03-02-2018, 08:04 AM
(This post was last modified: 03-02-2018, 08:07 AM by Diabolical Artificer.)
I tried to track down any parasitic oscillation by using my distortion meter's notch filter to filter out the AF, then scoped the OP from the meter, up to 80mhz, nada. So I used my frequency counter that can go up to 2gig, again nothing, though para osc could still be there. However I was convinced that the problem lay, if only in part, with the fact that the CF's are de-coupled by caps and that the grid stopper's are more of a hindrance than help.
I've been reading up on this, hope I have this right. As an PP OP stage is over driven, there is "diode action" between the OP valves grid and cathode, so voltage is dropped across the coupling cap, making the bias go more negative. As we are essentially over driving the 2C34, this could be the case. Voltage would be dropped across the grid stopper too. See - http://sound.whsites.net/valves/analysis.html ....part 9. He puts it better than I can.
The 2C34 needs 500mW to drive them into pos grid current, I boo boo'd by using a potential divider to reduce the neg power supply which the CF's use, using a 10k fixed R and 5k pot, this meant the cathodes were starved of current. Another contributing factor. The neg voltage supply is from a separate tfmr which is 140v DC when rectified and smoothed, so I used a 82v zener and 2k4 resistor (20mA for the CF's and 5mA for the zener) to drop this to 80v ish.
Sussing out what happens when an OP valve goes into grid current is also relavent when driving "normal" AF valves in AB1 Chris, as they go into or near grid current before clipping. See Keibert - System Design Facrors for AF Amplifiers below. Bit hard to follow in places but.
Well I had some success yesterday, with about 9v RMS OP, no fall in Ik. This is after spending a day or so trying to get a CF with CCS working. I tried to bias the CF using a voltage divider, which is nigh impossible as of coarse the cathode follows the grid, in this case by about 13v.
It then occured to me trying to draw a loadline as per usual is a waste of time. I'm sure the grid lines are nearly horizontal too and have a feeling that if this is the case, then the load line will be near vertical rather than slanting ; a bit like a class A PP SE stage.
So I used fixed bias to bias the CF's and since I was doing this I though I may as well remove the coupling caps and direct couple the CF's as per Tricomp's design. I've been reading more on this approach from vintage sources and think it's the only way to do it.
Got the cathode's sitting at -15v, signal in, scoped the OP of the CF's.... horrible! Looks like someone had punched the bottom peak on the nose and the peak was shoved inside, rectification? Also low amplitude. Gave it some more current on the CCS, sine slowly "re-forming", amplitude up. to get the OP of the CF right I found I had to set the CCS to 180mA! Not sure this is right but the MJE340 is within SOA as ce is around 60v, so something like 11w.
Still not 100% sure what the craic is with the CCS "driven" (should that be sourced, more like sunked) CF, but will investigate further today if I get time. When I get it working I'm going to put a coupling cap back on and grid stopper, I want to get to the bottom of this AB2 lark.
Andy
I've been reading up on this, hope I have this right. As an PP OP stage is over driven, there is "diode action" between the OP valves grid and cathode, so voltage is dropped across the coupling cap, making the bias go more negative. As we are essentially over driving the 2C34, this could be the case. Voltage would be dropped across the grid stopper too. See - http://sound.whsites.net/valves/analysis.html ....part 9. He puts it better than I can.
The 2C34 needs 500mW to drive them into pos grid current, I boo boo'd by using a potential divider to reduce the neg power supply which the CF's use, using a 10k fixed R and 5k pot, this meant the cathodes were starved of current. Another contributing factor. The neg voltage supply is from a separate tfmr which is 140v DC when rectified and smoothed, so I used a 82v zener and 2k4 resistor (20mA for the CF's and 5mA for the zener) to drop this to 80v ish.
Sussing out what happens when an OP valve goes into grid current is also relavent when driving "normal" AF valves in AB1 Chris, as they go into or near grid current before clipping. See Keibert - System Design Facrors for AF Amplifiers below. Bit hard to follow in places but.
Well I had some success yesterday, with about 9v RMS OP, no fall in Ik. This is after spending a day or so trying to get a CF with CCS working. I tried to bias the CF using a voltage divider, which is nigh impossible as of coarse the cathode follows the grid, in this case by about 13v.
It then occured to me trying to draw a loadline as per usual is a waste of time. I'm sure the grid lines are nearly horizontal too and have a feeling that if this is the case, then the load line will be near vertical rather than slanting ; a bit like a class A PP SE stage.
So I used fixed bias to bias the CF's and since I was doing this I though I may as well remove the coupling caps and direct couple the CF's as per Tricomp's design. I've been reading more on this approach from vintage sources and think it's the only way to do it.
Got the cathode's sitting at -15v, signal in, scoped the OP of the CF's.... horrible! Looks like someone had punched the bottom peak on the nose and the peak was shoved inside, rectification? Also low amplitude. Gave it some more current on the CCS, sine slowly "re-forming", amplitude up. to get the OP of the CF right I found I had to set the CCS to 180mA! Not sure this is right but the MJE340 is within SOA as ce is around 60v, so something like 11w.
Still not 100% sure what the craic is with the CCS "driven" (should that be sourced, more like sunked) CF, but will investigate further today if I get time. When I get it working I'm going to put a coupling cap back on and grid stopper, I want to get to the bottom of this AB2 lark.
Andy
