Ld91 mk2 Signature

[kbergsson]

Hi i bought an unbuilt kit last weekend. Its an older (2007-2008?) Lady Day 91 Mk 2 Signature, aluminium chassis with Tentlabs module.
I have assembled one of the monoblocks.
At first I measured the filaments i.e pin 2 to 8 on rectifier (5v) and 310a pin 1 and 6 (slightly over 10v), As well as rectifier pin 4 to 6 (sligtly over 800v). All ok so far.
Connected old car speakers an shorted input, put in the tubes... No nasty surprises , but some hum over speakers, tentlabs providing 4.95v to the 300Bs, ...but only 4.4volts over the adj. resistor (set at 885ohm). 300b pin 2 to ground only 220v Dc, pin 2 to 1 also 220v. 310a pin 2 to ground 247v and
B+ 273v, B+1 264v and 318v after choke to ground and 354v from rectifier pin 8 to ground.

I have retraced everything, checked soldering, re soldered etc.. And cannot find the error.
Please can you give me a hint on where to look for build errors and what to check further?
Best regards
Kjartan
Norway

[kbergsson]

Update to status. I have now got both monoblocks built. Monoblock nr 2 is playing musik, no hum at all on my Altec 604s. Measures ok but one side of tj300b plate is quite red ( red plating? ). Had unexpected quality issues. For the tentlabs i had connected 7.5v and 0 from the bundle. When tentlabs vas not providing the expected 5 v to the 300b i measured the 7.5 vire it was only 2.5 and the one marked 2.5 vas 7.5. Corrected and this was ok. The 400vac bundle ( same colour vire entvined ) also measured differently one 400+ vac and the other 300+. Measured the 300vac bundle (another colour code) and found the other 400+. Fixed this and all was ok.

I found an error in number 1 monoblock. My inexperience had overlooked that one of the tags used was grounded to chassis. Corrected this, all ok with the exception of the 310 which only measures 6.5v from leg 2 to ground. Plays without hum, but at much reduced volume. Tried swiching 310s (i was afraid I had damaged it) but no change. Must be a cold soldering somewhere.
The main thing is that the build is moving in the right direction. Cannot wait to audition once i get both monoblocks working.
Best regards
Kjartanb

[kbergsson]

Found a cold soldering, both amps playing musik now
Regrettably one of my tj300Bs is probaby defect. It has tvo red dots on the plate regardless of which amp is used.
But initial audition of cold amps is wery positive. Now i need to play music and run them in.

[Jogi]

Do you know what current is going through the TJ? They are not able to stand much more than 30 watt; if current is too high, they get red cheeks.
I also have Ladydays mK2, but changed the driver to C3m pentode, very good and beefy driver.

[kbergsson]

Originally Posted by Jogi

Do you know what current is going through the TJ? They are not able to stand much more than 30 watt; if current is too high, they get red cheeks.
I also have Ladydays mK2, but changed the driver to C3m pentode, very good and beefy driver.

Hmmm I have 75v over the 885ohm catoderesistor, this should result in c.a. 85mA. With approx 420v on the anode (plate) its around 35w (am I correct?). But this is essentially the default design of the LD91 as I have not changed any component values.
Im using tentlabs so the filament is stable on 4.90v

What is your experience with the C3M in stead of 310?

[kbergsson]

Originally Posted by kbergsson

Hmmm I have 75v over the 885ohm catoderesistor, this should result in c.a. 85mA. With approx 420v on the anode (plate) its around 35w (am I correct?). But this is essentially the default design of the LD91 as I have not changed any component values.
Im using tentlabs so the filament is stable on 4.90v

What is your experience with the C3M in stead of 310?

According to the 30watts mentioned by Jogi, and if my calculation above is correct, one should probably not use tj300b mesh plate in WE91 derevatives. The 1992 Joe Roberts article on WE91 for home use, runs the 300b at 425v and uses 880ohm catode resistor. There are many of the same component values in the LD91 as are mentioned in JRs article. Another example is Herb Reicherts Flesh and Blood there 425v and 80mA are used.
Is the tj 300b mesch plate maybe a 2A3 derivative with a 5v filament?

[kbergsson]

I must say that the LD91 mk2 signatures, even if not run in at all, are wery fine amps and match my Altec 604-8Gs extremely well.

[Jogi]

O.k., this should be the reason for the red cheeks. All mesh tubes have less dissipation capabilities, because there is less surface for dissipating the heat. So you should consider solid plate 300B or burn the TJ very quickly. The "normal" 300B is up for 40 watts. I run the TJ at a quite low Op at 350V/60mA, which is about 21 Watts only. They last already 6 years...and the tone is very nice, relaxed and musical.
I never tried the 310a driver, as it is also running only with a little more than 2mA, which is too low in my book. The 300B needs a nice kick in the butt, and C3m does that very nicely at 17mA. Very dynamic, smooth and stable during complex passages. And as it is a pentode like the 310a, there is also a cancellation of distortion, particularly the higher order, so that mainly second order THD of the output tube remains.
I can imagine, that also the 310a produces a nice tone, but I doubt its dynamic capabilities, especially with complex music.
I tried the terrible 6sl7, and 6sn7, e180f, E810F, D3a, E81l as drivers in various circuits, but I like the C3m best (in my case a C3o with 6 volt heaters).
But just let the amp play for a while, maybe you like it so much that you leave it like this. Or, if the itch arises, change to C3m and compare...

[kbergsson]

Im getting some EH300Bs next weekend, they should be better able to handle the environment provided by LD91. I really like the sound of the LD91, and on my Altecs they seem to handle and resolve complex music quite well, at least as well as my Hypex nCore NC400 monoblocks. Maybe the 300Bs at 350v/60mA need the "kick in the butt" from the C3O
With regard to the LD91 amps I will give them time to show what they can in current configuration. In my experience it takes time and several small adjustment to a system for it to deliver its best. It has taken me 3 years and 3 very different crossovers, component trials etc. to get my Altec 604s to sing to my liking. Have been on the werge of selling them several times underway The time will come however that change will be mandatory ...simply to breack the Status quo and then it will be fun to experiment with the LD91s.