nois problem, but I took care of that by replacing a pair of capacitors.

My 200S has started blowing fuses. I modified it several months ago, using the SDS Labs Mk3/Mk2 Capacitor Board (http://www.triodeel.com/dynaco.html) and two 1N4007 diodes replacing the rectifier tube. It worked fine for some time, but now blows the fuse after a relatively short time, varying from a few minutes to a few hours.

I was using my Sentura II as a replacement, until the heaters all went out in the tubes. Power light is on, but no glow in the tubes.

I opened up the 200S and made a few changes, taking the old can cap out of the circuit and removing a resistor to ground I'd installed. I figured that the old cap might be leaking DC, and putting the coil after the standby switch made the resistor unnecessary. I thought that would take care of the problem, but no such luck. It worked fine for one rehearsal, but died in the middle of the first song at the next.

I haven't done any troubleshooting since, but, if anyone has any thoughts, I'm open to suggestions. I fear I'm looking at a new power transformer for the Sentura.

[Soundmasterg]

On the 200S, does it blow the fuse if you power it on with all the tubes out? If so, then disconnect all secondary leads on the power transformer, make sure they aren't touching anything or each other, then power it on. If it still blows fuses, the power transformer is bad.

If it doesn't blow a fuse with the tubes out, then a tube is bad, or something related to the circuitry surrounding the tubes is bad. You may also replace the 1N4007 diodes since they are cheap, and bad ones could be the cause of the problem.

For the Sentura II, pull the tubes and check the heater string for voltage on each leg. To ground each leg should be about half the 6.3v, and if you check from one leg to the other instead of ground, it should be around 6.3v. If you have voltage at the first stage in the string after the power trans taps, then follow the string down the line to see if there is a break somewhere. If your heater tap is dead, it would be cheaper and easier to just add a filament transformer. You can get one pretty cheaply from AES. Just add up the heater current requirements for the tubes in the amp, and add some fudge factor of a couple ma. The primaries get hooked up to the same place as the existing power trans, and the secondaries get hooked up in place of the existing power transformer's heater taps.

Greg

[Isaac]

Thanks, Greg. I probably won't get to any of this until next week. But this, especially, was what I needed to hear:
"If your heater tap is dead, it would be cheaper and easier to just add a filament transformer. You can get one pretty cheaply from AES."

[Soundmasterg]

Glad to help Isaac. If you get to the testing, or need some help, let me know the results. Feel free to call. If you need my number send me a PM.

I've used an AES filament transformer on my 3 in 1 project amp. (Remember that one?) It was under $20 and works just fine....just make sure it gets hooked up right. It wouldn't be vintage correct to drill some holes to mount it, but it sure is a lot cheaper than getting another transformer.

greg

[Isaac]

The 200S has been powered on without tubes for 15 hours, so the transformer seems to be okay. I hit the standby switch a couple of hours ago, and so far, so good. I got to the Sentura II this morning, and I think there's nothing wrong with it at all. Fuse was blown, and someone had wired it such that the power lamp was before the fuse, not after, which is why the light was still on. One of the power tubes had cracked and burned out, which I imagine caused the fuse to blow.

But here's another thing I learned: I noticed that the Sentura II had a slo-blo fuse, so I checked the schematic to make sure that was correct. It is, so I checked the 200S schematic, and it calls for a slo-blo fuse as well. Maybe that's my whole problem with the 200S. Off to buy 3 amp slo-blo fuses to power up the Sentura.

[Soundmasterg]

You should always use a slo-blow fuse in the AC line position because otherwise, fluctuations in the AC line will cause a fast-blo fuse to blow out. That could be the problem.....since the amp has been running ok without tubes for quite awhile, the power transformer is good. Still could be a bad tube...you'd have to test it with the proper fuse in place and the tubes in and see if the fuse blows again. If so, then you may have a bad power tube.

greg

[Isaac]

Moved the power on light wire on the Sentura, and put in an old set of power tubes I took out of the 200S when I retubed it. No light with no fuse - how could there be? - powered up with new fuse and played fine.

Put the tubes back in the 200S and replaced the fast acting fuse with a slo-blo. So far, so good...

[cromag]

the feeling you get AFTER everything is a simple fix is fantastic, huh?   but before the fix, when everything breaks, is pure misery!

[Isaac]

Yes, the feeling after a fix is always good. In fact, I find that the harder the fix, the better the feeling.

The 200S was fine through rehearsal last night, but it's done that before. I expect I got it, though.

[Soundmasterg]

Hope so!

Greg

[Isaac]

Used the Sceptre at a rehearsal last night. So far, so good!

[Isaac]

200S made it through another rehearsal. I'm starting to feel confident.

[Soundmasterg]

I'm starting to feel confident.

Better knock on wood Isaac. 

Greg

[Isaac]

That makes my head hurt!