Forgive me in advance, for the very long post. But I am simply copying and pasting, what I posted at another forum...and they suggested I might get some good advice/feedback here.
Here goes:
I need some advice. There's no right or wrong answer; just opinions...though you do have to put yourself in my shoes a bit.
I purchased this used ARC VT-50 tube power-amp; as I am wont to do. You know how I roll: something catches my eye, or I have a need; I buy, I try...and some stuff sticks, and some stuff flips...lol.
Now...this is an "older" unit; point being, it has "manual" biasing. I'm not the world's foremost expert on tube biasing; so before I even unpacked it, or fired it up...I brought the amp into my office...and had some of the "techs" help me with it. We turned it on, waited 15 minutes...and tested the voltage. Left side was stable and spot-on; right side was a little wobbly. So we waited another 45 minutes or so (per manual...you should measure at 15 minutes and 1 hour).
At the 15 minute mark, I had the "tech" do it...so I could watch, and make sure there wasn't much to it. You just take your voltmeter leads, and touch them to each side of a test resistor. But at the 1 hour mark...I decided to give it a try. I swear...it seems all I did, was touch the 2 voltmeter leads to the test points; just as I had seen the other guy do it. But I must have shaky hands or something...because I got a pop that nearly made me sh*te me pants!
So I shut it down right away, and packed it right back up to be repaired at ARC. It looked to me...the untrained eye...that the damage was minimal; some noticeable burn, down on the tracer circuit...where things arced from the test point. And I talked to the previous owner, and he said he's arc'ed a tracer board before (not necessarily on the same unit); so I didn't feel like a total idiot.
Of course, I was hopeful and optimistic...that the repair, and therefore cost, would be minimal. After all: I had just bought this piece used, to "try out"; and now was having to shell-out for a repair bill...before I even had a chance to hear it, and/or flip it.
I didn't hear from ARC for a spell; so I just dropped them a polite line, to see if they had an update for me. Word came back "technician says unit has extensive PC board damage and will need a tube set. [tech name] is in the process of working up an estimate. Should have more info later in the week”.
Yikes; not, at all, wanted I wanted to hear. I'm especially surprised...by the need for a new tube set? Describing the incident that occurred; it didn't seem possible I could have blown all 8 tubes!!?? I asked the ARC forum, from another board...if ARC is pretty reputable, for giving it to you straight? Not that I had any doubts. I mean...ARC has been around forever, and I happen to love their kit.
One guy in particular said "they ain't cheap; but...", and I'll paraphrase here...they're not going to rip you off, by padding the bill or claiming unnecessary repairs.
OK, well; with a statement like that...maybe I shouldn't have been surprised. I mean...I was prepared for the "worst", but hopeful for the best. Well...you think I'm making this post, if the news turned out to be the latter?
I don't usually get into numbers...but for this discussion; the numbers are important. $1100 is what I was quoted for the repair...AND new set of tubes. Now I get it; a technician at a "reputable" house, like ARC...doesn't want to leave anything to chance. They want to replace ALL damaged parts...go ahead and put in a set of tubes, they KNOW they can trust (even though this incident occurred, when I was biasing the right channel. So even if I blew EVERYTHING on that side...which I wouldn't even bet on; I would be very surprised, if the 2 input tubes and 2 output tubes on the left side, were also "blown"); bias, tune...and send it back to you, knowing it's in tip-top shape.
Out of curiously, I asked the gal who is the admin for the Support department; if she could give me a "line-item" cost, on the tube set. Now again...let me reiterate: ARC...fine, fine company; and their policy and cost, is their policy and cost. I do not post this here to denigrate them; in fact...I usually keep my "dealings" pretty private. But they want $560 for the tubes!
Now...when this amp was on its way; I was already thinking about rolling something into the 6550 slots. Of course, the "best" 6550 known to be going right now...is the Svetlana "Winged" 6550C. But I already had a problem with them of late...and even the guy from Upscale said "smart to stay away"; because evidently, as they were stopping production, they really cut corners on QA at the end...so it's a dice-roll. He recommended Sovtek 6550WE, and even said "that's what ARC is using...now that the Svetlana Cs are out of production". Cost? $32/per; and that's retail, from Upscale! Let me do the math for ya; that's $128, for the Quad...American!!
Now...I don't know what ARC is using in the input slots (4 x 6922s); but I doubt it's anything "exotic". A) "stock" tubes usually aren't, and B) those "tech/engineer" types; they usually don't even think it makes any difference, lol. They often choose, based on reliability; not any audiophile mumbo-jumbo about NOS glass magic, lol. So let's say...they were going to go with something, like the EH Gold Pin? Cost? $25/per; and again...that's retail, and that's at Upscale, etc., etc.
So how can ARC justify charging me, $560; for $228 "worth" of tubes...retail? Well, like I said; they do what they do...and if you want to play the game, which is have them stand 1000% behind the work. You have to pay the price of admission.
So I asked them...is there any way, for them to repair this thing...without the fresh set of tubes? I mean...I understand they need tubes to test it, after repair; but couldn't they use some kind of house "testers", and then send it back tube-less? I mean...this MIGHT end up staying (I actually bought it, because I have a Wyred STP-SE pre-amp...that I think has potential; but needed some warmth, in the way of a tube amp). OTOH...I might just want to sell this thing (as I am again, wont to do).
On top of everything else; I actually already have...(4) 6922s I could use. So I'd cut my repair bill in HALF, and at worst...I'd be looking at ~$130 for a new quad of 6550s. Or...I might just cut bait, sell it advertised as "repaired by ARC; sans tubes...new owner, roll your own".
Sure...if I bought this new, treasured this amp, and were going to leave it to my kids; I'd let ARC do their thing, and know I had a finely-tuned machine...that I wasn't going to have to turn around and bias again (at least not right away). But this ain't that.
I wasn't even sure ARC would consider it; but I got word today..."Per our technician, you will receive NO SERVICE WARRANTY WHATSOEVER, if you choose this course of action. If the unit is damaged as a result of your tubes, you will be fully responsible for another repair and shipping both ways.” CYA and fear of god accomplished, lol.
I don't know; it's sure attractive...to cut that repair cost in half (for the record...the full cost of the repair, is damn near what I paid for the amp to begin with!)...get some tubes, AND some HELP! And just very carefully, try to get this thing back in good working order; without losing my shirt.
Now...another suggestion I've been given, is asking ARC if they will repair, and bias; with MY tubes. The 4 existing 6922s I have...and a new Quad of (reputable) 6550s, I would purchase. I think that's an excellent compromise; but IDK. Is that a little like bringing your own steak to a restaurant...and asking them to cook it for you? You know...because their mark-up is too high, lol.
It has also been suggested to me...that it is a SURPRISING amount of damage; to have "slipped" (if that's even what I did)...off the test resistor.
Trying to make a decision, one way or another, quickly; just feel confused and conflicted about my best course of action. Thanks for any feedback or advice!
Its a common problem for ARC. The same thing happened to my VT-100. When it blew, a piece of the resistor hit me right in the face while sitting in my listening chair.
I don't know if I would spend big money on a VT-50. I had one of those as well. It was OK, but the VT-100 is a lot better. Not just for power, the sound quality is at a higher level. I see them selling for low prices, so if it was my choice, I would spend the extra money and get a VT-100.
If ARC says the main board needs to be replaced than it does. There may have been problems with the board to start. ARC is not cheap but they do not do unnecessary repairs. They will go through the amp with a fine tooth comb and make sure it work like new. They will even do listening tests to make sure it sounds right. They want it to work perfectly. They will not half ass anything.
The fact that the tubes would not hold a steady bias after 15 minutes suggest that the tubes were getting old and probably needed to be replaced. When you buy a tube amp you assume it will most likely need new tubes. It doesn't matter what the sellers says. ARC amps are not auto bias.
Tubes from ARC are not cheap. But to be fair they hand pick the tubes after testing and burn them in before matching them. This takes time and effort. I don't buy my tubes from ARC but understand why some people do.
At this point you can either have the amp repaired with new tubes so they can test it. Having the board replaced without warranty doesn't seem like a good idea. Or you can have ARC return the amp and sell it cheap because it does not work. Either way you are going to loose money.
This is not just a simple protection circuit resistor, that is an easy fix. The board needs to be replaced which means there are far more serious problems.
I do agree though that the VT-50 would not be my first choice. There are far better ARC amps for a little bit more money.
Thanks so far gents. For the record...I think I was pretty clear, about not questioning ARCs integrity. And (now), I guess I see why their tubes; are more than double maybe comparable sold at retail.
Suggestions for the board were never in doubt. But when a quote is $1100...and half of that is for the glass; you start thinking outside the box. Especially...when as you guys have said; it may not even be a "keeper".
They've agreed to repair and bias, with new tubes...which will save me a few hundred bucks anyway :)
As for why so "much" damage occurred...from a simple slip at the test-resistor; IDK? I have to take the seller at good faith; he has good feedback, and says the amp was in working order when it left.
It could have a) been ready to throw something, and I just pushed it over the hump b) had an old, unstable tube...and after an hour, she was ready to blow. C) something could have happened in shipping; etc, etc.
I just want to give it a spin, without losing my shirt (of course I'm going to lose money; it's just a matter of how much).
ARC is known for being very expensive with repairs and re-tubing. Esp. re-tubing. As for getting the amp repaired with no warranty, doesn't sound like a good idea to me. As for the extent of the damage, IDK, but if ARC amps take out a resistor when a tube goes bad, imagine how much damage you can do when you short a circuit, which is what it sounds like you did (admittedly just guessing). I had a tube integrated once that needed to have the main PCB replaced at; it cost me about $1K 10 years ago.
Agree with Lostbears' posts. Just checked the ARCDB website. The VT50 production run started in 1998 and ended in 2002. Not sure when your VT50 came off the line, but it's got to be at least 13 + years old.
That said, 13 years is not a super long time for ARC tube gear. Kinda curious why you need so much work on your amp. OTOH, ARC has been producing much better tube gear since then.
Don't know what your budget is. But you could buy a lot more quality ARC pre-owned tube watts. For example, you might want to consider the ARC Ref 110 or the ARC VS-115.
If you decide to hold onto your VT50, I recommend letting ARC bring it back to its former glory. And yes, ARC charges twice as much tube-for-tube as most 3rd party tube vendors.
Last point ... be very careful when buying Svetlana Winged C 6550 tubes. Kevin Deal of Upscale Audio declined to sell that brand for a while because of quality issues.
"08-27-15: Bifwynne Agree with Lostbears' posts. Just checked the ARCDB website. The VT50 production run started in 1998 and ended in 2002. Not sure when your VT50 came off the line, but it's got to be at least 13 + years old."
Are you sure you were looking at the right amp? The VT-50 I had was made well before 1998.
At the 15 minute mark, I had the "tech" do it...so I could watch, and make sure there wasn't much to it. You just take your voltmeter leads, and touch them to each side of a test resistor. But at the 1 hour mark...I decided to give it a try. I swear...it seems all I did, was touch the 2 voltmeter leads to the test points; just as I had seen the other guy do it. But I must have shaky hands or something...because I got a pop that nearly made me sh*te me pants!
Hiendpawn,
Just guessing you shorted the B+ 420Vdc to the silver color Phillips head circuit board mounting screw just above the right hand side of the bias resistor.
When you get the amp back from ARC I suggest you buy a pair of hook on clips for your multimeter. Examples of.
As for the tubes it sounds like the 6550 tubes were near their end of life. Good chance so were the 6922 tubes.
You can't just drop in new 6922 tubes in the VT50 amp. The new 6922 tubes need to be biased.
And you MUST have closely matched 6922 tubes. Especially the driver tubes. Not only matched pairs for each channel but each section of the dual triode tube must be closely matched. If each section of the tube is not closely matched you will never be able to bias the input and driver tube for each channel. (Behind each channel side panel there are 3 bias trim pots.)
The VT50 is no where near as hard to bias as VT100 MK 1 and Mk 2 but it still can be a pain if you do not start out with closely matched 6922 tubes.
You may complain about ARC tube pricing but you would probably be surprised how many 6922 tubes they go through to get closely matched triode sections let alone matched pairs. VT50 setup instructions with complete retube. .
One day I brought my Volvo to techie to as how mow will be to fix my AC. They took a look and told $850 coz I needed to replace whole compressor and lines as well. Instead I repaired compressor myself for $15 and saved $835.
Another day I had dropped spinning grinder onto my big toe and cut it all the way down to the bone. After the doctor checked he decided to remove the toe. Instead of going under the blade or saw, I started applying bacterial ointment that 'eats out' infection and got myself healed instead of cut.
Bifwynne, Both of those are very nice amps. The Ref 110 is getting to be a real bargain at a little over $4000. Put KT120 tubes in it and you have a seriously great sounding amp for a really nice price. I know, I own one.
The VS 115 is also a nice amp and better sounding than the earlier 110. I like the look of the "flat tops" with the tubes out in front. Not as easy to find as the Ref 110 though. Both are really easy to bias with bias ports.
One of the things with the VT50 is that it is just not that powerful. Even for a 50 watt tube amp. You need efficient speaker to get the best sound. I prefer a bit more power even with fairly efficient speakers. My speakers are around 93db and I use a ref 110 to drive them.
When you get the amp back from ARC I suggest you buy a pair of hook on clips for your multimeter.>
I think you're exactly right, about hitting that screw. I have a picture of the resistor and damage, from right after; and that's the only thing I could think of as well.
As for the "alligator clips"; there is NO question...lol.
In hindsight, I'm not really complaining as much; and I was never "accusing". Let's be honest: most guys, who can afford kit like ARC...could give a sh*t, about a few hundred bucks. And as I've said...if this was an amp, I was SURE was staying; I probably would bite the bullet, and let ARC do their thing (because I now understand, at least a bit better...why they charge much more than a tube dealer).
But I've been clear about why that's not the case here.
For the record...they have agreed, to let me send them new tubes; and repair and "try" to bias from them. I say "try", because they have warned me...they will be testing the tubes, and reserve the right to reject anything they don't feel is up to snuff.
I'm down with that; I mean...I don't want "junk" in the amp either, keeper or not. I just hope they don't go CRAZY stringent in this case. I'm getting 4 Sovteks and 4 EH Golds, and having them sent right to ARC.
As for the many suggestions...I could do "better", than the VT-50. First of all...duh, lol. I'm a guy, who's been an avid box-swapper; and I'm trying to hone in on something to just "be happy" with.
Like right now...after many, many other boxes, have passed through the rack; I have a Luxman L-550AX, Ayre AX-7e...and I have a Wyred STP-SE, that I think might have potential.
I usually go the other way around; tube pre into SS muscle. But for the exceptionally clean STP...I decided to try tube power. So I wasn't going to break the bank to "experiment"...and, I've heard the VT-50 is a sweet, little guy :)
But trust me...if this combo worked out, and I was going to move up the ARC ladder; I'd buy a Ref 75 in a second!
Unless you listen to music really loud 45 watts per channel should be fine. At present I am running EH 6922 tubes and a quad of SED Winged C 6550C tubes I bought from Upscale Audio back in 2007 I had stashed away until about a year ago when I then installed them in the amp.
Before the SED 6550 tubes I ran all EH tubes.
If you are buying the new 6922 tubes from Upscale Audio tell them you need the two pairs closely matched and the two sections of the 6922 MUST BE closely matched. If the 2 sections of each tube are not closely match ARC will not be able to bias the tubes within the circuit design specs of the amp and will reject the tubes.
Closely matched pairs or a quad of the 6550 tubes is the easy part.
Also worth mentioning I doubt ARC will be installing a new right channel circuit board. Even if they still had one in stock I would be willing to bet they would want at least $1500 for it plus installation. They will repair the damaged caused by shorting the B+ 420 Vdc to the B- chassis connected power supply rail. The B- is connected to the chassis through a 10 ohm resistor that was more than likely also taken out by the short.
The basic circuit of the VT50 is the long-lived ARC ultralinear, partial-cathode-coupled design. TheVT50/100/200 series, which debuted about four years or so ago, saw the addition of FETs to the traditional circuit, employed as a constant current source. According to ARC, this allows for greater linearity, most especially when using the single-ended inputs (the design is both balanced and dual mono from the power supply forward). Other changes in layout and design allowed for better heat dissipation, with a payoff in greater reliability and longer life. Energy storage was also increased, with the VT50 boasting 354 joules worth.
The VT50 does have a hefty power transformer and DC power supplies. When turning the amp on or off don't dally around when flipping the rocker switch on or off. Flip it in a quick fast motion. Jim
Another thing to consider after the fact is in the future, unplug the amp before you connect and disconnect the voltmeter leads. Anytime you are reaching into an electrical device, de-energize it first before you do so.
That way, you completely avoid any "mistakes". it is an added step, but all it takes is one time touching high voltage and if you survive, it is something you don't want to do again.
I'm an Electrical Engineer and have many times reached into energized amps/pre-amps/and other equipment and was momentarily distracted to the point where I touched something and shorted it.
So, I "learned" to turn it off and unplug it first. Despite my experience and expertise.
^^ So...with the amp off; you clip your leads to the test points. Turn the amp on...let it run-up to operating readiness; and just see what the voltmeter reads?
Wow; not to be ungrateful...but I sure could have used, a simple solution like that a few weeks ago :(
"08-28-15: Czarivey One day I brought my Volvo to techie to as how mow will be to fix my AC. They took a look and told $850 coz I needed to replace whole compressor and lines as well. Instead I repaired compressor myself for $15 and saved $835."
I all fairness, you're an auto mechanic.
"Another day I had dropped spinning grinder onto my big toe and cut it all the way down to the bone. After the doctor checked he decided to remove the toe. Instead of going under the blade or saw, I started applying bacterial ointment that 'eats out' infection and got myself healed instead of cut."
^^ So...with the amp off; you clip your leads to the test points. Turn the amp on...let it run-up to operating readiness; and just see what the voltmeter reads?
Wow; not to be ungrateful...but I sure could have used, a simple solution like that a few weeks ago :(
Live and learn. 08-28-15: Hiendpawn
Life is not that easy..... You should not be afraid of electricity but you should respect it.
You should remove the top cover with the amp turned off. When biasing is done turn off the amp and wait about 5 minutes for the caps to bleed off before replacing the cover.
I assume you have the plastic adjusting tool for biasing that came with the amp. If not you need to buy one from ARC.
The Amp needs to be powered up to make the adjustments. The amp needs to be powered up for about 20 minutes, with no signal applied. This allows the amp to stabilize. Next connect the hook on test probes across the bias resistor on one channel. Make sure the hook on probes as well as meter leads do not come into contact with the HOT to the touch 6550 power tubes. Next set the blue adjustment pot to 65 mvdc. You will find it is a lot easier to set the bias having both hands free. As you probably already know it does not take much of a movement of the pot to swing the bias voltage one way or the other. The adjustment pot is quite touchy.
After you get the one channel set to 65 mvdc move the hook on probes of the multimeter to the other channel. Repeat the bias process.
Now go back and check the first channel where you started. You may find you will need to slightly increase the bias voltage again. Next back to the other channel again and check it again. You may have to repeat the process a few times.
Hiendpawn,
When you get the amp back from ARC you will need to install the new tubes. The tubes will be marked and must be installed in the corresponding numbered tube sockets. Leave the top cover off.... Though the Amp was set up and biased by ARC it would be a good idea to check the power tube bias again after a few hours of play time. For one thing the AC line voltage at ARC may not be the same as at your home.
Stop playing music and let the amp stabilize for about 20 minutes or so. Caution! The 6550 tubes will be quite HOT to the touch! Check bias voltage on each channel and readjust if needed. .
Zd542 ... no, I am not positive about the time line of the VT-50 production run in light of Jim's (Jea48) post.
My post was based on the ARCDB website: http://www.arcdb.ws/VT50/VT50.html Ordinarily, I find the ARCDB website to be very accurate. That said, Jim (Jea48) said he bought his VT-50 in 1997.
In any case, my main point is that the OP's VT-50 could be a bit long in the tooth. Based on numerous comments I recall reading about the VT series amps, many said they were good. But later models (e.g., the Ref 110 and the VS-115) attracted more enthusiastic reviews and A'gon comments.
FWIW, I previously owned the VS-115 and thought it was a terrific and reliable amp. Easy to maintain too. Shortly after ARC cleared the VS-115 for KT-120 tubes, I dropped in 2 quads. Pretty impressive improvement.
I currently own a Ref 150 SE which uses KT-150 tubes. WOW!!
Out of curiosity, I read the Soundstage review that Jea48 mentioned. Did anyone see this part of the review?
"Frankly, I’m surprised at the quality built into the VT50. This really is engineering and build quality that impresses. An example. I had two VT50s over the review period. The first broke in well and everything was progressing nicely until, in the middle of a quiet listening session, one of the bias-control resistors literally blew apart. The bang was quite loud and accompanied by a flash and smoke, and yet everything shut down cleanly. There was no damage to any ancillary equipment; in fact, except for the fuse and the tube, there was no other damage to amp. The issue was obviously a faulty resistor and not ARC’s doing, but the superb engineering confined the problem and saved my gear."
For the record...they have agreed, to let me send them new tubes; and repair and "try" to bias from them. I say "try", because they have warned me...they will be testing the tubes, and reserve the right to reject anything they don't feel is up to snuff.
IMO, high risk and hassle to save a few hundred. Buying and shipping from Upscale, what if tubes are rejected ... GL!
Well...I am very happy to report, a new wrinkle to my sad saga.
A member...who I know from a previous sale, and have stayed friendly with; read my tale of woe. He is an ARC nut, and has graciously agreed to donate a quad of Winged 6550Cs, he had as spares.
They're not recent production crap either; they came from ARC years ago...and while he says they have a little bit of use, they still test at better than new.
I'd love to name him, and sing his praises; but he asked to remain, an anonymous benefactor.
With this wrinkle...he also agrees, to just let ARC do their thing; in regard to the driver tubes.
More than even the money...a gesture like this, really restores my faith; in what can sometimes be a pretty snarky hobby :)
Here's another alternative ... I own an ARC Classic 60 that I use for a spare amp. I've had it since new. When I sold my REF-75 I had to wait for the new REF-75se to be built. I took the CL-60 into the Tech to have it checked over and any necessary repairs made. When I hooked it up, it was the first time I heard the amp in many years. I can honestly say ... WOW! What a musical amp. The Ref amps kill it of course, but we're talking about maybe $1800. on the used market for a good CL-60? Now that has to be the bargain of the year!
The VT50 is a breeze to bias compared to the Classic 60. This is a superb sounding amp but not easy to bias, especially for a beginner. Both the Classic and V series are some of my favorite ARC amps
Just curious, Is your Classic 60 balanced? The very late CL60s had no holes in the face plate and were balanced. Much like the later V series.
08-27-15: Hiendpawn So even if I blew EVERYTHING on that side...which I wouldn't even bet on; I would be very surprised, if the 2 input tubes and 2 output tubes on the left side, were also "blown."
08-28-15: Jea48 Just guessing you shorted the B+ 420Vdc to the silver color Phillips head circuit board mounting screw just above the right hand side of the bias resistor....
As for the tubes it sounds like the 6550 tubes were near their end of life. Good chance so were the 6922 tubes.
If B+ was momentarily shorted to ground via that screw, it's also possible that the power tubes in BOTH channels were damaged as a result of high voltage spikes caused by "inductive kickback" from the output transformers. Especially if the amp wasn't connected to speakers, or an equivalent load, when the procedure was being performed.
Inductive kickback is the principle by which the ignition coils in automobiles generate the thousands of volts needed to fire the spark plugs. When the current through an inductance, in this case the primary of the output transformers, is abruptly changed (as it would be if B+ were momentarily shorted to ground), voltage spikes at extremely high voltages can result, especially if the transformer is unloaded on the secondary side.
I'm assuming, btw, that the B+ is generated by a power supply circuit that is common to both channels, which I suspect is the case but don't know for a fact.
Al, that is all over my head...as I'm no EE; but it makes sense.
I will tell you this; the flash and pop, was pretty damn scary. And maybe I should look at the bright side; which is...I didn't get shocked at all. So maybe I got lucky in that sense?
I'm assuming, btw, that the B+ is generated by a power supply circuit that is common to both channels, which I suspect is the case but don't know for a fact. 08-30-15: Almarg
Hi Al,
VT50 Schematic and parts list. Note the two R30 10 ohm resistors on the schematic. Especially the one on the power supply diagram that connects the common B- DC rail of the power supplies to chassis. I would bet that resistor blew apart and opened like a fuse and limited the damage. I am not an expert on power supplies but isn't 354 joules a hell of a lot of DC storage power?
You made a good point about having a load connected to the output transformers. I just assumed the OP had speakers or a dummy load connected to the speaker output terminals of the amp. When I spoke of the power tubes being near their end of life it was because the OP said his friend could not get the bias of the right channel to hold steady and not drift. That was before the OP tried to set the bias himself and damaged the amp. . Jim
A) It is correct, I had no speakers connected to the amp at the time. I guess that was my first mistake?
B) When I say the right channel was "drifting" a bit; the amp had just been unboxed...from it's journey. Started cold, and this was the "15 minute" reading.
I don't know we can condemn it for that...and it was the tech who was like "it's drifting a bit". Chances are, it was probably well within tolerance...and I could have just left it the hell alone. But...hindsight is always 20/20; and I've been gifted some relief, so...
Thanks, Jim (Jea48). So, yes, the 420V B+ is supplied to both channels by the same power supply circuit.
And yes, 354 joules is a goodly amount of energy storage, especially in relation to the 45W/channel rating of the amp. But some of the more powerful ARC amps provide considerably more than that. For example the Ref150, rated at 150W/channel, is indicated as providing 1040 joules of storage. One joule, btw, equals one watt-second, or the ability to provide 1 watt of power for 1 second, or 2 watts for 0.5 seconds, etc.
What led me to think of the possibility that speakers might not have been connected was the statement in the original post that "I brought the amp into my office...and had some of the 'techs' help me with it...."
Your guess is as good as mine as to whether the protection provided by the 10 ohm resistor would have been fast enough to be effective, with respect to the inductive kickback issue I described.
What led me to think of the possibility that speakers might not have been connected was the statement in the original post that "I brought the amp into my office...and had some of the 'techs' help me with it...."
This picture shows the Phillips head circuit board mounting screw on the right channel that I think the multimeter probe came into contact with causing the short circuit.
Note the full wave rectification is mounted on the left channel circuit board. Also note each channel has it's own power supply electrolytic filter/storage caps. . Jim
Thanks, Kng. And thanks Jim for providing the info. I suppose it's possible that the physical placement of the rectifier circuit and the filter capacitors could have provided some reduction in the likelihood that a short to that screw would have affected the other channel, due to the small amount of resistance and inductance in the interconnections within the amp. But given that all of those things are electrically connected together for both channels, my point is simply that it is not implausible that the momentary short could have affected the output tubes of both channels.
Hiendpawn, not having the speakers or an equivalent load connected would not affect the bias adjustments. It would also not result in a risk of damage, UNLESS a "signal" or some kind of equivalent is put through the output stage of the amp. In this case the momentary short that apparently occurred might have constituted such a "signal." Also, a significant turn-on or turn-off transient that might be generated by some amps, either as a result of its design or as a result of some issue relating to its condition, might constitute such a "signal." For example, I once had a high powered solid state amp that produced a wicked thump in the speakers about 40 seconds after it was turned off, although of course in the case of that solid state amp inductive kickback would not have been an issue since there was no output transformer.
Wow Jim. Just looked at the close up pic showing the Phillips screw and the bias resister contacts. Small wonder the OP shorted his amp. You need 4 hands to bias that old clunker. My ARC amp has multi-meter probe sockets. Avoids (or at least mitigates) the chances for a short. So far, never had a problem.
Tell ya one thing about the Ref 150's 1040 joule power supply. A couple of months ago, I accidentally got shocked while putting the top back on my amp. Hit a power supply cap with my finger. My hair curled and I am lucky that that's all that happened.
Yes, that is why a pair of hook on probes should be used when biasing the amp. Sometimes I think the guy that designed the layout of the amp had a sick sense of humor.
Also as I stated in an earlier post the bias adjust pots are really touchy. The slightest movement can really swing the bias voltage one way or the other. Both hands are needed to adjust the pots. LOL, Heaven help the guy that has the shakes.
And when biasing the 6922 input and driver tubes the hook on probes are a must. (Though those pots are not touchy.)
As for the amp being an old clunker, LOL, even though it is 18 years old it has served this old clunker well, lol. I would still put it up against some of the new stuff on the market today. . Jim
I suppose it's possible that the physical placement of the rectifier circuit and the filter capacitors could have provided some reduction in the likelihood that a short to that screw would have affected the other channel,
Al,
That was not a thought in my mind when supplying you the pictures. I take you at your word that the OP could have caused damage to the power tubes. I just thought you might want to see what was under the hood of the VT50. The amp is built like a tank, but with that said, jmho, it is not layman maintenance service friendly. . Jim
Jim, yup, got the same problem with my bias pots. It appears that my ARC amp uses the same type of blue bias pots for the power tubes as those in the pics you kindly provided. At least they look the same.
I've gotten used to accepting close but not perfect when setting the bias pots. ARC recommends biasing the set tube at 65 mV. The slave tube is considered within spec if between 57 and 73 mVs. I am happy if I get between 64.5 and 65.0 mVs on the set tube ... and that doesn't even address AC line voltage variations.
Good suggestion about using alligator clips. But as I mentioned, not necessary for more modern, less clunky, ARC amps.
Jim, any idea why the 6922 bias pots are less weird than the power tubes pots?
And my sincere apologies for disparaging your amp as a clunker. I'm getting long in the tooth, so I admit being a clunker too. LOL!!
Jim, any idea why the 6922 bias pots are less weird than the power tubes pots? 08-30-15: Bifwynne
The 6922 bias pots are blue in color only and are not the same pots as used for the power tube bias pots. I believe they are a whole different animal in their design/construction. They look like the same pots that are used in the VT100 mk2 amp. Picture courtesy of Abe Collins and his former VT100 mk2 power amp.
And my sincere apologies for disparaging your amp as a clunker. I'm getting long in the tooth, so I admit being a clunker too. LOL!!
No need to apologize. But thank you. I took the comment as meant in fun. . Jim
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