[hpamdr]

Ok, enough tube rolling for today. I got and idea about what bias I have to set for several types. My measurements revealed this and the surprise of my KT88 biasing at the same value like KT66 remains. I would be curious to know in the future other Eternity users what bias will find for KT88. For me this is very comfortable, since I can roll now several nice types without changing the bias. At the moment I set the tube rolling switches to OFF, I went on the fixed bias of -6.2V input and -21.4V output and just put my KT88 and the L63 clear glass to burn in.
.....

Why KT66 and KT88 work at -21.4V? Hell knows, KT88 has to go around -22V or -23V and I doubt these were so heavily used or something. I have a pair of GE 6550A which measured around -25V. We will see, maybe others will come and test more KT88 in the future.

Hi @OctavianH about bias, I think that some differences are due the gyrator stabilization and tube changing due to heating.

-- If you like a bit of Maths--
Ohm low is : U=RI (in our case R is 470)

The measured 200V is not exactly what is constantly supplied to the tube. @SonicTrance Tomas explained the voltage drop balance from firm 210V and with 470R resistor to do the drop in accordance to Ia. But as a retro reaction Ia is linked to Va (also Vg and filament heat stabilization) following Tube curves.
On the 7-14V range to have gyrator happy we have a load in the 470R resistor from 14.89 to 29.79mA for a resulting 196 - 203 Va. The medium value is 10.5V drop so 22.34mA for 199.5V
--
I do not have how works input design so cannot do "calculation/guess" !

During Bias we only change Vg, Gyrator and tube do the remaining.... The goal is to listen music at this point, the measure is to get a good idea of tube values and be able to repair tube set in the proper usage range.

 

[OctavianH]

For sure that's the electrical explanation, from user perspective it happens what I described.

There's something fishy with my Psvane KT88T2. As I said yesterday KT88 should bias at a bigger negative voltage.

According to the GEC KT88 datasheed the value is -25V. This is similar to what I measured for 6550A (KT88 equivalent) and KT150. I think also KT120 should be the same.

 

[SonicTrance]

Doesn't psvane have a datasheet for KT88? You should look there.

 

[OctavianH]

Doesn't psvane have a datasheet for KT88? You should look there.

Maybe it has but I was not able to find it. I am still searching. Soundwise they sound exceptional. According to my measuremens, several new production KT88 bias like KT66.
I have 2 pairs, one Psvane and one Ayon (most probably Shuguang) and both measured around -22V. There were not bought new by me, but since they were good deals I took them. If I put these at -25V according to the specs I don't think they'll reach 15mA.

 

[hpamdr]

Maybe it has but I was not able to find it. I am still searching. Soundwise they sound exceptional. According to my measuremens, several new production KT88 bias like KT66.
I have 2 pairs, one Psvane and one Ayon (most probably Shuguang) and both measured around -22V. There were not bought new by me, but since they were good deals I took them. If I put these at -25V according to the specs I don't think they'll reach 15mA.

Hi Octavian,
I have a pair of GL KT88 Russian Made reisue and both measure around -26.0V Vg 200V Va -20mA Ia. (It needs 15minutes to have measure stabilized on these tubes continiously heating them)
Also what i was told by a friend is that new production of KTxxx does not have exact same curve than old production. Spec are very close on higher Anode voltage >400V than the one we use.

 

[OctavianH]

Hi Octavian,
I have a pair of GL KT88 Russian Made reisue and both measure around -26.0V Vg 200V Va -20mA Ia. (It needs 15minutes to have measure stabilized on these tubes continiously heating them)
Also what i was told by a friend is that new production of KTxxx does not have exact same curve than old production. Spec are very close on higher Anode voltage >400V than the one we use.

Many thanks for this. Exactly what I was expecting. Well, in my case I have 2 pairs of KT88 biasing at -22V and I can use my fixed output bias for them and KT66.

I gave today another chance to 6L6 which sounded not quite well yesterday. I tried them with warmer drivers (GEC L63) and also with more neutral ones (Fivre 6N7G) and the sound is thin, lifeless. So 6L6 and variants will not be on my priority list anymore. However, 6V6 sounds very good for my ears with 6J5. I will roll several pairs of 6V6 just to see how much of a difference I can hear. Strange about 6L6, I was hoping to have them close to KT66 or KT88.

6V6 on Eternity:

GEC KT66 are warm, very rich in tone especially in the mid frequencies area. I feel that GEC L63 are somehow similar. Combined might be sometimes too much. I went on the KT88 path and reduced the mid assault but added that strong bass and sub bass. It was fine, but again on some specific albums too much. 6V6 are still in the area of KT66 but somehow "weaker" if I might say, the reduce a little bit the richness of KT66 and might be relaxing on some cases. The kings remain KT66 and KT88, looser is for sure for me now the 6L6 but 6V6 is a pleasant and nice surprise. Of course, things might change while rolling drivers but at the moment I want to keep more these L63 I quite like. For these L63 the sound would be for me creamy, rich, warm and detailed. A very nice tone, an incredible tube for around 40EUR/piece. A must have in every collection.

 

[hpamdr]

For me EL32 have also a very nice sweet tone for romantic, chamber music and where few voices are in front (not sure that it is your kind of music you like).
If you have the adapter and want anyway to give a chace to grid cap
Will you plan EL11 or EL3N as driver ?

 

[OctavianH]

For me EL32 have also a very nice sweet tone for romantic, chamber music and where few voices are in front (not sure that it is your kind of music you like).
If you have the adapter and want anyway to give a chace to grid cap
Will you plan EL11 or EL3N as driver ?

I liked EL32 as drivers for 6AS7 in Elise to bring some nice tone to voices. However here I can put them only as output. I plan to do this sometimes, but I am still in the mood of "no adapter when I have so much to try". I have EL32 -> 6V6 adapters, for EL3N I have that monstruous chinese double one -> 6SN7. For EL11 I do not have to 6J5. So What I could try is the EL3N in double adapter. We will see, no prio for me.

 

[UntilThen]

Brimar 6L6GA black glass is very good but I can’t use in Odyssey. I let my friend buy it instead and he’s smiling every night now.

 

[hpamdr]

Take your time Octavian and do not get stucked to first switch impression.
For me, what is difficult with tube rolling is that it takes time to get accomodated to a combo and only the most colored one can have quick like / dislike.

Does the T1 is still very musical with Eternity ?

 

[OctavianH]

Does the T1 is still very musical with Eternity ?

I have not tried T1.2 yet. For me the quantity of bass is quite at the limit with Verite. I will try it later and of course I will spend time on listening, now I just check several pairs to see how they sound as a first impressions. Then I'll focus on the ones I like more.

Later Edit: @hpamdr Just tried T1.2 with Forza AudioWorks Noir Hybrid cable with Eternity, with Sylvania 6V6GT/G and GEC L63 (a combo more neutral and airy than with KT66 or KT88) and they sound very good, nice and as I knew them from Elise. Much more bass compared to Verite, same tone as I always knew them. For rock they sound good, but Verite is better in some areas being a little bit more aggressive. For your kind of music I guess they are very musical and you will like them.

From 300Ohm->600Ohm I had to increase the volume, on low gain setting, from 12AM -> 2PM. So around 40% more available on the volume knob. I switched to High Gain and to have the same volume I went back to 12AM. I think I hear a little bit more warmness on High Gain compared to Low Gain, but maybe I am wrong.

 

[OctavianH]

A small discussion about temps:

The amplifier is very silent, the case does not exceed around 30C (ok, in the picture we have 31.5C) in a room temperature around 23C.
Tubes as seen, KT66 around 80C, L63 around 50C, 6SN7 around 60C. Similar values with Elise.

 

[hpamdr]

So let say that is is a cool amp (did you measure also on the back near the cooling holes ? )

 

[OctavianH]

So let say that is is a cool amp (did you measure also on the back near the cooling holes ? )

Yes, same around 30C. It is cool.

 

[hpamdr]

From 300Ohm->600Ohm I had to increase the volume, on low gain setting, from 12AM -> 2PM. So around 40% more available on the volume knob. I switched to High Gain and to have the same volume I went back to 12AM. I think I hear a little bit more warmness on High Gain compared to Low Gain, but maybe I am wrong.

Tomas @SonicTrance could better answer.

Octavian, i think that is is not like for SS amp where High/Low are used for gain stage but rather High/Low Z are different tap on the output transformer so from 0-100 Low-Z is the best 100-600 High-Z should be the best. If your 300 Ohm is very sensitive, you can prefer low Z position specialy if you have High amplificcation factor tube.