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Old 28th December 2010, 07:19 AM   #21
rmyauck is offline rmyauck  Canada
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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?

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Old 28th December 2010, 10:56 AM   #22
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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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Old 28th December 2010, 03:11 PM   #23
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So George, that means that a distortion analyzer is critical to getting the optimum performance out of pentode outputs
.

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.

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was the minimum high order distortion point also the point that minimized HD3 in general?
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.

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It is somewhat discomforting that different versions of the same tube show such variation.
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.
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Old 4th April 2012, 12:27 AM   #24
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So, someone here mentioned that 6V6's run the same voltage on plate and grid. That seems to be born out by the datasheets I've seen. is there an optimal voltage for the 6V6? I've seen amps that run them as high as 350 volts. Does that sound about right?
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Old 4th April 2012, 05:40 AM   #25
Yvesm is offline Yvesm  France
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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.
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Old 4th April 2012, 06:41 AM   #26
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Originally Posted by Yvesm View Post
"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.
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Old 4th April 2012, 11:59 PM   #27
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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?
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Old 5th April 2012, 12:23 AM   #28
AJT is offline AJT  Philippines
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i use a mosfet/zener regulator to supply 285 to the screens of my 6V6 pp amp, plate is around 310 volts.....i do not know distortion percentage, i used a combination fix and cathode bias scheme...
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Old 5th April 2012, 05:24 AM   #29
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Originally Posted by mr2racer View Post
So, someone here mentioned that 6V6's run the same voltage on plate and grid. That seems to be born out by the datasheets I've seen. is there an optimal voltage for the 6V6? I've seen amps that run them as high as 350 volts. Does that sound about right?
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.

Last edited by Ronsonic; 5th April 2012 at 05:27 AM.
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Old 5th April 2012, 05:51 AM   #30
AJT is offline AJT  Philippines
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could that be the reason why the jj 6v6 sounded anemic running off a 6bq5 power supply?

the jj6V6 looked like a 6L6 to me....
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