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Old 7th September 2011, 06:00 AM   #131
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Hi!

I posted an article about the 45 on my blog within my 'Tube of the Month' series. There are also pics of a recently finished 45 amp:

VinylSavor: Tube of the Month: The 45

Best regards

Thomas
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Old 7th September 2011, 09:24 PM   #132
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Quote:
Originally Posted by Vinylsavor View Post
Hi!

I posted an article about the 45 on my blog within my 'Tube of the Month' series. There are also pics of a recently finished 45 amp:

VinylSavor: Tube of the Month: The 45

Best regards

Thomas
Excellent post Thomas, very informative and interesting.
Simple question, I'm looking to change driver, but with space and heater limitation so two options on the table given the existing constrains are 6J5 or 6N7 with CCS load.
What would you recommend between the two?
Have done some successful tests on the 6j5 at 9mA and -8V achieving less than 0.35% THD at 100Vpp. Need to try 6N7 now, any recommendation on the best operating point for this tube to achieve full output drive? BTW: HT supply is restricted to 350-370V

Thanks
Ale
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Old 8th September 2011, 05:56 AM   #133
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Hi Ale,

the choice depends on your gain requirements. I always go for just as much gain as necessary. So if the 6J5's gain is enough, I would use that.

What would be really interesting to compare is a different driver topology. Like choke loaded or transformer coupled. For those you would need to reduce the B+ for the driver stage to 300V.

I personally prefer the transformer coupling since it offers the possibility to use the ultrapath connection (cap from B+ to cathode).

Best regards

Thomas
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Old 8th September 2011, 07:20 AM   #134
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Hi Thomas,
With the current 6SL7 driver I have sufficient gain when using CD input, however, when I use my turntable gain is too much as you can notice some clipping on transients when playing at high volume - this is due to the turntable RIAA preamplifier I have which I don't know the gain it has. Evidently the 58-60 gain this stage has is too much when playing vinyl.
I'm planning to build a DHT preamplifier on a separate enclosure using probably a 26, 30 or CX112 or 01a, so this stage will provide some gain around 7-10.
My question was more focused on the linearity of the 6SJ5 compared to the 6N7 as one has a mu of around 20 and the other one around 30 if my memory is correct - sorry I'm not using my home pc at the moment so can't check this.
If I need to drive the 45 at 100vpp to get max output, I was looking to find the optimal bias point to achieve max swing with minimum distortion.
I think as per my previous point I found it for the 6j5, but not for the 6n7 as I don't have a good datasheet showing the transfer curves of 6n7 for anode voltages over 250v
Will have to do some test
Thanks
Ale
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Old 9th September 2011, 07:44 PM   #135
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Here is a summary of my latest tests of various driver stages to achieve 100Vpp.
All are CCS loaded and LED biased with a 15uF PIO bypass capacitor. LED array varies depending on the bias voltage:

6J5: Tested 8 tubes
Vg=-8.2V, Ia=7.1mA and Va=253-260V
Tube THD
6C2C 0.44% (best)
6J5GT Brimar 0.65% (worst)

6N7: Tested 8 tubes
Vg=-4.7V, Ia=6.5mA and Va=260V

Tube THD
6H7C XI-68 Foton 0.34% (best)
6H7C XI-68 Foton 0.46% (worst)
JAN-CRC-6N7GT RCA 0.60% (best)


So final verdict goes for the 6H7C (Foton -Tashkent, Uzbekistan) with a good 0.34% THD figure. Will be no issue to modify my 6SL7 driver stage to fit the 6H7C. Stage gain will now be around 30, which is more reasonable. To produce 100Vpp, I need a maximum input of 3.1-3.2Vpp which sorts out the issue stated in my previous post...

cheers,
Ale
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Old 9th September 2011, 08:09 PM   #136
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Hi Ale,

how do all these variants compare soundwise?
How is the distribution of the harmonics? The absolute THD figure tells very little about how it sounds.

Best regards

Thomas
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Old 9th September 2011, 08:20 PM   #137
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Hi Thomas,
You are right, haven't done a proper test with music. Just 1kHz sine test on my breadboard.
Clearly 6SL7 is a no go because of the gain. Although sounds really nice with my CD.
I was doing these test to find out wether to use one tube or the other and to find best operating point from a distortion point of view at full output voltage. Actually I could do both tests and see the results. Both tubes offer a THD below 0.5%, so a good starting point for a listening test.

Will try to make changes this weekend and post some impressions
Cheers,
Ale
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Old 9th September 2011, 08:24 PM   #138
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Hi!

I'd suggest to also compare the measurement at other voltage swings like 10Vpp, 25Vpp and 50Vpp. Most of your listening will be at those lower levels so they are much more relevant. Also look at the distribution of the harmonics since that telss much more about the sound.

Of course also good to check which set up gives the most headroom

Best regards

Thomas
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Old 10th September 2011, 05:27 PM   #139
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Default At last!

Today I managed to replace the 6SL7 driver with 6J5. The latter has a similar heater requirement, so worth giving it a try given my current constrains. Have to say that sounds superb. Managed to listen to vinyls properly now with a reasonable gain that doesn't clip the amp. Played "Take Five!" from Brubeck Quartet. My special part is the drums solo, where you can appreciate properly dynamic response and bass as well. This amp surprise me so much, can't believe there is no trace of hum at the output even with listening with my grados. Evidently this amp marries very well with my Fostex 167E full-range drivers. Better bass than my previous EL34 PP.
As I said before, can't believe what a 2W can deliver.

Had an interesting situation when I tried a different pair of 45. I found some random noise with no signal at the output. Apparently one of the two tubes are not working properly. Will trace the curves to see which one is :-(

So final test of my Sylvania JAN-CHS-6J5GT metal base was:
Vo THD
25Vpp 0.11%
50Vpp 0.22%
100Vpp 0.51%

Played with a combination of different LEDs to get the 8V bias required. Anode voltage around 260V achieved lowest THD at maximum headroom (100Vpp) and CCS current set to 7.8-8mA. Gain stage is now around 18-20 depending the tube.

Despite Pumas lost today against England, will now open a proper bottle of wine to celebrate a great sounding amp!

Thanks Thomas for final tips!

Cheers,
Ale
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File Type: jpg 6j5 DRIVER.jpg (152.7 KB, 283 views)
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Old 10th September 2012, 09:40 AM   #140
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Default 2 Watt output

Just browsing through the threads and the limited output of the 45 struck a bit of a chord, as did the exchange with Thomas.

Agree 100% with

"My special part is the drums solo, where you can appreciate properly dynamic response and bass as well. This amp surprise me so much, can't believe there is no trace of hum at the output even with listening with my grados. Evidently this amp marries very well with my Fostex 167E full-range drivers. Better bass than my previous EL34 PP.
As I said before, can't believe what a 2W can deliver."

Its exactly the same with the 46SE I acquired just over a year ago...sublime for 1.5 Watts
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