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Hello everyone!
Nice forum.....seems like you all know what you're doing.
However, I don't seem to know what I'm doing, as I just built a screechy-squealy amp that is one side AC30/4 (EF86) and the other side is AC30/6 (12AX7) but with a Master Volume and a full EQ section. Trannies are Mercury Magnetics Woden clones, custom aluminum chassis, etc... I'll post pics when I get home.
The amp sounds GREAT on the EF86 side....warm, articulate, bright when needed. SUPER COOL!
HOWEVER, the 12AX7 side has some issues when the preamp volume (gain) is turned up past 3/4 of the way up. Screechy-squealy-phaseshifty nastiness....bummer, as I'm so closed to being finished that I can taste it!
I'm pretty sure the tubes are not microphonic. I've tried them in other amps, and have had no problems. Also, I have tapped on them and there are no signs of microphonics.... My instinct says that the Tung Sol 12AX7s should be just fine (Top Hat has been using them in their King Royale amp with great success, and my amp is very similar to the King Royale...and the tubes came from a reputable dealer who tests them thoroughly), so I'm inclined to consider the layout to be a more likely suspect. The cathode follower 12AX7 seems to be the most sensitive, as if I install an EH 12AX7 in place of the Tung Sols the oscillation is better. It seems that the oscillation is connected to either higher gain or more top end, so that makes sense that the brighter, gainier Tung Sol 12AX7 would exacerbate the problem.
I did some work on it to clean up some things that I didn't get to in the initial build phase, such as:
I ran some shielded wire from the master volume pot to the input cap on the PI, moved/cleaned up some grounds in the tone stack section, re-ran some eq wires, and straighted up some of the shielded input cables. Now, the noise is even more present and obnoxious than before. So, perhaps I'm on the right track but headed in the wrong direction. TOTALLY frustrating...
So, here is what I've found to be the case now:
1) The EF86 channel sounds absolutely KILLER. Perfect. Volume and tone knobs, and that's it. No troubles, no nothing, just great sounding tone for days. Crank it or keep it low, it is GOOD GOOD GOOD!
2) The 12AX7 Master Volume side sounds great, too, until you crank the (500k) volume pot (gain) up past 3/4 open, then it gets phase-shifty and thin, and the squeal-scratch-oscillation starts happening on the top end. Gain down, no issues...
3) Turning the Cut and Treble knob down helps tame the squeal a bit, but the pot that has the most affect on reducing the squeal is the MIDRANGE pot. Interesting... Turning the mids all the way down almost takes care of the problem, though a little bit is still present. Don't know how to interpret that...
4) When the amp is in high power (switch in or out a 121 ohm 10w from between the recto and the fuse to either lower or raise the voltages downstream) and high filtering modes (32uf instead of 16uf in various places...), the squeal is more noticeable. Mostly, it is with higher voltages and with higher preamp filtering (1st gain stage...V1) that the squeal is more prominent, as raising the filtering in the power amp section doesn't affect the squeal at all.
So, it seems like the higher voltages, higher preamp filtering, and higher midrange settings are the straws that break this oscillating camel's back. Personally, I'm considering shooting this camel in the head and burying its body in the back yard, but then I can tell that I'm THIS close to having a truly fine amplifier on my hands....so I'll hold off on that.
Any suggestions, given the above? I've not tried switching the grids on the PI or the primaries on the OT yet, as my instincts say that the problem lies in the preamp somewhere.... Your help and input is greatly appreciated!
Sorry about the long first post....THANKS SO MUCH!
Nice forum.....seems like you all know what you're doing.
However, I don't seem to know what I'm doing, as I just built a screechy-squealy amp that is one side AC30/4 (EF86) and the other side is AC30/6 (12AX7) but with a Master Volume and a full EQ section. Trannies are Mercury Magnetics Woden clones, custom aluminum chassis, etc... I'll post pics when I get home.The amp sounds GREAT on the EF86 side....warm, articulate, bright when needed. SUPER COOL!
HOWEVER, the 12AX7 side has some issues when the preamp volume (gain) is turned up past 3/4 of the way up. Screechy-squealy-phaseshifty nastiness....bummer, as I'm so closed to being finished that I can taste it!
I'm pretty sure the tubes are not microphonic. I've tried them in other amps, and have had no problems. Also, I have tapped on them and there are no signs of microphonics.... My instinct says that the Tung Sol 12AX7s should be just fine (Top Hat has been using them in their King Royale amp with great success, and my amp is very similar to the King Royale...and the tubes came from a reputable dealer who tests them thoroughly), so I'm inclined to consider the layout to be a more likely suspect. The cathode follower 12AX7 seems to be the most sensitive, as if I install an EH 12AX7 in place of the Tung Sols the oscillation is better. It seems that the oscillation is connected to either higher gain or more top end, so that makes sense that the brighter, gainier Tung Sol 12AX7 would exacerbate the problem.
I did some work on it to clean up some things that I didn't get to in the initial build phase, such as:
I ran some shielded wire from the master volume pot to the input cap on the PI, moved/cleaned up some grounds in the tone stack section, re-ran some eq wires, and straighted up some of the shielded input cables. Now, the noise is even more present and obnoxious than before. So, perhaps I'm on the right track but headed in the wrong direction. TOTALLY frustrating...
So, here is what I've found to be the case now:
1) The EF86 channel sounds absolutely KILLER. Perfect. Volume and tone knobs, and that's it. No troubles, no nothing, just great sounding tone for days. Crank it or keep it low, it is GOOD GOOD GOOD!
2) The 12AX7 Master Volume side sounds great, too, until you crank the (500k) volume pot (gain) up past 3/4 open, then it gets phase-shifty and thin, and the squeal-scratch-oscillation starts happening on the top end. Gain down, no issues...
3) Turning the Cut and Treble knob down helps tame the squeal a bit, but the pot that has the most affect on reducing the squeal is the MIDRANGE pot. Interesting... Turning the mids all the way down almost takes care of the problem, though a little bit is still present. Don't know how to interpret that...
4) When the amp is in high power (switch in or out a 121 ohm 10w from between the recto and the fuse to either lower or raise the voltages downstream) and high filtering modes (32uf instead of 16uf in various places...), the squeal is more noticeable. Mostly, it is with higher voltages and with higher preamp filtering (1st gain stage...V1) that the squeal is more prominent, as raising the filtering in the power amp section doesn't affect the squeal at all.
So, it seems like the higher voltages, higher preamp filtering, and higher midrange settings are the straws that break this oscillating camel's back. Personally, I'm considering shooting this camel in the head and burying its body in the back yard, but then I can tell that I'm THIS close to having a truly fine amplifier on my hands....so I'll hold off on that.
Any suggestions, given the above? I've not tried switching the grids on the PI or the primaries on the OT yet, as my instincts say that the problem lies in the preamp somewhere.... Your help and input is greatly appreciated!
Sorry about the long first post....THANKS SO MUCH!
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