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Old 27th November 2010, 08:08 AM   #31
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by mltube View Post
Just a sample of tubes I have rolled into this experiment...
These are matched pairs (quite close)..
Hi Mike,
I managed to get hold of a set of 26s, on their way from the US at the moment
Was keen to see your latest circuit updates and results particularly on the driver stage?
Cheers,
Ale


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Old 27th November 2010, 09:23 AM   #32
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Quote:
Have you measured your stereo separation?
Is this an over estimated feature ?

I looked the specification of hi-end MC-cartridges and those have some 25 dB stereo separation in average.
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Old 27th November 2010, 09:42 AM   #33
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Hi!

The 45 belongs to my favorite tubes. The 27 is a great choice as driver, albeit maybe a bit low in gain, but if the system has enough gain it's great. I had a 27/45 in the past and spent a step up input transformer to get a bit more gain. I have a 45 amp in my commercial offering and there I use a 6N7 (both sections paralleled). That gives just the right gain. The 6N7 is a great tube which gets less attention than it deserves, maybe because the two halves need to be used in parallel (they have a common cathode). Good for me because that keeps the prices low

I plan to take a 26/45 into my amplifier lineup. Will start construction soon.

One caveat about mesh 27s. I tested many on a curve tracer and found that a lot of them have very irregular plate curves. Often the electrodes are misaligned which causes this. The ST shape 27s I measured all had very linear curves.

here some pics of my 45 amp, stereo with external PSU:

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Best regards

Thomas
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Old 27th November 2010, 09:47 AM   #34
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Hi!

Quote:
Originally Posted by artosalo View Post
Is this an over estimated feature ?
IMHO yes! I get often the question why I rarely build mono amps but rather stereo with external PSU. As was mentioned before MC cartridges typically have very low channel separation, so that a few dB less in the amplification does not really matter.

I think the sound degradation which people observed with common cathode resistors between channels is not related to stereo separation. I always spend separate cathode resistors even in PSE amps for each tube. So each tube finds it's own optimal op point. With shared cathode resistors the tube with more emission hogs most of the current and the other one is left 'starving'

Best regards

Thomas
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Old 27th November 2010, 04:10 PM   #35
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by artosalo View Post
Is this an over estimated feature ?

I looked the specification of hi-end MC-cartridges and those have some 25 dB stereo separation in average.
Yes, in my opinion too. I started out building amps with separate power supply or supply finals for each channel. Later testing showed a small amount of crosstalk with a single supply, but I could not hear the difference. If I split supplies now, it's between stages, not channels.

Same is somewhat true for bias supplies for a fixed bias amp. I use a single supply, and split it before the final RC if I need to reduce crosstalk. But I no longer try to reduce it to zero.

It's simple to test. Run an input into one channel and measure the signal level. Compare that to the undriven channel. If you are 35-40dB down, there's not much to gain by trying for 60dB.

Sheldon
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Old 27th November 2010, 05:45 PM   #36
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Quote:
I think the sound degradation which people observed with common cathode resistors between channels is not related to stereo separation. I always spend separate cathode resistors even in PSE amps for each tube. So each tube finds it's own optimal op point. With shared cathode resistors the tube with more emission hogs most of the current and the other one is left 'starving'

This makes good sense and is probably a major contributor to why I tend to dislike the sound of SE amps using this arrangement.
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Old 5th December 2010, 09:05 PM   #37
mogliaa is offline mogliaa  United Kingdom
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I managed to get hold of some 27s, they don't have a lot of gain though, what is the best driver circuit for the 45?
Thanks
Ale


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Old 6th December 2010, 07:27 AM   #38
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Hi Ale,

as I mentioned in my post above, 6N7 works nicely for me.

Best regards

Thomas
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Old 6th December 2010, 09:03 PM   #39
mogliaa is offline mogliaa  United Kingdom
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Hi Thomas,
Yes 6sn7 is a great valve for phase spliter, problem is that I have only one, and can't find any cheap Similar ones around.
I originally considered a 6sl7 in a symmetrical totem pole configuration or similar which can easily give a gain of 27db
Regards,
Ale
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Old 6th December 2010, 09:19 PM   #40
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Hi Ale,

I meant 6N7, not 6SN7.

Thomas
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