NEW TRANNYS

[4krow]

What I am always looking for is an increase in micro detail, and from my experience, that requires equipment that is well designed, and of course, quality parts. I would have to think that better quality trannys can help bring on the micro detail I like. Having said that, it does make sense to look at the amp as a whole. The tranny swap may be down the road at this point. It seems that there are some valuable suggestions by the members here that I must take into consideration first.

[2wo]

Hi Greg,
To convert to triode mode, on each 6V6 there will be a wire from the output transformer connected to pin 4. Disconnect this wire and tape or heat shrink the end to insulate it and tuck it out of the way.

Now take a resistor 150-200 ohm or so 1/2 W, this is not at all critical. Connect one end to pin 4, the other to pin 3. There will be another wire from the transformer on pin 3, that stays.

All done 

[chard]

Hi Greg,
To convert to triode mode, on each 6V6 there will be a wire from the output transformer connected to pin 4. Disconnect this wire and tape or heat shrink the end to insulate it and tuck it out of the way.

Now take a resistor 150-200 ohm or so 1/2 W, this is not at all critical. Connect one end to pin 4, the other to pin 3. There will be another wire from the transformer on pin 3, that stays.

All done 

   I wouldn't recommend doing this mode without rebiasing your amp. If you don't know how to rebias your amp for triode operation don't do this mode. Also triodes operate cleaner then pentodes, they don't need ultra-linear transformer taps or negitive feedback. If your amp has these things it may sound sterile in triode mode.

[4krow]

Points well taken gents. I will be looking into these things, and then determine whats next. As a matter of fact this amp is run in ultralinear.

[4krow]

I just have to ask; Does it matter if the amp is self biasing?  I'm betting not.

[2wo]

I respectfully disagree with Clifford. Unless the tubes are run to there absolute max, triode mode should be safe. Easy enough to check. What is the value of the cathode resistor and what is the voltage across it?

[chard]

When you get a self biased amp from a manufacturer you assume competence and don't worry about how the amp is biased as long as it sounds good. However when you do a major modification to a amp like changing from pentode to triode mode you should make sure the amp is biased optimally for the new mode of operation. If you look at the tube data sheets for the load line of the 6v6 in both triode and pentode mode they are not the same. For a given plate voltage and current draw a 6v6 in triode mode will generate a larger voltage drop across the cathode then in pentode mode, therefore a larger value cathode resister is needed in order to zero in on a spacific current draw and a chosen bias setting. Why hope for a good result when the tools are available to plan for one.   

[4krow]

Ok, there is 24VDC at the cathode pin, and the resistor is 680 ohms, +-5%.  I think I should take photo of the circuit itself. There is something I would like you to see. I can try and send through here, but if that fails, email always works for me.

[chard]

What's your plate voltage its pin 3?

[4krow]

PLATE VOLTAGE= 354 VDC

[4krow]

NOTE! CORRECTION OF VOLTAGE READINGS! CATHODE VOLTAGE = 22.7        PLATE VOLTAGE= 339    THE REASON FOR THIS CORRECTION IS THAT I ONLY HAD ONE CHANNEL OF 6V6 TUBES PLUGGED IN AT THE TIME OF MEASUREMENT.   SORRY ABOUT THAT

[4krow]

A couple of photos of the 6V6 circuit.

[2wo]

With the new numbers, I get a plate V of ~316 at 33 ma. Getting to the upper end but doable. 6V6 are tough. try it. I think you will like the sound. measure the voltages that way and if it looks like you are killing the tubes we can adjust it. not hard...John     

[chard]

Based on the information you supplied your amp is biased at about 87%, 339vdc minus the cathode voltage drop of 22.7 = 316vdc times your current of 33 miliamps equals ( 22.7 divided by 680 ohms) equals 10.428 watts divided by max dispensation or 12 watts = 87%. Its probably biased a little lower because screen current wasn't factored in. Based on the rather high cathode voltage drop of 22.7 and the fact that your amp delivers 12 watts per channel it can be conclued that your amp is operating in ultra-linear mode which is a mode that includes the best features of both triode mode and pentode mode. Your amp has the clean low distortion advantage of triode mode without sacrificing the greater power available with the pentode mode. Your output transformer is designed to operate in this mode. Try the mode John suggested if you like it great if not you can go back to stock.

[Doc B.]

I'm wondering if you have asked about these mods on the Audio Nirvana forum.