Is my pre a tube killer?

[dogwan]

Years ago I rebuilt my Dynaco PAS into the Norman Koren Spiced PAS.

http://www.normankoren.com/Audio/Spice_preamp.html

At the time I was lucky enough to have a nice stash of vintage 12ax7's. Mostly Telefunken's and Siemens from Germany. I really love this pre. The sound is killer.

Now at one point I thought about I would re-tube with modern issue Mullards and save my vintage ones. That was a bust as I had 2 out of 6 tubes fail within a couple days. They were returned and I happily returned to the Tele's.

Flash forward to this year when I bought a used ARC SP-16 that came with good Electro-Harmonix in the line stage, and Sovtek LPS in the phono stage. I was using the ARC for a while and decided to try the Tele's in it. SO, I moved the EH's and Sovtek's to the Spiced PAS. I used the PAS a couple times in another set up and all was fine when it was stored later. Then I decided to switch all the tubes back and when I tried to fire up the ARC with the EH's and Sovtek's I had no sound. Problem was traced to one Sovtek that tested completely dead on my tube tester. Nada, nothing, zip. WOuldn't even register on the emissions test.

So, this has me thinking that maybe the B+ in the PAS is killing modern tubes. The B+ is supposed to be 320v on the CF of the linestage (TU6) , and 280v on the CF of the phono stage (TU5). I actually did drop it down a touch and it actually measures 308v and 275v respectively.

Is this to high for modern issue tubes? The Tele's and Siemens just keep happily trucking along. Am I going to be stuck using vintage tubes for the life of the unit?

 

[primosounds]

So, this has me thinking that maybe the B+ in the PAS is killing modern tubes. The B+ is supposed to be 320v on the CF of the linestage (TU6) , and 280v on the CF of the phono stage (TU5). I actually did drop it down a touch and it actually measures 308v and 275v respectively.

Is this to high for modern issue tubes? The Tele's and Siemens just keep happily trucking along. Am I going to be stuck using vintage tubes for the life of the unit?

What voltage are you measuring? Is it the plate voltage at the 12ax7 pins? That is the voltage the is significant for the tube.
But it is not surprising that the vintage tubes are outperforming the currently made ones. At the time that the Telefunken and Siemens were made, tube technology was at its zenith.

 

[nj pheonix]

Sort of shooting in the dark.
Tubes are pretty resilient.
I would test all voltages on all pins (including filament)
Also if you could do peak hold from cold test.
Perhaps they're seeing some kind of surge at power up or down.
Last thought (long shot)
This doesn t leave unit powered (fully / partially) when powered off? I'm just thinking (maybe?) Tubes being harmed during off (standby?) Cycle

 

[gadget73]

Its also not impossible the tube was just a dud and being moved broke some marginal mechanical connection. Unless this becomes a habit, I don't know that I'd worry about it just yet.

 

[dcgillespie]

Was the tube lighting up in the tester? If it tests completely dead (no emission at all) in both sections, that more sounds like an open filament, as many testers test the tube with the heater operating in 12.6 volt mode. Therefore, if the heater in either section is dead, then both sections will test dead. As suggested, I'd check the heater voltage, but in BOTH units to make sure there isn't an issue there causing the heaters (and therefore cathodes as well) to run abnormally hot.

Dave

 

[primosounds]

So, this has me thinking that maybe the B+ in the PAS is killing modern tubes. The B+ is supposed to be 320v on the CF of the linestage (TU6) , and 280v on the CF of the phono stage (TU5). I actually did drop it down a touch and it actually measures 308v and 275v respectively.

Is this to high for modern issue tubes? The Tele's and Siemens just keep happily trucking along. Am I going to be stuck using vintage tubes for the life of the unit?

One tube going bad is not really something to get worried about, especially starting to doubt the working condition of the preamps. It would be more worrisome if you had a bunch of failures from the same equipment. Like others suggested take voltage readings and if they fall within 5 to 10% of the "bogey" value then your equipment fine and a random failure is to blame.

 

[triode17]

I have a question, how are you getting gain from a cathode Follower?

 

[GordonW]

I have a question, how are you getting gain from a cathode Follower?

There's two stages of gain, then a CF on the output.

http://www.normankoren.com/Audio/Spice_preamp_2.html

Regards,
Gordon.

 

[Bob]

had about 6 pas2/3s in the past. the 12ax7s were not overstressed. in fact, the Telefunkens
kept their specs for a long time - IOW pulling them out of an PAS meant they were good as is.

the PAS ran the tube heaters at the bottom of the range - I recall two heaters run in series at
about 22-24v not the mean/median 25.2v called for by spec.

a long time ago the Sovteks were not as reliable as they are now.

lastly, I also recall the ARs (audible illusions models) ran their tubes hard.

when I worked on the PAS, I'd always first lowered the B+ by adjusting the dropping
resistors in the PS but all 6 always had higher than dynaco spec on the B+ feeding
the line stage and the phono stage. BUT all the tubes in these were always OK.

I suspect its the Sovteks being run a little harder in the ARs. I think AR can send you
tubes that will work better than PAS pulls. However, I'd have that AR inspected for
in-spec voltages.

 

[dogwan]

Thanks everyone, maybe I won't worry about it.

FWIW, I have measured all the voltages on all pins in the Spiced PAS several times since the original build/conversion and it has always been within spec of that article.

I also didn't know that ARC was known for running their tubes hard. I thought it was the opposite, but I don't know where I got that idea.

 

[Bob]

all this stuff about hoarding tubes began in the era of the AR sp8/10 preamps where
the owners opined how great certain tubes were and the hoarding began. certain
tubes were bought and stashes created. and we're not talking about buying one or
two - we're talking about buying hundreds - and back in that day hundreds were
a lot cheaper than a NOS quad today.

a lot of designs from that era drove the tubes at design centers and beyond,
so a handy supply of replacements was mandatory. the 6922 was also used
hard in some preamps.

then folks realized that other off-mainstream amps using 7868/etc, another tube
of out production created yet another vacuum.

then the wars in 1990s destroyed tube factories in the
former Yugoslavia. and the venerable 7199 went out of Sovtek production
didn't help the 300K+ Dynaco ST70s and however many were left.

in turn the telefunken 12ax7s were the next target - folks found out the
Teles were worth far more than the PAS and they were being harvested
and sold. then there was that one guy who claimed the smooth plates
were better and the panic was full-on. I found the ribbed plates better
and there are certain markets where they are worth 2x over smooth plates.

then the Mullard el34 with XFx etchings (like Picassos) were the rage. and
still are.

I may have remembered some of the above details wrong but it's due
to age ( where were you in 1982? ) but my lessons were
1. if you're going to panic over things - make money at it in different markets
2. I am super happy that today's modern re-pros/re-makes sound better
than some totally used tubes sold as NOS, are cheaper, more available,
more consistent, more reliable.
3. always have a plan B so you're not left holding that 70# bag (amp)

so for Dynacos and others that are older than most grandfathers:
1. no sense in keeping it in original condition - upgrade the boards, parts, etc
2. on the new board mount resistors on the top with at least one component clearance
3. go higher voltage caps (I recall PAS using 450V can caps and the voltages reached 465+)
get the JJ 500v versions, wire underneath if necessary or do #5 and pray
4. replace the doh! selenium anythings
5. bring the PS B+ taps back down to schematic spec or even lower (like for heaters)
6. place a (new lo noise computer) fan powered by wall wart at the back and suck the heat out
7. get really good sockets if you're going to roll the tubes
8. buy several sets of tubes, listen, cull, and buy more of that same good sounding set
9. then if you really have energy, replace CC resistors with CF/MF/foils/etc and film
caps with poly/styrene/propylene/Teflon/mica/c0G until you reach the limits of
your ears/music/equipment/budget

in all cases, enjoy the music