[DecentLevi]

Hey guys, joining a bit late here. Though I'm not lacking in experience, I've got a couple dumb questions trying to better grasp about my upcoming custom Infinity amp. This was meant as a PM to @SonicTrance but I thought I'd post it here so the answers can help others too.

You can run the output tubes from 40mA up to 60mA. But 50 is the sweet spot.

after you have adjusted the grid voltage without tubes installed, turned off the amp and waited 5 min, inserted the tubes and power on. You watch the mA meters on the amp as the tubes warm up and start conducting. You can then fine tune the bias point with the trimmers while watching the mA meters so the tubes draw the amount of current we want, which is 4mA for the input and 50 for the output tubes, except for the EL32 which you need to run cooler at 40mA.

Hi Tomas, I've been meaning to get back to you to clarify a few things.

So do you mean that with all the diverse types of tubes I specified (EL34 series stock and others with adapters), the only thing that needs to be adjusted for optimal performance is the grid bias from 40-60mA? I thought I was adjusting the voltage not currernt? And this range should even provide close to the high voltage demands for the EL12 Spez down to the very low current draw of EL32?

For adjusting the bias, here's what I understand. Please correct anything that's wrong.
Turn on the amp without tubes and measure with an external DMM (with its' max voltage set to at least 100v) a voltage for the desired tube to roll in according to the sheet you provide. After turning it on with tubes in, fine tune to make sure the meter on the amp is in the center at around 50mA. This trimmer adjustment changes the grid voltage bias but not the current so the external DMM meter will show the difference of voltage while the mA meter will stay the same... except for extreme adjustments needed like for the EL32 this also subsequently causes the mA meter to fall down to 40 mA.
But then if I fine tune the bias with the voltage trimmers on top, how does this correlate with the current on the mA meters? Also after it's on am I supposed to make sure those needles are in the middle, or that the external DMM doesn't drift to the wrong voltage?

If an old tube were to arc / explode, would having the volume down prevent damage to the headphone drivers? If it happens while listening, does the amp have any circuit that prevents or minimizes such loud boom from reaching the headphones?

For the switches, since mine would have a pre-amp, speaker and headphone connections, mine would have a pre-amp and output impedance switch, but not selector switch, right?

Finally, about the 5-min. thing. What's the shortest amount of time you could still recommend between cool down & then waiting time after voltage adjustment sessions? I'm a bit impulsive or ADD and can't see myself waiting so long consistently, LOL.

 

[UntilThen]

Something tells me I must be nuts to buy a NOS pair of GEC KT66 but I am.

 

[OctavianH]

No, you are not. They are incredible. But before doing that I would suggest to try the new production KT66 and KT88 to be sure which ones you like more. When trying them in Elise I preferred the KT66 but many on the F.A. threads prefer the KT88. And from GEC KT66, the smoked grey glass are warmer and more holographic while clear glass are brighter, cleaner and a little bit more detailed (at least how I see them).

 

[UntilThen]

For my amp, I already have Genalex Gold Lion KT88 and Electro Harmonix 6L6GC. I like the 6L6GC - it's warmer and nice bass. KT88 is more aggressive but very exciting with rock. I'm getting a pair of NOS GEC KT66 and a pair of NOS Philip Miniwatt EL34 double O rings. Both clear unfortunately or fortunately depending on how you look at it.

There are a lot more tubes I can roll. The tubes will cost more than the amp.

 

[hpamdr]

Before taking the decision to write Tomas and ask him if he can build me a headphone only amplifier based on 6V6 as outputs and, if possible, 6SN7 as input, because I wanted to use a part of my tube collection on it and see where it goes, I searched several months for tube amplifiers based on these. I looked a lot at WBA Virtus-01, ALO Studio Six, Cayin HA-6A or several Prima Luna models. It seems that even if 6V6 are mostly used in speaker amps, everyone who tried them on headphones had nice words for them. In the end I decided to try a full custom build and Eternity took form. We will see where this will go, I am fully convinced it will be interesting for all of us.

I'm almost in the same position I have a collection of tubes that i want to use but want to get a new amp with different topology (SET) and no rectifier. I auditioned dna starlett/stratus and found the sound really great but wanted a more versatile amp (not as Glenn OTL). I was also looking on some DIY tube design and found gyrator concept quite interesting. I do not need a speaker amp as today i still use an heavy SS amplifier for my main loudspeaker system.
This is why I'm so interested by infinity and then the eternity with tube rolling options. I finally jumped in the queue.
I will have to wait a bit after @OctavianH but can benefit from his experience (we are both owner of F.A. amp and T1 headphone)..

I will probably keep my Euforia as i like a lot the sound with gec/mullard 6080 and 5998 and it is for me well built and service is first class (at least in Europe).

 

[UntilThen]

Hahaha @hpamdr why did you make the last line font so small. Congrats on getting in the queue for Eternity. I'm waiting for you guys to tell me the tone. This used mint Elekit TU-8200 is just so amazing with it's tone. Love the KT88 and 6L6GC on it and the 12AU7. I'm getting the adapters to roll EL12 spez on it.

 

[UntilThen]

Tell me which you prefer - 6L6 GC vs KT88 vs EL34

 

[hpamdr]

No, you are not. They are incredible. But before doing that I would suggest to try the new production KT66 and KT88 to be sure which ones you like more. When trying them in Elise I preferred the KT66 but many on the F.A. threads prefer the KT88. And from GEC KT66, the smoked grey glass are warmer and more holographic while clear glass are brighter, cleaner and a little bit more detailed (at least how I see them).

What i guess is that we cannot really predict from F.A. OTL experiences how the tube will sound in Tomas SET amps..
Anyway IMO on Euphoria KT88 (Genalex G.L ) is a tube able to drive many headphone and bring them to life with dynamic, details and sound-stage. It is a bit bright but a wonderful experience with electric blues, rock and electronic music. I had some NOS KT88 and sold them to only keep the GL which cost between 50$ and 50£ and are for me very well performing.
I also have a friend who get Cayin HA-6A and love it with same KT88.

 

[OctavianH]

This is why I'm so interested by infinity and then the eternity with tube rolling options. I finally jumped in the queue.
I will have to wait a bit after @OctavianH but can benefit from his experience (we are both owner of F.A. amp and T1 headphone)..

So it seems we will be the first Eternity users. This is great, however I am not very good at reviews or describing what I hear and my musical preferences are different than what others consider "audiophile". I also hate this kind of categorization, I am just a guy who likes beer, music and headphones (in this order, LOL). Even in these conditions, I will do my best to describe to all of you what I hear and let's hope you will not be disappointed. I was also enrolled in the Glenn-OTL queue for 9 months but during this time people on the Euforia thread started to discover the KT66. Later they moved on to KT88 and GU-50 but I remained stuck at KT66. I never heard an electrical guitar sounding more real and better than on the GEC clear glass KT66. So I realized that the Glenn-OTL, being not able to use 6V6 type of tubes, is not maybe what I need, and since the things were progressing slow on the queue I have decided to quit and focus on finding an amplifier which can accomodate 6V6 and, if possible, to be shipped from Europe. And this is how I reached UltraSonic Studios, which I was constantly "monitoring" because Oblivion seemed to me something interesting.

 

[SonicTrance]

So do you mean that with all the diverse types of tubes I specified (EL34 series stock and others with adapters), the only thing that needs to be adjusted for optimal performance is the grid bias from 40-60mA?

Yes! I'll make it easy for you and say that all output tubes, except for the EL32 @ 40mA, should run @ 50mA.

I thought I was adjusting the voltage not currernt?

Your are adjusting the negative voltage applied to the grid of the tubes. Which in turn sets the operating point. So when you have adjusted the grid voltage, without tubes, and then fine tune with tubes in place you don't need to watch the grid voltage anymore. Only the current is important in the end. Just watch the panel meters while adjusting.
The thing is that the gyrator plate loads are set to run at a specific current and are not happy otherwise.

Turn on the amp without tubes and measure with an external DMM (with its' max voltage set to at least 100v) a voltage for the desired tube to roll in according to the sheet you provide. After turning it on with tubes in, fine tune to make sure the meter on the amp is in the center at around 50mA.

Correct! 50mA for the output tubes and 4mA for input tubes.

This trimmer adjustment changes the grid voltage bias but not the current so the external DMM meter will show the difference of voltage while the mA meter will stay the same...

Not correct! The adjustment changes the grid voltage AND the mA meters. This is a very sensitive adjustment, especially for input tubes! That's why I'll use multiturn trimmers to make it less sensitive.

Also after it's on am I supposed to make sure those needles are in the middle, or that the external DMM doesn't drift to the wrong voltage?

Once you set it it won't drift. No need to measure anything. Have a look at the mA meters every time you power on the amp just to make sure all is good.

If an old tube were to arc / explode, would having the volume down prevent damage to the headphone drivers? If it happens while listening, does the amp have any circuit that prevents or minimizes such loud boom from reaching the headphones?

I've never had a tube arc/explode and it would depend what was actually happening inside the tube. I can't guarantee it won't affect the headphones but the output transformers and parafeed capacitor should provide some protection.

For the switches, since mine would have a pre-amp, speaker and headphone connections, mine would have a pre-amp and output impedance switch, but not selector switch, right?

If by selector switch you mean the "6SN7 switch" that @OctavianH will have, then no. Your amp will have impedance switch and a speaker/pre-amp switch. So you can have either headphones/speaker, headphones/preamp on at the same time. Or just one output active.

Finally, about the 5-min. thing. What's the shortest amount of time you could still recommend between cool down & then waiting time after voltage adjustment sessions? I'm a bit impulsive or ADD and can't see myself waiting so long consistently, LOL.

You can measure yourself with the DMM set to at least 300VDC. Put black probe in black banana jack and red probe in pin 3 of the output tube socket. You count the pins counter clockwise from the guidepin. When the voltage is down to a couple of volts you're fine.

Hope that helps.

 

[OctavianH]

Yes! I'll make it easy for you and say that all output tubes, except for the EL32 @ 40mA, should run @ 50mA.

This is valid also for Eternity?

You can measure yourself with the DMM set to at least 300VDC. Put black probe in black banana jack and red probe in pin 3 of the output tube socket. You count the pins counter clockwise from the guidepin. When the voltage is down to a couple of volts you're fine.

I have a rule:
- count the pins as you look to the socket of the amplifier counter clockwise starting from the guiding pin
- count the pins as you look to a tube base clockwise starting from the guiding pin (what you see on datasheets)
Confusing stuff, this is why I prefer others to make adapters because I am sure I'll do something wrong.

 

[hpamdr]

have a rule:
- count the pins as you look to the socket of the amplifier counter clockwise starting from the guiding pin
- count the pins as you look to a tube base clockwise starting from the guiding pin
Confusing stuff, this is why I prefer others to make adapters because I am sure I'll do something wrong.

I have the yellow jacket adapter drawing as a reference printed to have top view socket and bottom view tube.
When you see pin it is clockwise when you see hole it is anti-clockwise.
Also if you want extensive info on socket you can use this radio museum documentation.

 

[OctavianH]

There are many invaluable resources, one of them is this one:
http://www.4tubes.com/navigation/index-datasheets.html

A very nice article related to KT88 but not only those is this one:
http://www.jacmusic.com/KT88/kt88.htm

I think a section with links would be useful in the first posts.

 

[SonicTrance]

This is valid also for Eternity?

Eternity will run all output tubes @ 20mA. Input stage will run @ 4mA just as Infinity.

 

[UntilThen]

A very nice article related to KT88 but not only those is this one:
http://www.jacmusic.com/KT88/kt88.htm

A useful article thanks. I have Genalex Gold Lion KT88. Just bought Gold Lion KT77. Wonder why KT77 is not mention there. I also have the EH 6L6GC. KT77 is like a cross between KT88 and 6L6GC.