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hi guys, i have just finished a homebrew amp, that is based on an ac15 normal channel. there are 2 channels, one is a ef86 going into the volume pot and so forth, the second is a 12ax7 wired so one triode feeds into the next via a 500k vol pot to control the gain. this feeds into the same volume control as the ef86 (acting as a master volume on this channel). because they use the common vol pot, i know that there is an issue with the ef86 stage.
plugged into the ef86 i get alot less gain, even with the gain pot on the 12ax7 on very low levels. i can turn the vol up full and not have it particularly loud, where the gain channel is that volume much earlier with only a bit of gain coming through on that channel.
before i start, ill mention that when probing for voltages, i found that i used the wrong voltage tap on the tranny for the input voltage, so the amp is actually running 50v hotter on the input than expected (there are 2 red/white cables on this one and i believe someone at weber put the sticker on the wrong one for wire #6, or my extreme care on cutting the unneeded (now needed one shorter first was making a mistake). either way, the amp is designed to run on lower voltages anyway, so the extra voltage shouldnt have done anything serious, and should increase the gain im worried about, not decrease it. ill fix that once i go into it again.
one thing i think may be an issue is the voltage. the gain stage ax and the pi are each running off the same voltage so that is fine. the voltage after the dropping resistors for the ef86 are lower than i would expect.
the voltage on pin 6 (the anode) was 98, and grid no. 2 was 125. it strikes me that the 98 on the cathode is deffinately too low, and im not sure about the grid no. 2 voltage. what do you think the cause of this would be? its a jj ef806 that has just recently come out, could this be a tube fault, partially shorting something out? ill doublecheck that there arent any wires around the pins that are sticking out where they shouldnt (maybe one from the anode to cathode would give that kind of voltage passing) but i should have seen something like that. ive checked the values of the different resistors, and noticed that i had put a 2m2 in the grid no.2 dropping resistor place (from a matchless schematic), and replaced it with a 1m(whats on the datasheet) which didnt make a noticeable (after a few minutes soldering and such) difference.
everything else seems to be to spec, ive put a 6position turning switch to change the value of the cathode bypass cap and resistor combo (from around 2k5 to 1k8 or so by adding a second resistor and cap in series to a 2k7 resistor that is fixed in place. the values are within this region as i checked to make sure i didnt put a 27k resistor in place) these changes make very little differences to the sound at the moment (hopefully they will make a nice change once the amp is working correctly)
essentially the tube is wired up as per the datasheet (the second set of values for the higher gain), aside of the cathode resistor switch, which is the correct value at one setting.
is there anything that i should check aside from what i noted above before blaming the tube and getting another one?
plugged into the ef86 i get alot less gain, even with the gain pot on the 12ax7 on very low levels. i can turn the vol up full and not have it particularly loud, where the gain channel is that volume much earlier with only a bit of gain coming through on that channel.
before i start, ill mention that when probing for voltages, i found that i used the wrong voltage tap on the tranny for the input voltage, so the amp is actually running 50v hotter on the input than expected (there are 2 red/white cables on this one and i believe someone at weber put the sticker on the wrong one for wire #6, or my extreme care on cutting the unneeded (now needed one shorter first was making a mistake). either way, the amp is designed to run on lower voltages anyway, so the extra voltage shouldnt have done anything serious, and should increase the gain im worried about, not decrease it. ill fix that once i go into it again.
one thing i think may be an issue is the voltage. the gain stage ax and the pi are each running off the same voltage so that is fine. the voltage after the dropping resistors for the ef86 are lower than i would expect.
the voltage on pin 6 (the anode) was 98, and grid no. 2 was 125. it strikes me that the 98 on the cathode is deffinately too low, and im not sure about the grid no. 2 voltage. what do you think the cause of this would be? its a jj ef806 that has just recently come out, could this be a tube fault, partially shorting something out? ill doublecheck that there arent any wires around the pins that are sticking out where they shouldnt (maybe one from the anode to cathode would give that kind of voltage passing) but i should have seen something like that. ive checked the values of the different resistors, and noticed that i had put a 2m2 in the grid no.2 dropping resistor place (from a matchless schematic), and replaced it with a 1m(whats on the datasheet) which didnt make a noticeable (after a few minutes soldering and such) difference.
everything else seems to be to spec, ive put a 6position turning switch to change the value of the cathode bypass cap and resistor combo (from around 2k5 to 1k8 or so by adding a second resistor and cap in series to a 2k7 resistor that is fixed in place. the values are within this region as i checked to make sure i didnt put a 27k resistor in place) these changes make very little differences to the sound at the moment (hopefully they will make a nice change once the amp is working correctly)
essentially the tube is wired up as per the datasheet (the second set of values for the higher gain), aside of the cathode resistor switch, which is the correct value at one setting.
is there anything that i should check aside from what i noted above before blaming the tube and getting another one?
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