I've helped my sole tubular buddy in town, for years, for free. He passes on old and or broken equipment to me, and I'm fine with that. I got his VTA ST-120, when it broke. I'm not a mental giant with an engineering degree, etc. Some folks here are. I was tech trained years ago at a JC, but learned much more here. I grasp the cool fix for protecting a tube rectifier from breaking down under reverse voltage, by adding a pair of diodes. Very cool! Here is my problem: The rectifier tube was toast, but it diminished the % of 'good' rating of that right PP pair of reissue Genelex KT88s to 30% each. The pair on the left register virtually 100%. I stuck a 5R4WGB in as a rectifier, using the 30% tubes (it's less risky), and the KT88 lit up properly, and no fireworks. My friend saw the T.S. 5AR4 sparks (fireworks in side that tube, and the also the pair of KT88s on the right, and turned it off immediately). He bought new gear. I loved the amp driving his panel speakers! I tested these tubes on my Amplitrex AT1000A. From this description, which could have been MORE concise, would it be safe to bet that a protected rectifier tube, would be a proper and complete fix? 2 New Genelex KT88s too, (ouch). With rectifier tube in place, and if all 4 of the KT88 sockets match voltage, pin for pin, am I probably OK? The preamp parts, sockets and virtually all the bits are on a circuit board (other than the adjustment pots), and though this amp was preassembled, it's a 'kit' so info is available, but I DO love prefer to point wiring. I do have a vacuum solder sucker-adjustable soldering station, so if I have to, I will dive in, I will. Appreciate your helpful feedback, which I've received here before. Thanks in advance for your support, and good luck with your projects! :>)
How would the 5AR4 damage he KT88s? Couldn't the primary fault be somewhere in the KT88 circuit ? Bias, maybe ?
Thanks
Thanks
Well, I hear what you are saying, and thank you. VTA-120s ruined many reissue Tung Sol 5AR4s, in other folks amps, and was considered important enough to be addressed by the company. Adding diodes blocking Reverse Voltage before the tube is considered the fix by the company, which was purchased by another, when Bob Latino retired. It was his vision of the Dynaco ST-70 with bigger trannys, stainless steel chassis, individual power tube bias pots etc. It could be the bias for both tubes were adjusted wrong by my friend, precipitating that event, and the stress broke the known weaker link in this amplifier design, overstressing the 5AR4. My buddy is pretty careful, from what I've seen. Heck I used a TS 5AR4 in my Scott LK-72 as well, without problems, and it was 'best tube to use, the built right, solid choice.' That was the word. As the failure rate of these rectifiers in VTA-120s is endemic, I think amplifier design is more the issue, as regards the TS which is still a good tube. Thanks for your attention here!
As Hearinspace says (greetings Ian), the KT 88 grid bias (negative) voltages should be carefully checked.
Arc-over in the Power tubes is a sure sign of something badly wrong, and should be fixed before trying new tubes.
Bad bias can mean excessive current demand in the power tubes (one or more of them), and this in turn can overload the rectifier. (So the rectifier may be the victim, not the culprit).
Other design faults in commercial amps that can destroy power tubes include: large positive voltage from the driver stage at power-ON; (need an oscilloscope to check; get a low cost battery powered one, if need be).
Use of paper in oil coupling capacitor to the power tubes (these often break down and force positive voltage on to the power grid).
In brief, look for excess current in the power tubes, either full-time, or at startup (before the cathodes are fully warmed up).
Arc-over in the Power tubes is a sure sign of something badly wrong, and should be fixed before trying new tubes.
Bad bias can mean excessive current demand in the power tubes (one or more of them), and this in turn can overload the rectifier. (So the rectifier may be the victim, not the culprit).
Other design faults in commercial amps that can destroy power tubes include: large positive voltage from the driver stage at power-ON; (need an oscilloscope to check; get a low cost battery powered one, if need be).
Use of paper in oil coupling capacitor to the power tubes (these often break down and force positive voltage on to the power grid).
In brief, look for excess current in the power tubes, either full-time, or at startup (before the cathodes are fully warmed up).
When changing power tubes always turn down bias pots to lowest possible value ( most negative voltage
at the grid) Then after power up slowly adjust bias up to the recommended value, alternating
left and right channel.
If any of the tubes current/measuring points don't react as expected ; stop and fix the fault before proceeding.
at the grid) Then after power up slowly adjust bias up to the recommended value, alternating
left and right channel.
If any of the tubes current/measuring points don't react as expected ; stop and fix the fault before proceeding.
I concur with your suspicion that the 5AR4 might be beyond it's capability. It's output (DC) current is rated at 140 mA, being well exceeded by two pairs of KT88's in class AB when driven. The 5AR4 might suffice for bias current only.
Best regards!
Best regards!
The 5R4 is a NOS?
The 5R4 and 5AR4 are two different rectifier.
Same base but different specs
With the 5AR4 russian you have to take a maximum care on voltage and current delivered. In some case also with the value of the cap on Vdc
I have seen some fireworks with this type of tube while the Mullard ( p.e.) works great in same position.
It helps because it is indirect heating so you can read the current flowing ( with a proper set up)
And the suggestion to put the -Vbias at the maximum negative then turns slowly is good.
Walter
My 5U4WGA, b, etc are 'potato mashers' from a few companies, but I favor Chatham versions. Cetrons, and such are fine too. They are a heavy tube, all with a thick, high sleeve, thick glass, and a flat top. Mine are NOS or test as NOS. The first time I saw some, was a 4 tube bridge in converted Altec 1570s, with Electra-Print output transformers, and a giant military surplus cap on top, (amp laying flat on its front), with SE GM70 output tubes through Doc Edgar speakers at a demonstration. They sort of looked like ice cream cones, to me(5u4WGAs). I asked and was told what's what. Too heavy horizontally for my upright 1568s and 1569s. I used them in DIY 1625 monos, and 7591 monos. I've used them because my transformers were 115 VAC, I have 120 VAC at the wall, and 5U4s have a greater voltage drop, and run the amps at a 'normal' Voltage. I have a couple 5AR4s, like Philips. So I use them for safety and I like how my gear sounds, although a reissue Tung Sol 5AR4 ran fine in my Scott LK-72. >>> Putting bias pots - Voltage, and then turning them up to spec is very good advice!
Thank you to each of you who replied to my yelp for help! I really appreciate your answers, and the time you took to respond to my issues. I will follow up on all of them. My test equipment is a Gen-Rad Inductance, Capacitance, Resistance tester, table top multimeters, adjustable power supplies, and while I own a decent Oscilloscope, though I haven't used one for years, (but I will begin to do so), and my ears too 🙂. Regards, Robert, aka buffstreo1
Great! When you figure it out, please post. Details back from one who actually did the work are helpful all 'round. Thanks.
This is how I killed my rectifier tube. It even says to do this in the manual, which I had not read. Expensive mistake.