Little Dot Tube Amps: Vacuum Tube Rolling Guide - Page 51

Today the Sylvania 6CB6As arrived:

I did a quick A/B with my GEC CV4014s and yes, these Sylvanias seem to have the same level of gain as the EF91s. I think the GEC's bass is a bit bigger, but not by much, and the GEC's upper midrange is a bit brighter. However, I find these Sylvanias very balanced and musical. I think it is great to finally have a good selection of cheap tubes available that can be used with the EF91/2 settings.

Oh.. The blue line you see on the tube is some kind of reflection. It is not on the tube itself.

Thanks for the tip! That'll definitely keep me awake for a few nights...

After a bit of fact checking about all those tubes, and others I've read about, I decided to try a pair of Brimar 6BH6 I had lying around, since they seemed to have a compatible pin out and were similar to the 6BJ6 mentioned in that thread (150 mA current draw, so less than a 6AK5 and half of a EF91).

It didn't explode, actually it worked pretty well for a mystery tube. I was surprised by how airy and detailed it sounded, but the bass was crappy and it wasn't as musical as my favorite tubes. At least, I don't think I heard distortion but I could be wrong.

Ok, I'm convinced. I'll be rolling new families of tubes in my LD amp and reporting back. The families I'm most interested in are all the high current draw high gain ones, >300 mA. I see the guy in the other thread tested two or three tubes that draw 400 or 450 mA, I'd be curious to see if the amp can sustain that without massively overheating (as it is feels quite warm with EF91 in the winter as it is), and how it sounds.

Great, this is getting interesting! So, would you say these sound somewhat like a higher gain CV4015 (balanced and musical, but maybe a bit more punchy)? Out of curiosity, did you order any other brands too? Any date code on these, just to get a feeling of what time period they were made in?

Acapella, yes you understood what I'm trying to do here. I will make my MK IV drive orthos properly, whether it wants to or not! Well, at least I'll try... Since there is no doubt my CV4015 just aren't giving enough power to the power tubes in my amp to drive HE-500 correctly (I get all the nice OTL perks, holographic soundstage, dynamics and liquidity, but I can tell the bass is just not keeping up like it was even on the HD650 or even the HE-400). Whereas, my GEC CV4014 seem to be *just* enough to drive the HE-500 to maybe 80-90% of what it can do, and still has all the OTL goodness.

Anyway, interesting endeavors. I feel a whole dimension just opened before me.

Great, this is getting interesting! So, would you say these sound somewhat like a higher gain CV4015 (balanced and musical, but maybe a bit more punchy)? Out of curiosity, did you order any other brands too? Any date code on these, just to get a feeling of what time period they were made in?

Acapella, yes you understood what I'm trying to do here. I will make my MK IV drive orthos properly, whether it wants to or not! Well, at least I'll try... Since there is no doubt my CV4015 just aren't giving enough power to the power tubes in my amp to drive HE-500 correctly (I get all the nice OTL perks, holographic soundstage, dynamics and liquidity, but I can tell the bass is just not keeping up like it was even on the HD650 or even the HE-400). Whereas, my GEC CV4014 seem to be *just* enough to drive the HE-500 to maybe 80-90% of what it can do, and still has all the OTL goodness.

Anyway, interesting endeavors. I feel a whole dimension just opened before me.

Unfortunately, I don't see any date code on these tubes and the seller didn't provide any info either. I have also ordered a pair of Tung-Sols and the Russian military equivalents, 6J38P-EV (1978), but they haven't arrived yet.

I haven't had a chance to directly compare them to my Mullard CV4015s. Going only by memory, I would guess these Sylvanias to be more similar to the Mullards than the GECs. When I get time I will try to AB them.

6J38P-EV aren't the same as 6CB6 from what I've read (0.19A vs 0.3A), but closer to a 6BH6 tube (0.15A) like the one I just tried, yet still different, even visually. I keep seeing some confusion on google and ebay between the 6CB6A, EF190 and E90F 6BH6 types, mainly on all the Russian ads. I think I'll hold off on the Russian "equivalents" for the 6CB6A tube for now (there are other Russian tube families I can try first and that are clearly identified).

Anyway, I'm still curious to hear how it sounds (and if it works as I can't find the pin out for these tubes).

Oh... I so was looking forward to these Russian military tubes!  But from your experience, it seems that I will soon receive some worthless tubes.... :(   And I have learned a valuable lesson.  When I am dealing with relatively unknown tubes, I should not simply trust the posting language, I need to check specs as well. If you do find some Russian tube families that work well, I trust you will let us know. :)

About the time period for these Sylvanias. On the box, the address for Sylvania is 1740 Broadway, New York 19, N.Y. Postal zones were replaced by the zip system in 1963, but of course, this did not happen overnight. So I would hazard a guess that these tubes were made before 1970.

I found a datasheet for these : http://www.prekyba.biz/datasheets/download?f=6J38P.pdf

These actually seem to be closer to a 6AK5 tube in terms of specs : http://frank.pocnet.net/sheets/049/6/6AK5.pdf

With my limited (non-existent) Russian skills, I'm seeing compatible pin-out with a 6AK5 or most other tubes I've been investigating for the last few days (if I consider the c1, c2 and c3 to be the same as grid 1, 2 and 3 in English, and so on for the other letters for the heater, cathode and anode), though there is still a mystery sign on pin 7 which could/should be the shield, so OK. 6.3V 190mA, pentode, similar specs compared to 6AK5, slightly different useful frequency band but I can't read what it's technically used for though. The plate also looks closer to a 6AK5-ish one.Any Russian speakers here?

Well, if it explodes when you try it, do tell us... I can't say I'm quite confident on this one.

Thanks for the datecode estimate!

This is interesting! The 6AK5 is 175ma, the Russion tube is 190ma and the EF92 is 200ma. It is my understanding that the EF92 has the same pin out as a 6AK5, but with 200ma instead of 175ma. This suggests that I might try both ways, with the EF91/92 jumpers on and off. Thanks for digging into this!

You know, this is actually something I was wondering about earlier. What is the actual difference between the EF95 and EF92 setting? Pin out (doubtful), plate voltage? This isn't exactly trivial as apparently, using EF92/91 with wrong setting makes sound like schiit (as in not enough power and flat), but using EF95 with the wrong setting seems to be more problematic (too much voltage on *something* in the tube?).

Again, don't take my word for it considering compatibility, a few more informed opinions wouldn't be too much here.

My source is a posting by "Dept_of_Alchemy" in the Little Dot 1+ tube rolling forum:

http://www.head-fi.org/t/364043/little-dot-i-tube-rolling   (See post number 3 on page 1)

~~~~~

Yup, EF92 (and even more so EF91) has more gain, and I understand there is a pretty linear correlation between anode current draw and essentially "power" output in the amp , but that still leaves me wondering what the jumper settings actually change. When you look at the specs for many of these tubes, few settings actually vary a lot, and that is especially true between EF92 and EF95 (despite having totally different constructions), yet they won't work properly on this amp with the wrong jumper setting, just wondering why.

Take the 6BH6 I tested for example. It should be very similar to the EF95 and your (future) 6j38p-ev. But I tried it on the EF92 setting and it seemed to work fine, despite having a 150 mA current draw, which is less than a 6AK5/EF95 tube.

I've gone on a binge ebay tube buying, I've ordered 35-40 (decently priced) tubes already, and I'm not quite done yet. You can expect some major feedback in the next couple of weeks! My wallet is weeping, but oh well it's not that much money compared to an amp or pair of orthos (although it amounts to a lot over a few months).

35-40 individual tubes?  Or 35-40 pairs? I have about 25 pairs of tubes and I sometimes think I have gone way overboard! ahaha.... And as I have an LD 1+, 7 of those pairs are 408As. Anyway, I am really looking forward to learning what you end up buying and reading your impressions. And of course, when I finally get those soviet tubes, I will report back.

Also, I have been trying to A/B my CV4015s with these Sylvanias and to my ears they sound very similar. That said, it is pretty difficult to match volume while switching back and forth. Further my headphones are nothing to brag about. But again, I am quite surprised how similar they sound given their construction is so different.

Great fun! :)