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Re: 6bq5 triode configuration? (Longer and longer)

213.191.128.202

Posted by Damir on March 10, 2002 at 09:19:46

In Reply to: Re: 6bq5 triode configuration? (Longer) posted by chopper87 on March 10, 2002 at 06:24:03:

Well, I found schematics. First, maybe you have too much attentuation in the passive preamp - CD players, for example, have about 2 V rms output - good enough for most of the amps.
Yes, raising the value of the feedback resistor increased the amplification. In the case of the Eico HF-86, you have neg. feedback resistor from the secondary (33 k) paralleled with little cap of the 68 pF. This resistor is connected on the cathode of the first triode (pin 8) - the half of the 7247. Second triode (second half of the 7247) is the phase splitter. In that way, amp is "closed" in the feedback loop. You can raise the value of the 33 kOhms resistor in the small steps. Try 47 kOhms, don`t forget the capacitor (68 pF).
Amp will have more gain now - you can like it, it can "open" the sound, etc. But, dealing with feedback loops isn`t 100 % safe. Amp can become unstable, noise and distortion can rise. But, it`s easy mod, and you can always go back. Don`t forget the same on the other channel.
The Pilot SA-232 is little more complicated. You can do the same as above, raise the value of the neg. feedback resistor from the secondary of the OPT to phase splitter - we have 6k8 paralleled with 250 pF cap. Try enlarge it to say, 8k2 or up - you`ll see (hear). Also, you have the local feedback loop around the first triode - 1/2 of the 12AU7A. It`s 1MOhms resistor from the output of this tube to input - from pin 2 of the 12AU7A to pin 2 of the 12AX7 (input of the phase splitter). You can try to desolder this resistor, and in the same time change the resistive divider on the input - 150 k in series, 470 k parallel. Change the 150 k to say, 10 kOhms.
As you can see, it`s not so easy to inexperienced. And actually, I can`t recommend mods like that if you not really shure what you are doing.
P.S. Is the PILOT (or EICO) the amp you want rewire to the triode mode? Looking in the schematics I don`t know what`s the problem - disconnect screen grid supply to pins 9, and solder one 100 Ohms resistor on the every EL 84 between pins 7 & 9???

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