KT88 in place of 6550?

Panicos K · 1st February 2009, 04:02 PM

Has anybody retubed any audio research amps with KT88 in place of 6550?I think I've read somewhere that it is not recommended,still some KT88 tube manufacturers say that they are a direct replacement for all 6550.Any advice will be much appreciated.A friend is thinking to retube his arc amp.

kkillebrew · 1st February 2009, 09:37 PM

They are a direct replacement but if your amp is fixed bias it may need adjusting.

Panicos K · 1st February 2009, 09:58 PM

Thanks.On his amp bias is adjustable so it won't be a problem.

moonshine · 1st February 2009, 11:11 PM

Fixed bias means that the tubes are biased by applying a negative voltage direct to grid, not that the bias couldn't be adjusted. Cathode bias means that there is a resistor on the cathode that raises cathode from ground, therefore grid (at 0 volts) becomes more negative than cathode.

Usually amps that have fixed bias can be adjusted with a trimmer, so you will need to rebias after swapping tubes, although you have to do this every time you change tubes of the same type as well.

Panicos K · 2nd February 2009, 12:08 AM

Hi moonshine,I can't remember where,but I have read a report on what you say,and if I remember well it was not so simple as a trimmer adjustment.Maybe there was a replacement of that resistor or something more?

duderduderini · 2nd February 2009, 12:10 AM

Hi Panikos
I posed the same question and got many replies which you my be interested in.
Here is the url
KT88 and the 6550
Hope this helps
Nick

kkillebrew · 2nd February 2009, 01:49 PM

Quote:

Originally posted by Panicos K
Hi moonshine,I can't remember where,but I have read a report on what you say,and if I remember well it was not so simple as a trimmer adjustment.Maybe there was a replacement of that resistor or something more?

That is quite possible. Often a cathode resistor, or even values withing the fixed bias circuitry need to be tweaked to allow correct bias adjustment. It is usually a pretty simple procedure.

kkillebrew · 2nd February 2009, 01:57 PM

If your amp is of high quality (meaning power trans is not over stressed among other things) and if you are interested you can usually replace 6550s with the EI KT-90. I've only done this in the Citation II but it makes a big difference in power and sound. The challenge is finding good KT90's. With the KT90 you are more likely to have to mod the amp. In the Cit II you change the value of the cathode resistors from 15 to 18 ohms, if I remember correctly.

I don't know your amp, but the KT90 is a great upgrade if you can do it.

Panicos K · 2nd February 2009, 02:07 PM

Hi,thanks for this information too.The amp is not mine but my friend might be interested to check this possibility too.

Jim McShane · 2nd February 2009, 02:13 PM

Hi Kevin!

Just a couple comments...

The change from 15 to 18 ohms in the Cit II is not required for a tube change. It allowed us to use the stock bias meter and original bias adjusting procedure while lowering the idle current from the original 100 ma. (!) to 80-85 ma.

Second - sadly, the supply of good Ei KT-90s is pretty much non-existent. And the EH is NOT a tube I recommend.

There is no need to mod an amp if you were somehow able to find Type II or Type III KT-90s. The Type I biased cooler, so sometimes a bias circuit mod to accommodate the cooler bias is needed. But otherwise they are plug-n-play.

1st February 2009, 04:02 PM	#1
Panicos K diyAudio Member Join Date: Oct 2006	KT88 in place of 6550? Has anybody retubed any audio research amps with KT88 in place of 6550?I think I've read somewhere that it is not recommended,still some KT88 tube manufacturers say that they are a direct replacement for all 6550.Any advice will be much appreciated.A friend is thinking to retube his arc amp.

2nd February 2009, 12:10 AM	#6
duderduderini diyAudio Member Join Date: Mar 2004 Location: Albury NSW Australia	Read This thread Panikos Hi Panikos I posed the same question and got many replies which you my be interested in. Here is the url KT88 and the 6550 Hope this helps Nick __________________ "Better to say nothing and keep them guessing than to speak and remove all doubt."

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1st February 2009, 09:37 PM	#2
kkillebrew diyAudio Member Join Date: Jan 2009	They are a direct replacement but if your amp is fixed bias it may need adjusting.

1st February 2009, 09:58 PM	#3
Panicos K diyAudio Member Join Date: Oct 2006	Thanks.On his amp bias is adjustable so it won't be a problem.

1st February 2009, 11:11 PM	#4
moonshine diyAudio Member Join Date: Nov 2008	Fixed bias means that the tubes are biased by applying a negative voltage direct to grid, not that the bias couldn't be adjusted. Cathode bias means that there is a resistor on the cathode that raises cathode from ground, therefore grid (at 0 volts) becomes more negative than cathode. Usually amps that have fixed bias can be adjusted with a trimmer, so you will need to rebias after swapping tubes, although you have to do this every time you change tubes of the same type as well.

2nd February 2009, 12:08 AM	#5
Panicos K diyAudio Member Join Date: Oct 2006	Hi moonshine,I can't remember where,but I have read a report on what you say,and if I remember well it was not so simple as a trimmer adjustment.Maybe there was a replacement of that resistor or something more?

2nd February 2009, 01:57 PM	#8
kkillebrew diyAudio Member Join Date: Jan 2009	If your amp is of high quality (meaning power trans is not over stressed among other things) and if you are interested you can usually replace 6550s with the EI KT-90. I've only done this in the Citation II but it makes a big difference in power and sound. The challenge is finding good KT90's. With the KT90 you are more likely to have to mod the amp. In the Cit II you change the value of the cathode resistors from 15 to 18 ohms, if I remember correctly. I don't know your amp, but the KT90 is a great upgrade if you can do it.

2nd February 2009, 02:07 PM	#9
Panicos K diyAudio Member Join Date: Oct 2006	Hi,thanks for this information too.The amp is not mine but my friend might be interested to check this possibility too.

2nd February 2009, 02:13 PM	#10
Jim McShane diyAudio Member Join Date: Dec 2006 Location: Southwest Chicago Suburbs	Hi Kevin! Just a couple comments... The change from 15 to 18 ohms in the Cit II is not required for a tube change. It allowed us to use the stock bias meter and original bias adjusting procedure while lowering the idle current from the original 100 ma. (!) to 80-85 ma. Second - sadly, the supply of good Ei KT-90s is pretty much non-existent. And the EH is NOT a tube I recommend. There is no need to mod an amp if you were somehow able to find Type II or Type III KT-90s. The Type I biased cooler, so sometimes a bias circuit mod to accommodate the cooler bias is needed. But otherwise they are plug-n-play.

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