100s Help

[mattelliott2]

Hi all,

Joined the forum because I recently received a Sunn 100s Head and I'm having a bit of trouble with it.

I turn it on and it sounds awesome, until about a minute and a half after I turn it completely on. Then, it loses volume quickly and goes silent.

Replaced the Filter caps when I got it. What's yall's diagnosis?

-Matt

[EdBass]

It's pretty hard to diagnose via an internet forum, and the only proper answer would be to advise taking it to a qualified tech, but...

My first guess would be the filter caps, but those are new. Did you upgrade from the stock can cap?

Rectifier tube going bad?
Intermittent issue with the standby switch?
Bias supply issue, one or both of the 50uf 150V caps going south?

Keep in mind; I am NOT an Electrical Engineer, I just play one on the internet.

[Isaac]

I'm thinking a tube. Rectifier, preamp or splitter/inverter. That's the GZ34, 12AX7 and 6AN8 tubes, respectively.

[loudthud]

I'd be looking for a bad solder joint that occured when the cap was replaced. Basic troubleshooting with a DVM checking for B+ in the preamp would confirm or eliminate a problem in that area.

NOTE: This is a dangerous procedure and should only be performed by trained persons.

[EdBass]

Regretfully mattelliott2 hasn't checked back in since a few minutes after posting the question so we may never find out what it actually was causing the issue.
I was pretty curious, hopefully it wasn't just a "drive-by posting".

[mattelliott2]

Nope, not drive-by posting. Just got caught up in some other stuff.

Ordered new set of tubes just to be safe. I know the power tubes are good, I saw them come out of the package. But the preamp and rectifier tubes look older than all get out. On that note: I had no idea Peavey had tubes lol.

Local guitar place had the preamps, but no rectifier, so I just ordered a full set.

Pretty confident that this will be the fix, but you can never tell for sure. What are some of the reasons that the tubes would behave this way?

[Soundmasterg]

Old GZ34 rectifier tubes are in almost all cases going to be better quality than new ones, AND when you change a rectifier tube, and when you change power tubes, you should rebias the amp, so keep that in mind.

If you had a scope it would make it much easier to troubleshoot as you could follow the signal with your scope probe as it goes through the amp and see where it is going. you can make up something that will sort of work for the same purpose....rig up a 600v cap on a chopstick and leave the bottom lead hanging. Tape the body of the cap to the chopstick, then solder a wire to the other lead and then wrap tape over the whole connection. Use electrical tape please. Then take that other wire and connect it to a speaker, and connect the other lead on the speaker to ground. You should be able to hear the signal going through the amp. Then you can have someone else play, or run some music through the input of the amp, and follow the signal through the amp by touching the bottom lead of the cap to the input grid, then the plate, etc. Under no circumstances should you touch the leads of the cap, or anything else inside the amp while it is running as the high voltage is very dangerous, and if you are not sure what you are doing, take it to a proper tech to debug.

I don't think it is the tubes myself. If there is a problem in the circuit somewhere, say with the bias supply, then that would cause this problem, but you would also get a loud hum and your power tubes would begin to red plate. After that happened, then the tubes would be bad very quickly. My first guess would also be filter caps, but you said you replaced them.....did you replace all of them? Including the electrolytics in the preamp? There could also be a leaking coupling cap somewhere in the circuit. Its hard to theorize over the internet, but there are a number of things to check. Posting voltages at all the relevant tube sockets would be helpful. When checking the heater voltages, check for AC from each side of the heater winding....it should be at least 6.3v. The other socket pins should be checked from the pin to ground, but please be careful.....only put one hand in the chassis at a time with your meter probes, and clip the black one to ground before you turn it on.

Greg

[mattelliott2]

Greg,

Thanks for the informative post! I regret to say that I have not changed all of the filter caps. I thought that only the cap can needed to be replaced. As far as all of the other caps, why would they work for about a minute then fade out, I would think that they would just not work at all?

Regardless, it looks like I will have a bit more work in front of me than I thought. But, harder things have been done in the name of tone.

[EdBass]

Greg,

Thanks for the informative post! I regret to say that I have not changed all of the filter caps. I thought that only the cap can needed to be replaced. As far as all of the other caps, why would they work for about a minute then fade out, I would think that they would just not work at all?

Regardless, it looks like I will have a bit more work in front of me than I thought. But, harder things have been done in the name of tone.

I would certainly start there. Those big old electrolytics do odd things when they get old, from slowly fading out when they get warm to literally blowing up at start up. One thing is almost for certain though; they have a finite lifespan. Old caps, particularly electrolytics, should generally be replaced. In an amp as old as yours, it's nearly guaranteed that all the filter caps need replacement, and that could well be your issue.
I use the terms "almost" and "nearly", because every once in a while old caps stay good. I recently retired a '68 200S, and the original can cap still tests as new. That is an aberration though, and even though it's a great sounding 200S the reason I stopped using the amp is because I figure it has to be on borrowed time; probably 20+ years of borrowed time!

Good Luck, and keep us posted!

[Soundmasterg]

Greg,

Thanks for the informative post! I regret to say that I have not changed all of the filter caps. I thought that only the cap can needed to be replaced. As far as all of the other caps, why would they work for about a minute then fade out, I would think that they would just not work at all?

Regardless, it looks like I will have a bit more work in front of me than I thought. But, harder things have been done in the name of tone.

Yeah, no problem! First thing then is to change every single electrolytic cap in the amp. After that is done, see what happens. The coupling caps I try to not change unless they leak DC voltage to following stages. There is a way to test that that works well, but again you are working in a live chassis so be very careful. The way to do this is to unsolder and lift the lower voltage end of the cap...so if one end is conencted to a palte and the other to a grid, you unsolder and lift the grid end. The put your meter between that lifted end of the cap and ground and turn the amp on. The voltage will spike up to a level, and then gradually move its way down to some lower level and will stabilize there.....this takes about 30 seconds and whatever level the cap stabilizes at will be the DC leakage. A modern cap may have .001v leakage when hooked up to 300 V on the other end of it, whereas some of the old paper caps that are in some of the Sunns can be as much as a couple volts! If they are, that throws the bias off on a following stage and causes it to work and then slowly fade out, or to get all distorted. So this could be the issues, but maybe not...the only way to know for sure is to check every single coupling cap, or to use a scope or the method I noted above to track down where the signal is actually disappearing and troubleshoot only that stage.

Greg