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In Reply to: Re: 6bq5 triode configuration? (Longer) posted by Damir on March 10, 2002 at 00:17:13:
I will try again at a later date. I greatly appreciate your knowledge and assistancein this matter. I do have one more question. I use a passive preamp with my tube amps and barely have enough gain from both my pilot SA-232 and Eico HF-86 amplifiers. I also had this problem with my Dynaco stereo 70 until Uncle Ned suggested that I double the value of the two feedback resistors on the circuit board. Doing this dramatically increased the available gain from the amplifier. Any ideas how to increase sensitivity/gain with the other two tube amps?LAnce
Follow Ups:
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???
I was trying to run the Eico HF-86 triode (I rewired it with 100ohm resistors between pin #7 & #9) output. The Pilot appears to be a far more complicated circuit with little room to work on. The eico offered only a hum through the right speaker when I disconected the jumper and resistor from the filter CAN capacitor and conncted 100ohm resistors between pins #7 & #9 on all 6bq5s.
I am going to experiment wih reducing the feedback on the HF-86. I have been able to locate the 33k resistors comming off of the 32 ohm speaker taps. Do I also need to change the value of the capacitors that you mentioned running in parallel with the feedback resistors?Lance
Murphy`s law: "If anything can go wrong, it will". It`s such a simple circuit and this mod is so easy that it`s probably some stupid mistake like bad soldering, short circuit, disconect B+ for 7247 tube together with screen grids supply, soldering the wrong pins, forgot to remove jumpers between pins 9, etc. I too "dicovered" the schematic too late. Try later, easy, and looking in the schematic.
Feedback loop - try desolder 33k resistors and solder 47 k. For now, don`t change the value of the 68 pF condensers - solder them parallel with 47 k resistors (like it was before with 33 k). The purpose of this cap is to make high frequency neg. feedback different from the low frequency feedback, and prevent oscillations on very high frequency. We will change the feedback resistor just a little, and I hope that old value of the cap will be OK. Again, it`s no easy task without some knowledge and test equipment. If it is necessary, the "right" value is often sought with experiments in small steps. Good luck again!
P.S. Don`t hestitate to ask if something should go wrong again, ha...
Damir, I have ended up with 100k ohm resistors in the feedback circuit in place of the 33k. I have yet to evaluate the sonic change, but there is lots more volume. I did rewire as suggested for triode operation but got the same results as before (only a hum from the right speaker).Lance
I can't thank you enough for your valuable assistance with my amplifiers. I plan to try rewiring the eico HF-86 for triode output again today and will let you know what happens. As you noticed, the HF-86 is a very simple design with a wide open under chassis for access. I am aso going to increase the size of the feedback resistors to increase gain potential and hopefully sound quality. I will definately keep you informed.
Lance
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