zigzagflux said:
This is the picture of the underside of the tube amp.
I have replaced the white color audiospace cap to Mundorf Supreme. Immediate wow factor, espacially on C5. It seems C5 is the preamp signal.
I will try to put grid stopper on V1 and V3a,b at 1k/1W Riken.
The amp is rated at 220Vac. But in Malaysia, the voltage is 240Vac. So I bought a step down to 220Vac.
But when I measure the filament, it is 6.0V ac. The incoming supply is 224Vac. At the right side, can see the transformer pinout wiring. Can I get a typical wiring of the transformer. There seem to have some tapping but not sure. I measure the pin beside the blue and brown wire and it gives 235V and 245V ?? What should be the correct way of measurement ? Looking at the transformer pinout, there are 4 pair of tapping (blue and brown is on 1 pair)
I think I should connect to the tapping with '230V' to give the additional 5% to get 6.3V filament voltage right.
Ccschua,
I come rather late onto this, but if I may a few comments:
Regarding R5 and little C in series; together with R8C3: We must be sure about this. If R5+little C is across R4, you then have a dual phase compensating network there, purpose as explained earlier. Is that so? Two does not make sense, that's why I ask.
Then, feedback resistor Rf1, Rf2: You do not give values, so it is difficult to calculate NFB. From the previous posts, you will perhaps realise that the value of R5 (if we forget about R8C3 for now), is required to take the loop gain of the amplifier (that is gain before NFB, divided by gain after NFB) to less than one so as to avoid inter-loop oscillations. With a loop gain of 20 (quite usual), R8 would be 10% of R4 or 10K. But the optimal value would be different for other feedback factors. If you can give a value for Rf1, I could calculate and explain further.
(I seem to see in your photo that both R5 + C and R8 + C exists; not quite clear. Perhaps you can retrace and correct this - as said, it does not make sense to have both.)
Then, I see you give a voltage for G1 of V3B. One cannot really measure that with an ordinary voltmeter as the resistance of the meter in series with a high R11 will lower the value. That might be why you get a lower voltage there than anode of V1A. Actually they are the same, as no current flows through R11. The easiest way to check whether matters are in order is to check that the anodes of V3A and V3B are about equal. (At this point I must point out that the values of R13 and R14 are reversed in your diagram; R14 is usually the larger because the gain of V3B is slightly lower than V3A in this sort of circuit.)
I do not quite get the voltages you state when calculating. As said by others I would go to a slightly higher voltage on anode(V1), say about 80 - 90V. You can do that by decreasing the 220K between B2 and B3 to about 100K. (This is a guestimate; I can calculate a better value later - it is now 01:30 in the morning here and bed-time!).
With respect, I cannot see much sense in placing stopper resistors also in series with 'other grids', as these already have large other impedances in the circuit. 1K there would not achieve anything. I notice from your photo that R17, R18 is 1K; you could rather make them 2,7K - 3,3K. Also with regard to Ca - again with respect to others: Yes, omitting it gives some local feedback. But it also increases the rp (internal impedance) of the ECC83 to still higher than it already is, and with a fair Miller capacitance from V3 can cut into the audio band high up. The effect of this in connection with R5 + little C etc. needs to be examined together with the stability of the whole circuit for best effect.
Lastly, you made the puzzling remark that replacing C5 by an exotic made a large difference. Ccschua, this is only possible when the other C5 was faulty. I can assure you that any polyester/polycarbonate capacitor of the necessary rating (at least 450V) will be perfect, as there is no audio signal across that capacitor. As said above, check the V3 anode voltages for approximate equality (the two triodes are not necessarily exactly the same; there is manufacturing tolerance) to assure that C5 does not leak. If you can give the requested feedback resistor values, I can check for you in the morning.
The purpose is not to confuse you; no previous advice was really WRONG; I simply tried to explain matters.
Good Luck!
I come rather late onto this, but if I may a few comments:
Regarding R5 and little C in series; together with R8C3: We must be sure about this. If R5+little C is across R4, you then have a dual phase compensating network there, purpose as explained earlier. Is that so? Two does not make sense, that's why I ask.
Then, feedback resistor Rf1, Rf2: You do not give values, so it is difficult to calculate NFB. From the previous posts, you will perhaps realise that the value of R5 (if we forget about R8C3 for now), is required to take the loop gain of the amplifier (that is gain before NFB, divided by gain after NFB) to less than one so as to avoid inter-loop oscillations. With a loop gain of 20 (quite usual), R8 would be 10% of R4 or 10K. But the optimal value would be different for other feedback factors. If you can give a value for Rf1, I could calculate and explain further.
(I seem to see in your photo that both R5 + C and R8 + C exists; not quite clear. Perhaps you can retrace and correct this - as said, it does not make sense to have both.)
Then, I see you give a voltage for G1 of V3B. One cannot really measure that with an ordinary voltmeter as the resistance of the meter in series with a high R11 will lower the value. That might be why you get a lower voltage there than anode of V1A. Actually they are the same, as no current flows through R11. The easiest way to check whether matters are in order is to check that the anodes of V3A and V3B are about equal. (At this point I must point out that the values of R13 and R14 are reversed in your diagram; R14 is usually the larger because the gain of V3B is slightly lower than V3A in this sort of circuit.)
I do not quite get the voltages you state when calculating. As said by others I would go to a slightly higher voltage on anode(V1), say about 80 - 90V. You can do that by decreasing the 220K between B2 and B3 to about 100K. (This is a guestimate; I can calculate a better value later - it is now 01:30 in the morning here and bed-time!).
With respect, I cannot see much sense in placing stopper resistors also in series with 'other grids', as these already have large other impedances in the circuit. 1K there would not achieve anything. I notice from your photo that R17, R18 is 1K; you could rather make them 2,7K - 3,3K. Also with regard to Ca - again with respect to others: Yes, omitting it gives some local feedback. But it also increases the rp (internal impedance) of the ECC83 to still higher than it already is, and with a fair Miller capacitance from V3 can cut into the audio band high up. The effect of this in connection with R5 + little C etc. needs to be examined together with the stability of the whole circuit for best effect.
Lastly, you made the puzzling remark that replacing C5 by an exotic made a large difference. Ccschua, this is only possible when the other C5 was faulty. I can assure you that any polyester/polycarbonate capacitor of the necessary rating (at least 450V) will be perfect, as there is no audio signal across that capacitor. As said above, check the V3 anode voltages for approximate equality (the two triodes are not necessarily exactly the same; there is manufacturing tolerance) to assure that C5 does not leak. If you can give the requested feedback resistor values, I can check for you in the morning.
The purpose is not to confuse you; no previous advice was really WRONG; I simply tried to explain matters.
Good Luck!
Hi Johan,
I post the updated circuit diagram so that it is easy for discussion.
1. U are right about R13,R14 and so I put that in Blue.
2. Ca and R8 and C3 Are most likely not there. Again I have to say I cant trace the capacitors. The only last component I cant trace is the unit betwee the 2 6v6GT and the 0.1uF capacitor (white smaller cap). This electrolytic cap is right beside the yellow cable (the top mid side) if u refer to the photo. I suspect this electrolytic is connected to the Cathode of 6v6GT. I may be wrong.
3. Rf1 set for low negative feedback (NFB) and Rf2 is high NFB. Rf1 is 680R and Rf2 is 150K. there is an empty pad beside this resistors (perhaps for cap feedback). I may be wrong with the order but the value is correct. To be frank, I would rather throw away this high NFB thingie. it makes the sound so dull.
4. I will try replace C5 with other cap or just some normal cap and see the effects. I was sure to get better sound due to the fact I change the cap step by step, first with the 6v6gt grid coupling cap then by C5.
5. Filament supply has a diode which I am not sure what it is for. I have to open up and measure the tube filament supply using AC to confirm. AC filament is better than DC filament right ?
6. What is the recommended CCS connection point ?
A little bit of side track, (this is not supposed here) I put this little amp to a Dynaudio Focus 140 which has sensitivity of 86dB with recommended 200W into 4ohm. What I hear is mid and high is ok but the low is muddled. The step atenuator is almost 80% to drive this speaker. Looks like I need the high NFB to drive it or a bigger input impedance say 100k step att? phewwww.
I post the updated circuit diagram so that it is easy for discussion.
1. U are right about R13,R14 and so I put that in Blue.
2. Ca and R8 and C3 Are most likely not there. Again I have to say I cant trace the capacitors. The only last component I cant trace is the unit betwee the 2 6v6GT and the 0.1uF capacitor (white smaller cap). This electrolytic cap is right beside the yellow cable (the top mid side) if u refer to the photo. I suspect this electrolytic is connected to the Cathode of 6v6GT. I may be wrong.
3. Rf1 set for low negative feedback (NFB) and Rf2 is high NFB. Rf1 is 680R and Rf2 is 150K. there is an empty pad beside this resistors (perhaps for cap feedback). I may be wrong with the order but the value is correct. To be frank, I would rather throw away this high NFB thingie. it makes the sound so dull.
4. I will try replace C5 with other cap or just some normal cap and see the effects. I was sure to get better sound due to the fact I change the cap step by step, first with the 6v6gt grid coupling cap then by C5.
5. Filament supply has a diode which I am not sure what it is for. I have to open up and measure the tube filament supply using AC to confirm. AC filament is better than DC filament right ?
6. What is the recommended CCS connection point ?
A little bit of side track, (this is not supposed here) I put this little amp to a Dynaudio Focus 140 which has sensitivity of 86dB with recommended 200W into 4ohm. What I hear is mid and high is ok but the low is muddled. The step atenuator is almost 80% to drive this speaker. Looks like I need the high NFB to drive it or a bigger input impedance say 100k step att? phewwww.
It can go as high as the B+ you have available, 300V. Problem is, since you are DC coupled to the next stage, you upset the idle conditions of that next stage. That's where you want to run this in your simulator, and optimize each stage as best you can. The general idea is to run the triode in a linear region (which requires consulting the curves), ensuring you have proper grid voltage to handle the input signal, and verifying your B+ and plate load allows for the calculated swing you expect/want.ccschua said:
Your triode will begin to pull grid current when you approach 0V, depends on the valve. Your source will likely not be able to provide this current, and will therefore distort. Distortion in, distortion out. You want to steer clear of 0V grid voltage with your input signal applied. This would imply something in the range of 0.4/1.414 = 280mV rms.
Yes it is. The tube never enters cutoff, where it would stop conducting current.
http://en.wikipedia.org/wiki/Class_A_amplifier#Class_A
Ccschua,
Time difference between us create problems. I planned to later tonight (i.e. local time) quickly hook up your circuit (I have a protoboard suitable for that) and if I succeed will post here, perhaps sometime in your evening. I myself have never used simulation for tubes because of uncertainty with certain tube types. For me easier to work off spec sheet graphs and experience with tubes.
Provisionally I would look for about 90 - 100V on first 12AX7 anode and see how that falls in the operating area. The 12AX7 is not ideal for this application because of the relatively high anode saturation voltage and other pitfalls, but you have it now. Perhaps others come up with an ideal solution in the meantime.
Time difference between us create problems. I planned to later tonight (i.e. local time) quickly hook up your circuit (I have a protoboard suitable for that) and if I succeed will post here, perhaps sometime in your evening. I myself have never used simulation for tubes because of uncertainty with certain tube types. For me easier to work off spec sheet graphs and experience with tubes.
Provisionally I would look for about 90 - 100V on first 12AX7 anode and see how that falls in the operating area. The 12AX7 is not ideal for this application because of the relatively high anode saturation voltage and other pitfalls, but you have it now. Perhaps others come up with an ideal solution in the meantime.
ccschua said:
You indicate a measured cathode voltage of -21 on the 6V6GT tubes. So, you can have an input signal swing of 42v peak-peak before the output starts clipping. Another way of saying it is a 21 V signal, depending on how you like to say it. That assumes that the cathode stays at -21 volts, but it won't because the resistors are not bypassed with a capacitor.
As the input voltage increases, the current in the tube increases, which flows through the cathode resistors. The voltage increases across the cathode resistors and the grid bias point changes. I believe the technical term is cathode degeneration. Using a bypass capacitor which is large enough to be below the audio frequencies will keep the cathode at a stable voltage.
You could backtrack through the design, assuming a 21 v signal going into the 6V6 tubes and figure out the gain of each stage before it. That would yield a calculated maximum input signal strength before driving the output into distortion.
Your .4v 12ax7 bias sounds way too small to me. Either it is a bad measurement or a design problem. An input signal greater than .4v would send the first tube into distortion.
OK,
I have hooked up a experimental model, and must say that I did not find too large a change in values necessary. The main thing I reckoned without yesterday was that at B3, two x 12AX7 triodes are of course drawing current! (The other channel also.)
My only change was R12 to 18K. I give the voltages I then got, but you must allow some leeway; 12AX7s can have quite a tolerance
. Thus:
V1A anode = 85V
V1A cathode = 0,443V
V3 cathodes = 90V
V3A anode = 208V
V3B anode = 202V
VB3 = 125V
VB2 (from ext. power supply) = 298V
Gain (input - R15, R16) = 217 (with Ca)
Bandwidth (input to R15, R16): 5Hz - 46kHz
Max. output before clipping (over R15, R16) = 122Vp-p
Then, I checked up to R15, R16 - no 6V6s. As I am not familiar with your output transformer, it is senseless to check stability. There you will have to fend for yourself, judging with a square wave response. Thus, I also did not go into values for R5 and C2. I would hope that you do not require R5 <12K about; lower and the taper-off at high freuqency will start eating into the audio band. One can of course also use a small capacitor in parallel with Rf1. Oh, I also used Ca = 100µF. Without that, unless you use very little NFB, the gain might be too low. (I will not now go into fallacious arguments re 'bad' effects of NFB!)
Depending on the OPT, for 20dB of NFB you will need Rf = 10K approximately, and pro rata higher for less NFB. That will give an input sensitivity of about 600mVrms or lower. (I do not know about the value of Rf2 = 680 ohm in the diagram; that will be entirely too high for any stability. An error perhaps?)
I hope these few figures will steer you in the right direction. Good luck!
I have hooked up a experimental model, and must say that I did not find too large a change in values necessary. The main thing I reckoned without yesterday was that at B3, two x 12AX7 triodes are of course drawing current! (The other channel also.)
My only change was R12 to 18K. I give the voltages I then got, but you must allow some leeway; 12AX7s can have quite a tolerance
. Thus:V1A anode = 85V
V1A cathode = 0,443V
V3 cathodes = 90V
V3A anode = 208V
V3B anode = 202V
VB3 = 125V
VB2 (from ext. power supply) = 298V
Gain (input - R15, R16) = 217 (with Ca)
Bandwidth (input to R15, R16): 5Hz - 46kHz
Max. output before clipping (over R15, R16) = 122Vp-p
Then, I checked up to R15, R16 - no 6V6s. As I am not familiar with your output transformer, it is senseless to check stability. There you will have to fend for yourself, judging with a square wave response. Thus, I also did not go into values for R5 and C2. I would hope that you do not require R5 <12K about; lower and the taper-off at high freuqency will start eating into the audio band. One can of course also use a small capacitor in parallel with Rf1. Oh, I also used Ca = 100µF. Without that, unless you use very little NFB, the gain might be too low. (I will not now go into fallacious arguments re 'bad' effects of NFB!)
Depending on the OPT, for 20dB of NFB you will need Rf = 10K approximately, and pro rata higher for less NFB. That will give an input sensitivity of about 600mVrms or lower. (I do not know about the value of Rf2 = 680 ohm in the diagram; that will be entirely too high for any stability. An error perhaps?)
I hope these few figures will steer you in the right direction. Good luck!
I was already typing my previous post when Flysig posted. Good post there.
But ....
The measurement roughly corresponds with my own. One must remember that the 12AX7 is a very sharp cut-off triode. Also then, that is the maximum allowable peak input without NFB. If you take my measured gain of 217 up to the 6V6 input grid, the required input will only be 87mVp before the 6V6s are already driven into overload, never mind previous stages. With NFB the cathode naturally follows the input signal.
I just went back and measured; actually the gain without Ca (causing a little local feedback) does not fall by much; only 13%. But the quoted bandwidth then goes down to 33kHz because of the increase in 12AX7 plate impedance. That might affect NFB stability, depending.
.
But ....
flysig said:
The measurement roughly corresponds with my own. One must remember that the 12AX7 is a very sharp cut-off triode. Also then, that is the maximum allowable peak input without NFB. If you take my measured gain of 217 up to the 6V6 input grid, the required input will only be 87mVp before the 6V6s are already driven into overload, never mind previous stages. With NFB the cathode naturally follows the input signal.
I just went back and measured; actually the gain without Ca (causing a little local feedback) does not fall by much; only 13%. But the quoted bandwidth then goes down to 33kHz because of the increase in 12AX7 plate impedance. That might affect NFB stability, depending.
.
flysig said:
Sorry to be 'talkative', but I missed that point by Flysig. That is true, and the reason why fixed bias will give a higher output.
But one can create a quasi-fixed-bias condition by bypassing the 6V6 cathodes to common with a very high capacitor. Since the mark-space ratio of music is normally low, for much of the music such a cap will tend to keep the cathodes at constant voltage. (This is called 'constant bias under musical conditions'.) By high I mean 6800µf or such; with the small sizes electrolytic caps have shrunk to, this is not a bulky component any more. It will not work with maintained low notes, but one is better off with it than without it.
Johan Potgieter said:
That is amazing. I guess your prototyping works like JIT. I have copied your infor and take it with me like a bible, just in case the information gone missing.
So all this while I have been listening to distorted sound without realising. Just before u did this circuit, I have put in 1K1 Riken resistor at the grid of 12AX7. The sound really improve, I guess brough about by reduced current draw ? (I dont have any 3K3 to 10K resistor at the moment)
Since it is a marginal change to bring up the voltage at V1a and V3a, I guess the resistors are able to handle the power dissipation.
I dont have to touch the 220K.My only change was R12 to 18K. I give the voltages I then got, but you must allow some leeway; 12AX7s can have quite a tolerance
That looks like a big sausage 18k/2W resistor. I will give it a either Caddock or Kiwami. It looks like the 12AX7 is operated really close to the most left of the I/Va curve.
Will check after hooking up the 18k/2W.
It looks like V1a cathode is still very low at 0.443V. Under very loud listening using the KEF cresta 10 speakers in a medium room, the voltage at V1a grid is 0.42V rms depending on listening environment(Normally CDP will output 6V PP divided by a factor of 10 which is 0.6V PP. this works out to 0.42V rms) So I am operating real close to grid current flowing. But isnt that AB1 amp design has some grid current flowing ?
Probably I will just assume the stability is there. Ca should be there. I saw a 100uF/100V right at the C5 and R1910R area. I hope that cap is more for Ca than the 6v6gt cathode.
Should I just remove R5 and try.
My feedback circuit uses 150k so that is about 20/15=1.3 dB FB. I will pull out Rf2 and measure to measure its resistance.
How about adding a cap like silver mica 100pF across Rf1 to improve the high frequency. how will adding the cap affect the Rf1 value?
Johan Potgieter said:
Me too just cant keep the mouth shut. At this point, I dont think I can furnish any infor regarding the OPT capacity which is important for the 'stability' concern.
I am a little confused over CCS. What I know CCS can be sinking or sourcing and normally sinking is quite widely used in solid stage. In tube, wouldn't that the CCS sinking at 6v6 cathode a choice? Before the situation gets 'complicated' with CCs, can I just drop in a 100uF/110V electrolytics at the 6v6 cathode.
What I found between high and low negative feedback is that it affects the sound characters.
When high NFB, the sound stage is bigger, bass response tighther and more dynamics. When Low NFB, the stage is narrower, mid more focus and warm, especially the guitar acoustics and better resolution.
For most listening, I always use Low NFB, until I play the track of PEPE Romero (flamenco). I have to switch to high NFB in order to 'enjoy' the feet thumping, knocking effects. Only high NFB can cope with the dynamics and sound stage of the music.
What I notice about this amp operating point is serious lacking in bass response. It is not giving the thunderous, deep low and vivacious feel of the flamenco.
When high NFB, the sound stage is bigger, bass response tighther and more dynamics. When Low NFB, the stage is narrower, mid more focus and warm, especially the guitar acoustics and better resolution.
For most listening, I always use Low NFB, until I play the track of PEPE Romero (flamenco). I have to switch to high NFB in order to 'enjoy' the feet thumping, knocking effects. Only high NFB can cope with the dynamics and sound stage of the music.
What I notice about this amp operating point is serious lacking in bass response. It is not giving the thunderous, deep low and vivacious feel of the flamenco.
With Rf1 = 150K, you will have about 5dB of nfb. I would guess that Rf2 is perhaps 68K and not 680 ohm showed in the sketch. With that there will be about 10dB of nfb (all these with Ca in ). That should make some difference in audible distortion .
I do not understand why you experience serious lack of bass response - certainly not as it stands. I am not sure if you can get hold of an oscilloscope and signal generator; that should provide evidence of where this takes place.
A CCS in place of R12 will render balance of that stage independent of tube age. I have never considered it personally because of the extra complexety, even if slight. For me the resistor suffices.
A CCS in the 6V6 cathodes (seperately) will take care of those tubes' aging and balancing, but must then be bypassed by a suitable high value capacitor(s). Series (degenerative) feedback there is undesirable with UL.
The nature of the input attenuator does not really matter - within limits. Firstly too high, and capacitances there (especially the Miller-C of the input tube) will take their toll. Too low and obviously the preceeding control unit may not be able to drive the amp. Another possible factor could be the non-tracking of many ganged pots, especially log types, to affect balance.
A 100µf capacitor or lower for 6V6 cathode bypassing, will not make much of an audible difference. But if you cannot have a large cap as discussed, it will be better than nothing.
About the 12AX7 operating point. I repeat that it is a very sharp cut-off triode; in operation such as this, about Vc = 2V will cut it off. I have checked with an oscilloscope and also repeat that the 6V6 grid input will be overloaded before the 12AX7 runs out of 'grid-room', all else being equal. Bias of about 0,5V for 12AX7 is nothing new.
I do not understand why you experience serious lack of bass response - certainly not as it stands. I am not sure if you can get hold of an oscilloscope and signal generator; that should provide evidence of where this takes place.
A CCS in place of R12 will render balance of that stage independent of tube age. I have never considered it personally because of the extra complexety, even if slight. For me the resistor suffices.
A CCS in the 6V6 cathodes (seperately) will take care of those tubes' aging and balancing, but must then be bypassed by a suitable high value capacitor(s). Series (degenerative) feedback there is undesirable with UL.
The nature of the input attenuator does not really matter - within limits. Firstly too high, and capacitances there (especially the Miller-C of the input tube) will take their toll. Too low and obviously the preceeding control unit may not be able to drive the amp. Another possible factor could be the non-tracking of many ganged pots, especially log types, to affect balance.
A 100µf capacitor or lower for 6V6 cathode bypassing, will not make much of an audible difference. But if you cannot have a large cap as discussed, it will be better than nothing.
About the 12AX7 operating point. I repeat that it is a very sharp cut-off triode; in operation such as this, about Vc = 2V will cut it off. I have checked with an oscilloscope and also repeat that the 6V6 grid input will be overloaded before the 12AX7 runs out of 'grid-room', all else being equal. Bias of about 0,5V for 12AX7 is nothing new.
Johan Potgieter said:OK,
I have hooked up a experimental model, and must say that I did not find too large a change in values necessary. The main thing I reckoned without yesterday was that at B3, two x 12AX7 triodes are of course drawing current! (The other channel also.)
My only change was R12 to 18K. I give the voltages I then got, but you must allow some leeway; 12AX7s can have quite a tolerance
V1A anode = 85V
V1A cathode = 0,443V
V3 cathodes = 90V
V3A anode = 208V
V3B anode = 202V
VB3 = 125V
VB2 (from ext. power supply) = 298V
Hi Johan,
I changed R12 (originally 22k) to Kiwami 18k. After that, I measure at DC condition the voltage at V3 cathod remain the same as before at 73V. Pls advise how u get V3 cathode at 90V.
Mmmmm...
This is the poblem with a tube drawing current through a large anode resistor In this case including the 'dc' dropping resistor of 220K. It needs a variation in tube and resistors of only 6% to account for the voltage difference we have between us.
I have dismantled the test rig for this circuit but could reassemble in the morning. One might want to change the series 220K (B2 - B3) to 180K, or R2 from 330 ohm to 390/470 ohm, but the problem will remain at a future change of 12AX7s. I did not want to change the circuit too much (my own version would be slightly different). If you can get to this before I do (time zone difference) and you have spare resistors, you could try the above suggestions yourself. Also keep measuring voltage over R12 as you did; measuring directly on V1A anode could change the voltage there because of meter impedance.
As the present Governer of California said: I will be back.
Regards
This is the poblem with a tube drawing current through a large anode resistor In this case including the 'dc' dropping resistor of 220K. It needs a variation in tube and resistors of only 6% to account for the voltage difference we have between us.
I have dismantled the test rig for this circuit but could reassemble in the morning. One might want to change the series 220K (B2 - B3) to 180K, or R2 from 330 ohm to 390/470 ohm, but the problem will remain at a future change of 12AX7s. I did not want to change the circuit too much (my own version would be slightly different). If you can get to this before I do (time zone difference) and you have spare resistors, you could try the above suggestions yourself. Also keep measuring voltage over R12 as you did; measuring directly on V1A anode could change the voltage there because of meter impedance.
As the present Governer of California said: I will be back.
Regards
Sorry for the late reply that cause you to remove the test circuit.
I will change 220k dc drop down to 180K.
Another thing I notice is the filament voltage varies between day and night. I get 6.35V at night and 6.5V at day. wonder if this affects the 'glow' of tube or the biasing.
Also I am very confused by the feedback resistor. I am pulling my hair off becos of that switch from High to Low NFB. The feedback point starts from the 8ohm to the R3. Between that, I can see 2 resistors in the path. one is 150k and the other 120k. the trouble is when I probe between 8 ohm and R3, I get 680R no matter of the switch position. It leads me to think there is some smd comonent below the pcb.
I will change 220k dc drop down to 180K.
Another thing I notice is the filament voltage varies between day and night. I get 6.35V at night and 6.5V at day. wonder if this affects the 'glow' of tube or the biasing.
Also I am very confused by the feedback resistor. I am pulling my hair off becos of that switch from High to Low NFB. The feedback point starts from the 8ohm to the R3. Between that, I can see 2 resistors in the path. one is 150k and the other 120k. the trouble is when I probe between 8 ohm and R3, I get 680R no matter of the switch position. It leads me to think there is some smd comonent below the pcb.
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