I have spent some time playing with the bias of the stock KT88's with some excellent results. When the bias was set to the mid-point on the R8 meter, the actual bias current was just under 40mA and the sound was a bit thin. In reading the very large R8 thread over at the Aussie site, I found that the recommended bias for the KT88's was 60mA at 440v which is about 75% of the max dissipation. The bass was a tad too much for me and I dropped the bias down to 55mA which sounds just right to me in my system. Experiment with current between 45mA and 60mA to see what sounds best in your system.
As you've done all the hard work, can we now pinch the easy stuff ? If, roughly, the mid-point on the R8's meter is 40mA, are you able to say where the readings of 45 / 50 / 55 / 60 sit on the R8's meter? That would be really helpful and negate the "need" for others to purchase external mA meters.
So not a lateral move then? My Vincent just keeps sounding better and better now that the new tubes are breaking in on both that and the phono stage. However this just makes me keep thinking if some tubes are great, what would all tubes be like!
The problem is it could be different for different amps. It would be best that everyone that has one of these use actual bias meters to set the current accurately. The one pictured above was from Amazon and was right around $50.
It will likely end up someplace around the S in BIAS, but as Rockin' said it would be best to use a bias meter (and thanks to Rockin' for posting the link to the meter back a few pages).
Does anyone own a Line Magnetic LM-508ia or a 300B SET amp here? I am thinking of trying out my first push-pull amp, probably a Willsenton R8, in the near future and wondering how do you describe the sonic or other major differences between these two? After doing much research, I think I am more biased towards EL34 due to its more laid back sonic character. Any input is very appreciated.
I will be receiving a quad of Gold Lion KT77's on Friday and have a question about switching between KT88 and EL34 modes. Currently I have bias set at 60mA for the KT88's. Do I need to reduce the bias to the 35mA range before swapping the tubes?
So you're going down the rabbit hole? I will be very curious to hear what you hear after swapping tubes.
I have a Line Magnetic LM-218ia -- an 845-based SET amp. I also have a Cayin A-60T -- a 6L6-based push-pull amp that also allows remote control of Ultra-linear and Triode modes. They are both wonderful amps that are different. I prefer the SET sound but every time I think "I don't need two tube amps" as a justification for selling the Cayin, I listen to it for a few days and forget about selling it. The 218ia is really a wonderful amp that seems to have flown under the radar of its bigger brothers in the SET line-up. I would be surprised if a push-push pull EL34-based amp is more laid back than a SET amp. -Chris
I also have a quad of Gold Lion KT88's arriving on Friday. Just more interested in The KT77's for now since the amp came with KT88's. I want to hear the difference between the EL34 and KT88 types.
It doesn't really matter, the point is to avoid overload on the new tubes accidentally. Best is to make sure the bias setting is at minimum when u put on new power tubes n turn on the amp for the first time. For me, I would let the new power tubes warm up for 5-10 minutes before adjusting them to the required level.
Well my R8 just smoked itself. Something lit up bright enough in the chassis that I could see it below V3. I had just received a new quad of KT77's. Bias was turned all the way down before starting. I had V1 and V2 set to 40mA then I moved the bias meters to V3 and V4. As soon as I turned it on I had a crackle and smelled smoke. Gonna call Viva Tubes and let them know what happened. I guess it is back to the Vincent SV237
After rethinking this, I don't think the tubes had anything to do with the failure as all 4 were in when I was setting bias on V1 and V2.
Absolutely did and reduced all the bias pots to minimum before starting. Got through V1 and V2 with no problems. As soon as I started on V3, the amp smoked and could see that something lit up real bright below the V3 socket.