zerocharisma
07-29-2008, 12:45 AM
OK so I've tried to research for answers in regards to this but would like to get some more definitive information from those more knowledgeable than myself. I consulted 2 very knowledgeable amp gurus about the possibility of replacing the EL34s in my '68 50 watt plexi with NOS Genalex KT66s. They each told me that a proper rebias would accommodate this change and that I had the option of running the amp at either 16 ohms or 8 ohms into my 16 ohm 4x12 loaded with greenbacks. Both noted that the mismatch would be more suitable for the KT66s. Here is a quote:
"The stock output tranny in your amp, the Drake 784-139 is a 3.2K primary
impedance. Some folks say 3.4K...close enough. Your KT-66's will work
nicely into this load observing stock impedance settings ie: 16 ohms cab,
amp set at 16 ohms, etc. You can double primary impedance as you outlined by setting the amp for one setting below the cabinet impedance. So, 16 ohm cab, amp set at 8 ohms. This doubles primary impedance to 6.4K or 6.6K. 6L6 family tubes are more comfortable playing into a high primary impedance than an EL-34. Really the amp could be used at either impedance setting 8 or 16 ohms into a 16 ohm cab. The amp will be slightly less dynamic playing into the higer primary impedance and less efficient,
probably prodcue less power also. The feel is softer at this setting to me
and is notciable. I say try it both ways and go with what works for a
given application."
For my tastes, the KT66s sound INCREDIBLE in this amp and I personally prefer the matched impedance. However, I'm apprehensive about the safety of my amp at either setting. So I just want to be clear that if I leave the amp at 16ohms into the 16ohm cab will I be safe? And the same will be true if I decide to go down to 8ohms on the Marshall? Because even though both amp guys tell me this is safe, I'm still cautious. Despite the awesome tone I'm thinking of scrapping this whole experiment and swapping out the KT66s for other tubes that suit the OT better, maybe KT77s, KT88s or 6550s. Any help or advice would be much appreciated. Thanks!
"The stock output tranny in your amp, the Drake 784-139 is a 3.2K primary
impedance. Some folks say 3.4K...close enough. Your KT-66's will work
nicely into this load observing stock impedance settings ie: 16 ohms cab,
amp set at 16 ohms, etc. You can double primary impedance as you outlined by setting the amp for one setting below the cabinet impedance. So, 16 ohm cab, amp set at 8 ohms. This doubles primary impedance to 6.4K or 6.6K. 6L6 family tubes are more comfortable playing into a high primary impedance than an EL-34. Really the amp could be used at either impedance setting 8 or 16 ohms into a 16 ohm cab. The amp will be slightly less dynamic playing into the higer primary impedance and less efficient,
probably prodcue less power also. The feel is softer at this setting to me
and is notciable. I say try it both ways and go with what works for a
given application."
For my tastes, the KT66s sound INCREDIBLE in this amp and I personally prefer the matched impedance. However, I'm apprehensive about the safety of my amp at either setting. So I just want to be clear that if I leave the amp at 16ohms into the 16ohm cab will I be safe? And the same will be true if I decide to go down to 8ohms on the Marshall? Because even though both amp guys tell me this is safe, I'm still cautious. Despite the awesome tone I'm thinking of scrapping this whole experiment and swapping out the KT66s for other tubes that suit the OT better, maybe KT77s, KT88s or 6550s. Any help or advice would be much appreciated. Thanks!