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Old 21st March 2011, 10:49 AM   #21
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Originally Posted by imnewbie View Post
Seems like the PCB do not have any place to solder the UL taps.
The 13EM7 have no screen grids. You can't run a triode in UL.
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Old 21st March 2011, 07:25 PM   #22
CSlee is offline CSlee  United Kingdom
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Can I ask what the 100ohm resistor from OT secondary to ground is for?

Thanks
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Old 21st March 2011, 08:03 PM   #23
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I'm curiously looking forward for people commenting on it's headphone performance. I'm looking for a headphone amp and I'm wondering which one I should go with..Millet Minimax or Jonokuchi? I need amp for headphones only which makes this even harder decision.
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Old 21st March 2011, 09:24 PM   #24
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Can I ask what the 100ohm resistor from OT secondary to ground is for?

Thanks
Well, let's just call it a "paranoia resistor". It probably does not need to be there at all.

My concern was that it's possible to have the secondary of the OPT unloaded, either when no speaker is connected, or as you switch between speakers and headphones. The plate resistance of the 6EM7 is so low - it should control the primary voltage swing well even without any load - that I can't envision any transients big enough to hurt anything, but for the price of a couple resistors, I put them there.

Pete
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Old 21st March 2011, 09:31 PM   #25
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The plate resistance of the 6EM7 is so low - it should control the primary voltage swing well even without any load - that I can't envision any transients big enough to hurt anything, but for the price of a couple resistors, I put them there.
It has been stated that running a tube amp with no load can blow it up. This can be very true, I have seen guitar amps catch fire when the speaker gets unplugged or blows (open voice coil).

In my experience an SE amp under normal conditions does not experience this type of failure since there is always current flowing through the primary. However driving an SE amp to clipping with no load causes the primary current to abruptly cease, which generates a kilovolt level spike, possibly zapping an OPT. A resistor across the secondary is good insurance at the cost of a few milliwatts of output power.
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Old 21st March 2011, 09:59 PM   #26
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Ahh yes I see, thanks.

At a cost of just a few milliwatts of power, I'm surprised not to see it in most designs.

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Old 22nd March 2011, 12:16 AM   #27
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At a cost of just a few milliwatts of power, I'm surprised not to see it in most designs.
A simple resistor across the speaker terminals is good insurance especially in a low or moderate powered SE amp. It is not sufficient to save the OPT in a high powered P-P amp that is capable of being severely clipped. This means most guitar amps over 40 watts and maybe Petes other red board when someone hot rods it to 125 WPC. I have seen diodes, zeners, transient suppressors, gas discharge tubes, and resistors used alone or in combination in attempts to save OPT's from their users. Nothing is 100% foolproof though.
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Old 24th March 2011, 04:18 AM   #28
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I'm still debating which output transformer to get. Anyone know any other brand that offers affordable(around $100/pair) 8K SE output transformers? Transcendar only have 3K and 5K versions available. Electraprints are out of question since I wish to make this amp under $300(excluding chassis etc).
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Old 24th March 2011, 11:28 PM   #29
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Anyone know any other brand that offers affordable(around $100/pair) 8K SE output transformers?
Good luck. The only guys I might think would be able to deliver something in that price range would be vt4c.com. Look in their "Mars" single ended line. I don't see anything on their web site, but an email might be worthwhile. They're made in China, and I'd expect they could wind up anything you might need if sufficiently motivated.
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Old 4th April 2011, 08:38 AM   #30
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Just won a new pair of Transcendar's 5K version on ebay. But they will be used with simple SE soon. I wish they had 8K version available for Jonokuchi.

Last edited by imnewbie; 4th April 2011 at 08:44 AM.
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