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Old 21st May 2008, 11:29 PM   #1
neazoi is offline neazoi  Greece
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Default 6sn7 direct coupled and transformer coupled

Hi I was wondering what are the changes that must be done in the second version (direct coupled) of the next preamplifier in order to use a 6-7K transformer at the anode of the second tube instead of the 5mf capacitor coupling
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Old 22nd May 2008, 04:55 PM   #2
kevinkr is offline kevinkr  United States
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Default Re: 6sn7 direct coupled and transformer coupled

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Originally posted by neazoi
Hi I was wondering what are the changes that must be done in the second version (direct coupled) of the next preamplifier in order to use a 6-7K transformer at the anode of the second tube instead of the 5mf capacitor coupling

You can't use such a transformer in the anode circuit of a single 6SN7, you need ~ 3X rp for good performance - you can get away comfortably with 15K primary impedance if the primary inductance is high and the secondary is lightly loaded. (i.e. > 600 ohm load)

Why you would want to is beyond me.. A single 6SN7 or pair of 6J5 with transformers should provide enough gain for most line stage applications. (About 6dB - 12dB depending on load at the secondary for a 15K:600 transformer.)
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Old 23rd May 2008, 06:13 AM   #3
neazoi is offline neazoi  Greece
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Sir,
I am thinking of using a pair of UTC A-34 vintage transformers sold on ebay. They can be taped to 25Kohm or 6.5Kohm primary and 500Ohm or 125Ohm secondary.
I see you suggest 15K primary but from your experience do you think it should be ok with 25K?
Best regards.
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Old 23rd May 2008, 03:30 PM   #4
kevinkr is offline kevinkr  United States
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Yes, 25K:500 will be fine. Ratio is about 7.1:1 , gain should be somewhere in the vicinity of 6 - 9dB depending on loading. Again this is with just a single triode per channel. Set bias so that plate current is about 1 - 2mA lower than the transformer's rated primary dc current to avoid saturation.

Should sound quite good.

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