B+ fuse blown on VTL ST-150

Maritan · « **on:** 9 Apr 2013, 04:16 pm »

Hello all,

I was really enjoying some music last night, playing some Steven Wilson, Pink Floyd (and SteveFord, if you're reading this, Alan's psychedelic breakfast was quite the trip. My system DOES sound good), and then on to some live Porcupine Tree.

Well, I stepped away from the listening area for just a minute and I heard something odd (that I can't describe now because I wasn't paying that much attention). When I went back to check, the right channel was quiet. I switched everything off and I pulled the B+ fuse and sure enough it was bad.

1. I assume a B+ fuse blows when an output tube (SED 6550C in this case) goes kaput?
2. Where do I buy a B+ fuse for the VTL ST-150 (older model)? Do I have to buy it from VTL or can I go somewhere else?
3. If I find the bad tube, once I replace that and the B+ fuse everything should work well again, correct?

I won't be taking any chances, obviously. I already checked the bias (cathode) resistors and they're all within spec. I'll also be checking the screen resistors. I'm planning on taking the bottom panel off and I will visually check of all the components for any obvious burn marks/ melted insulation etc. If things don't measure nonsensical values, I will try to do an in-circuit check of various components between the good (left) and bad (right) side channels to compare and ensure things are okay.

4. Any other tips/ tricks that folks are willing to share so I can minimize the chances of this happening again?

medium jim · « **Reply #1 on:** 9 Apr 2013, 04:45 pm »

Not sure the problem, probably a tube that was old and arched. I'm not sure about the protection circuit, but typically you will need to replace a bias resistor as well as the bad tube. Check with VTL as they may use a fast blow fuse that might have saved the bias resistor and or other parts. I urge you to call VTL or maybe Steve Ford who is a long time VTL owner before doing anything. That said, Pink Floyd's Atom Heart Mother is one of their best!

Jim

Maritan · « **Reply #2 on:** 9 Apr 2013, 06:05 pm »

Jim - I don't know if you remember, but this was the amp that arrived with shattered tubes. The tubes in the amp are all completely new from www.tubesandmore.com. Thanks for your input though. Now, it makes even more sense to visually look at the bias resistor to make sure it is ok.

I'll call them in a little bit and see if they will test the tubes to find a bad one. And you're right - I'm hoping VTL owners (and definitely Steve Ford) will chime in and help out a little here. I'm hesitant to call VTL because their customer service is supposed to be absolutely awful (and I was warned by Steve himself about this).

And yes, Atom Heart Mother is an awesome album. I hadn't listened to it in a long time and I'm glad I got around to it.

SteveFord · « **Reply #3 on:** 10 Apr 2013, 12:21 am »

That's the type of fuse which was used in old microwave ovens - a local tube shop should be able to help you with that one.
If you don't have a copy of the VTL Book, let me know and I'll make a copy for you. It's the closest thing you'll ever get to a service manual for them.
You should be able to snag one on eBay if you want an original.
I only had a fuse blow once in 10(?) years and that was when a tube committed Hari Kari and it took out a resistor. It sounded like a giant was banging on the front door with a telephone pole but it was only a 3.6 going berserk.
BAM! BAM! BAM!!!! which will get your attention.

Look for cold solder joints after all this time, that could be your culprit right there.
DO NOT ELECTROCUTE YOURSELF!

Electrical components usually fail early on or last a long time without problem so it could be a new tube that wasn't up to snuff. If it popped the fuse, I would replace that resistor.
Check the bias pots, those things are junky little Radio Shack grade hunksa crap.

Ericus Rex · « **Reply #4 on:** 10 Apr 2013, 01:19 am »

Check with whoever sold you the tubes, there might be a 30 day (or more!) warranty on them.

medium jim · « **Reply #5 on:** 10 Apr 2013, 01:32 am »

Martian:

Sorry that it happened. It could be a bad tube, but maybe not. As Steve notes, cold solder joints are a reality in older amps. That said, new tubes that test good can go bad when burning in. It is rare, but does happen. It happened to me when I retubed my amp, but happened within 30 seconds.

I hope you get it up and running sooner than later!

Jim

FullRangeMan · « **Reply #6 on:** 10 Apr 2013, 01:54 am »

Seems this amp use PCBs, a new one will solve the prob or re-solder.

SteveFord · « **Reply #7 on:** 10 Apr 2013, 02:10 am »

Don't forget that amp took a monster shot in shipping, it could be something really stupid related to that.

Maritan · « **Reply #8 on:** 10 Apr 2013, 03:14 am »

All of you have made some great suggestions.

www.tubesandmore.com has a 90 day warranty on new tubes. Question is how are they going to handle this? I got very busy at work today and didn't have a chance to call them. I'm almost positive one of the tubes went out and took the fuse with it. I'm going to take all four of the 6550s on that channel to get tested.

I will be checking for cold solder joints, and any glaring issues etc. over the next few days. I'll keep you all posted on my findings.

Steve - the sound I heard definitely made me look up, but what I heard wasn't the Jolly Green Giant banging at my door with his trusty green bean. I do have the original manual that came with it, so I'll check to see if it has anything close to a schematic. If not, I'll let you know.

Thanks everybody.

Maritan · « **Reply #9 on:** 10 Apr 2013, 06:41 pm »

Quick update. One of the tubes is definitely bad. I can just look at what used to be the chrome top and see it. I called www.tubesandmore.com and they are replaced the entire quad of 6550Cs from the right side channel. Awesome! I'll have the brand new tubes in hand either today or tomorrow.

I called around locally but couldn't find the Limitron KTK-1 fast blow fuses that went out on the amp. So, I've ordered 3 just to have a couple lying around as spares. They won't get here till next week.

So, in the down time that I have, I'm going to start looking over the amp and make sure the resistors, capacitors etc. look good.

The bias resistor for the blown tube measured ok... So, I'm wondering if I should take the chance and not replace the resistor especially since the fuse was a fast blow. Thoughts?

SteveFord · « **Reply #10 on:** 10 Apr 2013, 09:51 pm »

Can you put your hands on the resistor easily enough? If so, replace it.
Check the bias pot for that tube, those things are untrustworthy little perverts.

Maritan · « **Reply #11 on:** 10 Apr 2013, 11:40 pm »

Good point. I'll be opening up the amp today or tomorrow to check. Will keep you folks posted. Anybody want to see pictures? If so, I'll take terribly blurry ones to document the process.

Do you have any info on the bias pots by the way? Ratings? Values? Makes?

I don't mind changing out the pots, but that might be more labour intensive than I'm willing to tackle at this point - just because the pots are accessible from the top side - which means I'll have to take the PCB out of the amp to exchange those - which in turn means that I will have to desolder and solder back a ton of wires...

And once I've done that - what's to stop me from changing out the capacitors? This could get expensive really quick.

SteveFord · « **Reply #12 on:** 10 Apr 2013, 11:59 pm »

Negatore on the bias pot, it must be an internal VTL number that I have.
434-0157-103 if that's any help. That number is off of an invoice from my MB250s so that may not be the same deal for yours.
Give VTL a buzz and tell them you need 8 for your amp for backups just in case.
Yes, it is a chore to replace those goofy things so maybe it's just a duff tube. You'll know shortly after you stick new ones in there.
Great tube vendor, by the way!

You might be able to take a bias pot to your tube vendor there and see if they can't match it up with a heavier duty piece if it turns out that's the culprit. It's just a little plastic square job and the heat must get to them over time.

Maritan · « **Reply #13 on:** 11 Apr 2013, 04:14 am »

Ok, so I was able to take the bottom off and look around a little bit. All components look in great condition save for a little dust here and there.

It looks like a fairly intricate and delicate job to take the PCB off enough to get to the pots, and the rest of the top side of the PCB. I didn't want to tackle that in the evening with artificial light. I'll wait for this weekend so I'll have daylight also to light up my "work area" well.

The pots, by the way, are Bourns 3386 series pots. Value is a little trickier because they're in circuit and cannot be easily removed to check the value. Since I don't have a schematic, I'm not sure what all it's connected to, but it measures 7.2k ohms end to end. I checked Bourns, but they don't seem to have that value.

More updates to follow.

Maritan · « **Reply #14 on:** 11 Apr 2013, 04:30 pm »

One more thing. The resistors for bias and screen are 10 ohm 5W and 1K 5W Ohmite wirewound 20 series resistors (25J10R and 25J1K0) respectively. Both are axial with through hole soldering. Soldering looks good, but I might be able to change the resistors out without having to remove the PCB from the case...

Hmmm... decisions to make.

Ericus Rex · « **Reply #15 on:** 11 Apr 2013, 11:47 pm »

So did the silver 'getter' go white in your bad tube?

Maritan · « **Reply #16 on:** 12 Apr 2013, 12:21 am »

Here's a pic of it:

I know the picture isn't the best, but not exactly white, but some of the chrome top (which I assume is the getter) is now missing, and a large part of what's remaining is blackish.

Does the chrome top turning white indicate anything specific?

SteveFord · « **Reply #17 on:** 12 Apr 2013, 12:46 am »

I don't know about white but I have it's twin in a drawer upstairs.
That one really lit up a dark room for a few moments due to the bias pot giving up the ghost.

Ericus Rex · « **Reply #18 on:** 12 Apr 2013, 12:55 am »

The getter turning white means a new tube has lost its vacuum. Not exactly sure what a darkened getter means.

Maritan · « **Reply #19 on:** 12 Apr 2013, 05:49 pm »

Alright, Steve, you've scared me enough.

I poked around the amp a little more last evening and I think I know how to get to the trimpots. As I mentioned earlier they are Bourns 3386 series pots. I think they're the "P" version based on the pin spacing/ configuration, but I didn't take pictures of the layout of the pots specifically so I can't confirm it right now.

The 3386 are cermet, sealed, single turn (270 degrees) pots that provide bias current to the 6550C. What we measure is the voltage drop across the bias resistor (10 ohms +/-10%) and adjust the trimpot to get close to 275mV.

With the single turn pots, your adjustment range is pretty compressed. I found the 3339 series of trimpots which are 4 turn pots and have the same pin spacing and adjustment screw on top as the 3386 series. They are identical electrically and power ratings-wise. With 4 turns from min to max, that should give me a lot finer control of where I bias the tubes.

The catch? Two actually. 1. Each 3339 pot is $4 more than the 3386 pot. 2. The 3339s are ever so slightly taller - 4.83mm vs 6.35mm. I think there's enough clearance in the amp though.

Once I check the pin spacing of the current pot on the PCB, I'll know what I will be ordering soon.