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ChetJettison
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    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

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    • Post n°1

    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by ChetJettison Sun Feb 25, 2024 3:20 pm

    A few days ago, while running my ST-120, I heard a commotion from the speakers, then silence. Blown fuse on the amp. Replaced the fuse, turned it on again and heard strange noises, popping, scratching... alarming sounds. Switched it off right away.
    Since I was in another room when the fuse blew, I didn't see if there was any arcing in the tubes. But they look OK, getters are still silver.

    Opened the amp up just now to take a look inside. Everything looks OK, save for the quad cap resistor. A bit of the ceramic housing is gone from one of the legs. Possibly blown?

    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Img_0711
    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Img_0710

    What do you all think, does that look blown? Is there a way for me to test it in situ?

    If so, what does that indicate? Especially when considering the blown fuse and all. Bad Quad Cap?

    If it helps narrow things down any, I was running Sovtek 6550 tubes, and a vintage Mullard GZ37 rectifier tube. I've got the auto bias board installed. I learned just yesterday that my mains voltage is a bit high at 123v, so I bought a variac to set it down to 117 per Bob's advice in another thread.

    I'm looking forward to getting this amp up and running again, so I eagerly await and much appreciate any advice and insight from the community here.
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    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty Re: VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by ChetJettison Tue Feb 27, 2024 7:51 pm

    Just bumping in case this got lost in the millieu.
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    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty Re: VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by Rec Wed Feb 28, 2024 9:37 am

    You can measure in situ. discharge the quad cap first. These can get hot under stress and I have replaced with a 5W resistor. You do want to ensure the minimum correct rating on this part.
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    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty Re: VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by ChetJettison Thu Feb 29, 2024 2:40 pm

    Thanks for the reply! The only resistors I can find at that value are 5%, will those work? Or do I need to special order some 1%
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    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty Re: VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by Rec Thu Feb 29, 2024 2:45 pm

    That's up to you. Considering variation in your line voltage, I think you'll be fine at 5% tolerance!

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    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty Re: VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by ChetJettison Thu Feb 29, 2024 7:46 pm

    OK sounds good. I think I'll put the 5% in, just for testing purposes, and then install the 1% when they arrive, assuming everything else is working alright. Thanks for the help
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    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty Re: VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by ChetJettison Fri Mar 01, 2024 8:40 pm

    Replacing that resistor doesn't seem to have solved the problem. I'm going to try swapping some tubes around to see if that changes anything. But I'm still getting loud popping/crackling shortly after turning the amp on. It was working so well for such a long time! What could have gone wrong!? What are the symptoms of a bad quad cap? Anything else I should look for?

    Put in some different power tubes, and checked the bias during warm up. Doesn't seem to biasing properly, even with the Auto Bias Board. One tube was hovering around 65mv, and starting to glow an eerie shade of blue after just about 5 seconds of the amp being on. I immediately switched it off for fear of causing damage to anything.

    I thought, perhaps my rectifier tube was bad? So I swapped in a different one, the only other one I had. I'm not sure if it was a good one or not... Things seemed OK at first. Tube bias was holding steady for a few seconds, music was playing... but after that, there was a bright flash from the amp and loud pop from the speakers! It scared me so much I jumped out of my skin! In the commotion, I couldn't tell where exactly the flash came from.

    I'll be honest, I'm flummoxed. I thought I had worked out all the kinks with this amp, and everything was going so well for ages. It's discouraging to encounter such a catastrophic series of failures. In my moment of despair, I'm torn between cutting my losses and ditching this amp, or taking it to a professional. If there's anybody out there in Dynaco Land who can help me figure this one out, it'll pull me out of my hole of Tube Amp Sadness.
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    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty Re: VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by Rec Sat Mar 02, 2024 10:42 am

    https://dynacotubeaudio.forumotion.com/t274-quick-troubleshooting-guide-for-dynaco-tube-amps

    This may be a good place to start. You can verify quad cap with a multimeter (one that has capacitance measure or watch resistance grow at the meter charges cap). Web search.

    Verify the voltages carefully on the power supply. This will rule out this area of the amp. Check all solder joints even on VTA board. Verify 10ohm resistors.

    This is a good a guide as I’ve seen. Good luck!


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    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty Re: VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by ChetJettison Sat Mar 02, 2024 12:18 pm

    Thanks for passing that along. One of the symptoms in that guide does seem to match what’s happening to my amp. I’ll perform the corresponding test.
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    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty Re: VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by ChetJettison Sat Mar 02, 2024 2:42 pm

    After looking at the guide again, it doesn't seem to actually explain HOW to perform the test. So I'm wondering, how do I test the quad cap to see if it has gone bad?
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    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty Re: VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by Rec Sat Mar 02, 2024 3:20 pm

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    • Post n°12

    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty Re: VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by Bob Latino Sat Mar 02, 2024 6:08 pm

    ChetJettison wrote:After looking at the guide again, it doesn't seem to actually explain HOW to perform the test. So I'm wondering, how do I test the quad cap to see if it has gone bad?

    The best way to test the quad cap is to remove all the wires/resistors that connect to the quad cap. Make a diagram on paper where the wires/resistors go so you can get the wires/resistors back where they belong after testing. Get out your capacitor tester. (Note - Some multitesters have a capacitance range built in but most do not). Set the capacitance range on the 0 to 200 Microfarad range. Connect the ground wire on the cap tester to any place on the chassis that is grounded. I would use the main grounding point just to the left of the quad cap. Connect the red wire on your cap tester to each of the four tabs on the quad cap one at a time. The tab the faces to the left towards the rectifier socket should test about 40 uF, the tab facing the rear of the amp should test about 80 uF, the tab facing the right side of the amp should test about 20 uF and the tab facing the driver board should test about 30 uF. See photo below ..

    Note - The reading that you get should be + or - 10 % of the actual value.

    Bob

    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? VTA-Quad-cap-setup-sequence

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    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty Re: VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by ChetJettison Sat Mar 02, 2024 7:03 pm

    A reply from the man himself! Thanks for that info, Bob. I did the "Resistance" check you mentioned in another thread, measuring the resistance of each leg of the quad cap. Each one was relatively stable around 2.5 MΩ.
    Desoldering all of those wires, especially in the "half moon" section of the quad cap, is a little daunting. But I'm willing to give it a try, since I'm at my wits end with this amp.
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    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty Re: VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by Bob Latino Sat Mar 02, 2024 8:26 pm

    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? VTA-Quad-cap-setup-sequence

    One minor correction on the quad cap photo above ... The resistor on a VTA ST-120 on the quad cap should be a 4.7K (4700 ohm). What I posted was a quad cap arrangement for a VTA ST-70 that uses a 2.2K (2200 ohm) resistor.

    Bob
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    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty Re: VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by pavlikkkk Thu Mar 14, 2024 3:30 am

    ChetJettison wrote:Thanks for the reply! The only resistors I can find at that value are 5%, will those work? Or do I need to special order some 1%

    Also check if the AB is damaged by shorting any of the 6550 tubes.
    Unplug the 6550 from its sockets and measure the resistance of pin 8 (Cath) to ground with an ohmmeter. It should be 10 Ohms on all four AB pins.

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    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty Re: VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by Wotan Thu Mar 14, 2024 4:28 pm

    ChetJettison wrote:A few days ago, while running my ST-120, I heard a commotion from the speakers, then silence. Blown fuse on the amp. Replaced the fuse, turned it on again and heard strange noises, popping, scratching... alarming sounds. Switched it off right away.
    Since I was in another room when the fuse blew, I didn't see if there was any arcing in the tubes. But they look OK, getters are still silver.

    Opened the amp up just now to take a look inside. Everything looks OK, save for the quad cap resistor. A bit of the ceramic housing is gone from one of the legs. Possibly blown?

    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Img_0711
    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Img_0710

    What do you all think, does that look blown? Is there a way for me to test it in situ?

    If so, what does that indicate? Especially when considering the blown fuse and all. Bad Quad Cap?

    If it helps narrow things down any, I was running Sovtek 6550 tubes, and a vintage Mullard GZ37 rectifier tube. I've got the auto bias board installed. I learned just yesterday that my mains voltage is a bit high at 123v, so I bought a variac to set it down to 117 per Bob's advice in another thread.

    I'm looking forward to getting this amp up and running again, so I eagerly await and much appreciate any advice and insight from the community here.

    I can tell you (from experience) if you were running the amp at higher than normal line voltage the cap may be breaking down at voltage (even if it looks good using the ohm meter tests Bob outlined). Before you unsolder everything (a PITA, I know, but you'll have to anyway if it needs replacing) try ramping up your voltage gradually with your Variac. You may find, starting out where the tubes just barely light up, the amp runs sort of normally. You'll have to replace the quad cap, but that would be a good indication that is where your problem is. Also, is there a way to disable your auto-bias board? If you start off with your power tubes biased at or near cut-off and the fuse doesn't blow, that's a data point. That plus no red plating probably means your tubes are OK. Whereas if you see fireworks inside a tube before the fuse blows, it's that tube. You're more likely to see it before the fuse blows with the voltage reduced. A Variac can be a valuable troubleshooting aid, especially with tube gear.
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    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty Re: VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by ChetJettison Thu Mar 14, 2024 4:41 pm

    Thanks everyone, for the replies! In the end, the trouble seems to be a blown rectifier. Which is a shame, as I was using a NOS mullard GZ37, and those puppies aren't cheap!
    Prior to this incident, I was running power straight from the wall, which at my house is pushing 123v. I've since acquired a bucking transformer that outputs around 114-115v. I'm not expert enough to say, but I suspect that will be a bit gentler on the amp, and perhaps prevent future failures.
    My backup rectifier is just a little Sovtek 5ar4, but I have another Mullard GZ37 that I'll swap in after a while. I want to get a couple dozen hours on the amp, just to prove it's running steady, before I risk another $200 rectumfrier tube.
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    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty Re: VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by HDroller Thu Mar 14, 2024 5:33 pm

    ChetJettison wrote:Thanks everyone, for the replies! In the end, the trouble seems to be a blown rectifier. Which is a shame, as I was using a NOS mullard GZ37, and those puppies aren't cheap!
    Prior to this incident, I was running power straight from the wall, which at my house is pushing 123v. I've since acquired a bucking transformer that outputs around 114-115v. I'm not expert enough to say, but I suspect that will be a bit gentler on the amp, and perhaps prevent future failures.
    My backup rectifier is just a little Sovtek 5ar4, but I have another Mullard GZ37 that I'll swap in after a while. I want to get a couple dozen hours on the amp, just to prove it's running steady, before I risk another $200 rectumfrier tube.

    If it turns out to be a blown rectifier tube you may want to consider a rectifier with greater current capacity. IMHO, a single GZ37 barely meets the current demand of the ST120. I had a couple of flash overs with mine, so I've gone to SS rectifier, choosing rectifiers that can easily handle maximum current draw the ST. Tubes 4 HiFi does offer the dual tube rectifier option--an admission of how little headroom there is IMO. Best of luck!
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    • Post n°19

    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty Re: VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by Rec Thu Mar 14, 2024 6:48 pm

    I think the transformer has two primary taps on the newer 120s

    You can use this with higher line voltage. This will drop the voltages down the line.

    Also worth building or getting a hold of a ss rectifier , minimally for trouble shooting.

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    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty Re: VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by ChetJettison Thu Mar 14, 2024 9:55 pm

    If that is the case, will using 115v cause any harm?
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    VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems? Empty Re: VTA ST-120... blown fuse. Quad cap resistor blown? Bigger problems?

    Post by Rec Thu Mar 14, 2024 10:52 pm

    Nope
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