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Old 14th October 2012, 04:00 PM   #2141
mach1 is offline mach1  Australia
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I'm wondering why there is an 330 ohm RC stage after the shunt reg (which would have op impedance of approx 90 ohms according to S. Bench's site).

Surely lower op impedance is better?
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Last edited by mach1; 14th October 2012 at 04:07 PM.
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Old 14th October 2012, 04:03 PM   #2142
10y is offline 10y  Italy
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Forgot to share filament supply: two Rod Coleman's board that work great!!

Ciao,

Anto
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Old 14th October 2012, 04:08 PM   #2143
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Anto -

Get a selection of 26 tubes. They vary a lot in how microphonic they are and how much they can hum. You probably need to choose some good ones. There should be a lot on ebay.com. There is usually plenty. I get mine from US dealers - I cleaned out my usual guy!

Andy
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Old 14th October 2012, 04:13 PM   #2144
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by 10y View Post
Problem to solve in the next:


1) great microphonic effect.... all noises generated near pre will sound on speaker

Ciao,

Anto
Ciao Anto,
Look at the rubber dampers here. DHTRob posted a link before if you are interested in contacting the supplier:
http://www.bartola.co.uk/valves/wp-c...0/IMG_8680.jpg

Try to starve the filaments. A slight starvation (e.g. 900mA) may help reducing the microphony. Also try using 26 instead of 226, as the latter tend to be more microphonic
Cheers,
Ale
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Old 14th October 2012, 04:14 PM   #2145
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by mach1 View Post
I'm wondering why there is an 330 ohm RC stage after the shunt reg (which would have op impedance of approx 90 ohms according to S. Bench's site).

Surely lower op impedance is better?
I just wanted to tweak the anode voltage and provide additional decoupling. Also wanted to try Morgan Jones' caps!
Ale
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Old 14th October 2012, 04:17 PM   #2146
10y is offline 10y  Italy
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Andy,

what is the less microphonic in your experience? It depends from brands, structure or between same type is random?

I too have bought from usa vendors on ebay.

Regarding hum I must separate the two woods around 1,5 metres for a inudible noise.

Ciao,

Anto
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Old 14th October 2012, 06:14 PM   #2147
10y is offline 10y  Italy
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Quote:
Originally Posted by mogliaa View Post
Ciao Anto,
Look at the rubber dampers here. DHTRob posted a link before if you are interested in contacting the supplier:
http://www.bartola.co.uk/valves/wp-c...0/IMG_8680.jpg

Try to starve the filaments. A slight starvation (e.g. 900mA) may help reducing the microphony. Also try using 26 instead of 226, as the latter tend to be more microphonic
Cheers,
Ale
Hi Ale,

I was thinking about similar rubber support, thanks for your confirmation.

Starving the filament could shorts tube life?

Ciao,
Anto
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Old 14th October 2012, 06:16 PM   #2148
mogliaa is offline mogliaa  United Kingdom
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I've been running preamps for nearly two years now and haven't had any valve issues
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Old 14th October 2012, 06:37 PM   #2149
10y is offline 10y  Italy
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Originally Posted by mogliaa View Post
I've been running preamps for nearly two years now and haven't had any valve issues
OK, will try tomorrow to reduce filaments voltage to 1,2-1,3 V.

Where do I find those nice rubber's base ?

Anto
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Old 14th October 2012, 06:44 PM   #2150
mogliaa is offline mogliaa  United Kingdom
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Anto,
Reducing filament current will increase Rp and impact the operating point. Don't know what circuit you are using, but try and listen. You may take the bias point far too out and the distortion may increase. I'd suggest to try adjusting the Coleman regulators and testing microphony and sound of the preamp. Then you can measure new bias point.

Re the rubber suspenders I got them kindly from DHTRob. There is a seller in Netherlands, they are not cheap though. I think Thomas Mayer may supply a different set of dampers if you fancy that alternative. Otherwise search this thread as the link of the supplier was posted time ago.
Ale
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