Time to stop talking about this or that problem, just built the amp as hundreds of others have in the past.
With a bit of care there will be no problems, the Sun will shine, the Birds will sing & all will be well! 👍
With a bit of care there will be no problems, the Sun will shine, the Birds will sing & all will be well! 👍
Speaking of birds....
Fisher's Logo sported a bird in flight carrying a music note, with the motto:
"Music Soars when you hear it on a Fisher"
Fisher's Logo sported a bird in flight carrying a music note, with the motto:
"Music Soars when you hear it on a Fisher"
The amp is designed for 110VAC not the 121VAC that I have. The very last thing I want is an amplifier that requires frequent power tube replacement. Therefore, I am addressing the voltage issues before continuing.
A Zener diode took the regulated voltage for the EL84 tubes down to 298V, exactly where it is supposed to be. I built the remainder of the PS circuit in PSUDesigner, and it indicates that changing R1 should fix the problem and bring that voltage down to spec. The new R1 resistor arrived today. Still waiting for some other parts to proceed.
The fix is simple & has been addressed here in this thread & many times in the DIY archive.
No need of another repeat on that item, When will you move ahead on a build?
😀
If you mean a bucking transformer, I reject that option, and I am proceeding to correct the voltages in other ways so I don't end up replacing output tubes more frequently than necessary. Thus, my original question at the beginning of this thread.
Tube voltages and power dissipation and tube life also are questions of general interest for those designing or modifying amplifiers, not just this one kit.
Changing R1 to 200 ohms should give me 360V out under load according to PSUdesigner. Spec is 330V to 350V, but we will how it works.
As soon as more solder arrives. Filament resistors arrived today, but no solder. 🙁
Also, thanks to everyone who has been helpful. I do appreciate it. I don't need another tube amplifier I guess, but it is an excellent learning experience. When I get it working, we will see how it sounds compared to my other one.
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I just finished testing that.
For the two 6N1 tubes, adding a 0.295 Ohm power resistor produces 6.54VAC with a dummy load attached. This voltage reduction was necessary not just because of the wall voltage (122.5VAC tonight vs. the 110VAC that the transformer was designed for) but also because I removed two "magic eye" tubes that were supposed to run their heater filaments from the same voltage source. Spec is 6.3VAC to 6.9VAC. So, I am exactly in the middle of spec at 6.54VAC.
For the four 6P14 tubes, a .050 Ohm power resistor produces 6.38VAC with a dummy load attached. Spec is 5.7VAC to 7VAC. So, I am exactly in the middle of spec. I may not need this little resistor as it doesn't make a whole lot of difference, but I have it on-hand now.
Filament voltages are now correct. The 300VDC regulated screen voltage is now correct. No bucking transformer needed to get this far, just one zener diode for the regulated supply and two small resistors for the filaments so far.
So, getting back to my original question: I will try replacing R1 with 200 ohms, which should produce 360VDC under load according to PSUdesigner. So long as screen voltage is held at 300, it may be OK to run at 360 volts. Spec is 320 to 350 volts, so I will still be over the max specified. When it's all assembled, I'll have to see if the output tubes are running over their rated 12 watt dissipation with 160 Ohm cathode resistors. The online calculator (cited on the first page) indicates that I may be at 94%. That is running them pretty hard if that turns out to be an accurate prediciton. However, a number of guitar amps do it without problems, so long as screen voltage is controlled at or below 300V, which is the case here. I also could swap another 100V zener for another 91V zener to drop the regulated screen voltage a bit more, if that is necessary. Guitar amp forums seem to target 275V in similar cases, but those are not hi-fi amplifiers.
Guitar amps don’t necessarily CARE about tube life. Run them hard (constant clipping exceeding screen dissipation rating is harder on them than idling at 20% over max plate dissipation), makes them sound right, and blowing things is a badge of honor.
Dropping the screen voltage further may or may not be the right move. If your speakers are current hungry (much lower than nominal in the low midrange) it’s not. For pretty much every other reason it is. Lowering g2 causes the plate current to max out earlier. If the max current is not needed you won’t miss it, if it is you do. Guitar amps spend 99% of their time driving impedances HIGHER than nominal. Even in the mass-controlled region the speakers don’t go much below 7 ohms. With hi-fi speakers anything goes - forcing the crossover to DROP the impedance seen by the amp is commonly used to equalize levels. Simpler speakers are better in that regard. Dropping g2 moves the ENTIRE set of plate curves down - so the quiescent BIAS required to get rid of crossover distortion also reduces. You do not HAVE to run them as hot for low distortion at normal listening levels.
As I alluded to earlier, high plate dissipation is NOT necessarily the worst case operation. Many old radio and hi-fi consoles run the output tubes in class A right at max plate dissipation rating and the only thing that ever fails is a weak rectifier or dried up power supply cap.
Dropping the screen voltage further may or may not be the right move. If your speakers are current hungry (much lower than nominal in the low midrange) it’s not. For pretty much every other reason it is. Lowering g2 causes the plate current to max out earlier. If the max current is not needed you won’t miss it, if it is you do. Guitar amps spend 99% of their time driving impedances HIGHER than nominal. Even in the mass-controlled region the speakers don’t go much below 7 ohms. With hi-fi speakers anything goes - forcing the crossover to DROP the impedance seen by the amp is commonly used to equalize levels. Simpler speakers are better in that regard. Dropping g2 moves the ENTIRE set of plate curves down - so the quiescent BIAS required to get rid of crossover distortion also reduces. You do not HAVE to run them as hot for low distortion at normal listening levels.
As I alluded to earlier, high plate dissipation is NOT necessarily the worst case operation. Many old radio and hi-fi consoles run the output tubes in class A right at max plate dissipation rating and the only thing that ever fails is a weak rectifier or dried up power supply cap.