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Little Dot Tube Amps: Vacuum Tube Rolling Guide - Page 275

post #4111 of 10498

On a slightly different tack, but after all we are all trying to squeeze the last ounce of performance by tube rolling are we not? And sometimes paying quite dearly for it, lol!

 

Something I had been mulling over for quite a while - brought to a head by a VERY annoying tendency for a channel to disappear when waggling the headphone cable! - and that was to see if I could get similar improvements to when I replaced the internal wiring of my Dynaudio speakers, and did away with the binding posts, soldering direct to the board. What a massive difference that made - using pure silver wire, mind you...Anyway, as I had to undo the Neutrik socket to see what was going on, my mind was made up there and then - so-called "upgraded socket" on the MKIV SE? - If this is the best Neutrik can do, I would advise EVERYONE to get rid ASAP!! Actually, when you think about it, it is rather silly to go spending a lot of money on upgrading cables/plugs/mains filters etc. etc. for the lovely new signal to get trashed by the headphone jack/socket combo.

So it is I duly removed said culprits (actually, the cable fitted nice and snugly through the hole in the socket) and soldered the silver darling direct to the board...gone is the aforementioned occasional loss of channel, and even before the new solder has settled in I am seeing positive results viz - the Tesla E88CCs (with gold pins and grids) that have just recently gone in, and which were sounding rather lacking in the bass department compared to the Telefunkens (mind you, it is early days of course...) are all of a sudden almost matching the TFKs!! So this is boding VERY well indeed - can't wait to get the latter back in and see what transpires...my lips are smacking already...

 

The upshot is, of course, if you have just basic soldering skills you might want to give this tweak a try...the front panel on the MKIV SE comes off easily enough (don't know about other models) and there's plenty of room to work on the solder points. (The volume knob does have a VERY tiny allen inside, but in the end mine was dud and the knob was actually being held on the spindle by a piece of insulating tape!!...Mmmm...)

 

I am a firm believer in having as few 'connectors' in the signal path as is possible - as you may have gathered...

 

Cheers!

post #4112 of 10498

I received a pair of CBS 5670 / 2C51 / 396A today. Black plates, D-getter. Squat little guys that remind one of a 9-pin 6AK5....

 

 

But first... While burning in the 1965 US-Amperex 7308 and the 1975 Voskhod 6N23P with the getter attached to a gray shield, I have been able to listen to them only casually. Even so, my impression is these are among the very best of the best, and I recommend them without reservation.

 

However, have put them aside in order to fire up the CBS 5670. With the breadboard 9-pin socket, even though I had to reconfigure all the connections but one, it was a relatively easy task. As with the 6DJ8, I found it necessary to connect the shield (pin 5) to cathode #2 (pin 8).

 

Like so:

 

9-pin socket        Left Vector     Right Vector

 

1 heater                                      4

2 cathode #1             2

3 grid #1                   1

4 plate #1                 5

5 shield to pin 8                           

6 plate #2                                   5

7 grid #2                                     1

8 cathode #2                               2

9 heater                                      3

 

So.... They light and play.... No distortion or weird noises..... In fact seem very musical....  But way too soon to say how good they are relative to the 6DJ8....

 

Anyone else feeling adventurous enough to follow me? lol :)

post #4113 of 10498

Finally got my sockets today after a 5 week wait ,so starting to assemble the 6v and 12v adaptors as for the octal adaptor thats on the way another 3 weeks possibly that adaptor is plug and play.   As you can see its simple but made to last.                                                                                                                                                                                                                                                                                                                                                                                                                                                                 

post #4114 of 10498
Quote:
Originally Posted by gibosi View Post
 

I received a pair of CBS 5670 / 2C51 / 396A today. Black plates, D-getter. Squat little guys that remind one of a 9-pin 6AK5....

 

But first... While burning in the 1965 US-Amperex 7308 and the 1975 Voskhod 6N23P with the getter attached to a gray shield, I have been able to listen to them only casually. Even so, my impression is these are among the very best of the best, and I recommend them without reservation.

 

However, have put them aside in order to fire up the CBS 5670. With the breadboard 9-pin socket, even though I had to reconfigure all the connections but one, it was a relatively easy task. As with the 6DJ8, I found it necessary to connect the shield (pin 5) to cathode #2 (pin 8).

 

So.... They light and play.... No distortion or weird noises..... In fact seem very musical....  But way too soon to say how good they are relative to the 6DJ8....

 

Anyone else feeling adventurous enough to follow me? lol :)

 

Nice! So the 2C51 type not only lights up and plays music (which we all kind of expected at this point) but also sound at least "OK" for now. Well, I guess that just gives a whole new field to investigate then!

 

Will follow on the 2C51 type at some point; right now I'm too busy to even follow up on my own ideas (yeah, that a conundrum for you lol), just like for octals, which I have to even test (though someone else had the same idea a week ahead of me!)... Also, the high prices of the 6922/E88CC & 7308/E188CC tubes I keep looking at are making me instinctively try to save money, or to at least rationalize my spending (they're definitely worth it though, as you, me, and a few more lads keep mentioning).

 

Again, anyone have a Western Electric 396A in his stash? So, we can just figure whether or not it's worth pursuing the 2C51 type (preferably before much collective cash has been invested)?

 

On a lighter and more general note, I use my MK IV SE on gain 10, because it both sounds the best to my ears (whether with my previous HD650 or current HE-500) and because it gives me the most "power" (read volume, but it's actually the least amount of feedback, as in "the least amount of reduced volume due to global negative feedback").

 

I've always found the gain x3 setting, while not unpleasant, to sound quite garish, with boomy loud bass and a overly tubey-warm sound. Gain x4 and x5 improved that quite a bit (I recall actually using gain x 5 with Senns for a few weeks, way back when, without feeling it was horrible), but ultimately, gain x10 gives me the most detailand hi-fi-ish sound, and, as some have said, let's the traits of all the tubes shine though. So, if you have bad tubes and cheap headphones, use any gain setting but the x3; if you have good or excellent tubes and high Z or low sensitivity headphones, use gain x10 (imho, of course). If you use the amp as a preamp, I haven't actually managed to figure out what sounds the best, despite trying. Then again, I hardly ever use my LD as a preamp anymore (I use to quite a bit, now only sporadically).

post #4115 of 10498
Quote:
Originally Posted by Audiofanboy View Post
 

 

Nice! So the 2C51 type not only lights up and plays music (which we all kind of expected at this point) but also sound at least "OK" for now. Well, I guess that just gives a whole new field to investigate then!

 

Again, anyone have a Western Electric 396A in his stash? So, we can just figure whether or not it's worth pursuing the 2C51 type (preferably before much collective cash has been invested)?

 

I am pretty impressed with the CBS 5670. Unfortunately, I am not familiar enough with this family of tubes to know if CBS actually made them, or if these are simply rebrands. They resemble some GE and Sylvania I have seen, but not exactly. Anyway, they sound good, with a balanced tonality and good detail and clarity. But as I have been listening to 6922 and 7308 lately, I don't think these CBS tubes are at that level. So for sure, I would like to try the best of the breed. And yes, if someone has a Western Electric or a Bendix  2C51 / 396A / 5670 they would willing to send me, that would be great! lol :)

 

Also, as the 9-pin socket has to be entirely rewired to switch between 6DJ8 and 2C51, it is not at all easy to compare them. However, there is an adapter which would make this nice and easy. The socket can be left in the 6DJ8 configuration and 2C51 could be used with this adapter.

 

http://www.ebay.com/itm/260723522614?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

post #4116 of 10498
Quote:
Originally Posted by TrollDragon View Post

Artsi are you building a DIY amp? If so check into the "I (Heart) Tube Amps" group on Facebook, those guys there really know their stuff.

After trying couple different schematics with my DIY 6SL7 6A6 amp i have come to conclusion that Little Dot MK II is very compact, powerful, clean sounding and hum-free device. It is hard to beat with any homemade contraption.

 

Most problematic with my DIY headphone amp seems to be variable humming when turning volume. 

Soldering continues...

post #4117 of 10498
Quote:
Originally Posted by gibosi View Post
 

 

I am pretty impressed with the CBS 5670. Unfortunately, I am not familiar enough with this family of tubes to know if CBS actually made them, or if these are simply rebrands. They resemble some GE and Sylvania I have seen, but not exactly. Anyway, they sound good, with a balanced tonality and good detail and clarity. But as I have been listening to 6922 and 7308 lately, I don't think these CBS tubes are at that level. So for sure, I would like to try the best of the breed. And yes, if someone has a Western Electric or a Bendix  2C51 / 396A / 5670 they would willing to send me, that would be great! lol :)

 

Also, as the 9-pin socket has to be entirely rewired to switch between 6DJ8 and 2C51, it is not at all easy to compare them. However, there is an adapter which would make this nice and easy. The socket can be left in the 6DJ8 configuration and 2C51 could be used with this adapter.

 

http://www.ebay.com/itm/260723522614?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

 

Well, we could go raid other 2C51-using gear threads, and mine information (or beg for 396A) there, like we've been doing with the Lyr thread (despite not owning the Schiit) lol...

 

Anyone got a foothold in the MDHT Havana thread lol?

post #4118 of 10498
Quote:
Originally Posted by Artsi View Post

After trying couple different schematics with my DIY 6SL7 6A6 amp i have come to conclusion that Little Dot MK II is very compact, powerful, clean sounding and hum-free device. It is hard to beat with any homemade contraption.

Most problematic with my DIY headphone amp seems to be variable humming when turning volume. 
Soldering continues...
The routing of wires seems to have a lot to do with hum, the faint hum in my MKIV these days I am pretty sure is coming from the heater wires and where they lay on the board as it was dead quiet when it first arrived. I am going to have to pop the back off and move the heater wires around with a chop stick to see if I can make it disapear. Failing that foolishness I'll see about a pair of 3A regulators and a couple of bridges to switch the heaters over to DC. There would be no hum once its 6V DC. biggrin.gif

On the FB group where they build the amps, all the wiring is point to point and very short, heater wires are specifically routed away from other lines etc... I don't think you could just fly wire up the amp outside of a metal enclosure and have it not hum, but carry on with your experimentation and post some info if you make any progress!
beerchug.gif
Edited by TrollDragon - 11/30/13 at 3:46am
post #4119 of 10498

Well folks, all I can say is getting rid of the headphone jack and socket is definitely worth the (minimal) effort. It is doing wonders for the Teslas - should have done it sooner.

Be bold, give it a go! And after peeking inside and seeing the piffling bits of cable emerging from the inputs, hopefully I shall be able to get at them also at a later date! (But suspect that won't be quite so easy...).

post #4120 of 10498
Quote:
Originally Posted by hypnos1 View Post
 

Well folks, all I can say is getting rid of the headphone jack and socket is definitely worth the (minimal) effort. It is doing wonders for the Teslas - should have done it sooner.

Be bold, give it a go! And after peeking inside and seeing the piffling bits of cable emerging from the inputs, hopefully I shall be able to get at them also at a later date! (But suspect that won't be quite so easy...).

 

I'm quite interested on this mod (I own an LD MK II). Any suggestion for a replacement jack ? I was thinking about the Neutrik one, but that's actually the one you removed :o

post #4121 of 10498

If you like 5670 / 2C51s try the LM Ericsson golds.

post #4122 of 10498
Quote:
Originally Posted by Nic Rhodes View Post
 

If you like 5670 / 2C51s try the LM Ericsson golds.

 

Sure! Do you have a source? I haven't seen any on eBay.....

post #4123 of 10498

Received my little adapter board for using octals :

 

 

....and these are the tubes I have to work with :

 

 

The first is unknown as most of the printing has worn off but, from what remains, I think it is a Westinghouse. Next we have a NOS Russian and then the Sylvania.

 

The interesting thing about these boards is that it allows you to take measurements which is something I have been wanting to do for a while. Here are my results :

 

                           Heater         Plate         Cathode

 

Westinghouse  (L) 6.38v           147.8v         1.052v

                     (R)                   147.1v         1.067v

Russian NOS    (L) 6.36v           151.7v         0.991v

                     (R)                   153.7v         0.950v

Sylvania          (L)  6.38v          150.6v         1.028v

                     (R)                   152.7v         0.982v

 

Things to note :

The heater voltages are very close to 6.3v . I live in 240volt land where I measure 238v at the wall and the Little Dots are designed for 220v so if the heater power supply were unregulated I would expect a much higher measured heater voltage. Several pages back someone showed a photo of the transformer and the driver tube heater windings were 6.6v @ 1 amp so looks like some sort of regulation is taking place. A good thing.

The grid voltage is down around 1v . This seems rather low and you might expect to hear distortion at higher volumes. Not surprising as the Little Dot is not designed for the 6SL7.

 

The wonderful thing about this adapter board I got is that you can easily add resistors in line to each pin so that is what I did. I had some 240 ohm resistors to hand so I soldered them in the cathode circuit and, together with the LD cathode resistor of 680 ohms, I got 920 ohms in total.

This is how it affected the results :

 

                           Heater         Plate         Cathode

 

Russian NOS (L)   6.36v           158.8v         1.189v

                 (R)                      160.6v         1.141v

Sylvania      (L)    6.38v           156.4v         1.233v

                 (R)                      158.0v         1.184v

 

Adding the extra resistor has increased the grid voltage by about 0.2v which should increase headroom and the plate voltage has gone up by about 6v as the current will have gone down a little. Not sure if the plate voltage is a problem as I haven't been able to measure the B+ yet but it does seem a bit high. Also you can see a difference in the right and left channels due to imbalance of the two triodes in the tube. If you used a trimpot instead of a resistor you could tune out that imbalance and optimise the grid voltage. Maybe next time ......

 

Be interesting to know if anyone else has taken measurements with different tube families.


Edited by Ctritical Bill - 11/30/13 at 8:54pm
post #4124 of 10498
Quote:
Originally Posted by gibosi View Post
 

 

Sure! Do you have a source? I haven't seen any on eBay.....


I use my private stash but some appeared on ebay in the last month (several lots) so stick with it but they do go for significantly more money than the rest of this designation. Sylvania gold brand are VERY good also (and RCA Command / GE 5*) but even when you get to the likes of Raytheon etc then the tube is still of excellent quality. The UK / US tubes in this designation are consistently high quality, more so than most tubes.

post #4125 of 10498
Quote:
Originally Posted by Ctritical Bill View Post
 

The interesting thing about these boards is that it allows you to take measurements which is something I have been wanting to do for a while. Here are my results :

 

[...]

 

Be interesting to know if anyone else has taken measurements with different tube families.

 

Indeed. While it's all fine and dandy that we managed to get all those definitely non-native tube types working on our LDs, it would be good and safe to actually measure whether or not everything is working properly.

 

One could imagine a premium double triode working fine and sounding great on an amp that wasn't made for it, but having a catastrophic failure after a few weeks because the electrical values were outside of the acceptable range; happens pretty often actually, even within the same tube family on one amp. An example I've read about - on Joe's tube lore and elsewhere - is that Audible Illusions preamps can use new production Sovtek-ish 6H23N & 6N1P tubes, but not NOS 6DJ8 types because it runs the tubes so hard that the NOS tubes fail quickly.

 

The problems we could encounter with 6DJ8 or 6SL7 tubes on our LDs aren't exactly the same, but I could totally imagine how it could be bad to run a basic 6DJ8 (intended for a meager 90V of plate voltage - correct me if my vocabulary is off) at a 130V+ plate voltage, while a 6922 or 7308 would tolerate those voltages without blinking. It all boils down to how large our electrical "margins" are.

 

So, yeah, any real world measurements on 6DJ8 types would be reassuring lol...

 

------------------------

 

Speaking of novals, after raiding ebay for about a week, and camping a few different auctions for days, I finally managed to get my hands on a nice pair of tubes I'd been looking for.

 

A pair of NOS NIB - that fact is rare in and of itself - Philips "Holland" E188CC, with factory codes VR2 and delta PO, so not the most common tubes - a fact that cost me a three-digit number in euros... But it's all fun and games I guess... Bear in mind these are the exact same tubes as the Amperex Holland E188CC/7308, the Valvo E188CC, Miniwatt E188CC, RTC E188CC, RT E188CC & Dario E188CC so long as they were made in Heerlen, Holland.

 

What is interesting is the factory codes here. The delta code and other writings confirm a Heerlen origin, so I'm not worried about that. But the lack of a proper year code (a "P" instead of number) kind of threw me off at first (I almost didn't bid because of that). Fortunately, I had the whole 6 days left on the auction to check out essentially everything I could about the E188CC type and production over the years lol.

 

While I couldn't make anything of the bottom codes (delta PO) to determine a production year, the top factory codes turned out to have all the necessary information, thanks to the change code - which I've hardly ever had to analyze up to now.

 

"VR" means E188CC/7308, no doubt here; but change code "2" meant that the tube was part of the third "batch" of this tube type ever produced (or of a third batch after they ran out of numbers, and started over, which seems unlikely). I could tell the tubes were from the 60's for a few reasons (labels and other cues) but I just didn't know when in the 60's; and I was looking for the earliest tubes I could find - or afford lol...

 

Turns out that after looking at a gazillion other E188CC tubes on google and ebay, code VR2 means that these were part of a batch made in 1960, since any tube made later has a higher change code (actually, almost all the Heerlen-made E188CC you find have VR6 to VR9 codes and were made in the mid to late 60's, so much later). But what's even better is that tubes with the VR0 (first batch ever) or VR1 are the famous very early late 50's production with either a D-getter or a folded D-getter (= extreme prices); almost prototype-like tubes in a sense. So the VR2 tubes should essentially be the first batch of large O-getter E188CC tubes to have come out of Heerlen (give or take a few months). And as gibosi and I realized with the US Amperex 6922, the earlier the better, and those VR2 are pretty early!

 

Anyway, hopefully they'll turn out to be great tubes, and I may have to settle for those production dates, since earlier tubes are both rare and expensive. Sorry for the long useless spiel about factory codes, but I figure exposing my thought process and findings on these matters could be of some use for a few people seeking the best 6922/E88CC & 7308/E188CC for reasonable prices. Do tell me if my rantings are getting annoying or purposeless lol!

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