[DaveC113]

The resistor pictured below went up in flames twice after switching to a JJ 5ar4 rectifier tube. The original resistor (100r, 1W) lasted about 3-4 weeks of daily use before it went, the second (a Kiwame 2W resistor) also lasted a few weeks. I'm not sure its the tube, but I'm leaning that way because its the only thing I changed. Since then, I added in the 3W Dale resistor in the pic and switched back to a Sovtec 5ar4. Also, I don't have the schematic, but I'm guessing the resistor is from the 5ar4 cathode to ground, the trace to the capacitor is highlighted in yellow, I think thats the resistor's bypass cap, its 68 uF, 400V.

Is there any way I can check the tube or are there any other possible causes?

TIA,
Dave

[JoshK]

I am assuming you don't have a tube tester?  Do you have a schematic?  Can you trace the first part of the power supply circuit if you don't?

Those white coffin resistors are made to act like a fuse and fail open when over current condition is approached.  Consider it a blessing, as it is probably saving expensive parts from going up in smoke.  Don't oversize the resistor in an attempt to circumvent the problem as it is a warning flag. 

Measure the current through the resistor (or voltage across it), when powered up (be very careful!).

[DaveC113]

I emailed partsconnexion about the schematic, maybe he will send it to me.

I measured the voltage across the resistor, its 4.45 V, and this value is close between the Sovtec and JJ tubes... so I'm thinking maybe its not the tube causing the issue.

So with 4.45 V and 100 Ohms, I get 44.5 mA, or about 0.2W. This shouldn't be enough to fry a 1w resistor, let alone a 2W.

Dave

[JoshK]

That PSU looks like a CRC, but it could be RC or something else too.  My guess is that the resistor is getting slammed on turn on when the capacitors pull current heavily.   Do you happen to have the datasheet for the JJ 5AR4...maybe its forward voltage drop is less than a standard 5AR4 which could result in a heavier current spike.   Does the voltage on B+ measure the same with either rectifiers?

[DaveC113]

Here's a link to a pdf of the jj rectifier:

http://www.jj-electronic.com/index.php?option=com_content&task=view&id=16&Itemid=12

B+ is 318V with either tube.

That makes sense about turn on, and maybe turn off too? I didn't think about it too much, because the preamp keeps the filliments on all the time, but the 2nd resistor went about 10 seconds after I turned the preamp off. Possibly an important detail  , but I wasn't thinking about the caps charging/discharging through the resistor...

[JoshK]

B+ is 318V with either tube.

That means the voltage drop is the same.  Shot down that theory. 

That makes sense about turn on, and maybe turn off too? I didn't think about it too much, because the preamp keeps the filliments on all the time, but the 2nd resistor went about 10 seconds after I turned the preamp off. Possibly an important detail  , but I wasn't thinking about the caps charging/discharging through the resistor...

Almost all bad things that happen to components, especially class a tube gear, which a preamp will be, happen on turn on/off.  When operating, because its class A, it is in a state of equilibrium, constant current and everything has settled into place.  Turn on/off is very choatic by comparison and why you don't see a lot of DC coupled tube gear.

Do you happen to have any chokes further down the PSU stream?  At turn off, chokes can create what is called inductive kickback. I forget the precise explanation but it involves the fact that chokes try to keep current constant through them but when voltage goes to zero and something else goes to infinity or something like that and kicks back.  This can cause problems in rectifiers and transformer secondaries I know, so its possible it can cause havoc in your resistor.  The easy thing to do is put a diode in parrallel, so that in normal operation it drops out of the circuit, but at shutdown the diode shunts the current protecting the rest of the circuit.  See tubecad for details.

[JoshK]

reference....

Blog 139, April 08

Now, think about it, a capacitor stores an electrical charge, which can easily prove dangerous, if shorted; whereas an inductor stores energy in the form of flowing current and can prove dangerous, when the circuit is opened, as the stored energy must be released somehow. That somehow usually takes the form of a big spark, as the voltage must climb towards infinity because the current has fallen to zero. Sometimes the spark occurs inside your output tube or other amplifier parts, depending how the circuit is arranged.

He always shows his power supply inductors with diodes in parallel. 

[DaveC113]

Josh, thanks for the info. I'm going on the assumption that the circuit I have was properly designed, and I have a component going bad. Adding the diodes you recommend may not be a bad idea, but even if it does work it seems like it's putting a band-aid on cut that needs stitches, because there is obviously a problem.

Also... I have had 2 fuses blow, both about 10 seconds after the unit is switched off since I last posted. I may unplug the preamp instead of using the power switch (which leaves the filaments burning 24/7). The only good news seems to be that the new 3W sand resistor is taking the abuse without blowing. I think the resistor is providing a path to ground for a surge, other components do not seem to be affected.

I emailed partsconnexion about the schematic, I hope they send it soon.

If I were to replace the three 68 uF 400V electrolytic ps caps, would motor run caps be a good upgrade? This value of cap (or anything close in size and value) isn't easy to find. 70uf motor run caps are easy to find, but I'd need a 3rd chassis to hold them, or maybe mount them on top of the current ps?...   

Dave

[DaveC113]

Here's a description of the ps. There are no inductors.

found here: http://www.anthemav.com/Oldsitev1/review/pre-1.html

Power-supply circuitry is quite extensive in this design. In the high-voltage supply, the tube rectifier is followed by a three- stage RC filter that feeds the main high-voltage regulator. In this circuit, which I call a zener follower, a constant-current source feeds a shunt regulator formed from a string of zener diodes. The output of the shunt regulator then feeds the gate of a MOS-FET series pass transistor whose source terminal is the circuit's final regulated output of 245 volts.

Following the main high-voltage regulator are six additional high-voltage regulators, three per channel. A 24-volt zener diode and a resistor are connected in series between each regulator's high-voltage input and ground; the voltage drop through this zener goes through a resistive divider to supply +12 and -12 volts for the regulator's error-amplifier op-amp. The op-amp's output drives the gate of a MOS-FET series pass transistor, whose source terminal feeds about 233 volts to the preamp circuitry; the source terminal's output is also applied to the negative input of the error op-amp. In each channel, separate regulators feed the phono section's first stage, its second stage, and the line output section. Each channel also has separately regulated tube-heater supplies for the phono section and the line output amp. B.H.K.

[JoshK]

OK, but this doesn't give us the details.  Unfortunately, cases like these are hard to solve on a forum.  I'd suggest posting your question on diyaudio's tube forum as there are lots of seasoned tubies that have seen every possible problem and can help narrow the causes more quickly than I.  I am going based on theory not experience.

I still think there is some issue with transient surges.  It could be due to the regulator or a number of other issues.  But obviously, if the resistor has gone up in smoke at turn off, it is because your are over-current taxing that resistor on the transient state.  It could be the rectifier, but it doesn't seem all that likely to me, but what do I know?  You haven't had this issue with the Svetlana tube though?

P.S.  that white coffin resistor, technically, should be soldered in the other way around, with the writing facing the board and the bathtub like part facing up.   This is because those type of resistors are meant to provide over current protection and fail open.  When they fail open they blow out that indented part.  If you face it up, you keep from getting all the stuff on the PCB and other parts. 

[JoshK]

If I were to replace the three 68 uF 400V electrolytic ps caps, would motor run caps be a good upgrade? This value of cap (or anything close in size and value) isn't easy to find. 70uf motor run caps are easy to find, but I'd need a 3rd chassis to hold them, or maybe mount them on top of the current ps?...   

Dave

I would imagine it would be an upgrade, yes.  However, I would first determine the cause of why you were blowing the original resistors before you proceed to change anything in the original circuit.  This is where the veterans of the diyAudio forum can hopefully help you out.

Once resolved, I think replacing the 'lytics with motor run caps can provide a significant upgrade, but not necessarily just a part swapping excercise.  The reason is the size constraint.  However, you can hopefully find the smaller motor run caps as 240VAC rated motor run caps should be sufficient for this purpose.  240VAC ~ 400VDC by my rule of thumb estimate, plus motor run caps are far more tolerant of transient overvoltage conditions than 'lytics and are designed for such cases.   First see what you can find in the ~60-70uf and ~240-300VAC MRC's and determine if they can be made to fit, if they can then simple part swap can be done. 

My presumption is you will have a hard time fitting them unless you put them out the top of your PSU.  In which case you can do some redesign and use smaller valued MR caps, using PSUDII as a guiding tool.  Its worth downloading PSUDII (its free) and playing around with it.  Download Bas Hornemann's guide on how to use it (short article with examples).  Play around and model your existing passive RC circuit and then what you would like to try.  Chances are you can redesign with smaller valued MR caps and obtain better damping properties (critical damping).  You don't want overshooting nor long response times (what you likely have now).  Look up what that means if you don't already know. 

[Poindexter]

Guys, the JJ 5AR4 rectifier does have an unfortunate tendency to go short; usually gradually and after some hours in-circuit.  I had two out of six do this, after the piece was in the client's hands, and so I no longer use the JJ part.  It could very well be as simple as this.  Try a Sovtek.

Poinz

[DaveC113]

Poindexter,

After not having any issues since I last posted (using a Sovtek rectifier tube), I have come to the conclusion its the JJ tube. It shorts after shut-down as the tube is cooling down, which has destroyed 3 fuses and 2 resistors. Another JJ rectifier tube shorted in a friends new tube amp causing minor damage, after several on-off cycles, so I will take your advise and not use them any more. Its too bad, they are a little juicier sounding than the Sovteks... maybe I'll get a Mullard when I'm feeling rich enough.... aa

Dave

[jrebman]

Another new production rectifier tube to look at is the TAD 5AR4 -- supposedly very good.  NOS RCAs are also good and nowhere near the price of Mullard/Amperex.

Dave,

Don't worry about getting exactly 68uF caps -- that first cap is the capacitive load seen by the rectifier tube if I'm following the description correctly, and 68 uF is at the upper limit for the 5AR4/GZ34 type -- a 55 uF or a paralleled pair of 30 uF ones will work fine.  Also, I don't know how old the preamp is but that electrolytic if it has enough hours on it is probably significantly off from 68 uF.

These zener-fed mosfet regulators are quite common and are basically the same as found in the Welborne Labs PS regulators and in the Van Alstine tube preamps.

-- Jim

[WGH]

NOS RCAs are also good and nowhere near the price of Mullard/Amperex.

If you can substitute a 5V4G, then Jim McShane has a RCA for $12.50 and a CBS for $13.50.
http://pages.prodigy.net/jimmcshane/tubes.htm
Nice big bottom end with NOS RCA 5V4GA's too.