• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Switching from ultralinear to triode

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I'm planning to build a 'Stoetkit Junior' with some parts I have lying around. That's a little pp amp ca. 9watts in ultralinear with four 6bm8's in total. The 'problem' is that my opts come from an old Lafayette receiver with the same tube complement, but don't have the UL taps.

Is there a general rule for converting ul to triode-mode? I know how it's done, but the big question is: which value should the resisor between anode and g2 have? If this depends upon the particular circuit, here's a link to the schematic (see last page of the file): http://www.fsaudioweb.com/images/stoetkit-j_construction_manual_en.pdf

You could just connect G2 straight to the anode, it might be ok... BUT often pentodes will oscillate if you do this, so normally a 100 ohm resistor is used.

Also you need to check the maximum G2 voltage, make sure you don't go over the ratings.

If your amp uses a negative feedback loop from the speaker terminal to the input stage, you could try running the output tubes in pentode mode, wil give you heaps more power. (Distortion might be higher, hence the negative feedback to correct it.)

To do this just connect each G2 of each tube to the B+ via a suitable resistor (around 1K or so is typical).

I'm gonna go for the triode-mode, low power is no problem + I think I'll prefer the sound. I'm always very surprised how uncritical some things seem to be! I guess it's a little like those grid stoppers then.

Are there formulas to calculate the 'ideal' value for a certain circuit, or just trial and error with audible differences as criterium? Or is the audible difference, once beyond oscillationproblems really that marginal there's no use in experimenting?

Thanks, shifty;)
It's a lot like gridstoppers- long experience has shown what ought to be used. In a large number of instances, it won't make any difference whether you use them or not, but... in some instances they are of enormous benefit, the rest of the time they don't hurt and don't cost much. Cheap insurance. 100 ohms is fine.

When going to triode, you may see more HF rolloff due to increased Miller effect.
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