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Little Dot Tube Amps: Vacuum Tube Rolling Guide - Page 277

post #4141 of 5851
Quote:
Originally Posted by TrollDragon View Post


Nice Job!
Except for the blue of course there is not quite enough shine for me... Just Kidding. tongue.gif

Now if you put two of those 3/4 to 1/2 tees end to end, you could put a 6DJ8 socket on one end and a socket for a 12AX7 on the other end ... One tube used at a time of course. Parallel wires from one B9A to the other with the appropriate changes at the socket end... biggrin.gif

Why do you put crazy ideas like that in my head ,would look great Hmmm. Damm you got me thinking about it :atsmile:.

post #4142 of 5851

I added a 36K ohm resistor on the plate of the 6SL7 to try and get the voltage down. Here are the results :

 

 

                                     Plate         Cathode

Stock setup - no additional resistors :

 

Sylvania          (L)          150.6v         1.028v

                      (R)          152.7v         0.982v

 

Added 240 ohm cathode resistor for 920 ohms total:

 

Sylvania      (L)             156.4v         1.233v

                  (R)             158.0v         1.184v

 

As above plus 36K ohm plate resistor for 69k total:

 

Sylvania      (L)             127.3v         1.024v

                  (R)             129.4v         0.994v

 

So the additional resistor on the anode has brought down the plate voltage but the cathode voltage has gone down again too. This is to be expected as there is now less current through the tube and therein lies the problem.

At stock setting there is about 1.5ma across the tube which is low to begin with and with the additional resistors it is now down to about 1.1ma . According to the datasheet, at 100v and 1v grid it would be down to 0.75ma . I don't think it is worth taking this further as it is fast approaching cutoff.

 

A better candidate in this circuit would be the 6SN7. Has anyone tried this as a driver ?

post #4143 of 5851
Quote:
Originally Posted by mordy View Post
 

Hi Ctritical Bill,

 

I just got the same breadboard as you.....

 

 

Would you and/or Ctritical Bill post a link to one of these breadboard sockets that allow one to add resisters? The 6DJ8 family, including the 6922 and 7308, could well be my end-game tubes. And while these tubes sound incredibly good now, it does make sense that if we can optimize the electrical values, or at least get closer to optimum, these tubes may actually sound better. Thanks! :)

post #4144 of 5851

Here you go :

 

http://www.ebay.com.au/itm/1SET-8pin-K8A-tube-test-diy-Experiment-prototyping-pcb-for-6V6-EL34-KT88-6SN7-EM-/190889423497?pt=R%C3%B6hren&hash=item2c71e57289

 

The seller has the same board with different sockets or just the board on it's own.

post #4145 of 5851
Quote:
Originally Posted by gibosi View Post
 

 

Would you and/or Ctritical Bill post a link to one of these breadboard sockets that allow one to add resisters? The 6DJ8 family, including the 6922 and 7308, could well be my end-game tubes. And while these tubes sound incredibly good now, it does make sense that if we can optimize the electrical values, or at least get closer to optimum, these tubes may actually sound better. Thanks! :)

 

It would be great if we could determine "optimized" values for all the resistors used for different tube types, whether noval or octal. But, looking at the 6DJ8 datasheets again - and listening to those tubes on my MK IV - I keep thinking that it's almost like the amp was made to use those tubes...

 

The cathode resistor values should definitely be pretty close to recommended specs for 6DJ8 types. At least moreso than for any of the 12Ax7 types. From memory, I think the values also fit a 6SN7 pretty well too, though they didn't seem totally out of spec for 6SL7 tubes.

 

Again, 6922/E88CC & 7308/E188CC types may actually be some of the best "fits" we can find for our LD amps, just because of their tolerance for higher plate voltages. The 6922 has both the best sound quality I've ever gotten out of the amp, and also the best "running conditions" I've ever managed to get, with low, low temperatures and low current draw.

 

Getting real world values for those tubes really would be great, if only just to know whether or not it's worth to actually invest in the best tubes, and potentially just "mod" the amp itself to use those tubes.

 

By "mod", I'm thinking more of socket modding: either cutting the PCB trace connecting pins 5 and 6 so two separate tubes can be used on each socket with a very basic adapter (or no adapter at all if pins were to be cut and the good old straps were to be used), or either completely replacing both B7G sockets with two B9A sockets with only 5 pins connected for each; or even alternatively, using a single B9A or octal socket, integrated in a new drilled central hole between the original driver tube sockets and soldered in all the right places on the PCB - without any of this being visible outside the amp. Best of both world, you'd still have the two original B7G sockets to do whatever you want, and also a single noval or octal socket to use a single double triode if you choose too.

 

Anyway, confirming the electrical validity of what we've been doing - despite the LD amps' apparent flexibility - would be a good idea.

 

Sorry for the rant again, I just have too many ideas late at night, and not enough outputs to actually use them or clear them lol... Just food for thoughts for all the hardcore modders on this thread. Hell, I actually seriously considering permanently modding my amp you know; just so I could use my fav' tubes without any adapter at all.

 

Seriously though, I'm just waiting for someone to try and add a third "central" B9A socket on their MK III or IV, or to try and replace the driver tube sockets with two B9A sockets...  I know someone will eventually try and dremel a third hole in there lol...

 

Edit: Look on the bright side, some people just don't have limits to their tube budget... I'd been following an ebay auction for about a week, knowing very well how it would turn out, but I didn't think anyone would be crazy enough to fork out $627 for a pair of essentially un-guaranteed E88CC. Yes, Philips Heerlen; yes 1957; yes D-getter; yes pinched waist; but still... $627? For driver tubes?? Color me appalled...

 

http://www.ebay.com/itm/matched-pair-E88CC-pinched-waist-D-getters-from-philips-miniwatt-NOS-/301026417318?pt=Vintage_Electronics_R2&hash=item4616929aa6

 

I do hope these turn out to be "god tier" tubes for the guy who bought them... Lol.


Edited by Audiofanboy - 12/1/13 at 4:42pm
post #4146 of 5851

I bought my bread board from the Chinese seller/manufacturer linked above- paid $6.90 with free shipping on auction (only bidder). If you wait a little I am sure he will put it up again on the US Ebay.

post #4147 of 5851
Quote:
Originally Posted by Ctritical Bill View Post
 

Here you go :

 

http://www.ebay.com.au/itm/1SET-8pin-K8A-tube-test-diy-Experiment-prototyping-pcb-for-6V6-EL34-KT88-6SN7-EM-/190889423497?pt=R%C3%B6hren&hash=item2c71e57289

 

The seller has the same board with different sockets or just the board on it's own.

 

Thanks much! I have put it on my watch list for now. So if I understand correctly, it appears that when you get this board, you have to solder some kind of connector, whether it be a simple wire or a resister / capacitor / inductor between the wiring blocks and the socket before you can use it? That would explain the short red wires visible on the picture you posted. Interesting!

post #4148 of 5851

I had a short face-off this evening between a 1965 US-Amperex 7308 and a 1964 US-Amperex 6922..... and I was unable to hear any difference between them...  At first, I was surprised. But perhaps it is not surprising at all. After all these were made in the same factory at about the same time. Further, from what I have read, the only difference is the 7308 is held to tighter specifications. So it may well be that these tubes were made on the same assembly line and the one that passed over the bar was stamped 7308 and the other was stamped 6922.

 

If this is so, then perhaps we can hypothesize that E88CC / 6922 and E188CC / 7308 made in the same factory at the same time will sound nearly identical..... 

 

So for now, my 1961 US-Amperex 6922 continues to hold the throne. Next up, a 1967 French-made La Radiotechnique E88CC.... 


Edited by gibosi - 12/1/13 at 8:00pm
post #4149 of 5851
Quote:
Originally Posted by gibosi View Post
 

 

Thanks much! I have put it on my watch list for now. So if I understand correctly, it appears that when you get this board, you have to solder some kind of connector, whether it be a simple wire or a resister / capacitor / inductor between the wiring blocks and the socket before you can use it? That would explain the short red wires visible on the picture you posted. Interesting!

 

Yes, that's it exactly.

post #4150 of 5851
Quote:
Originally Posted by gibosi View Post
 

I had a short face-off this evening between a 1965 US-Amperex 7308 and a 1964 US-Amperex 6922..... and I was unable to hear any difference between them...  At first, I was surprised. But perhaps it is not surprising at all. After all these were made in the same factory at about the same time. Further, from what I have read, the only difference is the 7308 is held to tighter specifications. So it may well be that these tubes were made on the same assembly line and the one that passed over the bar was stamped 7308 and the other was stamped 6922.

 

If this is so, then perhaps we can hypothesize that E88CC / 6922 and E188CC / 7308 made in the same factory at the same time will sound nearly identical..... 

 

So for now, my 1961 US-Amperex 6922 continues to hold the throne. Next up, a 1967 French-made La Radiotechnique E88CC.... 

 

That's what I thought until I read that there are supposed to be some actual physical differences between a 6922 or 7308. I can't speak for the late 60's and onwards models, but I have read that part of what made the original (Philips Holland) 7308 "premium" - or worth creating on top of the 6922 - is the anti-microphony system. On the top mica (not the third one with the getter stand), there's supposed to be one or two L-shaped "mica spring" cut-out from the mica and intended to keep the heater filament from "creating hum" or communicating noise to the cathode - or something along those lines.

 

Basically, the 7308 only exits because it's a lower noise version of the 6922, which should in and of itself, make it sound slightly better on paper. That, and even tighter specs, like even more matched triodes than a 6922. I would assume that the triode assemblies would be the same as the 6922 though, hence the similar sound. A noise-free 6922 made in the factory and time-frame as a anti-microphonic 7308 could/should/might sound the same. Even the 6922 is known to be or become a microphonic tube, so the 7308 was probably made to compensate for that.

 

But like I said, I think that the differences may have been more easy to hear on the earlier production runs, before the factories would have tried to cut costs and level their production towards the end of the tube era. I'd think an early 60's 7308 Amperex made in NY could sound better than our early 60's NY 6922, for example. I'd also expect more marked differences in the early to mid-60's Heerlen production, somehow.

 

I should get my E188CC by Wednesday, so I'l lhave a chance to compare tubes pretty quickly!

post #4151 of 5851

Hi.
This might be silly question with all this modding and other tinkering, but has anyone any good tips how to change jumper position easily :D I'm using sharp-nosed tweezers to fish them out. Not a big problem tho, because I only use one pair with EF92 setting. It's the Mk 3 with big brown nichicon capacitators? right next to jumpers.

post #4152 of 5851
Quote:
Originally Posted by Audiofanboy View Post
 

That's what I thought until I read that there are supposed to be some actual physical differences between a 6922 or 7308. I can't speak for the late 60's and onwards models, but I have read that part of what made the original (Philips Holland) 7308 "premium" - or worth creating on top of the 6922 - is the anti-microphony system. On the top mica (not the third one with the getter stand), there's supposed to be one or two L-shaped "mica spring" cut-out from the mica and intended to keep the heater filament from "creating hum" or communicating noise to the cathode - or something along those lines.

 

Basically, the 7308 only exits because it's a lower noise version of the 6922, which should in and of itself, make it sound slightly better on paper. That, and even tighter specs, like even more matched triodes than a 6922. I would assume that the triode assemblies would be the same as the 6922 though, hence the similar sound. A noise-free 6922 made in the factory and time-frame as a anti-microphonic 7308 could/should/might sound the same. Even the 6922 is known to be or become a microphonic tube, so the 7308 was probably made to compensate for that.

 

I can't see any L-shaped "mica springs" cut-out from the mica. But there is one very noticeable construction difference. On the 7308's top mica there is a metal plate covering almost the entire surface:

 

6922: The top mica is largely exposed. 

 

 

 7308: The top mica is largely covered by a metal plate

 

 

I have no doubt that the 7308 is quieter, the triodes are better matched, and, at least on paper, it should sound better. However, from my experience, I believe better test equipment and procedures are necessary to discern these differences. Sonically, these are so close that I cannot tell them apart. That said, if I could purchase a pair of 1961 US-Amperex 7308 for a good price, I would not hesitate. 

post #4153 of 5851
Quote:
Originally Posted by gibosi View Post
 

 

I can't see any L-shaped "mica springs" cut-out from the mica. But there is one very noticeable construction difference. On the 7308's top mica there is a metal plate covering almost the entire surface:

 

6922: The top mica is largely exposed. 

 

 7308: The top mica is largely covered by a metal plate

 

 

I have no doubt that the 7308 is quieter, the triodes are better matched, and, at least on paper, it should sound better. However, from my experience, I believe better test equipment and procedures are necessary to discern these differences. Sonically, these are so close that I cannot tell them apart. That said, if I could purchase a pair of 1961 US-Amperex 7308 for a good price, I would not hesitate. 

 

Pictures, great, very convenient!

 

Actually, on your third picture, I believe we can see one of the anti-microphony systems - what I have read are sometimes called "mica spring" - and indeed it looks like somekind of a shock absorbing mechanism, the same concept as on a car. I tried to outline the part below, but it's actually easier to see on the original picture once you've figured out where it is.

 

 

 

No doubt that there's more Schiit on the top mica of a 7308 than on a 6922; not that the specifics matter... But I like to know why things do what they do - especially if they're going to affect sound quality!

 

Edit: The only reason why I'm so specific on knowing about those different noise/microphony mechanisms is because I have read over and over that the 6DJ8 are pretty prone to that, and even the premium 6922. Even my own Amperex 6922 has made an odd noise once or twice; but a 10s weird noise on my premium tube is enough to make me paranoid for a week, listening for noise issues instead of music lol... Not that my 6922 is noisy, it's actually dead quiet - apart from that one-shot incident.


Edited by Audiofanboy - 12/2/13 at 7:39am
post #4154 of 5851
Quote:
Originally Posted by Audiofanboy View Post
 

Actually, on your third picture, I believe we can see one of the anti-microphony systems - what I have read are sometimes called "mica spring" - and indeed it looks like somekind of a shock absorbing mechanism, the same concept as on a car. I tried to outline the part below, but it's actually easier to see on the original picture once you've figured out where it is.

 

Yes, I can these "mica springs", and for sure, the 6922 doesn't have them. Further, perhaps due to all that extra metal, the 7308 feels noticeably heavier in my hand.

 

And now I am very curious to see what the Holland-made Amperex 7308/E188CC looks like....  (Tracking indicates that it arrived in NY from France in only four days... and it sat in NY for two weeks! lol  But finally, as of today, it is on the move again.... Soon I hope.... ) 

post #4155 of 5851
Quote:
Originally Posted by mordy View Post
 

So I thought that I snagged a bargain Tung Sol that just squeezed in at my $8 limit including shipping. A little research seems to indicate that they were two different factories and I could not find any information that Tung Sol acquired National Union (not to be confused with National which was the earlier name for the Japanese Matsushiita).

 

The only identification letters on the the tube are YT on the blackened glass, and KC on the base. KC  = December 1953? Would appreciate if anybody has information, as well as information if there is a link between NU and TS.

 

Would appreciate help in a possible TS-NU link, and the dating of this tube.

 

I'm not aware of such a link. NU was an independent tube manufacturer, in fact an amalgamation of a number of small early tube manufacturers. NU sold its newish second plant in Lansdale, PA to Philco not long after the end of WWII. I suppose NU basically diversified out of the tube making business in the 1950s.

 

I'm not able to help with the date codes.

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