Rating for KT88?

[leib10]

Just got my "matched" quad of KT88's in from a supposedly reputable store, and I'm going to have to send them back because they're unusable even though they guaranteed a matched quad. This time I'm going to specify the rating for the tubes. Seeing as I'm new to KT88's, what would be a good rating for them if I'll be using them in a Bogner Uberschall Twin Jet?

 

[glip22]

How does the TJ compare to the Rev Blue you had? You want your 88's rated at 63-66. Didn't the amp have EL34's in it? I still have the 77's in mine.

 

[leib10]

Well, they gave me two 61's, a 64, and a 65, so obviously it didn't sound that great, but it was a lot tighter and less flubby- exactly what I was looking for. Also a bit darker. I'm also not sure exactly what the bias should be on the Twin Jet. I knew it was about 32mA for the Rev Blue and the EL34's, but I'm not sure about the Twin Jet. Any idea about the range of bias?

 

[glip22]

Well, they gave me two 61's, a 64, and a 65, so obviously it didn't sound that great, but it was a lot tighter and less flubby- exactly what I was looking for. I'm also not sure exactly what the bias should be on the Twin Jet. I knew it was about 32mA for the Rev Blue and the EL34's, but I'm not sure about the Twin Jet.

You're ok and matched if you take one of the 61's and pair it with a 64 and one with a 65. These are close enough that it will not matter much. The OT doesn't see four tubes. It sees two pairs. Set one of the 61's to 30 ma and write down the bias for each tube. Pair the highest testing and lowest together and the other two tubes together and install the first pair in sockects one and two and the other pair obviuosly in four and five. For example if the tubes test at
31ma, 33ma,34ma, and 35ma. Put the 35 and the 31 together for your first pair Total of those are 66ma. The other pair's total is 67ma. Your OT will see this as matched within 1ma. Bias your hottest tube to about 40ma assuming your PV is close to mine at 525.I wouldn't use the 88's full rating to bias. Its best to play it safe and use 30 watts as your maximum dissipation when we are using new production tubes with high screen current. Does this help. The only problem is when the tubes are too far apart. You can usually get them allot closer with the above procedure but when one is too far off the bias will be too low for that tube. Your quad is not too far out at all. You just have to recheck for bias drift, especially if they are not burned in.

 

[leib10]

Is there any way to adjust the bias of each tube individually? I'm using the bias adjust screw and I've turned it all the way down and still the lowest two of the tubes will bias is about 45mA!

 

[glip22]

Is there any way to adjust the bias of each tube individually? I'm using the bias adjust screw and I've turned it all the way down and still the lowest two of the tubes will bias is about 45mA!

When you received the amp what tubes were in it? If EL34's then the piggybacked resistor in the bias circuit was removed. If not, I guess you need a much lower rated quad. Like 50. Mine works with the 63 rating.

 

[glip22]

It really depends how they rate their tubes. Obviously these are off.

 

[leib10]

Just talked to Charley at Bogner. He recommended that I try some colder tubes and, failing that, to have a tech change out a resistor. So we will see if I can get them to take their quad back and give me a colder one.

 

[6AM]

Every Bogner I ever owned had an annoyingly small sweep to the bias pot.

 

[protoplasma]

Is there any way to adjust the bias of each tube individually? I'm using the bias adjust screw and I've turned it all the way down and still the lowest two of the tubes will bias is about 45mA!

Are you reading a pair of tubes at 45 mA, or is that for one tube only?

 

[leib10]

2 are reading at 47-50, the other two are at 39-41. I'm going to try for some with a rating of like 50 and see how that goes.