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Malu3067

The risks of not using correct bias on valve amps

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Hi All

 

I have a conundrum. I have a set of Tung-sol KT120 Cryo matched valves.

 

Their specs are:

Ip: 88

Gm: 8,000

I must admit I am unsure of the significance of these specs.

 

I have a Richter Medusa amp that comes with KT88’s but is compatible with KT120’s.

 

When I install the KT120’s I can not adjust the bias enough to achieve the recommended setting of 30mA for KT120s, the closest I get is around 40mA.

 

They seem to work fine and sound awesome. However I am worried I am causing damage to either my amp or the valves by not having the right bias.

 

Am I paranoid or should I be worried? [emoji53]

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Hi, I may be wrong, and I defer to much more learned individuals on this forum, but running your valves 10ma above recommended spec is not going damage your amp but it will shorten the lifespan of your valves in the long run and you may get slightly less than optimum sound quality. 

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I think my understanding is also the above. My amp bias sets at 4 but I run at 3.7 for 6CA7

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30ma is actually biasing a KT88 pretty cold. Even 40ma is no drama at all. Even less of a drama for a KT120. It's barely coasting at that. The question is whether the amps power transformer is up to supplying the extra 10ma per tube. Best to check with Richter to be sure but I can't see too many dramas there.

Does the power transformer get overly hot with the 120's in?

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30ma is actually biasing a KT88 pretty cold. Even 40ma is no drama at all. Even less of a drama for a KT120. It's barely coasting at that. The question is whether the amps power transformer is up to supplying the extra 10ma per tube. Best to check with Richter to be sure but I can't see too many dramas there.
Does the power transformer get overly hot with the 120's in?


Richter recommend 28mA for the KT88’s and 30mA for the KT 120’s so perhaps they err on the conservative side for biasing?

The site I bought the valves from said they draw an additional 0.35mA each over the KT88’s, which in my case with four equals about 1.4A increase in current draw.

As expected the transformers do get hotter with the KT 120’s than with the KT88’s but I can still touch them without feeling like it will burn me. But I do not know if it is too hot.

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I have some kt120's here that tested at Ip:59 when new. I also once had some kt120 that were around 90, like your kt120's at 88, and couldn't pull them down because I ran out of adjustment on the trim pots in the amp. 

My amp uses 6550's and was built way before kt120's even existed but the power xformer has enough head room that the manufacturer has given kt120's the all clear. Your amp was designed with kt120's in mind so extra current draw has been factored in but still check with the manufacturer. You might need to track down a different set of tubes or maybe you can get the amp modified for a bit more adjustment. Either way it's worth it because KT120's are great tubes.

Or, it might be just fine the way it is. If you can leave your hand on the transformer without fear of getting burned it's probably ok but still with check the manufacturer. All care, no responsibility and all that.

 

 

 

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Just stating a recommended idle bias current avoids possible confusion, and allows a not-so-technical user to swap out valves and hopefully return an amp to acceptable operating condition. 

 

But you may pay a price for not going the extra step and assuring that the amp is actually operating the output stage appropriately.  It would be a poor amp manufacturer who didn't provide a servicing guideline with the nominal idle B+ voltage, along with the nominal plate or plate plus screen dissipation power level, along with a recommended maximum dissipation level and recommended maximum imbalance (if the output stage was push-pull). 

 

Valve options and ratings can change, and so can mains AC voltage, and a valve amp user should also be checking the idle heater voltage to ensure the total amp is not being operated inappropriately.  Sort of like owning a car but not being concerned about what tyres are fitted, or their pressure levels, until the car swerves off the road and needs a tow truck.

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Just stating a recommended idle bias current avoids possible confusion, and allows a not-so-technical user to swap out valves and hopefully return an amp to acceptable operating condition. 
 
But you may pay a price for not going the extra step and assuring that the amp is actually operating the output stage appropriately.  It would be a poor amp manufacturer who didn't provide a servicing guideline with the nominal idle B+ voltage, along with the nominal plate or plate plus screen dissipation power level, along with a recommended maximum dissipation level and recommended maximum imbalance (if the output stage was push-pull). 
 
Valve options and ratings can change, and so can mains AC voltage, and a valve amp user should also be checking the idle heater voltage to ensure the total amp is not being operated inappropriately.  Sort of like owning a car but not being concerned about what tyres are fitted, or their pressure levels, until the car swerves off the road and needs a tow truck.


Thanks trobbins now I am really worried. [emoji846]

You have raised my curiosity, I will need to look into the various specs you referenced but off the top of my head other than bias I don’t think they provided the rest in the manual.

Luckily I have a set of Prima Luna golds so I think I might stick with my KT88’s for general use and only swap in the KT120’s on special occasions.

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I have some kt120's here that tested at Ip:59 when new. I also once had some kt120 that were around 90, like your kt120's at 88, and couldn't pull them down because I ran out of adjustment on the trim pots in the amp. 
My amp uses 6550's and was built way before kt120's even existed but the power xformer has enough head room that the manufacturer has given kt120's the all clear. Your amp was designed with kt120's in mind so extra current draw has been factored in but still check with the manufacturer. You might need to track down a different set of tubes or maybe you can get the amp modified for a bit more adjustment. Either way it's worth it because KT120's are great tubes.
Or, it might be just fine the way it is. If you can leave your hand on the transformer without fear of getting burned it's probably ok but still with check the manufacturer. All care, no responsibility and all that.
 
 
 

I must admit I was thinking of getting another set with lower settings. Do you think the Ip setting is the one that affects bias?

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1 hour ago, Malu3067 said:


I must admit I was thinking of getting another set with lower settings. Do you think the Ip setting is the one that affects bias?

I think that may be the problem who knows? It could be the tubes or there may be a fault with the amp... the best advice anyone can give is to cover your backside and speak with the people that designed the amp and provide the warranty. It takes the same amount of time to write an email to them as it does to post a question on a public forum and it could save you some grief in the long run. Drop them a line, I'm sure they will be very happy to help:)

 

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I've taken a "robber" at bias setting on the 6L6 tube.
Because I'm good at the colors that apply at a certain temperature, I set the color of the grade (800 degrees).

 

It can not be too high current in any case.

 

 

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Should kt88 and kt120 have the same voltage (high volt)?
Do not know but that sounds a little unnecessary and if so ........

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