I'm thinking why wouldn't it work with a conventional PP output transformer with a 3.3- to 5K primary and 4 and 8ohm output etc. I guess one could try the 330 V screen with a separate PS trans. on any PP amp design too along with raising plate voltage to 620V. How would sound quality be affected?
Regards,
Randy
Regards,
Randy
With NFB around the whole amp, then distortion will be pretty low in the middle range of signal. Keeping distortion low at higher signal levels starts to become a complex mix of stage performances, and everyone will have their picky comment on each stage - such as a better PI configuration, or a new OT, or ditch the tube diodes. Reducing noise at low signal levels may be as simple as needing some layout rewiring, and heater elevation and humdinger tuning. If you can't fully isolate the chassis from vibrations then you may need to try isolating the tube sockets and making some tube dampers.
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I have three HP distortion analyzers several power supplies and several other toys at my disposal so I use them. In the end your ears are the end user so they should be the final judge. I have spent hours tweaking an amp on test equipment only to have it sound neutered! I am sure that an amp can be tweaked by ear, but I am trying to learn the relationship between measurements and sound.
I can't say for sure right now. The computer that I used for FFT analysis died over a year ago. I have built a new one and got it working but haven't integrated it into my test bench yet. I ripped my lab apart several months ago to redo everything. I was spending too much time seting up and tearing down test set ups. I have made switch boxes for swapping in test loads and speakers, test equipment and a CD player, and the HP8903A / FFT box combination will replace the kludge that I use now.
I currently daisy chain two HP331A manual distortion analyzers and feed the output into a scope and the 8903. That allows each 331A to notch out an individual harmonic using the 8903 to measure what's left. 331A's can be found at hamfests for $50. One can measure THD by itself. Two together can (with patience) measure IMD and selective harmonics one at a time.
To my ears, tweak the amp to kill the 5th harmonic at relatively high power (10 watts with my current speakers) then tweak the amp to lower THD at .5 to 1 watt. Listen, repeat. The levels I quote are based on my inefficient speakers and bad hearing. High efficiency full range horns are under construction which may change the levels, or the entire method.
That's because there are so many different types of the common audio tube types. There are dozens of tubes that say EL84 on them that are not true EL84's. Some are pentodes, some are beam tetrodes. How can they be the same. The same goes for the EL34. There are lots of "6L6GC's" out there too but they all seem to be beam tetrodes. All of the vintage 6BQ5;s that I have seem to be reasonably similar as does the JJ EL84. Some of the relabled Russian stuff melts at the levels that I feed the NOS tubes or the JJ's.
All pentodes or beam power are very sensitive to theyre screen voltage: increasing it increases the plate current and the slope also.
In an AB PP this necessitate to increase grid bias to do not ecceed plate and screen dissipation.
This means that you also MUST alter the load according to what is your target: more power or less distortion, your choice !
The main reason of tube failure is an excessive screen dissipation wich is also very influenced by the value of the load wich is not so easy to predict mainly because LSP impedance may vary considerably with frequency.
The safe side is to run those tubes at reduced screen voltage and accept a little less output power.
As a side effect this make'em a bit easier to drive
In doubt draw load lines
Yves.
In an AB PP this necessitate to increase grid bias to do not ecceed plate and screen dissipation.
This means that you also MUST alter the load according to what is your target: more power or less distortion, your choice !
The main reason of tube failure is an excessive screen dissipation wich is also very influenced by the value of the load wich is not so easy to predict mainly because LSP impedance may vary considerably with frequency.
The safe side is to run those tubes at reduced screen voltage and accept a little less output power.
As a side effect this make'em a bit easier to drive
In doubt draw load lines
Yves.
"The main reason of tube failure is an excessive screen dissipation which is also very influenced by the value of the load which is not so easy to predict mainly because LSP impedance may vary considerably with frequency. The safe side is to run those tubes at reduced screen voltage and accept a little less output power. As a side effect this make 'em a bit easier to drive
Totally agree. The urge to eke out more power usually ended up with tube failure or shortened lifespan. And that happens all the time with ultra-linear operation. UL design works well when the voltage is low enough for both plate AND screen to operate at ease. Too many amps suffer from stressing out the screen at high voltage.
Thanks Guys,
Right now I'm building a push-pull pentode amp using 6V6 outputs. I want to minimize distortion while extracting maximum power and tube life. In short, I want everything! Duncan amps data sheets say 285 plate/285 screen gives 14 watts at 3.5% THD. My question is can upping the voltages decrease distortion from that 3.5%? Or is that as good as it gets? I'm not talking about a lot here. 300, 325?
Right now I'm building a push-pull pentode amp using 6V6 outputs. I want to minimize distortion while extracting maximum power and tube life. In short, I want everything! Duncan amps data sheets say 285 plate/285 screen gives 14 watts at 3.5% THD. My question is can upping the voltages decrease distortion from that 3.5%? Or is that as good as it gets? I'm not talking about a lot here. 300, 325?
Tube manuals tend to be a conservative and somewhat dated. A lot of times materials and processes improved and the recommendations and maximum specs are not updated. This is especially severe with things like 60s NOS 2A3 and other early designs. The tubes were far stronger than the spec sheet shows. The 6V6 shows relatively low max voltages, but Fender guitar amps have run them with 400V on the plate and B+ on the screen* for decades. Some guitar guys are running the JJs at 460V.
Look for the best sound and try not to violate more than two maximums (Volts, Current, Wdiss) at a time and only one by a lot and only fudge the other a little and you're usually okay.
*Current limiting resistors do drop the voltage when it starts to draw.
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There are arguments about the tone of the JJs, like about everything else in music electronics. Some guys will get in an uproar that it isn't a "real 6v6." Some day I'll sit down with some and dig through the specs and maybe develop an opinion other than, "my customers like the sound and don't come back too soon."
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