Selecting a Primary Impedance

[Tim Hall]

Can someone tell me how one might go about choosing a primary impedance for a given OT (with variable primary impedances) for guitar? Is it determined in relation to the desired operating point of the output tubes? Any rules of thumb?

[bob p]

Steve Bench has an excellent tutorial on Loadlines on his web site. In the examples he goes through the calculations that are required to select all of the tubes operating parameters.

[Tim Hall]

Thanks Bob. I've learned how to draft out load lines and establish operating points...so I take it the impedance is chosen with respect to the desired plate load then??

[bob p]

For a pair of output tubes, the plate load (primary impedance of the OT) is defined by the slope of your load line. That makes sense from an Ohms Law perspective, since V=IR and the plate characteristic uses V on the X axis and I on the Y axis.

Loadlines are helpful if you're drawing something up from scratch and you're spec'ing an OT for the task. If you're designing an amp, you can spec the iron for the desired operating conditions, and you have complete control over all of the specifications (voltage, current, resistance).

More commonly, hobbyists have an OT available that they need to build an amp around. When that happens, the primary Z of the OT defines the slope of the loadline that you have to use, and you're left with Voltage and Current as the parameters that you can modify. You can shift the loadline to the left or the right, or move it up or down (within plate dissipation limits), but you can't do much about its slope.

Putting this into practice, if you use an OT that has mulitple primary Z taps, like a Hammond universal OT, you have a number of different loadline slopes to choose from. Its like opening a can of worms to design an optimal setup for multiple different primary impedance values. For a guitar application, I wouldn't worry too much -- I'd design for the middle of the road, or perhaps to tolerate a slight mismatch that would correspond to adding another speaker cabinet in parallel.

Is that helpful?

[Tim Hall]

Yes, very helpful. I was in fact looking at a Hammond with multiple taps. Thanks again.

I guess I have a new question then...any rules of thumb for grid voltage for the output tubes? What I have drawn up now would theoretically give me between -5 and -6V. After designing the preamp, this seems kinda high, but I suppose the the last tube gets hit with the hardest signal swing (?)

[bob p]

there are no universal rules to work with -- every tube is going to have a different grid signal amplitude requirement. that is to say, the grid swing on a pair of 6L6 won't be the same as EL34, which won't be the same as KT88, etc. And even for any given tube, that number will vary depending upon your operating conditions.

Your best bet is to either stay within the recommendations of the tube data sheets, or plod through all of the work involved in drafting the loadlines for each amplifier stage, working from the back of the amp to the front.

[Steve Conner]

In a push-pull amp the grids of output tubes usually sit at a fairly high negative voltage, relative to the cathodes. For EL34s it's around 30V. For 6L6s, around 45V. Hence they do indeed need large signal swings to drive them to full power: 60V peak-peak for EL34s, etc.

EL84s and 6V6s need considerably less bias and drive voltage.

[Tim Hall]

I'm starting to make sense of all of it now...you guys are a great help. The output tube in question is actually a 12AU7 (both triodes PP).

Cut sheets for the 12AU7 recommend grid at -8V.