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Little Dot Tube Amps: Vacuum Tube Rolling Guide - Page 279

post #4171 of 10489
Quote:
Originally Posted by Ctritical Bill View Post
 

So you can see the heater voltage is down quite a bit from the 6SL7 which was 6.36v . This is the unregulated power supply doing it's thing of dropping the voltage as the current draw increases. The 6SL7 draws 300ma and the 6SN7 draws 600ma so that all makes sense. Under-volting the heaters a little is no bad thing as it increases tube life.

 

The plate voltage is down below 100v which I think is the ideal in this circuit and it is a lot closer than the 6SL7 so a good result. Also the grid voltage is up around 2v where it should be.

So the technical results are all very good.

 

Speaking of ideals, in this circuit with a B+ of 200v , the ideal plate voltage would be 100v and grid 2v , which results in about 3ma through the tube. The Little Dot resistor values work to give these results as long as the tube is suitable. So if you are looking at graphs in the datasheets then you want to find tubes with graphs where the values of plate volts = 100, grid volts = 2,  and current  = 3ma roughly converge.

 

Doesn't the fact that the 6SN7 draws 600ma present a problem? I have been under the impression that the LD's limit is about 500ma? 

post #4172 of 10489
Quote:
Originally Posted by superdux View Post
 

Can it always be seen, if a tube is faulty if you take closeup fotos? I have a matched pair voskhod tubes and one seems to be faulty, especially after switching from left to right the dropouts followed especially when i had the volume on 12 O`clock with 5 gain setting. Does this sound familiar to anyone? I couldn´t see any faulty wires that were detached and the silver lining in the tube head seemed intact too.

The seller asked me if i could make some closeup fotos before he decides towards a refund.

 

I certainly do not possess the skills necessary to study close-up pictures of a tube and determine why it doesn't work properly, but maybe this vendor can? I very much doubt it...  Even so, it's easy to send him the pics he is requesting, so give him what he wants. However, if he decides there is nothing wrong, don't give up. Keep demanding a refund. Cheers.

post #4173 of 10489
Yes mordy the different oils would add an interesting flavor or longevity to the tube especially with some of the newer Synthetic's.

Toss a few bay leaves in there with some herbs and a garlic bud or two... This will increase the WAF (Wife Approval Factor) at least ten fold.

4 of the 5 on that site had leaked, so that would be a fine mess to clean out of the amp. A nice high temp silicone should keep the oil in place though.

Now this is an extension on the oil only for the most experienced modder...



Lava Lamp Bottle!

biggrin.gif
post #4174 of 10489

Thx for reply gibosi:)

post #4175 of 10489
Quote:
Originally Posted by gibosi View Post
 

 

Doesn't the fact that the 6SN7 draws 600ma present a problem? I have been under the impression that the LD's limit is about 500ma? 


No, not at all. Using a pair of EF91 in the conventional way draws 600ma total as well. I think it was TrollDragon who posted a photo of the LD transformer and the heater winding was labelled 6.6v @ 1 amp so well within the limit.

post #4176 of 10489
Quote:
Originally Posted by Ctritical Bill View Post
 

Artsi, you beat me to it. I have just tried a 6SN7 and, I agree, it works perfectly and still has that fabulous soundstage of the 6SL7. Mine is a NOS Russian from 1983 and of course I took some measurements ( stock values with no added resistors ) :

 

Heater  -  6.12v

Plate (R) - 86.0    Grid (R) - 2.13

Plate (L)  - 85.8    Grid (L)  - 2.13

 

Would you be willing to provide a primer on how to make these measurements? My system, with a breadboard 9-pin socket and 7-pin Vectors, certainly provides full access. However, I have no good idea how to make these measurements.

 

Also, in case it makes a difference, I have an LD 1+, which is a hybrid amp, and it is my understanding that the tube stage is configured as a cathode follower.....

 

Are these AC Volt measurements? or the DC?

 

If I put the one of the meter's probes on Vector pin 3, one of the heater pins, where do I put the other probe? Pin 4? Somewhere else?

 

In the same vein, if I put one of the meter's probes on Vector pin 5, the plate, where do I put the other probe?

 

And the same question for the grid?

 

Further, is there anything else you think a rank amateur should know?

 

Thank you!

post #4177 of 10489
Quote:
Originally Posted by Ctritical Bill View Post
 


No, not at all. Using a pair of EF91 in the conventional way draws 600ma total as well. I think it was TrollDragon who posted a photo of the LD transformer and the heater winding was labelled 6.6v @ 1 amp so well within the limit.

 

I see.... this makes sense.... the limit is really 500ma per side. And so with these double triodes, 600ma is equivalent to running two 300ma tubes. Thanks!

post #4178 of 10489

Hi superdux,

 

Before you return the tubes, try to gently scrape the pins on the bad tube. Sometimes there is a little oxidation or dirt on the pins and you don't get a good connection. A small knife blade or something similar will do - does not have to be very sharp.

 

I had a 6N6P power tube sitting for a year and a half thinking that it had died on me. One day I thought of scraping the pins and it came back to life again.

post #4179 of 10489

Hi Oskari,

 

Thanks for clearing up the separate origin of National Union tubes. My tube is in prolonged use and suddenly I realized that the softness in the bass has tightened up after some 60 hours. Based on the following, perhaps we can place this tube in the 40's:

 

National Union


These are all very old tubes as National Union disappeared shortly after the war.  Therefore most all of them were made during the 1940's.  They have the gray colored coating inside the glass so the plates are hidden from view.  I haven't had the heart to break one in order to describe the plate structure, so you will have to wait until I break on by accident!  Many of these tubes are in military packaging but do not carry the "W" designation as I believe that the "W" designation didn't come until after the war.  Great sounding......very short supply.  Only a few on hand today.

Type

Description

 

National Union

6SL7GT 

Early 40's version. Gray or Black Glass  
post #4180 of 10489

From looking at the label on the transformer...

 

Driver Filament - 3.3V*2/1A | Green Black Green.

 

If you follow the Driver filament wires back to the toroidal transformer, you will find that they seem to be two parallel windings going inside the transformer, with a single center tap wire connected to the board on a solder pad marked 1/2 f. This would lead me to believe that there is 1A shared between the two pairs or 500mA each.

 

The Power tube filament wires also go back inside the transformer for a 2.5A split between the two pairs or 1.25A each.

 

The only way to tell if these are parallel windings or a single winding with two pairs of wires attached would be to measure the Power tube filament pairs and see if there is zero ohms between any of them. If there is only a higher resistance between each wire in the pair, then it is a parallel winding. If there is a Zero ohm reading between one wire from the left socket and one wire from the right, then it is just a single winding with two pairs attached.

 

You can't measure the same way between the Driver filament pairs because of the single center tap wire.

:beerchug:

post #4181 of 10489
Quote:
Originally Posted by MIKELAP View Post
 

Use long jumpers like this way easier to change

Processed By eBay with ImageMagick, z1.1.0. ||B2

 

Much obliged, MIKELAP

post #4182 of 10489

Hmm.. I have problem with two of mine tubes


Tesla 6F36/6AH6/6J5P
6J4P/6AU6/EF94 no idea who made these, russian.

 

Both pentodes.. Are they incompatible with ld?
I run them with 2/7 wire mod, but all I get is this very faint sound when volume is turned to max.
Sounds bit like if I listened turntables cartridge without any amps on. +the teslas create this "fire-ish" sound on other channel.


I have also ran successfully 6K4P-EV /6BA6 tubes, which I rank very highly among my tubes, with 2/7 wire mod with no problems.
 


Edited by Kestovitutus - 12/3/13 at 5:46am
post #4183 of 10489
You don't have the jumper in EF92 mode with the 6AH6 and a 2-7 jumper as well do you? That would short the plate, grid and cathode all together.

6AH6's need to be run in EF95 mode with a 2-7 jumper, that is the only way they work.

Check the table on page 77 for settings.
beerchug.gif
post #4184 of 10489

Thx mordy, will certainly try that suggestion out!

post #4185 of 10489
Quote:
Originally Posted by TrollDragon View Post
 

From looking at the label on the transformer...

 

Driver Filament - 3.3V*2/1A | Green Black Green.

 

If you follow the Driver filament wires back to the toroidal transformer, you will find that they seem to be two parallel windings going inside the transformer, with a single center tap wire connected to the board on a solder pad marked 1/2 f. This would lead me to believe that there is 1A shared between the two pairs or 500mA each.

 

The Power tube filament wires also go back inside the transformer for a 2.5A split between the two pairs or 1.25A each.

 

The only way to tell if these are parallel windings or a single winding with two pairs of wires attached would be to measure the Power tube filament pairs and see if there is zero ohms between any of them. If there is only a higher resistance between each wire in the pair, then it is a parallel winding. If there is a Zero ohm reading between one wire from the left socket and one wire from the right, then it is just a single winding with two pairs attached.

 

You can't measure the same way between the Driver filament pairs because of the single center tap wire.

:beerchug:

 

Oh, I'd forgotten there was a entire 1A dedicated to the driver tube heaters, I kept thinking the values were lower for some reason...

 

Would - temporarily - plugging a since double triode with a heater current draw of around 850 mA - on only one side since it's a single tube - be a terrible idea then?

 

I keep reading how the 6NP6 & especially the 6N30P can make great driver tubes, but I haven't even dared to try a 6N30 in the driver position yet because of its high current draw; its low mu and very high gm being a separate issue. Would it unreasonable to draw 850 mA from only one of the two driver sockets?

 

Similarly, I'd never noticed this pretty important electrical difference between a 6SL7 and a 6SN7. The fact that the 6SN7 draws 600 mA from a single socket may be a cause for concern compared to its higher mu counterpart that only uses half that. Then again, many here have run two separate 450 mA tubes (so a 900 mA or more current draw) for months without issues, so I guess the amp could use the whole 1A if needed...

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