Load Line and Biasing for Class AB 6550 power amp

[polo]

Hi everyone,
I'm building a marshall-like bass amp (1991 super bass), i've got some problems on polarizing the poweramp. Can anyone help me on drawing loadlines for class ab operation?

B+: 490V
OT primary: 1750 ohm
Tube: 4x6550 EH
Expected output power: 100W rms

What's the correct bias current?

Thank you, Polo

[dcgillespie]

Polo -- I am assuming this is a fixed bias, PPP design. In any event, the B+ level you have chosen and the load you are using will ensure (i.e. force) class AB operation. Class A operation with those parameters would simply not be possible with these tubes, as they would burn up before you could even use it!

The load you are specifying implies that you are going to operate the screens at nearly the same voltage as that of the plates. If that is the case, 60 ma will be ample quiescent current for your design.

Dave

[Manni57]

Hi Polo

I am not sure what you mean by "polarizing'.
Perhaps you mean biasing. That would explain why you are looking for loadlines. I have been studying this lately and here are some links that I found helpfull:
Steve's Tube Pages
http://www.ax84.com/media/ax84_m225.pdf
http://www.freewebs.com/valvewizard1/se.html
And then of course there is the Radiotron designers Handbook that you can download for free from Pete Millet's website (thanks Pete).
I found all that confusing. Especially once you want to do class AB1. In the end I just stuck to an existing design and got going with that. It turned out OK.

Good luck with your project
Manni

[polo]

Thank you guys for you precious and quick help, forgive my english.

@dave: yes, it is a pushpull fixed bias design, my previous calculation give me about 45ma biasing, but i wasn't so sure about that, i'll try biasing at 60ma

@manni: these are very interesting links, i already know valvewizard but it get me confused about designing class ab with 4 tubes. of course i have to study deeply RDH, it seems to be an exaustive textbook

Thank you again

[Trout]

For the Marshall style amps, fixed bias,

You might find this useful to establish a good safe zone. Many of the guitar guys use this basic chart & calculator.

Weber Bias Calculator

60ma appears to be pretty hot, If memory serves my 72 Super Lead 100W with 6550's was set at 50ma

[dcgillespie]

The final current draw value will be based on your goals. 50 ma of quiescent current will favor a little extra tube life, and likely keep the output stage out of cross-over distortion territory. 60 ma will favor much lower distortion, with little impact on resulting tube life. In terms of quiescent plate dissipation, a total cathode current draw of 60 ma will still result in a plate dissipation level of just under 80% of design center ratings for the 6550, meaning that tube life will be extensive based on this fact alone in the proposed design. In guitar amp service, tube life is much more a product of how great and how often peak current demands are called upon over the life of the tube.

Dave

[polo]

another question, just to be sure, those 50-60ma are for each tube or for each pair?

[dcgillespie]

That would be for each tube.