ST120 KT88 bias for long life

My ST120 has Sovtek KT88s that sound plenty good to my ears. I sure don't need all the power of the 120, so what's the best bias voltage for longest tube life? I just ordered a TDR board will order a WZ68 soon. Thanks...

Gregg

Gregg, do a SEARCH here on KT88 BIAS. Several good reads; I just set mine @ .525 or there abouts. Dale

It's not all about power. It may surprise you how often the tubes are tickling the top end when pressed for headroom.

Also worth noting, most tubes are designed to sound best at about 80% rating anyway. That's taken into account when setting recommendations for bias values as well.

Be interesting to note what others are using for KT66's, KT90's, and other equivalent types? Doesn't seem as if there's much difference electronically. I do know I as well as others get more PUNCH and clarity out of my KT120's running them at 60mV, but those are a whole nuther animal ...

I'd be more concerned about the rectifier than the power tubes in any case.

I turned on the ST120 today and it blew a fuse 5-10 seconds later. I replaced the fuse and installed my spare Sovtek 5AR4, turned it back on and all seems well. Last week, my son and I blew all the dust off my speakers for almost two hours, so I wonder if the rectifier wasn't stressed a little. I have the bias set now at .525 and wonder if .500 would give any additional tube life....

Gregg

GreggW wrote:I turned on the ST120 today and it blew a fuse 5-10 seconds later. I replaced the fuse and installed my spare Sovtek 5AR4, turned it back on and all seems well. Last week, my son and I blew all the dust off my speakers for almost two hours, so I wonder if the rectifier wasn't stressed a little. I have the bias set now at .525 and wonder if .500 would give any additional tube life....

Gregg

well, I actually set my KT120's at 0.500V. Very conservative, but they will last a heck of a long time!

Just spent almost an hour looking around the forum about WZ68s, rectifiers, and KT88s; there's a lot of information out there. Besides trying a WZ68, it's time to try one of the new higher quality 5AR4s. In the meantime, .500 it's gonna be.....

Gregg

New "high quality" 5AR4?

There's always hope.

I hear good things about the TungSol version, but it's hard to beat an old Mullard. A tasty little '50's production with low miles ...



Then again, it's getting hard to FIND an old Mullard ...

Still lovin' my GZ37 (got a spare, as those are getting right difficult to find as well) ...



... but I also got a WZ68 for backup.



As Tom Lehrer says .. BE PREPARED!!!

PS ... I also got the Chinese tube that came with the amp ... that sucker's scary ...

I missed the fireball, smoke, and didn't get to see a light show, but now I'm out of backup rectifiers. Time to go shoppin' for a Tung-Sol........

Lots of info on using a WZ68 in ST120s. Has anyone used the WS1 with a TDR? Looks like the WS1 is good for an amp. Does the WZ68 just emulate a tube better?

Seems to me the WZ68 has slow start ... the WS1 does not?

Don't get too worked up about all the "Not recommended for audio amplifiers" warnings on their NEW! Improved? website. They're geared more towards instrument amps and they've always said that, just not in writing. The copper caps have a real good track record with our amps.

If you read the small print, the WZ34 is the recommended replacement for a 5AR4. Then again, you can put twice the current through a WZ68 ... and it's the same price. So, why not?

You're right, and the $5 price difference won't even buy a 6-pack of decent brew.....

sKiz'

Those GZ37's are about the sexiest tube around.

The rectifier corollary of the Eimac 3-500z made popular in that other domain where tubes still hold some sway.

They DO stand proud amongst the other big trees, don't they? Even taller than the mighty KT120 it is ...

Must be careful, sKiZ',

This last picture verges on tube porn -- you may get a following of fans wanting you to build them similar amps. Or just supply video footage on line of those sexy tubes in glorious low-light audio action.

I know one tube seller who says the KT120 can be set from .40 to .65mV. The lower you go the longer they should last (theoretically).

If I quit eating for a week, maybe I could afford a GZ37. Just the light show in a dark room would be worth it....

I don't imagine anything we can do to a KT120 on one of these amps can hurt them.

I do remember Bob mentioning he'd run his up to 90mV with no issues. Also, no discernible improvement in audio.

... and Bob replies ...

Re: sKiZo > I have run the Tung-Sol KT120's up to 75 milliamps each tube (.750 VDC bias voltage) for hours as a test with my own VTA ST-120. The KT120 tubes will take this no problem but the power transformer will run warmer. I never went up to the .900 VDC bias voltage point for each output tube that sKiZo mentioned above nor do I recommend doing so.

My bad ... I sit corrected.    

I (as well as a few others) find they develop more PUNCH at 60mV, and the ST120 has plenty of reserve to drive them properly at that level. I'd think that would definitely have an impact on a 5AR4's longevity though.

NUTTIN' beats a Mullard GZ-33 for sheer testosterone. Make the KT-120s look like novals.
My tube pal Andy Bowman advises starting with .45 VDC/tube on the bias pots for KT/6550 type tubes, then advancing up to where you aren't hearing an improvement, which is somewhere between .5 and .55 for my ears.

Dave, are you saying you can hear a difference related to rectifier used?

Tube Nube wrote:Dave, are you saying you can hear a difference related to rectifier used?

Absolutely. It pertains to the B+ voltage drop under a load, or what guitar amp players refer to as "sag." 5R4 tubes have a lot of voltage drop, or sag; 5AR4 types, and GZ33 and GZ 37 Mullard tubes and the solid-state Webers have much less of a drop and are better suited for our purposes, as we are into reproducing sound, not creating it, with hi-fidelity, "fidelity" to the source material being the operative word.
As to the difference between solid-state copper-tops and and an antique Mullard tube, well, that's one them intangibles. But the Mullards look cooler.

Jazz guitarists like very little sag, and high headroom for CLEAN sound, where most rock and blooze guys like sag and overdriven sounds. Thus, high powered solid state or tube amps are popular for jazz, where small overdriven amps are quite trendy for blooze.

audiobill wrote:Jazz guitarists like very  little sag, and high headroom for CLEAN sound, where most rock and blooze guys like sag and overdriven sounds.  Thus, high powered solid state or tube amps are popular for jazz, where small overdriven amps are quite trendy for blooze.

AudioBill, I apologize for lumping real guitarists (esp. such fine ones as yourself) in with the other lot.

sKiZo wrote:I don't imagine anything we can do to a KT120 on one of these amps can hurt them.

I do remember Bob mentioning he'd run his up to 90mV with no issues. Also, no discernible improvement in audio.

I (as well as a few others) find they develop more PUNCH at 60mV, and the ST120 has plenty of reserve to drive them properly at that level. I'd think that would definitely have an impact on a 5AR4's longevity though.

A 5U4GB would give you the needed extra current to run at 60mA.  Even better would be 5V3 or better yet a 5V3A which takes 3 amps heater current.  The 5V3 and 5V3A will have lower voltage drop at 250mA than a 5U4GB will have since they are rated at  350mA and are easy to find on Ebay.  You can run the 5V3 which takes 3.8A easily with the uprated 5V transformer windings. I verified this with Bob.

David, I was intrigued to learn this too!!!!

Amazing how much one can learn on this forum. I'm going to look for a 5V3A to try in the ST120 also. Ive got a Genalex 5AR4, WZ68, and a TDR on the way. I'm going to do an unscientific rectifier comparison with the expectation that all will sound really good. My youngest son just hijacked my 1963 vintage AR2a speakers, but I admit they sound much better in his larger room. So now I'm running a Velodyne powered sub and a pair of small Polk towers: the ST120 just loafs. I could probably blow the speaker cones across the room. Remember the scene in Back to the Future?