duncanamps.com

[jonte]

Hi out there! i own a mesa 50/50 power amp and my tubes
have reched their limit,since it ain't possible to change the bias on this amp i was wondering if there's any other brand i can use Sovtek,Svetlana,Ruby tubes 6L6's without shortening the tubes lives.
I've been told that i have to modify my amp to be able to use other
tubes than the Mesa str420 tubes that comes with the amp.anybody who
can help me on this matter.I'm using Sovtek 6L6GC's in my amp now
cause it's hard to find the original tubes in sweden where i live,or do i have to modify my amp so i can change bias to be able to use other tubes.
thanks for your help. Jonte

[fatboy135]

Hello I´m newbie and maybe my opinion have a less weight but I think that there are a lots of urban legends about the BIAS.
Well if you are working with a boutique amp or a high end hifi amp an accurate bias setting is needed, but If you plays a boogie and you trash like napalm death are you sure that you need a milimetrical bias setting??
In my little workbench I could prove with my hameg oscilloscope that the autopolarizate circuit of the boogie works so fine with several kind of valves.
Then I think that you didn´t need to do an extra bias circuit, but if you want it I suggest to you soldering a pot in parallel with the bias resistances.
If you don´t have an oscilloscope you could ask to mesa for the grid voltage value and with a multimeter see if the new valves are bad polarizate.