[attmci]

the problems with multiple parallel output tubes as i see it are:
a) the grids can interact with each other very easily. to isolate them, you can install grid stoppers for each tube, but this can also lead to other problems, described in c) below...
b) there is some merit to the long held hifi belief that a single tube sounds clearer than multiple parallel tubes. this has to do with the impossibility of having an exactly matched pair of tubes. as a result, the tubes will "fight" each other to some extent as to what exactly the output SHOULD be at any given instant. folks have said the parallel tube tends to defocus the sound when compared to the single tube.
c) including those grid stoppers described in a) can actually exacerbate the problem, since now the electrode voltages will be dependent on the electrode CURRENTS.

 

[attmci]

The main problem is proper load sharing between the devices. Tolerances of the tube parameters, such as transconductance, can be quite high(this could be good too.  2x 6bl7 close to one 6AS7 ) . Without special precautions, one tube will bear more of the load current than the others, and it will fail prematurely.

 

[Badas]

I've just observed another sound improvement on using quad 6BL7's. Speed.

I have a speed test track. Robert Palmer - Some like it hot. The drums are super fast and precise.

My Woo usually gets killed on this track. Virtually unlistenable. Usually my SS amp is far superior. Not now. What an improvement. Perfect. Woo killed my SS amp. Far more speed and impact.

I'm wondering if speed is a reason this tube is getting a lot of fans?

 

[Badas]

Does anyone know of a good tube seller that has decent stocks of 6BL7?

eSlay is okay for picking up a few for trial. However there is not many auctions for multiples if someone wants to stock a few for future use.

 

[leftside]

@attmci I haven't noticed any of those problems with my matching quad 6BL7's.
 
@badas I've noticed the impact for sure. I'll try and see if I can notice the speed compared to my BHA-1. I can drive both amps at the same time, so it's really easy to do a quick A/B comparison. Difficult to find matching quads of the 6BL7's in one go. Luckily there still seems to be quite a few singles available, so at least you can probably match the construction/brand/year.

 

[2359glenn]

  The main problem is proper load sharing between the devices. Tolerances of the tube parameters, such as transconductance, can be quite high(this could be good too.  2x 6bl7 close to one 6AS7 ) . Without special precautions, one tube will bear more of the load current than the others, and it will fail prematurely.

If you make your own adapters besides grid stoppers on every grid put a equalizing resistor on each cathode to balance 
the current in the tubes.    Or modify the adapters you have with these resistors.
This is what I do in OTLs that have six 6BL7s and the current is close to the same in all the tubes.

 

[gibosi]

  The main problem is proper load sharing between the devices. Tolerances of the tube parameters, such as transconductance, can be quite high(this could be good too.  2x 6bl7 close to one 6AS7 ) . Without special precautions, one tube will bear more of the load current than the others, and it will fail prematurely.

 
I choose not to let "perfect" be the enemy of "good". Yes, ideally, it would be best to have grid stoppers on every grid and equalizing resistors on every cathode. But even so, those of us using these inexpensive adapters are experiencing an SQ equal to, and in some cases, even exceeding the best 6AS7-type tubes we own. If the worst happens and a tube fails prematurely due to bearing more of the current load it's no great loss. These tubes are cheap and I have extras. So I am just going to pop in another and get back to the music.
 
Cheers.

 

[whirlwind]

 

  The main problem is proper load sharing between the devices. Tolerances of the tube parameters, such as transconductance, can be quite high(this could be good too.  2x 6bl7 close to one 6AS7 ) . Without special precautions, one tube will bear more of the load current than the others, and it will fail prematurely.

 
I choose not to let "perfect" be the enemy of "good". Yes, ideally, it would be best to have grid stoppers on every grid and equalizing resistors on every cathode. But even so, those of us using these inexpensive adapters are experiencing an SQ equal to, and in some cases, even exceeding the best 6AS7-type tubes we own. If the worst happens and a tube fails prematurely due to bearing more of the current load it's no great loss. These tubes are cheap and I have extras. So I am just going to pop in another and get back to the music.
 
Cheers.

I would rather not have to use adapters.... but if I have a tube die prematurely I will do the same as you gibosi....I will just throw another one in, it is not like losing a 5998 or a GEC 6AS7G....the price is right on these tubes.
 
I actually think I prefer the tone just a tad bit better with the 6BX7, but both are great sounding tubes...heck even one per channel sounds good with the HD800

 

[mordy]

Hi,
 
I apologize for my lack of knowledge regarding the electric parameters of multiple tubes, but as Gibosi says, I have not had any problem using multiple mixed brands/types of power tubes. Those combinations that don't work result in loud hum. Sometimes, even using the same tubes, there is one or two that cause hum, and you have to try another one to eliminate it.
 
Re what attmci says about single tubes sounding clearer and the difficulty of matching power tubes exactly, my personal impression using multiple 6BL7 tubes is increased clarity and instrument separation. However, and I want to stress this, one of the main factors re clarity seems to be the driver tubes. The power tubes serve up a foundation for the sound, especially bass impact, but the driver tubes execute the sound so to say. Having six power tubes elevates the performance of the driver tubes - even ones that did not sound so impressive before perform better with the multiple power tubes.
 
Some driver tubes produce a more musical presentation, others more analytical, and much of the detail and clarity is connected to the driver tubes.
 
Obviously, there is a lot of synergy involved between driver and power tubes.
 
Another five star tube that I use as my favorite driver is the EL3N. It is very quiet, runs very cool, 10,000 hr life, and has very good synergy with the BL and BX tubes and just about anything else for that matter.
 
It seems to me that using multiple power tubes is nothing new in power amps, although the smaller headphone/preamps we use are generally not designed for more than two power tubes. However, it occurs to me (and maybe I am wrong) mixing power tubes is a less known subject.

 

[pctazhp]

   
It seems to me that using multiple power tubes is nothing new in power amps, although the smaller headphone/preamps we use are generally not designed for more than two power tubes. However, it occurs to me (and maybe I am wrong) mixing power tubes is a less known subject.

 
My eyes tend to glaze over with the technical discussion of multiple tubes, other than knowing when external power is needed. As you probably know, I'm not necessarily a multi-tube fan (primarily because of the usual need for external power - but even more importantly because I have finally found a 4-tube combination that I absolutely love in my system). But it seems to me virtually all of the tube rolling we do in our tube based headphone amps is pretty much trial and error, regardless of whether multiple tubes are involved. And further it seems to me that whatever combination brings someone the most listening enjoyment is the best for that person, regardless of theory.

 

[leftside]

Originally Posted by mordy /img/forum/go_quote.gif
 
Another five star tube that I use as my favorite driver is the EL3N. It is very quiet, runs very cool, 10,000 hr life, and has very good synergy with the BL and BX tubes and just about anything else for that matter.
 

Going to be rolling 4 of those in soon.

 

[Aussie Lou]

Does anyone know of a good tube seller that has decent stocks of 6BL7?

eSlay is okay for picking up a few for trial. However there is not many auctions for multiples if someone wants to stock a few for future use.

 
Is Lamberto out of stock?
 
http://www.ebay.com.au/itm/2-X-6BL7GTA-TUBE-ULTRON-BRAND-TUBE-MATCHED-PAIR-CRYOTREATED-C6HV7F080115-/311248552049?hash=item4877dc0071:g:S5UAAOSwa39Urmij

 

[leftside]

Ha - they are getting more popular. His prices have gone up a lot.

 

[Badas]

Is Lamberto out of stock?

http://www.ebay.com.au/itm/2-X-6BL7GTA-TUBE-ULTRON-BRAND-TUBE-MATCHED-PAIR-CRYOTREATED-C6HV7F080115-/311248552049?hash=item4877dc0071:g:S5UAAOSwa39Urmij

Ha - they are getting more popular. His prices have gone up a lot.

Yeah. I contacted him. He is out. Plus his price quadrupled.

 

[lukeap69]

That's because of the rise in popularity of 6BL7 in Glenn's, Elise, and this thread. Let's start bashing the 6BL7 so the price will go down again.