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01-13-2008 Post does not mapped to Knowledge Tree
hagtech


Hawaii
Posts 117
Joined on 02-13-2006
Post #: 26
Post ID: 6341
Reply to: 6269
7788 ringing
I am having a LOT of microphonic trouble with the 7788s in a high gain stage.  I run a 40mV input signal into a 7788 triode at 15mA CCS.  Plenty of gain, nicel output.  But dang, if I bump the tube or board or chassis, the grids inside ring like a bell.  More of a shimmer.  I can measure one having a very noticeable 5kHz tone.  It's a shimmer with a strong 5k resonance, like a tuning fork.  This is not a ping that damps out quick.  It lasts for 10 seconds or more.  One sample is worse than the other.  On a scope it looks like I have a 5kHz carrier tone riding on top of the signal.  I checked the usual suspects (fluorescent lighting, etc.), but it came down to the tube itself.  No RF oscillations.

I'm not sure I can use this tube in such an application (like a phonostage).  Romy, how do you keep yours quiet?

jh
01-13-2008 Post does not mapped to Knowledge Tree
Lucian
Posts 3
Joined on 01-03-2008
Post #: 27
Post ID: 6343
Reply to: 6341
7788 Ringing
Hello,

While the question was addressed to Romy, may I be allowed to barge in :-)?...

A question before anything: do you feed the 7788 DC or AC on the filament?

Many tubes are perfectly able to oscillate mechanically on 1KHz--20KHz range, specially when they are very high gain (see your CCS load...). This self- or microphony- induced oscillation seems to be excited further by the 60Hz heater power (electromotor induction into the cathode). DC on the filament tends to minimize/eliminated this electromechanically induced oscillation.

Obviously, against microphony there is the normal array of measures that one can try: soft "lantern" shock absorbers on a separate tube plate and mesh shielding, but all this can only do so much... Microphony seems to be one unavoidable evil with tubes...

Lucian
01-13-2008 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 10,156
Joined on 05-28-2004
Post #: 28
Post ID: 6344
Reply to: 6341
My experience with 7788 is very different.
Jim, I do not know what to say: I never experienced any nose problem with it. A few years ago I built 2 mono phonocorrectors with 7788 at input as a pentode both of them were just nice and quiet and did not give me any problems in noise department. The phonocorrectors are still in service. Over the year I was trying a few different 7788 brands: Philips, Raytheon, Telefunken, Siemens, few otheres and none of them were noisy. I drive the 7788 with DC of course, typical for me SS rectifier with LCRC. The last cap is 80.000uF. The tube has 150V on plate and slightly less on screen, both drivers from the same 400V power supply, SS bridge – LCRC with an enormous last cap. The tube has one ferrite bid on grid, 50R grid stopper and uses Pearl’ shock-mount tube socket. I use as grounded tube cooler around the tube. Pretty much it is all that I did. I have no microphonic at all and noise at 150MHz (~200MHz in realty) scope looks very noise and random (I have no faster scope). I do pick microphonics if I hit the tube but it immediately dies. I never ever seen my 7788 get aroused itself in the way how 417A or 435A did. What is also amassing that all my 7788 were practically identical in gain, plate impedance and etc… Sorry, I am not useful in it as I never develop any experience to fight with 7788 as I never had any problems with it.

The Cat

"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
01-13-2008 Post does not mapped to Knowledge Tree
Paul S
San Diego, California, USA
Posts 2,657
Joined on 10-12-2006
Post #: 29
Post ID: 6345
Reply to: 6341
The trouble with tubes...
Why do we even bother with tubes for low levels?  Of tubes generally, I have used 6SN7s, 2A3s and EL34s with no known-to-me problems, but lots of various noise trouble with certain tubes, including 6DJ8 family, KT88s, and on and on, especially annoying in cases where I like other things about the tubes.  I have found quiet 6DJ8 tubes, but I have to go way out of my way for them.

One thing I have noticed, though, is that some problem tubes quiet down when operating points are lowered, where this is an option.  Of course, not all tubes are mechanically quiet, and most were not designed to be, either.  I use the old dark plastic (PETA, or something like that) Herbies dampers on tubes that are SUPPOSED TO BE quiet.

Jim, I know it's a stupid question, but how many 7788s have you tried, and from how many sources?  Haven't we all gotten/tried several bad tubes in a row at some point?

I don't know the 7788, but I am generally suspicious of too much gain from too low level, which is one reason why I prefer to jump start with a transformer.

I know you will find a solution!

Best regards,
Paul S
01-13-2008 Post does not mapped to Knowledge Tree
hagtech


Hawaii
Posts 117
Joined on 02-13-2006
Post #: 30
Post ID: 6346
Reply to: 6345
More Whine
I guess I need to try out some more samples.  Only have a few here.  And only one has a long time constant 5kHz ring.  Heaters are 6.2Vdc with 20mVrms sinewave ripple at 120Hz.  Fed from schottky - L - C.  Other side of heater is grounded.  Operating points are low, with only 80V on plate, at maybe 13mA.  Sort of in a bind here if the 7788 doesn't work. I'm re-using a low voltage B+ supply (only +105V) from another project.  With a choke on plate or CCS (line level output) I still get enough headroom.

Maybe I could use some step-up tranny.  Although they would be dangerously near the power supply chokes.  It's a small box.

jh
01-13-2008 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 10,156
Joined on 05-28-2004
Post #: 31
Post ID: 6347
Reply to: 6346
High transconductance and the Hawaiian archipelago.

Jim,

in my corrector (Dima’s design with R2=50R) I made many experiments adjusting the VR1. Dima proposed 150V on 7788 plate purely arbitrary, what he did it he never ever seen 7788 and it was just a concept RIAA for him. After it was built and I played with it turned out that 150V was a dead right voltage and I do swear that I was able to head what the voltage change to 160V and to 140V. So, you idea to “re-use a low voltage B+ supply”  might not be so economical after all…. BTW, I know that you are creasy person who like to put low bias tube at high-signal input… therefore I  hope you you’re your 7788 with it’s sub 1V of bias input at phono/mic preamp input not at line-level. At line-live that tube shell melt… It should not see more than couple hundreds mV AC on it’s grid…

Jim, I am very confident that you do something very wrong. I have practically no skills in the field of electronics DIY and the damn 7788 gives absolutely no problems to me – that makes this tube absolutely fool-proof even I was able to cook a phonocorrector with it. I mean you with you expertise shell be able to use the 7788 with no aggravations of any kind… Something you do is just wrong or you just have a bad party of the tubes.

BTW, I can testify that I did experience bad 7788. 5-6 year ego I bought on eBay a party of 150 tubes, they were all 7788 Amperex, all pulls and the price “Buy Now” was ridicules – like $40 or something like that. When I got the tubes a half of them were almost back – they were VERY severely burned out. I tested a few dozens of them that had clean glass and the almost all of them has no transconductance.  Among all party I pick 4 of 5 tubes that were more or less useable.  The rest I put back on eBay as pulls and went to somebody in Europe for $350. I do not know, I teach people for year never buy anything from me as whatever I sell is crap that I hate – people just do not listen. Anyhow, this good Amperex that I still have also had no noise problems. So, it is something on your side… You live on a volcanic island, perhaps the screen of the 7788 is too sensitive to those dally harmonic tremors…

The Cat

"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
01-14-2008 Post does not mapped to Knowledge Tree
hagtech


Hawaii
Posts 117
Joined on 02-13-2006
Post #: 32
Post ID: 6353
Reply to: 6347
Buy It Now!
I'll buy some more 7788 and give them a try.  I'd hate to change tubes at this point.  My triode connection has pin 8 (g3) to anode, not ground).  I wonder if that makes a difference.  Also, the power supply chokes are nearby, perhaps they are putting a 60Hz or 120Hz motor vibration into the chassis, which stimulates the microphonics.  Well, off to eBay.

jh
01-17-2008 Post does not mapped to Knowledge Tree
hagtech


Hawaii
Posts 117
Joined on 02-13-2006
Post #: 33
Post ID: 6368
Reply to: 6353
Too much gain
Maybe I was a little hard on these tubes.  I have this habit of listening to noise floor at extra high gains.  Maybe 20dB above loud.  It is part of the design process for me.  I like to hear all of the hiss, whine, buzz, hum, shimmer, and ping of everything going on.  I tap on all parts of the chassis and tubes.  This is much more effective than running measurements.

Anyway, I finally had the chance to listen to the tube.  The hum was not so noticable at normal playback levels.  And the tube microphonics were also much less than I had eluded to.  I could still hear the 5kHz tuning fork between songs.  However, then one channel went very bad.  The sound was like a hair stuck under a stylus while playing LP.  Huge distortions, most notably on the louder passages.  Funny, it took me awhile to cath on, as the other channel was crystal clear, and the brain was trying to sort things out and run real time corrections.  A quick swap of tubes from left to right and the problem went with it.  So I have a very baaad tube.  Sadly, the one with the 5kHz resonance is the good sounding one.

jh
01-19-2008 Post does not mapped to Knowledge Tree
hagtech


Hawaii
Posts 117
Joined on 02-13-2006
Post #: 34
Post ID: 6390
Reply to: 6368
Good News, Everyone
Romy, you were right.  I just got in another pair of 7788 from eBay.  This time, one tube has a little bit of microphony, not bad, really.  The other was dead quiet!  No ping at all.  Very impressive, that sample.  So now I guess I have to buy batches and select...

jh
01-19-2008 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 10,156
Joined on 05-28-2004
Post #: 35
Post ID: 6392
Reply to: 6390
The "art" of buying high transconductance tubes.

 hagtech wrote:
Romy, you were right.  I just got in another pair of 7788 from eBay.  This time, one tube has a little bit of microphony, not bad, really.  The other was dead quiet!  No ping at all.  Very impressive, that sample.  So now I guess I have to buy batches and select...

Well, it how it should be. The 7788, according to people who knows, at its max gain has the lowest noise among any other reasonable tubes. I have to admit that with exception the large party of the pulled Amperex tubes that I bought in the past (that were juts all severely burned out), and among all others 7788 that I have (perhaps 40-50 tubes of the few brands) I never come across any 7788 that was     microphonic. I did not plug all of my tube in my phonocorrector but I did plug one or two of each brand (perhaps a dozen brands and vintages).  Perhaps it was just luck or perhaps it was just the way how I buy them.

Talking about 7788…. I do not buy used tubes; I do not buy tubes that were measured; I do not buy tubes that have no original boxes. I do not buy single tube or a pair of tubes. I tend to buy the 7788 from a seller who has a party of NEW 10s or 100s of them. All of them should be sealed and then I buy my few tubes from that party. I tend do not buy the high transconductance tubes if a seller has a part of I would say 20 tubes and 2-3 of them without original box. To me it is bad sign saying that somebody who owned the tubes cared about them. Well, perhaps I am wrong, but not doing audio for living I know that if I sell anything then it is something that I really hate or do not need. Perhaps I am wrong… but I never seen the bad sounding or noise 7788. My experience with WE 417A/437A was very much different and at that times (~2001) I was buying variety of those tubes that did not comply with my “rules” above  and as a result was horrible – 80% of my WE tubes were horrible. I am not saying the exceptions are not possible but… still I would be very anal if I was in market for high transconductance tubes, particularly since the price for 7788 went up last years and a dozen of a good 7788 might cost over couple hundred dollars.

Rgs, the Cat

"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
01-19-2008 Post does not mapped to Knowledge Tree
N-set
Gdansk, Poland
Posts 617
Joined on 01-07-2006
Post #: 36
Post ID: 6393
Reply to: 6392
Other high-gm choices?
Guys, appologies for drifting a bit off topic (Romy fell free to remove the post).
Are there any other interesting high gm tubes?
Mu of 30-50 and Rk around 1.5k
is what I'm looking for.
Any experience with
the standard 6C45P, EC8010, etc?

Thanks,
jk

Cheers,
Jarek
STACORE
01-19-2008 Post does not mapped to Knowledge Tree
hagtech


Hawaii
Posts 117
Joined on 02-13-2006
Post #: 37
Post ID: 6394
Reply to: 6393
Other Tubes
That 8233 might do it.  Also noticed the 6CL6 in triode mode will get you almost there, but only a mu of 20.  The 6688 could be a substitute.

jh
01-19-2008 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 10,156
Joined on 05-28-2004
Post #: 38
Post ID: 6395
Reply to: 6393
High gain tubes...
http://www.pmillett.com/pentodes.htm

http://www.pmillett.com/images/pentodes.PDF

"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
02-17-2008 Post does not mapped to Knowledge Tree
Romy the Cat


Boston, MA
Posts 10,156
Joined on 05-28-2004
Post #: 39
Post ID: 6684
Reply to: 6368
Read my leaps: Absorb-GEL

Jim,

I do not know if you still fighting with 7788 noise but let me tell you something: those Allnic shock-absorbing tube socket adapters are insultingly affective. I act lay made some measurements and I was VERY surprised how much the Absorb-GEL eats vibration, I think it should a must for phonostage/mic first tube. In fact the effectiveness of the Absorb-GEL change the ways how the chassis design might be viewed…

http://www.goodsoundclub.com/Forums/ShowPost.aspx?postID=3333#3333


The Cat

"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche
Page 2 of 2 (39 items) Select Pages:  « 1 2
   Target    Threads for related reading   Most recent post in related threads   Forum  Replies   Views   Started 
  »  New  Where are our good phonostages?..  Omnigon Tubes...  Analog Playback Forum     61  665211  05-31-2004
  »  New  Shock-absorbing tube sockets...  Skinny pins and cheap-o sockets...  Audio Discussions  Forum     11  82126  12-21-2006
  »  New  The 6E5P tube data...  Bartola Valves: 6e5p beam tetrode SPICE model...  Melquiades Amplifier  Forum     44  491082  07-23-2007
  »  New  The Shielding Condoms on those tubes......  The Shielding Condoms on those tubes.......  Melquiades Amplifier  Forum     0  25858  09-21-2009
  »  New  An interesting Russian pnonocorrector...  Uber-tweeky phono topologies deconstructed...  Analog Playback Forum     9  99300  03-01-2010
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