Another Tube Amp Biasing Question: when changing entire tube set

[Valveglow]

My VTL MB 225 Triode monoblock amps call for manual biasing of the tubes between 275 - 300 millivolts and originally came with 6550 tubes.

They are being retubed with KT88 tubes. My questions are as follows:

• Should I keep the same values as 6550 and KT88 are interchangeable?
• Is it best to keep the bias low near the 275 or high about 300?

Thanks much!

 

[peterh]

1- yes
2- doesn't matter. 285 is in the middle.

 

[Valveglow]

1- yes
2- doesn't matter. 285 is in the middle.

Thank you. I didn't think it mattered very much.

However, being that the tubes are somewhat different in circuitry and power handling, I thought it might be beneficial to keep it either on the high or low side rather than in the middle where it is now.

 

[Tom Bavis]

I think you'll find that modern 6550s and KT88s from the same maker are identical inside - bottle is only difference. 275-300 is a fairly narrow range - actual value won't make much difference. Most important that they be equal.

 

[peterh]

Some vendors might not differentiate between kt88 and 6550.
Others ( JJ is one) does follow the original specs.

 

[Valveglow]

I think you'll find that modern 6550s and KT88s from the same maker are identical inside - bottle is only difference. 275-300 is a fairly narrow range - actual value won't make much difference. Most important that they be equal.

Some vendors might not differentiate between kt88 and 6550.
Others ( JJ is one) does follow the original specs.

Thank you both. Interestingly, I have heard for instance that Electro Harmonix 6550 and KT88 are the same. In reality, they are not. I contacted the company and they explained that though similar, the circuitry and values are not the same. A company rep went on to explain what he felt were the sonic differences between their 6550EH and KT88EH. To me, what is even MORE interesting is that the bottle, which used to look very different is what is actually the same (or extremely similar). They both have KT88 looking bottles.