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roberto

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Hola. I have an Audio Research VT-100 MKII that uses KT-88s as output tubes. I do know that these are a better tube model than the 6550s. I was talking the other day with a friend and he said that there are a lot of different tube options that you can just put without doing any harm and will get sweeter sound. Does anybody can share it with me if this is true? I tried to find the substitutes at thetubestore, but did not get any. Thanks
Roberto.
 
The KT-88 isn't a "better" tube than the 6550, it does have a slightly higher power rating and usually tends to sound a bit tighter and more extended on both ends of the frequency spectrum than the 6550.

However, some of the vintage 6550's are very musical tubes, just a bit spendy. Good ones can run in the 100-250 each range, which makes retubing your VT-100 a bit expensive.

Depends on how much time you spend listening! That amp usually goes through a set of output tubes every 3-5000 hours.

You can experiment a little bit with different types of current KT-88/6550's, but if memory serves me correctly, that amp needs 8 matched tubes, with only one bias adjustment. My ARC Classic 120s were the same way.
 
Buenos dias Roberto!
Your friend has given you a somewhat open-ended piece of advice! Does he mean simply trying different brands of 6550 or KT88? They'll all have different sound characteristics; some good, some not so good. Or is he being specific about your Audio Research amp? Some high-end tube amps in recent years have an automatic bias circuit, and conceivably, any octal output tube with the same pin-out as a 6550 or KT88 could be used; therefore, almost any tube in that "family", from the 6V6 to the KT88 could be tried.
HTH!:D
 
Nope, the VT100 will only use the KT88/6550 tube.
And it is NOT auto bias. It has to be set and again, I
can't remember the VT100's internals. Many of the ARC
amps have one bias control for all 8 tubes, while others
have individual bias settings for each tube.

Best bet is to look at your manual, or call ARC.

Overall, the 6550 tends to be a bit warmer sounding
than the KT88 though.
 
Hola. I have an Audio Research VT-100 MKII that uses KT-88s as output tubes. I do know that these are a better tube model than the 6550s. I was talking the other day with a friend and he said that there are a lot of different tube options that you can just put without doing any harm and will get sweeter sound. Does anybody can share it with me if this is true? I tried to find the substitutes at thetubestore, but did not get any. Thanks
Roberto.
Roberto, I suggest you read this thread:
http://db.audioasylum.com/cgi/m.mpl?forum=tubes&n=79033&highlight=gadsons
You must re-bias the power tubes in that amp any time you change tubes -- even if it is 6550 > different 6550! As for sound experiments (tube rolling), changing the (4) 6922 driver tubes will make a bigger difference than changing the power tubes. And if you are driving the amp single-ended, then changing the (4) 6922 input (splitter/inverter) tubes will change the sound also (however driving this amp single ended would be a crime IMO, since it's a balanced circuit amp, like a BAT only better!)

The problem is that doing these things requires 2 - 4 hours. For example, even if you are not changing the power tubes and only changing the 6922's to find what sound you like, you must still re-bias the power tubes again.

Many, many screws . . . . . . . . . .
 
Someone has just reported that the Gold Lion re-issue is very good on another thread, possibly BETTER than the original. It's also a very reasonable price.

In my KT88 based amp of old (Air Tight ATM2), the power tubes made far more difference to the sound than the driver tubes... but that is a different circuit, and Neil (nsgarch) may have specific ARC familiarity...
 
Someone has just reported that the Gold Lion re-issue is very good on another thread, possibly BETTER than the original. It's also a very reasonable price. In my KT88 based amp of old (Air Tight ATM2), the power tubes made far more difference to the sound than the driver tubes... but that is a different circuit, and Neil (nsgarch) may have specific ARC familiarity...
That may be very true with the Air Tight (they still make the ATM2) because they drive the power tubes right to the limit! And certain power tubes (like all NOS GEC/Genalex KT88, 66, 77, EL34) have a signature sound, sort of an unmistakable shimmer which it's easy to hear once youve been exposed to them. I haven't heard the Russian reproductions, but I bet they don't 'shimmer' :D As a matter of fact, if you want an really accurate copy of the original Gold Lion KT88, see the Pentalabs KT88SC. Doug's Tubes sells them both.
 
Gracias amigos!!!

Muchas gracias chicos!!!...I went to my friend´s house to look and listen what he had put into his VT-100MKII (same as mine), but a little bit older than mine. The tubes that he is using are 7027. His speakers are not ML, are Usher, and I did like what I did listen. Most of the time, we listened piano solos, classical and then some jazz, but what got me hooked to this was one recording that I had listenend a lot: Michael Garson, Serendipity. It is an old recording by Reference Recording, and I was very used to listen this thorugh my CLSs with Conrad Johnson electronics...I can say that our ML sound is cleaner, but I did get tons of magic here also. Truly 3D sound with the musicians playing for you. Does anybody knows anything about this tube? I do not have the guts to did what he did, took his old tubes and replaced them for this 7027 number and he said he did not re-bias or what ever. He got a truly good dammed walking from me!!! I told him that he could burn his amplifier if he does not adjust the bias...but he said to me, Hey, I have them running over three months by now...gee, I did not know what else to say...I had tried to see the 7027 specs, and seems that the power disipation is less than the 6550s, but they are running cool, and nice sound too. Guys, thanks again for all the response to this thread...you are making a crazy costarrican very happy!!! Regards, and happy listening,
Roberto.
 
A 7027 is actually a replacement for a 6L6. It can be substituted for a 6550 but I don't know why ;--) It is used a lot in guitar amps. If your friend wants the best sound from his ARC power tubes, (and he is rico!) let him find some NOS Tungsol solid blackplates or solid grayplates. The new Tungsol 6550's are supposed to be very good, but I don't know . . . . . . . .
 
A 7027 is actually a replacement for a 6L6. It can be substituted for a 6550 but I don't know why ;--) It is used a lot in guitar amps. If your friend wants the best sound from his ARC power tubes, (and he is rico!) let him find some NOS Tungsol solid blackplates or solid grayplates. The new Tungsol 6550's are supposed to be very good, but I don't know . . . . . . . .

thanks a lot nsgarch...will tell him regarding it...but the funny thing, the amp is working with those tubes...even that they are not a direct replacement...I do not understand this...again, I thank you so much!!! happy listening,
Roberto.
 
Roberto -

I tried 2 matched quartets of new Tung-Sol 6550s in my VT1000 III (I still have it, btw - now using Ref 110.) They really sounded great until one of them started glowing very bright orange-red. Fortunately I discovered this before the tube took out a plate resistor. (This is not an uncommon event with all incarnations of the VT100 amp, BTW.) I don't know if my experience was unique, but BEWARE!
 
Roberto - I tried 2 matched quartets of new Tung-Sol 6550s in my VT1000 III (I still have it, btw - now using Ref 110.) They really sounded great until one of them started glowing very bright orange-red. Fortunately I discovered this before the tube took out a plate resistor. (This is not an uncommon event with all incarnations of the VT100 amp, BTW.) I don't know if my experience was unique, but BEWARE!
It wasn't the *amp's fault. If you biased the quartets (and I'm sure you did) then somebody was asleep at the tube tester!! I'd send the whole quartet back and request a re-do ;)

*NB it is true that sometimes a driver tube can "run away" and cause the power tube to be overdriven but that usually results in immediate failure of the PT. So I'm sticking to my story :D
 
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It wasn't the *amp's fault. If you biased the quartets (and I'm sure you did) then somebody was asleep at the tube tester!! I'd send the whole quartet back and request a re-do ;)

*NB it is true that sometimes a driver tube can "run away" and cause the power tube to be overdriven but that usually results in immediate failure of the PT. So I'm sticking to my story :D

Thanks again amigos...got from ARC dealer a new set of 6550s matched pairs, my manual amplifier recommends that I should bias them at the resistor test points for 130mV. I do have a great tester (Fluke 867B) and it is factory calibrated certified. I am going to check my amp bias, just to be sure that I am in that range. I would like to bring down a little bit the bias, lets say in the order of 120mV for safety operation, I do know that I am sacrificing some watts at the end, and also they will run a little bit cooler, I do not play my music too loud. My room is very small, I do not need to...thanks my dear amigos!!! happy listening,
Roberto.
 
Thanks again amigos...got from ARC dealer a new set of 6550s matched pairs, my manual amplifier recommends that I should bias them at the resistor test points for 130mV. I do have a great tester (Fluke 867B) and it is factory calibrated certified. I am going to check my amp bias, just to be sure that I am in that range. I would like to bring down a little bit the bias, lets say in the order of 120mV for safety operation, I do know that I am sacrificing some watts at the end, and also they will run a little bit cooler, I do not play my music too loud. My room is very small, I do not need to...thanks my dear amigos!!! happy listening,Roberto.
some tips about biasing ARC amplifiers:

  • The tubes are biased in pairs, (not each tube separately.) That is why you must use matched pairs ;-)
  • Unless you have an excellent power company, or a constant voltage device or regenerator, variations in line voltage will cause corresponding variations in bias voltage.
  • To be safe (if you can't guarantee constant wall voltage) measure the wall voltage at different times of the day and night, and then bias your amplifier when the line voltage is highest, so you have no worries :)
 
some tips about biasing ARC amplifiers:

  • The tubes are biased in pairs, (not each tube separately.) That is why you must use matched pairs ;-)
  • Unless you have an excellent power company, or a constant voltage device or regenerator, variations in line voltage will cause corresponding variations in bias voltage.
  • To be safe (if you can't guarantee constant wall voltage) measure the wall voltage at different times of the day and night, and then bias your amplifier when the line voltage is highest, so you have no worries :)
thanks!!! a great tip...you are right!...it is better to adjust the bias at late night when we have clean and steady voltage. Yes the eight tubes are factory matched by ARC. They are 6550C Ip:63 Gm: 7.200 made by Svetlana (Russia) and each tube comes with its searial NO. too. Again, thanks a lot!!!
Warm regards from Costa Rica, happy listening,
Roberto.
 
Hola Roberto.....depending how much you'd want to spend, these have had a pretty good couple of reviews in the UK (HIFI+ iirc), but they sure ain't cheap!

http://www.euroaudioteam.com/kt88diamond.php

regards

thanks magoo...the ones that I just got are from ARC...did the bias adjustments yesterday, and wow!! I sure needed new tubes!...back to heaven...thanks a lot and happy listening,
Roberto.
 

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