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Old 30th November 2004, 12:30 PM   #1
Mark25 is offline Mark25  United Kingdom
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Default valve newbie; 6H1H pre-amp.

All,
A few years back my previous valve amp, an AMC CVT 3030 integrated, expired. Although i was aware that it had some good qualities, i have not bothered with tubes since, preferring the cast iron grip afforded by active SS amplification. However, recently a good (generous) friend of mine, Bas Horneman decided it was time for me to get back into valves, and threw me a 6H1N to play with. Here's the outcome:
Click the image to open in full size.

Here's the circuit;
Click the image to open in full size.

and the PSU's;
Click the image to open in full size.

It plays music, accompanied by hum!
Here are some of it's vital signs;

B+ 230 vdc
Va 145v
Ra 20K ohm
Rk 680 ohm
Gain 15

Here's the problem(s);
I have about 50 mV hum at the output. I'm aware that my PSU for B+ is not state of the art, but assumed that if it was good enough for 30w push-pull EL34's (AMC), it would support a pre just fine. The 390 uF cap certainly gives a marked improvement over a 100uF cap, (which gives 200mV hum on the o/p). The amp itself seems to give a PSRR of 0.66, the ohms law effect of Ra, i guess.

I guess the way ahead for me is to look into a more sophisticated type of psu, pi filter, choke i/p, cap i/p, etc ?????? Beyond the names and topology, i don't really understand the difference between these types of psu's BTW.

Also, the 390uF resovior cap is taking about 10 secs to stabilise at B+, i'm wondering if that is stressing my 4.9 va isolation transformers too much?

Any comments/suggestions welcome.

Regards Mark
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Old 30th November 2004, 12:59 PM   #2
moamps is offline moamps  Croatia
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Try this simple CRC filter...
(btw, PushPull apps have much higher PSRR)

Regards,
Milan
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Old 30th November 2004, 01:02 PM   #3
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Also you could change the 1k resistors in the LT Heater supply to 50R or so..
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Old 30th November 2004, 01:39 PM   #4
Mark25 is offline Mark25  United Kingdom
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Default thanks

Thanks moamps, i implemented your cct, but with a 2W 680 ohm R, it's all i had. Pre-amp now gives 5mV hum on o/p, that's a factor of 10 decrease - great! Although the hum sounds like it's higher in frequency now, more of a buzz than a hum, i'll check the F later.

Bas, how did i know you'd say that? i'll get round to it one day, thanks again for the valve -> fun.

Mark
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Old 30th November 2004, 01:51 PM   #5
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You could also try a HV snubber...
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Old 30th November 2004, 01:58 PM   #6
Mark25 is offline Mark25  United Kingdom
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Quote:
Originally posted by Bas Horneman
You could also try a HV snubber...
What's that then? Don't forget i'm a newbie!
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Old 30th November 2004, 02:03 PM   #7
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http://64.154.92.195/forums/bottlehe...ges/54580.html

And just in case you miss his followup link.
http://www.siteswithstyle.com/voltse...and_xfmrs.html
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Old 30th November 2004, 02:23 PM   #8
Mark25 is offline Mark25  United Kingdom
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right, lots of theory there, especially in the followup link. For SS stuff i normally refer to;

http://131.109.59.51/images/pdf/Calc...m_Snubbers.pdf

and have not had much success, are Snubbers more important in valve cct's?

What effect do they have on the sound?

I'd like to keep my cct as simple as poss.
Mark
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Old 30th November 2004, 02:33 PM   #9
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That was the science part....

Most of the time 2 * 10R resistors WW with little 0, 01uF caps....do the trick nicely...

Quote:
What effect do they have on the sound?
My suggestion would be to try it out for yourself...And generally I do find any changes in psu for a valve circuit more audible than in the psu for an opamp...with the massive feedback of the op amp..all detail is blurred into a homogenous cesspool anyway..and changes are hard to detect.



Quote:
For SS stuff i normally refer to;
Written by....a tube kinda guy!...Jim Hagerman
http://www.hagtech.com/
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Old 30th November 2004, 03:16 PM   #10
Mark25 is offline Mark25  United Kingdom
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OK, normally i'd wait for the snubber until the cct was finalised, but your general quick fix proposal can be applied at any time so i may give that a go.

The thing i'm having trouble understanding with tubes is:

Why isn't dual mono psu's an absolute must, being that the PSRR of the amp ccts are so low?
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