Short rectifier life when VTA amp is plugged into a line conditioner

Bob Latino wrote:I have had a number of customers report to me on short rectifier life in VTA amps when the amp is plugged into a line conditioner rather than directly into a wall outlet. The only thing that I can figure is that some of the line conditioners have some type of automatic voltage regulation built in. When the line conditioner senses the large current draw and subsequent voltage drop when you turn the amp on, it tries to compensate by jacking the voltage back up. When the voltage rises quickly from the conditioner, the voltage regulator in the conditioner may over compensate and cause the voltage to rise too high which takes out the rectifier. I can't be sure that this is the problem and it probably doesn't happen with all line conditioners - BUT - those that have had the problem and have gone back to plugging the amp directly into a wall outlet have Emailed me and mentioned that their issue of short rectifier life has gone away. To be clear, this does not happen with all line conditioners but is probably more common with less expensive line conditioners.

Bob

Hey Bob, I'm wondering if it might be related to the adjustment of the power curve. I had a similar issue when using lower quality UPS's where they would adjust the power wave to something closer to a square wave vs the sine wave from the mains.

It would cause certain equipment (like fridges) to run extremely hot, or not at all.

This post describes a similar issue:
http://www.electrondepot.com/electrodesign/ups-sine-wave-vs-square-wave-output-51733-.htm

The solution would be to get a line conditioner that advertises a sine wave power profile.

David

Rich wrote:The best way to get a proper "line conditioner" is to use a variac with an AC line filter

IMHO the "best" way is to use a constant voltage transformer such as those made by Sola.

I dont' think I've ever plugged a power tube amp into an outlet. Always into a variac. I like to start it at maybe 90v for a few seconds then crank it up to 120v.
Never ever blown a tube amp or rectifier tube.

Bob, do these amps in question use the relay board option?  It sounds like the rectifiers are blowing on initial power due to inrush current demands for a brief moment.  Plus they don't make rectifiers as rugged as they did many years ago.  That is a hefty load on the ST-120 B+ circuit and quite demanding upon the rectifier when power is turned on (relay board or standby switch engaged).   Big can caps, supplemental cap board, low-gain board cap upgrades, and two chokes in parallel yielding 1/2 their inductance.   The current demand is large at power up until filter caps charge and voltage stabilizes.   And an AC power regulator will only add to the rectifier loading by keeping the line voltage from dipping and power factor corrected.  

But I like the abundant B+ filtering for sound quality purposes and suits me just fine with the rectifier sub.  The GZ34 datasheets usually have performance curves with added resistance on the two rectifier AC inputs to limit current.  These amps may need extra resistance to limit current.  And the easy solution with a standby switch is to bypass the switch with a 100K ohm resistor to allow the caps to slowly charge before the switch is actual engaged.

My one and only tube rectifier experience in the ST-120 blew on first power up and took the fuse with it.  It has been solid state rectifier replacement ever since.  Anyone have PSpice circuit simulator SW?  Would be interesting to know if the peak B+ current exceeds the GZ34 limit of 750mA at power on.  We might be surprised by the simulation current demand as voltage ramps up.  Also can compare the results against the datasheets for a solution if needed.

I have never had to use any sort of power conditioning with my VTA ST120.  I guess that our power company is an anomaly.  They provide a fairly stable line power.  I have monitored the power over a few days time and our power is low at 116vac to a high of 119vac.   I have never had a rectifier fail on it's own.  It has always been in combination with a power tube failure taking the rectifier and the fuse.   It may also help that our power comes from the same substation as our local hospital which gets preferential treatment whenever there are power difficulties.

I'm pretty sure my 122+ VAC has something to do with the 40K transmission lines that feed my house. Couple housing developments went in down the road and they had to upgrade from the 4K service that had been working well for me for 30+ years.

Never thought to check what I got prior to the upgrade ... bit late to check now.

On the positive side, it IS rock steady. I left my Kill-a-Watt plugged in for several days to get an average. That also gives high/low readings - fairly useless usually over a long test, but there wasn't a lot of fluctuation. Glancing at the meter every now and then always showed around 121-123 ...

That's why I figure I can get away with a couple CL-90's in the circuit. One inline on the main hot, and another on the B+ should drop me down to around 117-119 more times than not ...

This shouldn't be an issue with a simple isolation transformer -- correct?