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Old 16th March 2006, 12:01 PM   #1
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Default My favorite preamp

I tried building this preamp, but only put in the MAGN and TAPE equilization networks. I used it with a record player for years, but never tried it with a tape recorder. I like it better than the Heathkit preamps I have and the built-in preamps in typical integrated stereo tube amps.

http://t000bs.topcities.com/schematics/preamp6267.htm
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Old 16th March 2006, 10:49 PM   #2
lndm is offline lndm  Australia
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Glad to hear it. Any future plans?

Welcome to the forum.
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Old 17th March 2006, 08:53 PM   #3
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Some people like using 6267/EF86 tubes in triode mode, but I never tried it. Also, I found that using DC on the filaments eliminated hum produced in the preamp. Before that I tried a hum balance control with 6.3 VAC on the filaments, but only one channel could be completely balanced out at one time.
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Old 17th March 2006, 09:25 PM   #4
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Quote:
Also, I found that using DC on the filaments eliminated hum produced in the preamp. Before that I tried a hum balance control with 6.3 VAC on the filaments, but only one channel could be completely balanced out at one time.
That is quite a sure sign that the heater circuit wiring is layed out suboptimal. EF86 has bifilar wound heaters, helping to cancel heater-induced (mind: not heater feed wire induced) hum out in the tube (heater construction) itself.

EF86 itself is easily capable of delivering hum-free amplification with AC-powered heaters in the lowish two digit to single digit Milli-Volt input sensitivity range when sticking to recommendations and good engineering practice.

Instead of using the DC-axe I would try rewiring the the heater feed wires following the usual and well know recommendations to prevent heater-feed induced hum.

Tom
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Old 18th March 2006, 02:44 AM   #5
anatech is offline anatech  Canada
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Hi 7n7is,
Yup, review your lead dress and routing. You can also bias the heaters positive from ground by 20~30 VDC. Lose the hum balance to ground first. Tap from the end of the B+ and stick a 10uF cap to ground from that point.

-Chris
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Old 18th March 2006, 09:11 PM   #6
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Yipes, this is where the difficulties come in. I suppose each of us has our own preference for a pre-amp circuit, even if not too different from the other fellow's.

First my contribution on the easy part: Welcome 7N7is. About d.c. on filaments, one must beware that it is smooth dc. Often not much more than raw rectified ac with a filter capacitor looks quite pious, but has such spikes that the noise is no better than with the "smooth" ac itself. I agree with Anatech here - BUT: with an input of 2 mV from a record pick-up, RIAA equalised, with the best of wiring it can be rather difficult to eliminate the effect of 18 000 mVp of 50 Hz in the vicinity to negligible proportions. I have managed this, but did find an audible (if not dramatic) advantage from using dc - but then, as said, smooth - in my case through an LM... regulator or such. The choice is yours.

To put it cautiously, my choice for a 2-stage pre-amp would have been for a pentode-triode or double triode equaliser (feedback to cathode) followed by a Baxandall + cathode follower (double triode). ..... but not to fault this circuit, if that is the choice. My greatest amazement is the second pentode stage tapped down to only 18% of the output in order to get a low enough driving impedance for the tone control - why not feedback?

But not to try sound too wise.

Regards.
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Old 18th March 2006, 10:40 PM   #7
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I tried using positive bias when using the hum balance control.

More on EF86 preamps here:

http://www.vintageradio.me.uk/amplif...lardpreamp.htm

http://vintage-radio.net/forum/showthread.php?t=7751
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Old 18th March 2006, 11:05 PM   #8
anatech is offline anatech  Canada
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Hi 7n7is,
Were you able to float the heaters up? Most hum balance pots need to be isolated from the chassis or the heaters will remain referenced to ground.

-Chris
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Old 18th March 2006, 11:29 PM   #9
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If I am allowed 2 further points:

Over tests on several dozen pre-amp tubes (not a lot, but still) I found an unexpected variation in heater-cathode influence of one sort or another. So I usually "chickened out" and made the heater chain positive, apart from the balance pot - that allowed me to use some tubes that were unsatisfactory with earthed heaters (when I did not use dc).

Then I just posted something in the semiconductor section that I might as well repeat here. With RIAA equalising of the feedback kind, I find it rather surprising that response below say 30 Hz is just allowed to continue on, instead of adjusting the values of components already there (coupling and cathode bypass capacitors) to give a handy rumble filter of some 15 dB/octave or better, especially in twin tube stages. It always appeared inside-out to me to pay hundreds of dollars more to get a turntable to do what could have been achieved with a few dollars of components.

Further regards.
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Old 25th March 2006, 01:02 PM   #10
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Default ef86 triode mode

You mentioned wondering how the tube sounded in triode mode. I've been using a pair of ef86's as a first stage voltage amp in my Dynaco st-70 for years. I wired it for triode instead of pentode because the amp already has enough gain for my cd player. It still has gain of around 35 or so in triode, if memory serves me.
The tube sounds wonderful in triode mode. It's very clear and balanced in my opinion. Probably as good as any 6sn7 or 76 to my ears. I have it on ac filament power, and can't hear any hum at all.

re/Bob
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