My new el34 + 12ax7 VCT amp. (Need Ideas For my first headphone vacuum tube amp Too~) - diyAudio
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Old 13th December 2011, 12:37 PM   #1
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Default My new el34 + 12ax7 VCT amp. (Need Ideas For my first headphone vacuum tube amp Too~)

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Old 13th December 2011, 11:10 PM   #2
mc352 is offline mc352  United States
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Looks like very nice work. In the first picture it looks like there are 2 on/off switches, if so why 2?
Was this a kit or one of your own creation.
Can you provide any details on the design or specs?

Thanks. I am looking to start building a tube amp myself, so I have been lurking and asking questions.
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Old 13th December 2011, 11:32 PM   #3
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oh that switch is used for the feedback and volume increase. This is really not a kit at all i drill all the part by myself include main power transformer>
But now i got a question cause the main transformer is getting hot when 4hrs later, why will it happen>?

Do any one have some ideas on headphone amp by tube?
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Old 14th December 2011, 12:42 AM   #4
prelius is offline prelius  United States
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Default Power transformenr looks a bit on a small side...

Nice work on the amp: clean and neat...
I might be wrong, but it looks like the power tranny is a bit small for your amp. It is rated only at 150mA on B+ (at least that's what I see in the picture). A single EL34 can draw up to 100-110 mA depending on the brand, and since it looks like you have a SE amp, the current draw is pretty much constant, even at idle it will draw 80% of allocated current, making transformer work all the time. With the HT secondary on the "lower" side (again, in the picture is says 300-0-300), your B+ looks like around 400V after rectifier, and with the load on about 340-360VDC. At that voltage your current draw is likely to be in 85mA + per tube range, which puts you above the tranny rating. Please post schematics to confirm.
Thanks, Paul.
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Old 14th December 2011, 01:00 AM   #5
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Old 14th December 2011, 01:06 AM   #6
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But i have a question> you have said that one side of the 300 - 0 will be 150ma but in both sides it will be 300ma cause it is full wave rectifier?
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Old 14th December 2011, 02:02 AM   #7
prelius is offline prelius  United States
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Current rating of the secondary winding has nothing to do with how many secondaries the tranny has, or whether you use a full wave rectifier. It is determined (mostly) by the diameter of the wire used on that winding. The larger the diameter (or cross-section, and ultimately, the surface of the wire) the more current it can pump without overheating. In a EI based transformer, the max current capacity is limited by the size of the spool/windows of the EI iron. For example, if the secondary calls for 600 turns of the coppers wire, it will take two layers using #24 wire to support 100mA of current, but if you want 300mA, it will need the same number of turns (600), but using #14 wire, and that would occupy 10 layers on the spool. So once you wind it, you will not be able to fit the spool onto the iron, and need to use a larger profile (which costs more).

Looking at the schematics, it definitely looks like you are either very close or over the max rating. With R8 off (lowers current consumption, you draw 67 + 67 = 134mA, plus 1.2mA for 12AX7 for the total of 135mA. That leaves you 10% spare capacity, which is not enough. With R8 on, you draw 146 mA, which is effectively puts the tranny at fully rated load. And that's not a good thing. You always want at least 25% spare current capacity.

Now let's look at the heater winding. It is rated at 4A. Each of the EL34 draws about 1.5A, and 12AX7, when using 6.3v draws about .6A, so your total heater current is 3.6A, That's also very close to the max. Based on my personal experiences, US transformer makers typically under-rate their trannies by 10%-15%. Chinese manufacturers rarely do so. You are extremely close to the tranny max rating on both HT and heater winding and it this is a single reason why your transformer gets hot.
Thanks, Paul

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Old 14th December 2011, 07:03 AM   #8
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hey really thanks but i have check the inductor of my amp the resistance of it is 101ohms and the voltage pass through is 11.86 so it got out a 100ma current and this is totally lower than the maximum current of the 150ma. I am not really same as the above data the 400 ohms in el34 is changed to 390 and the pi circuit is using 68uF and 220 uF
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Old 12th July 2013, 02:42 PM   #9
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Haa Prelius, what a long time, here you are 6550 with an old amp i have.

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Old 12th July 2013, 04:20 PM   #10
prelius is offline prelius  United States
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Nice!!! Does it overheat? :-)
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