I have build myself a preamp with 12B4A tubes. About 80v-ish on the plate. Dropping voltage from around 250v over a 11KOhm resistance. Pin 2 and 7(grids) have each a 2,2kOhm connected to the 100K volume pot. A 470 Ohm resistor on the cathode, no capacitor in paralell.
Sometimes the volume starts to increase, then goes "down" again?, witout touching the volumeknob. Can somebody tell me what is wrong. The preamp sounds great if I compare the preamp to other preamps I have12AU7 srpp, 6j5 single stage, but after about 15 hours use the preamp starts to behave this strange.
Sometimes the volume starts to increase, then goes "down" again?, witout touching the volumeknob. Can somebody tell me what is wrong. The preamp sounds great if I compare the preamp to other preamps I have12AU7 srpp, 6j5 single stage, but after about 15 hours use the preamp starts to behave this strange.
I would get yourself setup so you can see if the DC voltages move around when this happens.
Do both channels do this?
My suspicion would be that you possibly have a weak connection in the heater circuit, and thermal expansion and cycling may temporarily restore a connection that isn't 100%.
Do both channels do this?
My suspicion would be that you possibly have a weak connection in the heater circuit, and thermal expansion and cycling may temporarily restore a connection that isn't 100%.
Thanks!, I will check out the heater curcuit today. I also will disconnect the volume pot and check out if there is anything wrong there.
All the heather connections are ok, and I replaced the volume pot to an Alps blue 250K Log, and I think the problem is solved? The preamp sings like an angel! Do someone know which 12B4A who are the best?(no microphony, long "lifetime"). Today I use a pair of RCA, and there are no microphony with these tubes, and I feel they sounds great.
Most of them has more or less microphony, so vibration reduction (tube socket vibration damping, O rings etc.) is primary.
I only use Sylvania 12B4a tubes.
I only use Sylvania 12B4a tubes.
A slew of 12B4 designs can be found in the archives. I'm providing something I worked up to use what was on somebody's "pile". You definitely don't have to use the pseudo choke I/P filter PSU setup shown. Well filtered or, perhaps, regulated "juice" at roughly 150 V. for the B+ will certainly do. Also, something less pricey as the O/P coupling cap. is OK. The fellow I worked that design up for has a "thing" for "boutique" parts.
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That depends on how difficult the downstream load is. If it's a tubed power amp whose I/P impedance is >= 100 Kohms, a simple 10M45S is plenty good. OTOH, if driving the IHF "standard" of 10 Kohms is contemplated, some setup with cascoded FETs or BJTs is in order. Also, it may be necessary to capacitor bypass all or at least part of the cathode bias resistance.
A side question, without CCS, how the value of plate resistor affect the sound? Here the OP use 11K, I have seen as low as 4.7K, mostly 6.8K or 8K.
The 12B4 datasheet shows RP as 1030 Ω. The net load the triode experiences should be 3X RP or greater, for linearity's sake.