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Old 8th January 2013, 05:46 PM   #51
djn is offline djn  United States
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Can you explain swing. I never really understood what that is. How do I measure swing?
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Old 8th January 2013, 06:24 PM   #52
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by djn View Post
I assume so too E. I might want more gain from the pre if my amp driver could not swing enough for the gm70
Yes. Personally I'm more interested in the tone of the preamp valve and its load driving capabilities (e.g. cables, TVC, etc.). I'd expect most of the gain being provided by the driver so the role is more of a line-stage (impedance matching) than of high amplification.

Regarding your swing question: this depends mainly on the valve characteristics and not the gain itself. A valve can provide a lot of gain and may not be able to swing too many volts (or may not be that linear at large voltage swings)....
Swing=Vpp. You can measure with the scope but you may struggle to see minor distortion. You will only see cutoff impact on the signal when is very evident. You will need to somehow adapt your driving signal (e.g. Pete Millett's interface) to your PC and look at the THD. By doing this you can see whether your driver stage can swing your volts required whilst providing minimum distortion. You can use this as a basis to adjust the driver's operating point.
Ale
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Last edited by mogliaa; 8th January 2013 at 06:29 PM.
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Old 8th January 2013, 07:57 PM   #53
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Yes but if you find a driver that has the swing, drive and distortion you like and it has say a gain of ten, and you need 180vpp into your output tube then from cdp level sources you'll want more than a gain of 0-2 for example. if you had a gain of ten that could get you 20v rms out, give yourself 6db in hand for low recordings and you've still got enough output to swing your driver.
Not a problem with D3a etc but perhaps if you want to use a typicaly low gain DHT and stick with 2 stages it may be of use. holistic innit :-)

enzo
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Old 8th January 2013, 08:06 PM   #54
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Quote:
Originally Posted by mogliaa View Post
Yes. Personally I'm more interested in the tone of the preamp valve and its load driving capabilities (e.g. cables, TVC, etc.). I'd expect most of the gain being provided by the driver so the role is more of a line-stage (impedance matching) than of high amplification.

Regarding your swing question: this depends mainly on the valve characteristics and not the gain itself. A valve can provide a lot of gain and may not be able to swing too many volts (or may not be that linear at large voltage swings)....
Swing=Vpp. You can measure with the scope but you may struggle to see minor distortion. You will only see cutoff impact on the signal when is very evident. You will need to somehow adapt your driving signal (e.g. Pete Millett's interface) to your PC and look at the THD. By doing this you can see whether your driver stage can swing your volts required whilst providing minimum distortion. You can use this as a basis to adjust the driver's operating point.
Ale
This is where i am a long way behind guys like Ale, i'm still lookin at numbers and have a lot of listening to do!

But swing was what this thread was about, tubes that had the swing to drive gm70's and seeing which could do that at the lowest distortion. Finding the optimum operating points and sharing the info. These are the lowest thd points, not the best sounding per say. For example i've heard it said the 6c45pi sounds 'hard' 'metalic' and 'ss' at over ~ 20ma, well as i raised the currant you could see the 2nd harmonic come steadily down at the same rate the 3,4,5 etc came up. this is a great example of the subjective tallying with the objective. iI posted the low thd point where 2h & 3h ballanced

enzo
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Old 9th January 2013, 01:57 AM   #55
djn is offline djn  United States
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Ale and Enzo, what numbers that you are measuring that tell how much a driver tube can swing??? I can build anything, but I am still learning about this stuff.
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Old 9th January 2013, 09:12 AM   #56
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All I did was swing enough volts into the test subject to get the 180v out I needed and then adjust the grid bias and currant to find the lowest distortion point. Some rubes I tested were good at small signal swings but even if they could swing the volts they distorted eg pc86, nice low level performance but no good for swings over a few tens of volts pp

The eye opener for me was finding small tubes that performed well in a2, on paper they shouldnt have been able to swing lots of volts at low distortion but in practice they were excellent.

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Old 17th January 2013, 09:54 PM   #57
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I got some 6j52p's delivered today:-)

The interesting thing about this one is that it likes low currents.

Best figures for 67vrms out

Va Vgk Ik rms %
172 1.77 16.6 67 0.32

Best bias for max swing

187 -2.5 10 104 0.59%

so thats ~290p-p from a 340vht 8-0 @0.59

That was the best tube out of a small sample of four, one i didn't test as it's getter plate had made a bid for freedom and the other two did ~.6% at 67vrms.

From these few i.d say that its a good tube, though there are marginally lower distortion tubes that seem more consistent, as a D3a a-like perhaps if you spent the same on some of these as a single d3a you might find one to better the german?

Still, only 3 tested so perhaps a pinch of salt with these results.

enzo

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6p15p-ev and 6p1p to go, then i'll have a look at octals.
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Old 17th January 2013, 10:32 PM   #58
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Tested some 5687wb's, These are obviously much lower gain (x 18) compared to the 6p52p's (x 74) so my signal generator only got them to 36vrms, suprisingly these didn't get better with higher current and were as good at 10ma as at 20.
Infact unlike the triode strapped pentodes they were very stable over the biases I tested them at, as long as they were biased out of A2. Best i got was 0.6% @ 36vrms/100vp-p. I wont be using these, but their stability could be of use to some?

enzo
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Old 24th January 2013, 09:36 AM   #59
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I had some 6j9p-e delivered, they are a very obviously not the same as the 6j9p, seem the same design but built like tanks.

I had high hopes for these as perhaps they could take the extra dissipation needed to get the best out of the little 6j9p.

Well they dont like going into possitive grid voltages :-(

Both @67vrms, 6j9p best of 2, 6j9p-e best of 10.


Va Ik Vgk %
6j9p-e 220 26ma -3.6 0.55 5.72w

6j9p 229 26ma -2.4 0.057 5.95w
6j9p 204 21.5 -2.3 0.09 4.2w

0.057%

Click the image to open in full size.

Click the image to open in full size.

enzo

Last edited by enzoastro; 24th January 2013 at 09:43 AM.
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Old 24th January 2013, 10:05 AM   #60
AJT is offline AJT  Philippines
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Quote:
Originally Posted by djn View Post
Can you explain swing. I never really understood what that is. How do I measure swing?
basically the plate voltage of the driver tube, say you have 250volts on the driver plate, the driver swing therefore can be 200volts peak allowing for 50volt tube saturation voltage....
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