[Scotlynn]

Need some help from folks who know more about tube tech issues than I do (which is a pretty big universe of people, no doubt.)

I bought a used pair of Bob Carver Cherry 180's a few months back. They came with the original set of Silver Eagle KT88's, which I believe were re-branded Shuguangs. After I got the Carvers, I ordered a dozen Psvane KT88 Tii's from Grant Fidelity after reading so many great things about them. I got them about a month ago after waiting for shipment direct from China - 2 were dead, but I installed the other 10 and kept 2 of the Silver Eagles in (after checking with Carver that it would be ok to mix KT88s.) I sent the 2 dead tubes to Grant Fidelity in Canada for replacement, but then over the next few days started to experience blown fuses regularly in my amps. In 3 days  I went through about 5 fuses, 3 on one side and 2 on the other. Grant Fidelity told me I could ship them all the tubes for testing, so I packed them all up and shipped the remaining 10 tubes to Canada, and reinstalled my Silver Eagles.

Three weeks went by, not a single blown fuse with the Silver Eagles. I just got the Psvanes back this afternoon, installed them and, you guessed it, blown fuse. I replaced it, and a 1/2 hour later, another blown fuse. I'm giving it one more chance and its been playing for a couple of hours now with no issue, so I'm hoping (likely beyond hope) that it was some loose particles inside the tube(s) that caused a burst, or some other issue that is now resolved.

Rachel at Grant Fidelity says it may be a "compatibility" issue related to "screen current." I have never experienced any problems like this before with tubes after owning several of pieces of tube equipment over the years and engaging in a lot of tube swapping.

Long story short, its been a very frustrating experience for me since I paid for these tubes (and they weren't cheap!!!) almost 2 months ago and have had nothing but problems, even having to pay to ship the whole lot up to Canada for testing. Before I give up completely (if they keep blowing fuses) I was wondering if any of you had this experience or could shed some light on what might be happening here and how to fix it. Or is there really such a thing as amp/tube compatibility?

Thanks in advance....

[underdawg]

you might end up only being able to use nos usa tubes which I have to do in the past with certain amps, wait till you see the prices of those tubes.

[underdawg]

I think I would contact Carver and see if he can update them for new manufacture tubes.

[Scotlynn]

Thanks, underdawg. I figure that since I had no issues with the Silver Eagles (Shuguangs?) that can't be the problem. But I am going to drop Carver's shop a note to see if he has any ideas.

[adydula]

Yup,

I would talk to Carver for sure.

If the tubes were indeed electrical equivalents they shoudl work just fine. I cant believe that they are if your not having any blown fuse issues with the original KT88's.

Does the amp play and sound ok before they blow?

The circuit the tube is placed in should be designed to allow the operation on the tube within its operating characteristics.....either the circuit or the tube is "changing" for some reason.

Tubes that short will pull lots of current and blow fuses...grid resistors will burn up as well and act as a fuse or change reistance due to tube shorts etc...

You might have had a tube short and possibly damage a resistor and have some issue with these tubes...but the fact that the originals still operate is baffeling, especially if the new Psvane's are indeed KT88's and not mislabled etc..

Will be very interested to hear what you find out.

All the best
Alex

[Scotlynn]

Thanks, Alex. I find it hard to believe that it could be anything other than a defect in the tubes. I did read that during shipping, particles can come lose and create issues.

[FullRangeMan]

Thanks, Alex. I find it hard to believe that it could be anything other than a defect in the tubes. I did read that during shipping, particles can come lose and create issues.

Unfortunately oxide cathode tubes are prone to loose dust with vibrations.
The good news are DHT tubes use thoriated wire cathode and are free from this prob.

[Ericus Rex]

Dumb question:  are you rebiasing the amp with the new tubes installed?

[Scotlynn]

Yes, fully rebiased. There were actually only very minor adjustments needed switching from one set of tubes to the Psvanes. I'm going to let them run a while today and really hope it was just a dust issue, but am not hopeful.

[adydula]

Tubes usually can take a lot of abuse....but to blow fuses something akin to a short is drawing a lot of current to blow fuses...I know you spent a bunch of bucks on these tubes.

I think I would get a set of EH Kt88's for $60 or so and try them and see of they work ok....if they do....well I would  get rid of these....

http://www.boiaudioworks.com/kt88?kw=kt88%20eh&gclid=CIL6zJqv1LUCFQ6xnQodqgsAcA

Alex

[FullRangeMan]

I would like to ear Mr.Bob Carver opinion on that issue.

[weitrhino]

It's entirely possible there are as few as 2 flakey tubes in the bunch, notwithstanding the 2 dead tubes in your original purchase.  (that can't do much for confidence in a product)  Try mixing in just one new tube per amp at a time and test for stability.  Continue to replace the Carver tubes one at a time till you have cycled through all of them.  You may be able to narrow it down to a particular tube(s). 

You didn't mention it, but if you've seen a flash at the time the fuse blows then you know some contaminant got vaporized into oblivion. I had one fuse blow last year after running my 180's four solid years.  Once the fuse was replaced there's never been another problem, although they sound so good I've never wanted to roll in anything different.

[Scotlynn]

Thanks for all the responses and suggestions, folks. Very helpful.

Tony from Bob Carver's shop did get back to me and he checked the specs on the tubes and can't figure out what the problem might be. He say's the 180's aren't picky about what tube they use, so there is no inherent "incompatibility." He kindly offered to have a look at the amps.

No blown fuses since yesterday. I'm going to give it a couple of more days, and if I get even ONE more blown fuse I have two choices:
1. Send the amps and tubes all to Bob Carver to have it all checked out, but them I have a big expense on shipping and without my amps for a few weeks, or
2. Return the Psvanes and try to get my money back, chalk it up to a lesson learned, install the original Silver Eagles and not skip a beat.

I'm leaning heavily towards # 2....

[Ericus Rex]

Some manufacturers recommend easing the tube into its final bias point.  I don't know the exact particulars but something along the lines of setting bias at 50% or so current and running the amp for a few hours then raising it to 65-70%.  Again letting the amp run for a few hours.  Then 90% and so on.  I've heard this method is better for the tubes then popping them right in at full bias.  Again, let me stress that I'm not totally sure about the percentages, hopefully someone here has direct experience with doing this.  I think I read this in the Music Reference circle.  If Roger recommends it I'd suggest we all follow his advice!  But I do agree with the others; this shouldn't be happening.  Also, fuses are blown by current surges, I believe, so it is possible that you'll have to bias them a little lower than usual.

Given their expense I think I'd just send them back.

[FullRangeMan]

Carver circuit in this amp seems very stable.
It just pop a fuse even using non standard tubes.

[Freo-1]

I would suspect the PS Vane tubes.  I had a couple quads that were dodgy, and wound up sending them back.  Honestly, the only reliable KT88's I had good luck with were Penta KT88SC's.  They sounded OK, but not as good as the NOS GE 6550As (but THAT gets pricey!).