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Old 29th October 2002, 09:25 AM   #11
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Another excellent candidate for the driver stage is a 6N7 with its two triode sections wired in parallel. This is a very underappreciated valve.
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Old 13th November 2015, 12:02 PM   #12
waam68 is offline waam68  United Kingdom
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Blast from the past....I just built this simple 71A circuit from my spares stash really because I'd bought a cheap lucky dip box of tubes on eBay which to my surprise also included a 26, 71A and cx380 rectifier which I was looking to use some of and as I recently switched to Lowther Acoustas and a sub I wanted to see how flea power I could go. The build was on a surplus power supply chassis I sprayed blue, existing holes limited the layout a bit. As the power tx wasn't bought specifically I added a 5v tx for the rectifier and another 6.3v for the 6FA driver filaments. I also separated the bias and filament supplies for each channel. The amp works really well apart from 2 things I need a steer on. Firstly the 500k volume pot is acting like a hum pot with low hum at either and and progressively louder hum towards the middle. I'm running it with the pot full on as I'm using a preamp. The other thing is although the sound is very warm and generally very nice, the really high frequencies get annoying after a while like there is some very high frequency distortion and no it isn't the Lowthers....I swapped back to my Morrison micro 2A3 and while it is very neutral it doesn't have this problem on the lowther pm6A's. Is it likely to be caused by the el cheapo Xse output transformers or something else? The outputs are biased at 20ma which is recommended on the data sheet. It's only my second point to point amp and my first with fixed bias. I'd like to try and resolve these issues as it's got potential!
Mike
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Old 13th November 2015, 12:05 PM   #13
waam68 is offline waam68  United Kingdom
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Old 13th November 2015, 03:59 PM   #14
kevinkr is offline kevinkr  United States
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Nice!

I long ago designed and built a 71A based stereo amp with 5842/417A driver stage and fixed bias output stage. I used this mainly as a headphone amp, but it will also drive very efficient speakers as well.
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