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Old 12th December 2010, 08:13 PM   #11
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Well, I finally got my 'shop-amp' done. I skipped the plate stoppers, only used the usual grid stoppers, and separate cathode resistors, which I suppose act as 'cathode stoppers'. Being a flea power amp, it makes just under 1watt, I havent bypassed the cathode resistors since I dont need the gain. That gives some local NFB, which I don't mind, and ensures each triode operates within estimate.
I used what I had laying around, which is how most of the amps I build for myself end up being built.
I paralleled 3 5687 triodes in the output and used some 5k:8 Edcors I had. Unfortunately the only PT I had only gives me 170volt for B+. But the work shop only needs a watt so no real worries. With this low B+ I really don't need 3 5687s, but I don't like V-6s.
As is it sounds pretty damn good. Measurementwise it has nice low distortion till I pass 2.5V rms where the higher Hs start popping in. Max swing is +-4V into 8ohms. The 5687s wont let the plate voltage swing lower than about 80V without higher harmonics showing up. With some grid drive I imagine I can swing them a little lower for half a watt more, but I'm not that desperate.
Either way, sounds really nice, highly recommended but if doing it again I'd buy a new PT with proper B+ voltages, somewhere around 250-300volts.
The Edcors are great for the price, they are the real cheap XSE-15 types, and I measure -3dB corners at 15Hz and 54kHz. Not bad for those OTs.
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Old 12th December 2010, 09:01 PM   #12
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Btw. I forgot to thank all of those who answered this thread.
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Old 13th December 2010, 11:45 AM   #13
Cassiel is offline Cassiel  Libya
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Join Date: Sep 2004
Location: Madrid
It's nice to see something different. I also have a 6SN7 mini amp. Yes, 1 Watt is enough.
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Old 13th December 2010, 03:20 PM   #14
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Location: Alps:Tube amp designs over 150W, SMPS guru.
I find the ECC88 Cathode follower config is particulary prone to self oscillations around 470Mhz..I use small ferrite beads and carbon res at the grid pins and if a CCS is used on the cathode, an "output resistor of 50ohms".
As SY suggests, the shortest decoupling route to reduce self inductance from valve tag to chassis.
Ironically, the wire lead inductance inside the tube is nearly longer than the external with components !

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Old 13th December 2010, 05:22 PM   #15
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Hmm, my 'go to' scope has max 166kHz BW (It's an old trusty 16bit Picoscope). I guess I should unpack the old Philips scope, but that thing is a monster.

Just to keep this incredible thread going, here is the FFT of a 1kHz testsignal.
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