JJ KT88s and other octals

[kruesi]

I wonder if anyone else has had problems with JJ KT88s, or if it's just me...?

I built the amp below a little over a year ago- works fine, no evidence of any oscillation or other misbehavior, running 50 mA quiescent bias. Then suddenly one day- GAWDAFUL SCREEEEECH POP BANG! and one of the KT88s lit up with fireworks inside.

Impressive, but not quite what I had in mind with the design.

Ordered a replacement pair, set the bias, looked it over really closely with the scope- everything fine- Until about 6 months later (two days ago), the OTHER channel exhibited the same symptom.

&#!+!!!!!

Can't see inside the KT88s too well but it looks like the screen is sagging, mis-aligned, etc. Will break the envelope and take a closer look soon.

I'm well within the ratings, running 570V on the plates, ultralinear, plenty of grid stopper resistance (a bit TOO large if anything), 50 mA bias. (I know the schematic says 600V - I actually measure 565V.)

I've heard a few bad things about JJ's octal production -but then, I hear bad things being said about almost everything. Opinions are abundant... -But this got my attention.

Anyone else have any problems with JJ's *octal* line? -I just love their miniatures, especially their ECC83S (12AX7, spiral heater) and EL84. So when it came time to build a larger amp, I chose their KT88s. Mistake?

I'm about to try Genalex / Gold Lion. Opinions?

Anyone see any big horrible problem with the schematic?

[rknize]

I had some issues with a new set of JJ KT77 about a year ago. The cathode slide out of the mica and shorted against something down below. Destroyed the rectifier in the process. I have a couple of their 6L6GCs as well. One is several years old and the other is younger. They have been fine. I got a replacement quad of the KT77 and they have been working fine for a year now.

I think they have had periods of quality control problems in recent years, but if you look at them in the guitar world they have a good reputation. The Saratov tubes have a good rep. The EH KT88 is very well regarded and the Genelex that seems to have descended from it has received glowing reviews for most everyone that I have heard try them.

[kruesi]

Thanks, Russ-

It's good to know that the quality issues are sporadic, not constant (if my issue really is a quality issue) -And good to hear that Genalex is highly regarded- they're my next choice. -that, or the EH at about half the cost of the Genalex.

I do see that the glass envelope on one of the JJ KT88s is not square with the base- I wonder if they have a mechanical positioning error withe the internal structure as well...?

[audio845]

I had 2 JJ KT77s go up in smoke on me a couple of years ago and haven't used JJ power tubes since. I've been using the Genalex reissue KT66s, KT77s, and KT88s and haven't had a single problem tube. They may be a bit more expensive but they seem to be utterly reliable and sound great.

Charlie

[kevinkr]

I've had plenty of problems with exploding JJ KT88 over the years, some are good and some are not. This is by no means limited to JJ either, I've had recent problems with EH KT88 as well as Tungsol 6550 reissues.

Run them anywhere close to the historic ratings and all of these tubes seem more likely to fail. (Mine run conservatively in regulator service and still fail periodically.)

The EH KT88 does seem better put together than is generally the case with theor so JJ.

In the past I've had pretty good results from the Winged C (Svetlana SPb) version of the 6550C and they sounded pretty good. (Don't confuse with the current SED versions which are made in New Sensor's Expo Pul plant.)

As always IMHO and YMMV.. Caveat Emptor.. Etc!

My only concerns would relate to the CCS's, in particular the one on the 12AX7A based diff amp, there can't be much voltage across that transistor, a well filtered negative supply rail of even -5V (derived from a non-floating filament supply perhaps?) would result in significantly better performance. Check to make sure this is not a concern with the CCS on the 6SN7 as well. (I had a huge problem in a line stage with a ring of two CCS running on the bias voltage of a 6SN7 diff pair running at ~20mA - there was not enough voltage margin with audio present to prevent the CCS from cutting off on the negative portion of the audio swing. Sounded terrible! )

[kruesi]

Thanks, audio845 and kevinkr - It's beginning to look more like it's them than me. Comforting...

If EH seems to be built well, I'm tempted to try them next- about half the price of the Gold Lion.

Yes- SED != Winged C Svetlana. -Unfortunate.

Running at 800V seems to be tempting fate... though I'd think they should hold off 550 - 600V OK if they claim 800V. -?

[rknize]

I think Kevin was referring to current "Svetlana" tubes sold in the US. These are Saratov tubes (New Sensor somehow grabbed the trademark away from the real Svetlana). "SED"/"Winged-C" is the old Svetlana, made in St. Petersburg. It is easy to tell the difference from pictures. SED tubes have large, rectangular alignment holes in the plates while Saratov 6550/KT88s have small, round holes. I recently had a "Tung-Sol" (Saratov) 6550 fail for no apparently reason. The tube just runs away.

Real SED tubes are very nice, but the price shot up recently making the Gold Lion's more attractive. I have a set of SED EL34 and KT88s and they are excellent tubes. I have a pair of 6550C too, but I haven't used them much.

[kevinkr]

Originally Posted by kruesi

Thanks, audio845 and kevinkr - It's beginning to look more like it's them than me. Comforting...

<snip>

Yes- SED != Winged C Svetlana. -Unfortunate.
<snip>

Actually it doesn't - that is what I was trying to say. SED = New Sensor Saratov plant aka Expo Pul. Winged C = Svetlana St. Petersburg, different plant and company not affiliated with New Sensor. (Understandable relations are somewhat hostile) Svetlana lost control of its name in the West when the joint venture SED went bankrupt a decade or so ago and New Sensor acquired the rights at the bankruptcy auction.

[kevinkr]

Originally Posted by kruesi

Thanks, audio845 and kevinkr - It's beginning to look more like it's them than me. Comforting...

If EH seems to be built well, I'm tempted to try them next- about half the price of the Gold Lion.

<snip>

Running at 800V seems to be tempting fate... though I'd think they should hold off 550 - 600V OK if they claim 800V. -?

If you've got the bucks for the Gold Lion I would go for those as they are a large step up in performance and reputedly have better QC and process control than the cheaper EH. I have had EH tubes fail recently. I would try to get these from a vendor who hopefully has an ironclad policy on replacement for early defects. I generally recommend Jim McShane as he takes some serious care in testing tubes and is selective about what he carries - avoiding types and brands he knows to be unreliable. (Note I have no affiliation at all)

I would not trust any current production KT88 beyond 550V and run them at no more than 525V. This will get you ~ 70W into 3.8K pp and is still not going to be 100% reliable with current tubes. Idle current at these voltages probably shouldn't be much over 50mA despite the so called dissipation ratings due to shoddy screen grid geometry and the resulting plate hot spots in modern versions of the tube.

Maybe Rich Walters will weigh in with some useful advice as he has huge current experience with beam tubes in power amplifier output stages. (These days I relegate them mostly to regulator service.. )

[rknize]

Hmmm...I didn't realize that little detail. Most of the tube vendors still refer to "Winged-C", built in St. Petersburg, as "SED". They make it a point to do so because of the confusion over the "Svetlana" brand in order to justify the higher price of them.