Audio Research VT100 MKII retubing blew resistors

see responses below

1. I was replacing input and driver tubes – your originally Sovteks ended their life. I have used Sylvania JAN 7308 tubes.
->ok choice

2. I replaced small tubes and after few swapping I calibrated it closely step by step with ARC procedure and achieved quite good measurements falling within all tolerances. Imbalance at +160 VDC point was less than 10 Volts in both channels.

-> did you also get the +60 vdc where indicated on the PCB?
sounds ok so far. Did you also check to see if voltages on pin 5 of all power tubes around -50 (negative 50) VDC?

3. After finishing small tubes calibration I have installed original 6550C power tubes from ARC which worked fine before.
[food for thought: sometimes with tube circuits they all age together; things drift and current is compensated as a system. Since you put in fresh input tubes and rebiased for NEW settings, the weakest power tube may have been stressed]

Before I started biasing power tubes loud noise and flame appeared and one of the resistor blew and turned into a pieces. (But it was not the power tube biasing resistor). Those flame caused burning second resistor placed next to the first.

4. Broken are two vertical resistors 100 ohm and 1 ohm placed on the right side next to V12 power tube socket. One is black carbon type marked PRC SM186 100 +/- 5% (that blew first), and second is brown resistor marked with gold and blue strips placed vertically next to those black. I also have damaged (braked) the signal path under those resistors.

5. I would like to mention that resistors blew not right after powering on the amp with power tubes installed but after about 15 seconds of warming up. So probably it was not the normal short circuit.

-> sounds about right. it does take some time to build up the electron cloud...at this point the tube conducted a current large enough to pop the resistors, and heat up the copper trace connecting it to the rest of the circuit.

6. Before installing the tubes I cleaned up the sockets first with polypropylene alcohol and after that treated tube pins with Caig’s DeoxIt Gx contact enhancer. I am wondering if the rests of alcohol could be a reason of fire?

->no, it likely evaporated long before you turned it on. COntact cleaners should not be used on tube pins as they can turn (from heat) nasty and actually increase surface resistance.

I have cleaned the sockets about two hours before installing the tube and the alcohol should evaporate. I also do not thing if I could use DeoxIt so extensive to make short-circuit on the V12 tube base.

And now my questions:

- Is it possible that after achieving every measuring points within tolerances that input tubes still causing resistors damage?
-> there is another measurement that you should check (before the power tubes are in) and that is only listed on the actual schematic. It refers to the grid #1 bias. If it as MUCH more positive, say -30 or -20 vdc, then you put that tube in a condition for excessive bias current. Since you blew both the 1 ohm plate resistor and the 100 ohm grid #2 resistors and the traces under the resistors that tube was either put in that state by excessive positive bias on pin 5, and/or it was faulty and simply arced and took everything out (likely the case since you dialed in the input tubes so well)

- Can I still try swat the input tubes with hope to get good results, or should leave those Sylvanias and look for something else?

-> leave these alone. Just make sure to check the pin 5 on the empty power tube sockets. (use the 4 ohm tap as ground reference)

- Is the power tube from the socket where resistors blew still usable? How to check it in another way than just trying?
->If it were me I would not trust it. How Many hours are on the power tubes in general? If >1500 or so you might want to get 2 new matched quads. Don't mess around with anything but 6550 winged C from the tubestore, for example.

Dpac996,

thank you for your very detailed answer.

I will try to answer all your points:

1. Yes, to +60 Volts measuring point was OK.

2. I did not check any of the pin 5 power tubes measuring points. When I switched on my amp to check it another resistor start smoking - 5.11 Kohms before Q4 transistor. I thing that must be something wrong with those Sylvanias. I would like to try another set. I am considering Elecro Harmonix 6922 Gold pins current production tubes but with balanced pairs option (offered in Europe by jacmusic store). I hope that this way I can get perfectly matched octet of input tubes.

3. I think that ordering new power tubes is a good option and I also prefer winged C Svetlanas.

4. I have used older version of ProGold contact enhancer and it if fact become shellac, but now Caig have improoved version with extended temperature range specially designed for tube pins - it is DeoxIt GX if I remember correctly.

I asked my questions to ARC and we will see what they say.

Tan you or your inut.
Miimetr

What Jacmusic say about EX balanced tubes:
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Balanced means, both triodes within one tube are identical within a pre-defined limit. Only like this both tube halves will produce the same sound. We do this matching both for plate current, and Transconductance, using official Telefunken operating points. The system balancing was an official part of the Telefunken tubes like ECC802S etc. However, for 6922 this officially was never done by manufacturers. Still we offer it for this tube, because specially with this tube it is more important than with any other audio tube! This is the number one tube where people report sound differences quickly. We use: Balancing delta = Max 1,5mA for the plate current (Static parameter) and 10% maximum difference for the transconductance (dynamic parameter).
So before a tube can be called balanced, first both halves must first have the same plate current, and after that have the same Transconductance It must pass both tests in sequence. This tube is a high gain (50x) AND a low voltage (100V) tube type at the same time. These two requirements are very hard to meet AT THE SAME TIME. So this makes it very difficult for the manufacturer to produce those tubes. The question is what will they do with the tubes that are not so good. Good NOS E88CC is almost impossible to buy now. It is normal that within one glass, you can have two quite different tubes, also with NOS. So very few tubes pass the balancing test.
Special ECC88/6922 note for technicians: I think we can say, this is one of the most difficult tubes, if you need a good replacement. If any discussion comes up about about too much noise, or bad sound, it is with this tube. Where does this come from? There are many amplifiers using this tube in so called SRPP circuits. As you know, with a circuit like this, both halves of one tube are put in series, so must draw the same plate current per definition. Problems will come with an unselected tube, where both tube halves by itself actually would have liked to draw a different current. The result of such an unbalanced tube in an SRPP circuit is, both tube halves are in a "battle" about what current they can draw. One of the two halves will try to dominate, and determine the final current for the circuit. The voltages in the circuit are now not correct any more. Also the actual current drawn can be too high or too low, depending if the "higher" or "lower" current tube is the winner. If a stronger mis-balance occurs, the circuit will produce more noise than normal and add too much distortion. Also tube wear out can be the result of this, and an acceptable result in the beginning will soon be less good, and the complaints starts from the beginning. To prevent any problems like this, for an SRPP circuit ONLY balanced tubes should be used.
Balancing a "GB" single tube means: only like this, both halves are guaranteed to have the same plate Transconductance, the same bias and same gain and by this produce the same sound. This is done on a calibrated tube tester, which measures the tube data with a 1600 Hz test tone. (So not one of those Hickock testers, and nobody knows what the tester is really testing)

Hi guys,
I am now having the same problem like this, pls help me!
I changed the drive tube and when I was adjusting the bias of the diver tube to 10mVDC relative to each other at the two bias adjust point, A resistor blew up too. I don't know what kind It is and where to buy it.
PLS help me!I can not take it to AR agent, because not available in my country from Asia.
Pls note that this resistor connect to a transitor(may be) with the heating free on it.
Mny tks to your help. ASAP pls!
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[IMG]http://i5.photobucket.com/albums/y163/babyleaf1104/Cuong/IMG_0533.jpg[/IMG]

VT100 MKII's are a bit tricky to set up. They sometimes require 2 volt meters as trimming one trim pot will effect the others trim pot setting. I don't believe this should be done with output tubes in the unit. My guess is your shifting the DC operating points at the output stage causing instability, while trying to trim the front end tubes. Trimming the front end is not something I recommend customers do unless they are skilled at it and have experience doing it and have two volt meters. This is quite different then setting the output tube bias.

I have a VAC PHI 300.1 amp and never have a problem changing tubes. I had one KT-88 go cherry red but it just took out the fuse. No resistors have ever blown.