AudioCircle
Industry Circles => Sonic Craft => Topic started by: David Ellis on 29 Nov 2018, 03:04 am
-
Bias help appreciated. Bottom Line, what impact should adjusting bias have in a wonderfully modified Golden Tube SE40?
I found this tool used to calculate the bias for a vacuum tube. This was very illuminating for me:
https://robrobinette.com/Tube_Bias_Calculator.htm (https://robrobinette.com/Tube_Bias_Calculator.htm)
I soaked some time in the calculator and think I understand the objective impact of changing the bias voltage in my Golden tube SE40SE . I am using old stock Philips double halo 6L6GC vacuum tubes. I think these tubes were also offered by Sylvania.
I historically ran the bias at 90% ( or slightly higher ). This worked dandy for about 10 years. Then, I red-plated a tube last week.
I found managed to purchased another matching double-halo 6L6-GC :D
In any event, yesterday I setup my bias at 70% in the SE40SE and THINK the balance of sound is indeed cooler with less bass, more midrange and slightly more treble. At 90% the sound seemed fuller with more bass.
I changed the 6SN7 tubes also, so my bias change is not an isolated change.
Have other folks puttered with this? What is the bias impact for bass in the SE40?
My SE40SE continues to sound terrific and be extremely reliable. My output tubes are lasting about 10 years (or more). This is extremely good compared to my Jolida 302b. That amp would eat a Svetlana El34 every 3-4 months.
Anyhow, any input would be appreciated !
Thanks!
Dave
-
"Bias 70%, 80%, 90% ?" not really understanding this? % of what? Bias the tubes around 600-610mV. The factory spec is 640mV, but that is running the tubes pretty hot. The amp is spec'd around the Russian Sovtek 5881/6L6GWC. That tube is far tougher than any old stock 6L6GC. The SE40 B+ will hit over 500V at start up and settle in around 500V. That is higher than most OS 6L6GC's can handle.
Have the power supply caps been replaced? What mod's?
There is a wealth of GTA knowledge available on the Golden Tube Audio Yahoo group.
Cheers,
Geary
-
Looks he is referring to per cent of the plate dissipation imo.
Jolida seems correct, any tube running 90% of dissipation will lasts few hours, 10 years are surrealistic time life to me.
-
Thanks for the read Geary,
I will do my best to respond.
"Bias 70%, 80%, 90% ?" not really understanding this? % of what? Bias the tubes around 600-610mV.
Please look at the attached link in my initial post, and plug some numbers. At 600mv across my 11 ohm cathode resistors, the result is darn close to 90% cathode plate dissipation. This is a "heavy" load for the power tube.
I suppose it would be possible to bias the tubes at 640mv, but this might be in the zone of intentional distortion practiced by some guitar guys with 6L6GC tubes. I am not confident that cathode current is the mechanism for this, but it seems reasonable. For me, I'd like my old stock 6l6GC tubes to survive.
My SE40SE was completely done by Jeff G. about 12 years ago. It has a full compliment of the very best "guts" possible. Before mods, this amp sounded awful, terrible, detestabl :oops: :duh:e. It was really, really bad. After mods 8) :thumb: :D .
The Jolida 302b was my first vacuum tube amp. It sounded darn good, and better after mods. I suspect that it's hunger for eating vacuum tubes was the result of not having a slow turn on circuit or some way to pre-heat the tubes before the B+ voltage was applied. I suppose the Jolida might have had a very high B+ voltage too. I wasn't as smart about vacuum tubes back then. I ain't super smart about vacuum tubes now, but my knowledge has improved somewhat.
Thanks for responding gents !!
Sincerely,
Dave
-
David,
My apologies for the delayed reply. I'm rather busy these days, but I did not expect to be a week late :oops:
First, I find the 6SN7s considerably influential in the se40, so it is hard to say what all my have been effected.
There are 3 versions of the owners manual. One basically implies the biasing target is 640mV; one....635mV; another implies you simply never want to exceed 640mV. I side with the latter. You never want to exceed 640mV. With an unregulated B+ and Bias supply, these babies are all over the place. I have a long version of my bias procedure, and a short version. I find them to work well with the two major levels of modification that we perform. The short is what I offer to the general public. But, bear in mind that this applies to a modified amp that tends to swing less than a stock amp.
Warm the amp for 3 minutes for stone cold. After the initial 3 minutes, you only have a 7 minute window it adjust for 610-615mV. If you exceed the collective 10 minutes, turn the amp off. Wait until it returns to stone cold. Start again by warming the amp for 3 minutes. If this can not be obtained within two attempts, something is most likely wrong. If any new tubes, or mixed up order, are installed, rotate all the pots to the full clockwise position before powering the amp. This amp can damage tubes within seconds.
Biasing low seems like a great maintenance move for some, but I find the amp too dark, and the collapse in spacial qualities unexceptionable.
-
Bias the tubes around 600-610mV.
That is a pretty good rule of thumb for a stock amp provided the operator does not take too long to do the adjustment.
The amp is spec'd around the Russian Sovtek 5881/6L6GWC. That tube is far tougher than any old stock 6L6GC.
Because they were neither 5881s or 6LG anything... They are 6Π3C-E Russian Military tubes, and are tougher than current Sovtek offerings. Further Solo (Golden Tube Audio), hand picked a low current range of tubes for their se40. It should also be pointed out that 90+% of NOS tubes are in fact OS tubes. These OS tubes are the culls, of culls, of culls... Moved from here to there, and shelf to shelf for over half a century.
There is a wealth of GTA knowledge available on the Golden Tube Audio Yahoo group.
And a fair amount of disinformation... Well, at least a dozen years ago when I was banned :green:
-
Thanks for responding Jeff,
I have accomplished some practical education with my amp and believe I have learned something.
Quick Summary: 90% and 80% plate dissipation in my modified SE40SE sounds WAY better than 70% plate dissipation.
At 70% plate dissipation (about 500mv drop across the 11 ohm cathode resistor) my amp sounds somewhat anemic. I didn't measure the results, but it sounded like about 2db of spl was gone from 50hz to about 100hz. And, the midrange was slightly thin sounding too - not very dynamic. I didn't like how this sounded. I even purchased a Kenrad VT-231 6SN7 for more bass. It didn't work. I suppose if the listening room were very absorbent acoustically and/or the listener really enjoyed dry orchestral recordings this might be viable.
80% dissipation solved the anemic audible sound (550mv drop across the 11 ohm cathode resistor). I have been living with this for a few days now and it sounds MUCH better. It's more dynamic everywhere... fuller... richer... the bass returned. I am darn happy with this.
90% plate dissipation is what I historically setup (600mv drop across the 11ohm cathode resistor) and I will listen and post my sentiment here in a couple weeks. My vague recollection is that 80% and 90% may sound very similar, but... I have changed a few other things and haven't really tested this is isolation. I will post my subjective thoughts herein when I have an isolated opinion on this.
This has been a a terrific learning experience for me. I soaked a few hours of reading and research into the what bias actually does for a vacuum tube. There are certainly patterns of accepted practices, but learning what works for my amp may not exactly reflect prevailing wisdom for "all" amplifiers having a similar configuration.
Thanks for your comments and continued support!
-
At 70% plate dissipation (about 500mv drop across the 11 ohm cathode resistor) my amp sounds somewhat anemic. I didn't measure the results, but it sounded like about 2db of spl was gone from 50hz to about 100hz. And, the midrange was slightly thin sounding too - not very dynamic. I didn't like how this sounded. I even purchased a Kenrad VT-231 6SN7 for more bass. It didn't work. I suppose if the listening room were very absorbent acoustically and/or the listener really enjoyed dry orchestral recordings this might be viable.
Other idea is lower primary impedance, you would check the B+ for any freak.
How much is the pri Z?
-
Be aware that increased bias (reduced voltage drop across the cathode resistor), decreases power output on this type of class A topology. Also, I've got the most bass from Sylvania chrome domes.
-
Yup, I did learn that lowering the voltage drop across the cathode resistor and therefore lowering cathode plate dissipation would lower the output power from the amplifier. I am finding that I don't really need full output from the SE40. Thanks for this Jeff!
And, Fullrangman, I am very sorry, but I am primarily a speaker guy. Could you provide some educational links providing a basis for your questions:
Other idea is lower primary impedance, you would check the B+ for any freak.
How much is the pri Z?
I looked for "Primary Z" in the glossary of Valve Amplifiers by Morgan Jones, and online too. I didn't find anything remotely relevant. I also checked the glossary for my RCA Recieving Tube Manual to no avail.
I am aware there really is close to 495 volts of potential between the cathode and the anode pins for the 6LCGC vacuum tubes.
I am happy to continue my education, but I need help/links to get smarter on these matters.
Sincerely,
Dave
-
pri Z = primary winding of OPT?
Do I get a pri Ze? :lol:
-
Sorry Dave I asked what is the primary impedance at OPT to see if it can be reduced to more output power.
You could use the same core if it is hi quality or order a new OPT pair in traditional EI or C core for a more open sound.
-
Do I get a pri Ze? :lol:
:jester:
-
Sorry Dave I asked what is the primary impedance at OPT to see if it can be reduced to more output power.
Ahhhh, okay.
I will check the bias in a couple weeks and will spend some time with my multi-meter and try to find the primary impedance of the output transformer. However, I am really not inclined to tinker with the transformers. I don't need more power and I believe the transformer quality is already darn good.
Randomly...
I DO believe there is a qualitative difference among transformers, but I THINK the transformers in my SE40SE are at least good. I had a delightful discussion some years ago with Jack Elliano at Electra Print. I don't remember if I telephoned him or if he telephoned me. In any event, after chatting with Jack for about 10 minutes I was convinced there IS a qualitative difference among transformers. Both Quality Control and the interleaved winding technique are significant and I believe Jack winds the good stuff. My belief in this has not changed.
My modified Golden Tube SE40 already sounds extremely good - vastly better than my unmodified Cary 572SE MK2 SET. The Cary amp has very good parts internally and had a super fancy Sterophile Class B rating when introduced. Cary even used what appears to be POI coupling capacitor and a few other better quality capacitors under the hood. The Cary 572 should have good iron. The 572SE MK2 should sound better than my modified Golden tube SE40 or at least closely similar in quality, but the comparison is not close. Also, several years ago I had the chance to listen to a test project on the Golden Tube and a Atma Sphere 60 watt amp at a friends home a few miles from my home. Yes, there were differences, but the modified SE40 Golden Tube was darn close. Perhaps the Bass from the Golden Tube was better and the midrange & highs were slightly better from the Atma Sphere. I convey all of this to convey that my modified Golden Tube isn't a "project dog" needing significant help. The Modified Golden Tube SE40 is a wonderful amplifier and the transformers must be at least "very good" in terms of their quality.
Next, the output transformers in my SE40 are whoppin huge! They are the size of a large grapefruit. This has 2 impacts. First, they barely fit in the chassis. Second, they would surely be very expensive to replace with something from Jack Elliano. If an ST70 PA060 transformer from Triodeelectronics costs $165, a fancy one from Jack should probably cost $220. The "whopper" in my Golden tube would probably cost twice that amount and then... would it actually fit inside the chassis?
I will convey what I find for the primary impedance of the output transformers when I check the amp in a couple weeks. I plan to check the bias on the vacuum tubes to ensure there isn't any wandering.
Oh, I do have a related question... how often should I check the bias on my vacuum tubes - perhaps annually or every 6 months depending on use ?
When I red-plated a couple weeks ago I hadn't checked the bias for perhaps 2-3 years. I am guessing this is probably too long.
-
Also, I've got the most bass from Sylvania chrome domes.
Ditto in this regard. The Chrome domes were my first entry into old stock Vacuum tubes and the sound was indeed extremely good! I purchased them from Andy at Vintage Tube Services and more recently from Tyler and Chelsea at TC tubes. I don't recall what previous "new" production vacuum tubes I pulled, but the chrome domes seemed like an undiscovered hifi "boon". They were darn cheap and sounded darn good! I think I paid perhaps $22 each for my initial chrome domes :) .
My recent 6SN7 tubes, Kenrad 231 and 1940s RCA bottom getter, might sound slightly cleaner than the chrome domes, but the impact is quite minimal and they replaced some aging (50%) chrome domes.
Having the ideal bias setup on my properly modified SE40 was/is vastly more importantly than having "fancy" 6SN7 vacuum tubes!
Nonetheless, I purchased another fancy black glass VT-231 for Christmas. :xmas: :D
-
Solo (Golden Tube Audio) was a transformer company. From what I understand, they accepted the contract to build the SE-40 because they felt it was a good way to sell transformers. IMO, the transformers are the best part of the SE-40. Primary Imp., core gap, etc... is much less important second guessing a working proven design. Do what you are already doing... Just listen, and decide.
I feel that one of the biggest problems with this amp is that it is biased too often. If you come back 20 hours (collective play time) after meticulously biasing this amp, the bias will be off. You could bias this thing until the pots fall apart. If your amp has been modified by us, or with one of our kits, it will dwell less. In any event, bias it well once, and do not re-bias until it gets dark, or the soundstage collapses. On an amp that has our mods, good tubes, and has been biased properly, this should not occur in less than a year. It may not occur for years.
-
It occur me that it can be the output tube, not amp fault.
Some tubes are know to have a very stable bias as GM70,
other as 6C33 are know to have a instable bias as normal.
-
On an amp that has our mods, good tubes, and has been biased properly, this should not occur in less than a year. It may not occur for years.
Thanks much for responding Jeff! I will make this an annual check.
In retrospect.... yesterday I found my review on Audioreview for the SE40 dated 2005 - 13 years ago :o. Candidly, I forgot that I have been using this amp for 13+ years. There was certainly a span of time where I didn't do anything to this amp 5+ years. I haven't maintained meticulous track of tube replacement, but the maintenance and stability have been exceptionally good :thumb:.