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In Reply to: RE: I should have looked at the schematic before I answered your question. posted by Michael Samra on March 15, 2015 at 04:19:52
Let me clarify what I'm seeing with the 6P3S-E. First, this is not an issue of insufficient drive. Saturation of the 6P3S-E itself is what's limiting the power, nothing else. With the amplifier converted to fixed bias, the 6P3S-E makes 19W max, whereas the 6L6GBs make about 27W and the 7355s do 30W. Increasing the bias current does not remedy this, and after several more tests, I can see that only an increase in plate voltage will allow the 6P3S-E to output more. So, I'm convinced Steve O is right - the high Rp of these tubes causes them to saturate earlier than a 6L6 or 7355 with the 3.6K output transformers in this HK. This means - as I've suggested during our previous discussions on this issue - that the 6P3S-E is not a suitable sub for the 7355 in the HK A500 or similar amps. As you say, it wants more voltage and a higher output Z. This implies the 6P3S-E is also not a good sub for EL-34s, at least not in circuits that similarly use a low-Z transformer. In those amps, as in this one, the 6P3S-E will saturate early, limiting output power. As for its performance in circuits designed with, say, +425V or more and at least 6K output Z, that's something else. I do have a couple amplifiers here that meet that requirement, but I have a larger number of 25-30W/ch amps with low Z OPTs that I want to resurrect first. It's not a surprise to me that the 6P3S-E won't do the job, but disappointing just the same. OTOH, the really good news was that 6L6GBs perform extremely well in this application. Based on manufacturer's data sheets, that was totally unexpected.
I'll test these 6P3S-Es again in a more suitable amplifier when time permits. If that doesn't produce numbers I can live with, I'll have to thin the herd. :)
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Buy Chinese. Bury freedom.
Follow Ups:
"First, this is not an issue of insufficient drive."Actually it is a question of drive as well with the 6P3Se. You are comparing this to a 6L6gb which would be fair however,the 6P3Se tubes are somewhat lower dissipation when driven under the conditions you are experimenting with.Increasing bias current doesn't really increase power much but it should lower distortion a bit.
This implies the 6P3S-E is also not a good sub for EL-34s, at least not in circuits that similarly use a low-Z transformer.
Nobody disputes that.It is never a perfect replacement but it works provided you have at least 400v on the plate and have at least a 5k impedance with sufficient drive.Some EL34 circuits have this capability such as Bob Latino's amps and the AES six pack amps.
Now,try and match up the best pair of 6P3Se tubes that you can and run them in a circuit like the W5m and let me know what you find..The W5s have about 11k impedance but they also have enough drive to get the job done and the B+ is running around 390v plate to cathode.
Bottom line,you do lose some power with the 6P3Se but the fact that you can run them so much harder,that can be an advantage in some circuits. When I look at the data on the 7355,it takes about 22% less drive than a 6L6gc so I'm actually shocked that you got those good of numbers. Did you modify the driver circuit when you converted this amp to fixed bias?
Jack,since you have fixed bias in that HK amp,try and see you can red plate the 6P3Se by dropping the bias at G1. You will notice that it's quite difficult in comparison to a regular 6L6gc so that tells me if you have the capability to run the tubes into a higher impedance with sufficient drive,you can get pretty good numbers..Take a look at this. I did modify this Scott 280 but notice the B+ on the tubes and this is also a fixed bias amp. This simply shows the fact that you can run the hell out of these tubes and you almost have to get the absolute best numbers.
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
Edits: 03/16/15 03/16/15
Michael, when you say it "works" at higher voltages and Z, are you referring only to output power? Haven't you made distortion measurements? That's the only remaining question in my mind - how do they perform regarding distortion under those conditions? I don't doubt they'll make power at 592V. :)
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Buy Chinese. Bury freedom.
I have made many measurements but mostly in 6L6 and KT66 amps..Most of the disputed argument I presented in past posts was to reiterate the fact that these tubes can handle much more than 350v rating claimed in the Data by the Russian military.They do dissipate lower power than a 6L6gc but I have run them in a Mac Mc240 and I was able to get 43wpc...Of course the 6L6gc black plates made 51 watts but distortion was within 10% of the black plates but at lower power.If you don't match those tubes up at full voltage,you will have readings all over the place.
In the W5ms,the distortion was also a bit higher than with the black plates but again,we are .2% for one and .33% for the 6P3Se..Would we hear that under most conditions? Probably not.
Match these tubes up the best you can and then give it a try in a 6L6 based amp and let me know what you find.
Mikey
"If it measures good and sounds bad, it is bad; if it measures bad and sounds good, you have measured the wrong thing."
- Daniel R. von Recklinghausen
Thanks, that's good info. I'll look to see which of my 6L6 amps uses the highest B+, that'll be the one to test.
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Buy Chinese. Bury freedom.
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