Hi All,
Sorry to post with my woes, but I just thought someone might be able to point me in the right direction.
I have a 12ax7 based phono preamp that I am building point to point out of spare parts (basically, mostly) --thought it would be a fun project.
But I have been stumped by this one!
This is my first tube phono preamp. I have built some solid state preamps point to point with success. I also have a pair of PP EL34 amps I built and work great. Though, I admit my knowledge and experience are rudimentary at best.
See the attached schematic.
When hooked up to a cheapy little headphone amp for testing I get some 120hz (I think) buzzing noise with some harmonics when the volume is kept low. This is quieter than the music on the test record.
If I turn up the volume to a moderate listening level, a 60hz (I think) tone dominates and completely swamps the signal. The hum seems to rise disproportionately to the amount the volume knob is turned.
If I hook the phono pre up to a preamp with a stepped volume pot I get a chirping noise every time I change the volume. This noise gets louder with every step. (I can only raise the volume 2 or 3 steps before the chirping is insanely loud!) The chirping also happens when the input selector is swithched to or from the tube phono pre.
Also, I notice that the DC offset, with the inputs shorted and the phono pre warmed up, starts off around -3V and settles to .004V in about 3 seconds. On power up the DC offset climbs to about 13V before settling to .004V.
Electrically:
V1 (the input tube) appears fairly normal. The plate reads a little low at 74v, with the plate resistor drawing .66mA. the Cathode reads .74V, and the Cathode resistor seems to be pulling .47mA.
The coulping cap (C103) between V1 and V2 is passing -23V to the plate of V2, which seems odd to me. OR, that is to say, I get a reading of -23V at the plate of V2.
It looks like V2 is drawing too much current. I am getting a reading of 170V at the plate, with the plate resistor drawing 1.2mA.
(This, of course, means that the grid of V3 (cathode follower) is seeing roughly the same voltage as the plate of V2, or ~ 170V.)
The cathode resistor at V2 is only drawing .27mA of current. Cathode reads .58 ohm.
The cathode of V3 reads 185V, and the cathode resistor looks like it is drawing 12.3mA.
I've gone over the circuit a dozen times and can see no mistakes in my wiring. Measuring with a DMM I get the correct values from point to point.
The power supply is CRCRC giving me 280v B+, (checked it in PSUD2 and I see no strange behavior here.)
The filaments are 11.9V DC filtered with a CRCRC filter (again, checked with PSUD2) elevated 30V. All my filament wiring was done as though it would be AC heaters. Tightly twisted pairs running along the corners of the enclosure and enter at right angles to the sockets. They pass all other components at right angles.
I'm sure I've done something stupid, but I just can't find it!
Any clues?
Sorry to post with my woes, but I just thought someone might be able to point me in the right direction.
I have a 12ax7 based phono preamp that I am building point to point out of spare parts (basically, mostly) --thought it would be a fun project.
But I have been stumped by this one!
This is my first tube phono preamp. I have built some solid state preamps point to point with success. I also have a pair of PP EL34 amps I built and work great. Though, I admit my knowledge and experience are rudimentary at best.
See the attached schematic.
When hooked up to a cheapy little headphone amp for testing I get some 120hz (I think) buzzing noise with some harmonics when the volume is kept low. This is quieter than the music on the test record.
If I turn up the volume to a moderate listening level, a 60hz (I think) tone dominates and completely swamps the signal. The hum seems to rise disproportionately to the amount the volume knob is turned.
If I hook the phono pre up to a preamp with a stepped volume pot I get a chirping noise every time I change the volume. This noise gets louder with every step. (I can only raise the volume 2 or 3 steps before the chirping is insanely loud!) The chirping also happens when the input selector is swithched to or from the tube phono pre.
Also, I notice that the DC offset, with the inputs shorted and the phono pre warmed up, starts off around -3V and settles to .004V in about 3 seconds. On power up the DC offset climbs to about 13V before settling to .004V.
Electrically:
V1 (the input tube) appears fairly normal. The plate reads a little low at 74v, with the plate resistor drawing .66mA. the Cathode reads .74V, and the Cathode resistor seems to be pulling .47mA.
The coulping cap (C103) between V1 and V2 is passing -23V to the plate of V2, which seems odd to me. OR, that is to say, I get a reading of -23V at the plate of V2.
It looks like V2 is drawing too much current. I am getting a reading of 170V at the plate, with the plate resistor drawing 1.2mA.
(This, of course, means that the grid of V3 (cathode follower) is seeing roughly the same voltage as the plate of V2, or ~ 170V.)
The cathode resistor at V2 is only drawing .27mA of current. Cathode reads .58 ohm.
The cathode of V3 reads 185V, and the cathode resistor looks like it is drawing 12.3mA.
I've gone over the circuit a dozen times and can see no mistakes in my wiring. Measuring with a DMM I get the correct values from point to point.
The power supply is CRCRC giving me 280v B+, (checked it in PSUD2 and I see no strange behavior here.)
The filaments are 11.9V DC filtered with a CRCRC filter (again, checked with PSUD2) elevated 30V. All my filament wiring was done as though it would be AC heaters. Tightly twisted pairs running along the corners of the enclosure and enter at right angles to the sockets. They pass all other components at right angles.
I'm sure I've done something stupid, but I just can't find it!
Any clues?