Hello,
I have been putting together an AC30 style amp, pretty much following the schematic. I have it prototyped right now in an old chassis, as i wanted to experiment with the circuit before i finalize the design.
I have simplified the circuit down to 1 channel, which is the bright channel. I have a treble and bass control, as well as a cut control.
I am using Mercury Mag pt and ot. The power transformer i got is modeled after a higher voltage version that was supposedly pretty desirable for the cleaner more powerful tone due to a slight bit higher voltage. Consequently the voltages overall are a bit high.
My problem is that as i turn up the bass I am getting a pretty serious motorboating, and with the cut i am getting a high oscillation as well.
I am using new caps, slightly larger in PS than original AC30. My ground scheme has all tone and preamp grounds to one point on chassis. The big PS caps are all grounded to a seperate lug, and the PT grounded to yet another with the AC ground as well.
Since its a prototype box the wiring is not perfect, but i have been careful with my grid wiring and plate wiring, and the heater wiring is pretty clean. I have tried moving the grid wires around and haven't been able to change the oscillation that way either. I have tried other EL84's (Phillip's that sound great in my vintage AC30) with pretty much the same result
The voltages are listed below. There not super high, though V1 is a bit off(perhaps because i am only using one side of the triode with the same cathode-1.5k resistor?)
V1-pin6-153v pin 8 1.1v voltage coming into 220k plate 315
V2-pin1 338v pin2&6 202v
V3-pin1 and 6 249v
EL84's--@343 per plate. 338 G2. 11Vcathode
I know this is beyond the diss point of this tube but seems to be within spec of what this design has been known to handle..
Are there any glaring errors? possibly too high of current on V1 plate>? Maybe use higher cathode resistor since only using 1/2 the triode?
Any other ideas?
BTW, i have tried switching the OT wires
Thanks
Ian
I have been putting together an AC30 style amp, pretty much following the schematic. I have it prototyped right now in an old chassis, as i wanted to experiment with the circuit before i finalize the design.
I have simplified the circuit down to 1 channel, which is the bright channel. I have a treble and bass control, as well as a cut control.
I am using Mercury Mag pt and ot. The power transformer i got is modeled after a higher voltage version that was supposedly pretty desirable for the cleaner more powerful tone due to a slight bit higher voltage. Consequently the voltages overall are a bit high.
My problem is that as i turn up the bass I am getting a pretty serious motorboating, and with the cut i am getting a high oscillation as well.
I am using new caps, slightly larger in PS than original AC30. My ground scheme has all tone and preamp grounds to one point on chassis. The big PS caps are all grounded to a seperate lug, and the PT grounded to yet another with the AC ground as well.
Since its a prototype box the wiring is not perfect, but i have been careful with my grid wiring and plate wiring, and the heater wiring is pretty clean. I have tried moving the grid wires around and haven't been able to change the oscillation that way either. I have tried other EL84's (Phillip's that sound great in my vintage AC30) with pretty much the same result
The voltages are listed below. There not super high, though V1 is a bit off(perhaps because i am only using one side of the triode with the same cathode-1.5k resistor?)
V1-pin6-153v pin 8 1.1v voltage coming into 220k plate 315
V2-pin1 338v pin2&6 202v
V3-pin1 and 6 249v
EL84's--@343 per plate. 338 G2. 11Vcathode
I know this is beyond the diss point of this tube but seems to be within spec of what this design has been known to handle..
Are there any glaring errors? possibly too high of current on V1 plate>? Maybe use higher cathode resistor since only using 1/2 the triode?
Any other ideas?
BTW, i have tried switching the OT wires
Thanks
Ian
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