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 Post subject: Re: 12AT7 and a O.P.T.
PostPosted: Feb Fri 17, 2017 5:34 am 
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You can use a non-gapped transformer for single ended amps as long as your output current steers clear of the transformer saturation point. Bob Weaver pointed me to a way of measuring saturation that I found invaluable in comparing OPTs I had on hand when developing my 6BM8 headphone amp. I used an LM317 regulator configured as a constant current source to measure saturation. I ended up with a lovely sounding amp.

Scroll down about 2/3 of the page to the second transformer saturation chart and read around there at this link:

viewtopic.php?f=12&t=304402&start=20


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 Post subject: Re: 12AT7 and a O.P.T.
PostPosted: Feb Fri 17, 2017 5:34 am 
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michael w wrote:
This afternoon I built a couple of different single tube two stage amps. One with a 12AT7 and another using a 12AU7. Both worked as you all described in your posts above. The one thing I did notice was both these amps had more (louder) background noise than an earlier amp I built using a 6BM8 triode/pentode. Question: are pentodes typically quieter than triodes? I'm trying to come up with a low power headphone amp with minimal background noise.

Thanks for the heads up on the Fender Reverb OPT

Altereds were always my favorite cars to watch Mike. It takes some real balls to drive one of those.
Quite the opposite. All things being equal triodes are quieter than pentodes although, at these levels, you shouldn't notice much, if any, difference.


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 Post subject: Re: 12AT7 and a O.P.T.
PostPosted: Feb Fri 17, 2017 6:03 am 
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Elvirafan wrote:
The circuit I posted was probably intended to be used as a very low power amp to drive a small speaker, probably be good enough for a simple radio, but not for a headphone amplifier. Headphones are far more sensitive than speakers, so the noise and hum will be very annoying.

Google "headphone amp". There are lots of designs with tubes, transistors, and hybrids as well.

Bob
May be but the circuit posted only produces about 30mW. That's pretty puny for a speaker but right about an Ipod headphone output.

Hum would just need better supply filtering. Noise is just amplified like anything else.


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 Post subject: Re: 12AT7 and a O.P.T.
PostPosted: Feb Fri 17, 2017 7:57 am 
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I've had my eye on this one since I have many IRF510s and tubes.

Image


The forum has 160 pages on this amp.
http://diyaudioprojects.com/Solid/12AU7 ... 7-Headamp/


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 Post subject: Re: 12AT7 and a O.P.T.
PostPosted: Feb Fri 17, 2017 1:08 pm 
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The 12AT7 amp is from this site:

http://members.iinet.net.au/~cool386/amps/amps.html

There are some other simple tube amps there as well.

There's a Japanese site (notes in English) with similar designs here:

http://www.intio.or.jp/jf10zl/7vac.htm

The 6AW8A triode/pentode was a popular one tube audio amp for some of the cheaper communications receivers back in the 60s.

Rob


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 Post subject: Re: 12AT7 and a O.P.T.
PostPosted: Feb Fri 17, 2017 9:34 pm 
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That's what I found to be true Bob. The background noise was greater than with a triode / pentode design. You all figured this out long ago, but it's all new to me. Thanks for all the good advice. Going to try a 6AW8A today. Looking forward to reading the article on transformer core saturation.


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 Post subject: Re: 12AT7 and a O.P.T.
PostPosted: Feb Sat 18, 2017 12:31 am 
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michael w wrote:
That's what I found to be true Bob. The background noise was greater than with a triode / pentode design. You all figured this out long ago, but it's all new to me. Thanks for all the good advice. Going to try a 6AW8A today. Looking forward to reading the article on transformer core saturation.
There's something other than being a triode or pentode that's causing the noise because, as I said, triodes are quieter.


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 Post subject: Re: 12AT7 and a O.P.T.
PostPosted: Feb Sat 18, 2017 7:15 am 
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This evening I finished another amp this time using a 6AW8A. For whatever reason the background noise is quieter than with the triodes. When I say "background noise" I'm referring to the normal sound being produced by just having the set turned on. I'm not referring to a noise that shouldn't be there like 60 cycle hum. Is there a better term for this besides what I'm calling it?


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 Post subject: Re: 12AT7 and a O.P.T.
PostPosted: Feb Sat 18, 2017 8:02 am 
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michael w wrote:
This evening I finished another amp this time using a 6AW8A. For whatever reason the background noise is quieter than with the triodes. When I say "background noise" I'm referring to the normal sound being produced by just having the set turned on. I'm not referring to a noise that shouldn't be there like 60 cycle hum. Is there a better term for this besides what I'm calling it?
You mean background hiss from the radio?

I don't know what's going on because you've given literally no details at all, not the least of which is what OPTs you've used on each but bias, B+ voltage, circuit, gain, what radio, etc. are all missing. But, all else being equal, triodes don't have pentode screen-plate partition noise because they don't even have a screen. However, as I also said, that shouldn't even be detectable in the output tube so I can only conclude there's something else happening with the circuit configuration that's giving the false impression.

Ironically, 'noise' (if any) would be coming from the first stage gain triode and that's (I presume) a triode in ALL your little amps. Now PENTODE output amps have a rising frequency response due to speaker impedance (not so much with headphones), which would aggravate hiss, but you say it's the triodes. It just doesn't make sense. Just a wild stab in the dark, I suspect something OPT related, like core saturation, poor frequency response, or ?


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 Post subject: Re: 12AT7 and a O.P.T.
PostPosted: Feb Sun 19, 2017 6:46 am 
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Thanks for all the suggestions Flipperhome. I have a decade resistance box that I use when adjusting bias and an output transformer with different load taps. I put together every conceivable combination of ingredients imaginable while conducting these tests on the 3 amp circuits we've been discussing.

Today I hit the jackpot when I chose a 6BM8. When I turned on the set I thought I forgot to connect a wire somewhere because it was so quiet. I'm only driving at 40% of the rated power and this is the best I have found for use with my headphones. If ever I want to drive a speaker I can always adjust the bias for that.

I want to thank everyone for contributing to my limited knowledge on this stuff. This is a great forum and a great bunch of guys. We are all fortunate to have this..................... michael


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