A bright flash, a low dull thunk and diminished volume

This is my new ST120. I had the amp running about 4 to 5 hours. It was sounding great. Then there was a bright flash from the rectifier tube and a dull thunk from the speakers then lower volume. I shut it off immediately. There was no smoke or weird smells. Let it cool and put it back on the bench. Pulled the tube and I cant see any burnt or charred areas. It looks normal. No burnt pins. I took the bottom off. Everything looks okay. Nothing burnt. I haven't checked the voltages yet. So I'm assuming the rectifier blew. I may be doing the diode mod sooner then later. It looks like you use the 1N4007 or UF4007 diode. Any thoughts?

As I understand it those are the symptoms of a rectifier tube blowing.

A bit of a disappointment but you should be up and running once you replace the rectifier tube.

As for the diodes you I believe you are correct with the 1N4007, but you may want to ski that thread
I seem to recall someone using some higher rated diodes.

The XX4007 series of diodes are rated at 1,000 volts at 1 amp. Which should be adequate. They may also be run in parallel.

Please be sure to purchase from a reliable source, however. Counterfeits are becoming increasingly common.

C, always clean and retention the tube socket pins. I know its new but do that anyway. Then a new tube should do it. Good luck, Dale

Best solution is switch to a different rectifier. GZ37, GZ33 or Weber Copper Top are all more robust options. There are some 5AR4 that will work but mostly NOS tubes that have gone way up in price.

Mr C. What Brand tube was it that blew?

Thanks LeGrace. I'm going to be looking for a replacement. Thats good to know.

Harry, It was a TUNG-SOL 5AR4

Here's what my voltages look now:

Fuse is good
0 VDC on all 4 of the bias points.
Rectifier:
Pin 4 - 420 VAC
Pin 6 - 420 VAC
Pin 2 - 0 VDC
Pin 8 - 0 VDC

6550
Pin 1 - 0 VDC
Across pin 2 & 7 - 6.8 VAC
Pin 3 - 0 VDC
Pin 4 - 0 VDC
Pin 5 - -64 VDC
Pin 6 - -64 VDC
Pin 8 - 0 VDC

Quad cap
1 - 0 VDC
2 - 0 VDC
3 - 0 VDC
4 - 0 VDC

Bob is sending out another 5AR4. In the mean time I went down to the local guitar store and got a GT 5AR4. All they had where the Groove Tubes. With the new tube it blew the fuse. I replaced the fuse and tried again. The 5AR4 flashed and blew the fuse. Is something else going on for it to take out the 5AR4? Could guitar center have had a bad tube? Is there some trouble shooting to do before trying a another 5AR4? This amp worked for about 5 hours before loosing the tube. Bad quad cap?

Nope, problem is your average 5AR4 cant handle the load. The second one I tried was a NOS Sovtek 5AR4, probably 40 years old. Only lasted 3 weeks. Also blew the fuse. (edit: older NOS Mullard 5AR4 is supposed to be an exception) Meanwhile the GZ37's are 2 years+ and still going strong.

I can get a Mullard GZ37. Pricey but I found one. Now I'd hate to kill a buck fifty tube. But if it works I don't mind. How about a SS rectifier? I see the Weber mentioned. Will a SS handle the load?

Weber SS is a lot cheaper and some use it. I would never use one though. Grab 2 of the GZ37 and you should never need to buy another rectifier ever again.

Been running the same Tung-Sol 5AR4 for a couple years now with no issues. (knock knock) ST-120 with KT-120s biased at 55mA (.550 V). Have you tried powering the amp up without the rectifier tube but no output tubes and see if the fuse blows?

nmchiefsfan wrote:Been running the same Tung-Sol 5AR4 for a couple years now with no issues. (knock knock) ST-120 with KT-120s biased at 55mA (.550 V).  Have you tried powering the amp up without the rectifier tube but no output tubes and see if the fuse blows?  

Did you do the diode mod?

HarryY wrote:

nmchiefsfan wrote:Been running the same Tung-Sol 5AR4 for a couple years now with no issues. (knock knock) ST-120 with KT-120s biased at 55mA (.550 V).  Have you tried powering the amp up without the rectifier tube but no output tubes and see if the fuse blows?  

Did you do the diode mod?

Diode mode is not a panacea IME. I lost 5AR4's soon afterwards doing this mod. Because it does not reduce stress on the tube. Like I said the odd 5AR4 will do OK but most will not. You can roll the dice and take your chances.

No diode mod. I do have the TDR board though.

nmchiefsfan wrote:No diode mod.  I do have the TDR board though.  

Thank you

I'm just trying to gather data

Chiefsfan, It doesn't blow the fuse turning it on with the rectifier removed.

Harry, no, I have not done the diode mod. My understanding it's purpose is to protect the transformer if there is some catastrophic failure of the rectifier. I may be wrong but that's my understanding.

The diode mod is beneficial to the rectifier. Even more beneficial overall is installing AB module.

LeGrace is correct on all posts, the 5AR4 tube is very marginal when used on an ST120 amp (same with M125 monoblocks).
5AR4/GZ34 tubes are rated for 250ma MAX, but new product parts I don't even trust that rating.
Realize that each output tube is running at 50ma or more, that's 200ma, and the driver board is using 25ma,
so that's 225ma, or 90% of it's rating, assuming the tubes aren't pulling any more current than that.
Also be sure to never turn off the amp, and then it back on within 2-3 minutes. Wait at least 5 minutes before turning back on.

Thanks Mr. Admin
Sounds like a GZ37 is the ultimate solution.

Mr. C...try with rectifier in but no power tubes. That will help you rule out the quad cap. If fuse blows with no power tubes but the rectifier in then you really can't blame the rectifier as it is not supplying much current.

Mr C wrote:This is my new ST120.  I had the amp running about 4 to 5 hours. It was sounding great.  Then there was a bright flash from the rectifier tube and a dull thunk from the speakers then lower volume. I shut it off immediately. There was no smoke or weird smells.  Let it cool and put it back on the bench. Pulled the tube and I cant see any burnt or charred areas. It looks normal. No burnt pins. I took the bottom off. Everything looks okay. Nothing burnt. I haven't checked the voltages yet.  So I'm assuming the rectifier blew. I may be doing the diode mod sooner then later. It looks like you use the 1N4007 or UF4007 diode. Any thoughts?

Mr. C, I had this exact failure when I first built my VTA-120. I got the kit without any tubes because I was buying the tubes before the kit came. I had been using the amp if I remember correctly, for about a day or two and then I seen a flash on the rectifier tube. I immediately shut the amp down. I pulled the rectifier tube which was a JJ GZ34. I put that rectifier in my Woo Audio WA-6 headphone amp and it did the same thing after a few minutes. The NEW production rectifiers are not like the Vintage one and seem to not be able to handle the stress that the VTA-120 or some other tube power amps puts on them. IMO I don't think the new production GZ34's rectifiers even match their specs because I had read quite a few posts of other forums of others having the same issues with other gear (power amps). The JJ rectifier I had in there that went bad after looking at it closely had like a bunch of little metal fragments moving around loosely inside that tube.

I went over to AK and one of the members there and is a member here as well "SkiZo", he recommended I use a GZ37 as he has one in his VTA-120 as well. I found one and it wasn't cheap but compared to if I had to be replacing these NEW GZ34's it would come out the same anyways. I put the GZ37 in and fine tuned the bias and I never had that issue again with the rectifier. The GZ37 is a very robust tube and should give you years of problem free operation from the Rectifier section is every is working properly.

Also, while you are at it and depending on how long you have had your VTA-120, you may want to do some maintenance on the tube sockets. Cleaning and tightening them up. The reason I said this is because I had another issue when I heard a low level pop when I was turning on my amp one day. I had been using it like you 4-5 hours everyday until I heard that. It turned out to be the driver tubes in the front end of the VTA-120. The center tube was one of them and the right 12AU7 tube went bad. I cleaned the sockets and tighten up the connections for the pin. No problem since then. The tubes that went out were new production Northern Electric 6SN7 and a NE 12AU7. I replaced the 6SN7 with a Russian 6H8C for the center tube and received a new NE 12AU7 from the dealer and they have been worker great ever since. The amp now sound awesome and has a very wide soundstage

Just remember to drain the main power supply caps before doing any maintenance! that's very Important!

Hope this helps!

Thanks Bob. I haven't short cycled it. And it's been on and off just a few times with all it's tubes. I went over some of the solder joints this morning, specially the quad cap and SCM. This morning my line voltage is 119.6. I have seen it a little over 121. most the time its in the 120 range. My speakers are rated at 89dB, they are Polk monitor 10s. I did have it turned up a little at the time. It had been on for about 2 hours and I had it on for about 2 hours earlier. I cant do anything till I get the tube. Any preference between the Weber SS rectifiers?

Ah, bite the bullet and get a couple of GZ-33s. GZ-37s aren't built to handle the capacitance at  start-up. You can throw about anything short of a cannon-ball at the GZ-33 and it will come up roses. I have used the same GZ-33 on a variety of VTA products for the past decade or so, including some hugely inconvenient speaker loads, no worries. If you go the Weber route, open a few holes in the can near the top and one near the bottom to let it ventilate, otherwise don't expect more than 6 months out of one. I'd just as soon drop $75-$100 on a GZ-33 and be done thinking about rectifiers ever again.

Just to add, since your most recent post, don't use Webers or other diode type rectification until and unless you've installed the TDR. Whole reason I prefer toob rectification is that it ramps up the B+ gently instead of just slamming all that juice into a cold output tube when you hit the start button.