KT88 Bias Update for MTS Amps

MetalMajik said:

Thats some great info! I'm glad to see this resolved. Please keep us posted on the tubes, I'm really curious to see how the KT88's stack up against the GT 6l6 GE's. I must say however, this is quite a drastic change of direction from the 6v6's that you were considering

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trendkill1168 said:

MetalMajik said:

Thats some great info! I'm glad to see this resolved. Please keep us posted on the tubes, I'm really curious to see how the KT88's stack up against the GT 6l6 GE's. I must say however, this is quite a drastic change of direction from the 6v6's that you were considering

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Yeah, I change my mind alot....LOL. This is a pretty lethal combination though!! I'll be leaving it like this from now on. Right away I noticed that the KT88's added more power and "growl". They seem to thicken up the sound alot with a slight compression on the low end. This makes for great lows that are very controllable. Perfect for galloping palm mutes and fast riffing on the E & A strings without it getting muddy. The E34L's provide that nice mid-range punch which is perfect for leads and also contribute to the low end. Both tubes are great on the highs. Sounds very good on the high gain modules and perfect for metal. I wish I had something to record some clips with so I could post them. The KT88's are on the outside slots biased at 50mA and the E34L's are on the inside slots biased at 40mA. This is as good as I have ever heard this amp sound. I played a Marshall TSL half stack the other day and this amp just kills it. I have a friend that just bought a Mesa Dual Rec the other day and I'm interested to see how this head stacks up against it. I was almost getting bummed out with the RM and you just have to be patient with it because it is so versitile. Once you find the right combination of tubes and preamp modules it just rips. I'm finally pretty happy!!

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Songman68 said:

Actually you need to multiply it by .75 not .7. I know it is not that much difference but every little bit counts.

Here is the way it is figured according to Bob, and in his own words.

We will use the plate dissipation for a standard 6L6 for this example which is 22 watts. Example: 22 watts divided by 500 plate volts equals .044 or 44 milliamps of plate current draw at idle. The 44mA should never be exceeded! Then multiply the .044 by .75 or 75% which equals .033 or 33mA. This is where your power tubes will be out of crossover distortion and will perform very well. There is an acceptable window of correct bias and if you want a warmer tone or what some players refer to as a "brown sound" you can use up to 90% of the maximum current formula which would be 40mA rounded off. Your tube life will be a little shorter at this setting but only by about 20%. However, I would NOT recommend this for Chinese tubes or tubes that are sold by the Big guys that have been remarked unless you know for sure what tubes they really are. The JJ Electronic 6L6 is a very stout tube and JJ claims that it will dissipate 30 watts. Since I like to run my gear pretty hot I will routinely bias them at 50mA in an amp that has 485 plate volts. The tone is very warm and the tubes stand up very well. I tortured a quad in my Fender twin for a year, set at 50mA at 480 plate volts and they were still going strong until I swapped them out for a quad of KT88?s but that?s another story....

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Songman68 said:

Cool man I am going to try two KT88's in mine with two 6L6's and see how that sounds. My guess is it will be killer too. It will mix some of those KT88 mids in with the 6L6's and will probably kill.

My guess is that the multiplier of .7 and or .75 is probably not that big of a deal, but Bob's site does say .75. But like I said it probably is not going to make much if any difference at all.

Keep rocking dude,

Songman68

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trendkill1168 said:

Songman68 said:

Cool man I am going to try two KT88's in mine with two 6L6's and see how that sounds. My guess is it will be killer too. It will mix some of those KT88 mids in with the 6L6's and will probably kill.

My guess is that the multiplier of .7 and or .75 is probably not that big of a deal, but Bob's site does say .75. But like I said it probably is not going to make much if any difference at all.

Keep rocking dude,

Songman68

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I highly recommend it, they sound great. Using the factor of .75 will get them biased a little hotter at 53mA. I have mine at 50 right now so I guess it's just a matter of how you like the sound between the 2 settings. I have a set of 6L6's sitting around too so I may swap the E34L's out with them and see how they sound just for the hell of it. I wonder what it would sound like with a quad of KT88's? :twisted:

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Songman68 said:

trendkill1168 said:

Songman68 said:

Cool man I am going to try two KT88's in mine with two 6L6's and see how that sounds. My guess is it will be killer too. It will mix some of those KT88 mids in with the 6L6's and will probably kill.

My guess is that the multiplier of .7 and or .75 is probably not that big of a deal, but Bob's site does say .75. But like I said it probably is not going to make much if any difference at all.

Keep rocking dude,

Songman68

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I highly recommend it, they sound great. Using the factor of .75 will get them biased a little hotter at 53mA. I have mine at 50 right now so I guess it's just a matter of how you like the sound between the 2 settings. I have a set of 6L6's sitting around too so I may swap the E34L's out with them and see how they sound just for the hell of it. I wonder what it would sound like with a quad of KT88's? :twisted:

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If you try it with two 6L6's let me know how it sounds please.

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trendkill1168 said:

Songman68 said:

trendkill1168 said:

Songman68 said:

Cool man I am going to try two KT88's in mine with two 6L6's and see how that sounds. My guess is it will be killer too. It will mix some of those KT88 mids in with the 6L6's and will probably kill.

My guess is that the multiplier of .7 and or .75 is probably not that big of a deal, but Bob's site does say .75. But like I said it probably is not going to make much if any difference at all.

Keep rocking dude,

Songman68

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I highly recommend it, they sound great. Using the factor of .75 will get them biased a little hotter at 53mA. I have mine at 50 right now so I guess it's just a matter of how you like the sound between the 2 settings. I have a set of 6L6's sitting around too so I may swap the E34L's out with them and see how they sound just for the hell of it. I wonder what it would sound like with a quad of KT88's? :twisted:

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If you try it with two 6L6's let me know how it sounds please.

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No prob.....I have a quad of Groove Tube 6L6 GE's. I can check em out tonight.

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trendkill1168 said:

According to Bob at Eurotubes he uses this formula to figure the bias settings: Take the plate dissipation of the tube (35 watts for the KT88) and divide it by the plate voltage of 500 for the RM. Then multiply that times .7 (this figure will get you out of crossover distortion according to Bob).

So 35 divided by 500 X .7 = .049 so the KT88's would be biased at 49mA.

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chrishurley said:

trendkill1168 said:

According to Bob at Eurotubes he uses this formula to figure the bias settings: Take the plate dissipation of the tube (35 watts for the KT88) and divide it by the plate voltage of 500 for the RM. Then multiply that times .7 (this figure will get you out of crossover distortion according to Bob).

So 35 divided by 500 X .7 = .049 so the KT88's would be biased at 49mA.

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35 watts is not the max plate dissipation of a KT88. It is 42 watts. You'd be safer at 35 watts, obviously, but the tone may or may not suffer.

I wonder how much current the monster power transformer in the RM100 can supply. A quad of KT88's would be at least 200ma, with is a bit of juice. That transformer certainly *looks* the part. I guess Bruce Egnater should know, but I wonder if Randall will cover the warranty if you cook the PT by pulling too much current rhgouth it.

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