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.
4
4krow
Guest
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.
4
4krow
Guest
PLATE VOLTAGE= 354 VDC
4
4krow
Guest
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
4
4krow
Guest
A couple of photos of the 6V6 circuit.
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.
4
4krow
Guest
I am going to try this mod and see what it amounts to. While I'm at it, I may replace the output caps. Sorry if this is redundant, I sent a message earlier this morning, but it didn't seem to take. Probably should be mentioned that because I live out in the sticks, all supplies will have to be ordered. I've got a list already, so this is a good time to do it.
Grainger49
New member
Greg,
I didn't see if you have said that you have a schematic. Do you? If so, scan it and send it to me. I can make some suggestions.
Posting a schematic of a current production product might be in bad taste.
I didn't see if you have said that you have a schematic. Do you? If so, scan it and send it to me. I can make some suggestions.
Posting a schematic of a current production product might be in bad taste.
4
4krow
Guest
Grainger, I think that probably no one will ever have a schematic of this amp. David Dicks is not the kind of guy that gives things up easily. His idea when designing this amp was to use 'ordinary but decent' parts to achieve great sound. All in all, I think he achieved much. But of course, I can see room for improvement. For instance, the original tubes were Chinese, and not that they sound poorly, but one of them failed within a week. So I changed out the 6V6 tubes to JJ's and the 12ax7 to Electro Harmonex. The 6NS7 tubes are still Chinese. I will try this triode mode, but I think even after that there are a few improvements to be had.
4
4krow
Guest
UPDATE:
I have carefully considered the pros and cons to the idea of revamping an already good amp, and for the cost, I have decided that simply upgrading the caps in the signal path, and bypassing others will be a better bet. By doing this, I will have affected(for the good) the current design, but not pushed a lot of money towards it. The other decision was to upgrade the 6Sn7 tubes as well. I stopped there to see that leaves me, and so far, I cannot make a judgement given the break in time of the new tubes/caps. As always, it was of great value to me to have the help and advice given here. I will report back when there has been some time with the ears..
I have carefully considered the pros and cons to the idea of revamping an already good amp, and for the cost, I have decided that simply upgrading the caps in the signal path, and bypassing others will be a better bet. By doing this, I will have affected(for the good) the current design, but not pushed a lot of money towards it. The other decision was to upgrade the 6Sn7 tubes as well. I stopped there to see that leaves me, and so far, I cannot make a judgement given the break in time of the new tubes/caps. As always, it was of great value to me to have the help and advice given here. I will report back when there has been some time with the ears..
