[baronbeehive]

While looking at the 6080 tubes' data sheets for clues about temporal stability in operation, there actually were some to be found on the second page in the Tung-sol 6080WA's data sheet.

There in the section "Additional tests to insure reliability", it is stated that the tube's transconductance can change up to 10% from the specified 7000 micromhos in the first hour of use, may drop to 5800 micromhos in the first 100 hours of use and to 5500 micromhos in the first 1000 hours of use.

(I haven't found any data on transconductance variation by hours of use in other manufacturers' data sheets(!!))

This is the trouble, data sheets are all different.

Those changes would throw off the calculations of the Cavalli formula for the impedance mod quite a bit. If the tube spends most of its life near the 5500 micromhos transconductance rate, then the correct resistance resulting from the formula would be 280 ohms and not 220 ohms, which is the result with the specified 7000 micromhos rate.

And that 280 ohms is closer to the stock 330 ohms value than the 220 ohms value... so the best resistance value would likely be something other than the result from the spec transconductance.

I doubt if the exact figure is important, although it will be more important for 6AS7 types say, than 12AX7 types, as you have seen tubes vary over their lifetimes so it will always be an approximation, although the Cavalli formula tried to get it right it had to provide several corrections, even then we have this problem of varying over time.

Tubes will vary and there is even the problem of using matched pairs or quads, they in fact will not be matched forever, the only way to tell would be if you happen to have a tube tester.

It should be also noted that the 7000 micromhos specified transconductance only applies with specific operating conditions which are listed at the beginning of page 3 of the data sheet, which are Ef = 6.3V, Eb = 135V, Ec = 0, Rk/k = 250 Ohm. (What does that Eb voltage refer to?)

And on the last page of the data sheet there are curves of how the transconductance varies by DC grid voltage and some other variables. How should those be taken into account here?

(I've now removed the impedance mod completely myself but might put it back later on... with some values).

Don't know what Eb is. Here are a couple of references you may or may not find useful, the first explains some of the data like maximum and working values, the second gives in intro into tube load lines and what is important, sorry I can't help more.

https://wtfamps.com/tube-data-sheets/

http://www.diyparadise.com/tubeloadline/tubeloadlines.html

 

[baronbeehive]

Merry Christmas guys, lets hope that next year brings some improvements to all our lives .

Oh and Happy Thanksgiving... and hyvää joulua!

 

[CopperFox]

Merry Christmas guys, lets hope that next year brings some improvements to all our lives .

Oh and Happy Thanksgiving... and hyvää joulua!

Hyvää joulua ja onnellista uutta vuotta!

I believe thanksgiving is most often in November... but it is good to be early!

 

[baronbeehive]

Hyvää joulua ja onnellista uutta vuotta!

I believe thanksgiving is most often in November... but it is good to be early!

Thanks!

You're right, I'm getting in early for next year, sorry Maxx!

Say hello to Father Christmas for me, I believe he lives in Finland .

 

[Maxx134]

Say hello to Father Christmas for me, I believe he lives in Finland .

Haha merry Christmas

 

[CopperFox]

Well I've hit a bit of trouble now. I bent and broke one of the legs on a vu meter and so decided to replace the meter thinking that would be something that is simple and easy to do. However, after trying two different replacements, apparently neither was identical to the original one and the amp hasn't started up properly since.

The original meter seemed to have a wire inside it that connected the positive and negative current terminals (?). One of my replacements didn't have that. The other one did but after checking its data it is specified to have internal resistance of 1 ohm, while some others that look similar may have for example 680 ohms.

This is rather confusing especially as the replacement meter that didn't have the terminal-connecting wire had product photos that showed it inside a MkVI+.

(Perhaps I could try removing the remaining original meter and put two similar replacement meters to see if that makes any difference. Haven't tried that yet.)

I'm also a bit surprised that my amp won't start up right without those meters in the right configuration. How is the meters' circuit connected to the rest of the system and why would it matter so much?

Looking back at the thread there seems to have been succesful cases of vu meter replacement. Where did people get those meters - from some LD representative or somewhere else?

 

[Maxx134]

Probably can get a replacement from little Dot.
I believe the turn on delay is a cap charging circuit.
Just leave the unit on longer until it switches on.

 

[baronbeehive]

Well I've hit a bit of trouble now. I bent and broke one of the legs on a vu meter and so decided to replace the meter thinking that would be something that is simple and easy to do. However, after trying two different replacements, apparently neither was identical to the original one and the amp hasn't started up properly since.

The original meter seemed to have a wire inside it that connected the positive and negative current terminals (?). One of my replacements didn't have that. The other one did but after checking its data it is specified to have internal resistance of 1 ohm, while some others that look similar may have for example 680 ohms.

This is rather confusing especially as the replacement meter that didn't have the terminal-connecting wire had product photos that showed it inside a MkVI+.

(Perhaps I could try removing the remaining original meter and put two similar replacement meters to see if that makes any difference. Haven't tried that yet.)

I'm also a bit surprised that my amp won't start up right without those meters in the right configuration. How is the meters' circuit connected to the rest of the system and why would it matter so much?

Looking back at the thread there seems to have been succesful cases of vu meter replacement. Where did people get those meters - from some LD representative or somewhere else?

Bummer . Is the amp completely dead? Could you post a pic of the damaged meter.

I've checked back and the link I used to get my meters on ebay is dead and I have no other record as the purchase history on ebay only goes back a couple of years.

The fact that a meter isn't functioning properly shouldn't affect the amp it must be something else that happened before you did that. The meters just show the current nothing else. Before I put mine in one wasn't reading correctly, I had blown the circuitry inside, yet the amp was still functioning perfectly. The reason I blew the circuit inside the meter was I had installed a resistor feeding the meters with the wrong value. But your issue of the amp not starting is not the meters.

If I can find out any more information I will let you know but in the meantime the only thing I can suggest, apart from remembering what you did prior to the meter issue, is to search member kinglim on this thread, he might have put in new meters also, he put in virtually new everything. Apart from that try asking Little Dot what meters they use.

 

[Maxx134]

He should take the old broken meter apart to see what is going on. Maybe fix the leg and glue the meter back.
Find out if the new meter damaged or shorted the delay circuit from charging up, to turn relay on.

 

[baronbeehive]

Yeah, if the connector is broken off there should still be something to solder on to. This is where a DIMM would come in handy to check the resistance of the 2 meters to see if they match.

Those wires going into the meters are extremely flimsy.

 

[baronbeehive]

Chassis extension underway to accommodate film caps:

 

[baronbeehive]

Nearly finished:

In case you're wondering the string will be taken off.. that's if it doesn't fall apart LOL.

 

[baronbeehive]

Nearly ready:

... will be even better when it's the right way up...

 

[baronbeehive]

... Phew, now it's the right way up, and it doesn't get any easier, those 2 minutes spent waiting for both channels to come on is a nightmare. I thought I messed up again when no sound came on but then I remembered I moved the DAC and the connection on it is a bit dodgy so now everything is fine .

Following on from the excellent work of CopperFox I finally decided to go from endgame to beyond and that means chassis work as you can see. No more compact version, that's gone out of the window, this is all out for the best possible SQ! So lets see what can be achieved when you pull out all the stops.

First are WCF caps, I've paralleled my .33uF Mundorf SIO metallized polypropylene with some .68uF to take the total to around 1uF. So I will report back when I've burned them in for around 100 hours just to be sure. I'm interested not only in sound, and if there will be oscillations at 1uF, but I will also test whether the frequencies stay level, or will there be clipping on transient recovery after a heavy bass test by checking out the meters. With the existing there is a dip at max volume which I only just noticed, not having had the headphones on at max volume before ever, that's more Maxx's thing .

 

[baronbeehive]

SMLD MODS: (finished.. er.. sorry.. resumed.. temporarily )

Chassis extension -

Looks like the Little Dot just turned into the Big Dot, it dwarfs my Miniwatt, even with its own chassis extension:

You can follow my mods pages: 50, 53, 55, 57, 62, 65, 66, 68, 69, 70, 71, 72, 147, 179, 230, 231, 234, 253 which include pics. Also Mogos mods are here; 33-38, and SonicTrance here: 23, 43, including some great pics.