Hi, I have a problem ... but I don't know if it's a problem.on my prephono based on Audioresearch SP3/6...clone.
V1 = ecc83, V2 = ecc83, V3 = 6n1p.
When I measure on the V3 grid and on the V3 cathode to see the voltages and calculate how negative the grid is ... I notice that the voltage is not stable ... it moves ... it goes up and down ... it goes down and it goes up ... it goes up and down .. it is so unstable that I can't figure out how many -volt they are.
I find the unstable voltage on the grid and on the cathode of V3 and on the anode of V2.
The range of motion is about 1.5v on about 135v (Vk of CF).
Who makes who move? Whose responsibility is it?
Obviously the stages are continuously coupled. It's normal?
It seems to me that in the past, doing these measures, I had not established.
To be honest I do not remember well if in the past, when I measured, it moved .. I think not .. but I'm not sure.
I point out that the power supply voltage is perfectly still as I also have a mains stabilizer (voltage in the house 230v + -1%).
I tried to change both the V2 with other ecc83..che put in V3 another 6n1p, then a 6n23p then an e88cc ... it always does the same .. the voltage moves.
Can you give me information about it?
Thanks
V1 = ecc83, V2 = ecc83, V3 = 6n1p.
When I measure on the V3 grid and on the V3 cathode to see the voltages and calculate how negative the grid is ... I notice that the voltage is not stable ... it moves ... it goes up and down ... it goes down and it goes up ... it goes up and down .. it is so unstable that I can't figure out how many -volt they are.
I find the unstable voltage on the grid and on the cathode of V3 and on the anode of V2.
The range of motion is about 1.5v on about 135v (Vk of CF).
Who makes who move? Whose responsibility is it?
Obviously the stages are continuously coupled. It's normal?
It seems to me that in the past, doing these measures, I had not established.
To be honest I do not remember well if in the past, when I measured, it moved .. I think not .. but I'm not sure.
I point out that the power supply voltage is perfectly still as I also have a mains stabilizer (voltage in the house 230v + -1%).
I tried to change both the V2 with other ecc83..che put in V3 another 6n1p, then a 6n23p then an e88cc ... it always does the same .. the voltage moves.
Can you give me information about it?
Thanks
Your circuit has a low frequency oscillation, an instability that occurs due to phase shift in the
power supply and/or audio stages. The audio stage coupling capacitors may be too large,
or the power supply decoupling capacitors may be too small, or both.
This was called motorboating in the early days of audio.
power supply and/or audio stages. The audio stage coupling capacitors may be too large,
or the power supply decoupling capacitors may be too small, or both.
This was called motorboating in the early days of audio.
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Power problems are impossible as I have a solid power supply which has always gone well. C (335uF)-Gyrator(20H)-C (335uF) ... followed by 4 capacitor multipliers 6db (tip50 + R (colector-base) 22k + 100uf on the base and 100uf on the output of the multiplier, all the 100uF bypassed with 1uf Mkp).
One multiplier for LEFTv1+v2, one multiplier for RIGHv1+v2, one multiplier for CF-LEFT, one multiplier for CF-Righ.
The phono sounds good and is super quiet. No abnormal noise.
The cones of the woofers are immobile .. even with the volume at maximum, I don't think it's powerboating. But I don't know! I am not an expert.
With a spectrum analyzer (FFT) I have no problems. .... but the voltage moves.
So do you think it could be the 0.15uF Mkp (Miflex) cap between stage V1 and V2? It is the only possible cap.....
Could it be that it only moves when I touch the test leads? Only to measure I have to touch!
Thanks
One multiplier for LEFTv1+v2, one multiplier for RIGHv1+v2, one multiplier for CF-LEFT, one multiplier for CF-Righ.
The phono sounds good and is super quiet. No abnormal noise.
The cones of the woofers are immobile .. even with the volume at maximum, I don't think it's powerboating. But I don't know! I am not an expert.
With a spectrum analyzer (FFT) I have no problems. .... but the voltage moves.
So do you think it could be the 0.15uF Mkp (Miflex) cap between stage V1 and V2? It is the only possible cap.....
Could it be that it only moves when I touch the test leads? Only to measure I have to touch!
Thanks
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I did a stupid little test. I changed valve type to V2.
With 2 type of good 5751 = the tension moves..about 135v
With 2 type of ecc81 = the tension moves much less. I have a different different value..which is normal. One triode is almost stationary, the other moves a little more ... but less than with 5751 and ecc83..about 95v
Witch two type of ecc82 = the tension is immobile, very firm,..about 48v
I doubt there is any useful information but I wanted to do a quick test.
Thx
With 2 type of good 5751 = the tension moves..about 135v
With 2 type of ecc81 = the tension moves much less. I have a different different value..which is normal. One triode is almost stationary, the other moves a little more ... but less than with 5751 and ecc83..about 95v
Witch two type of ecc82 = the tension is immobile, very firm,..about 48v
I doubt there is any useful information but I wanted to do a quick test.
Thx
Try to measuring voltage on R6 (V2 cathode resistor).
It's only 2k2, so if there no fluctuation, V2 stable.
V3 direct coupled to V2, so if V2 stable it is so (assuming it works well).
If your voltmeter resistance not enough high, the V2 anode load (301k) or V3 cathode resistor (100k) voltage will be false.
It's only 2k2, so if there no fluctuation, V2 stable.
V3 direct coupled to V2, so if V2 stable it is so (assuming it works well).
If your voltmeter resistance not enough high, the V2 anode load (301k) or V3 cathode resistor (100k) voltage will be false.
This amp also has feedback, doesn’t it? Do not increase the values of the coupling capacitors but use the original values.
Best regards, Frank
Best regards, Frank
Hi 🙂
My cathode resistor is 23.5k because i use a 6n1p.
100k i when i use ecc83.
My Ra is 330k.
..unimportant differences..
I have misure on Rk, i have a little fluctuation, from 1.297v to 1.330v..random type.
When i put another multimeter on pin resistor (kiwame 330ohm) between V2 and V3 (V3 side)...the tension on Rk2 ...have a super fluctuation..and down to 1.170v....and stay down there .. floating between 1.170v and 1.200v.
At the same time, with the other multimeter I measure the usual fluctuations on Vg3.
I can say that if I measure only on Rk2 .... I have the fluctuation I wrote ... there is .. but maybe slight ... As soon as I touch the Rg3..the fluctuation increases a lot .. going down to 1.170v e 1.200v.
On V1 all tension is solid.
Could it be an idea to increase the kiwame from 330ohm (grid stopper V3) up to 1k? (honestly I think it's maybe useless)
Other beginner idea: Could it be a solution to insert a grid stopper between the 0.15uF cap (connection from anode V1 to grid V2) and the grid of V2?
Tomorrow i put the scheme. I go to sleep now. I'm in Italy ... different time!
Thanks!
in fact, I did the test because I was sure that the problem disappeared when the gain went down.
tomorrow post the scheme.
good night. 🙂
Did you use bigger values of the coupling C's?
A mistake many beginners make. Do not try to outsmart the guys of ARC (only if you really know what you are doing).
A mistake many beginners make. Do not try to outsmart the guys of ARC (only if you really know what you are doing).
...coupling ....do you mean between anode of v1 and grid of v2?
No, i used its 0.15uF mkp (miflex).
You say it's too high a value?
On sp3 there is a 0.1uF, on sp6 a 0.47uF.
But between grid v2 and gnd I have a 2.2mohm R. On AR sp3/6 it is 470k.
Could this R be too high and need a stronger gnd reference?
I put 470k and i ceck and i go to sleep!
Thanks
No, i used its 0.15uF mkp (miflex).
You say it's too high a value?
On sp3 there is a 0.1uF, on sp6 a 0.47uF.
But between grid v2 and gnd I have a 2.2mohm R. On AR sp3/6 it is 470k.
Could this R be too high and need a stronger gnd reference?
I put 470k and i ceck and i go to sleep!
Thanks
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Do not change values of the original (do not mix schematics but stick to only one design). Then test... then change. And then test again.
Best regards, Frank
Best regards, Frank
You cannot measure the voltage on the grid of a cathode follower with a multimeter/oscilliscope. Rather measure the cathode voltages of the valves and calculate if your current/voltage is correct
If you want to do trouble shooting on an amp then first remove any feedback loops. It makes finding issues much easier and excludes the feedback induced errors as well.
i do not poke tester probes into the grid, the test leads can have that antenna effect, what i do is monitor the cathode voltage instead....
besides, your tester paralleled the grid leak resistance introducing errors..
you did not mention what type of tester you were using...
besides, your tester paralleled the grid leak resistance introducing errors..
you did not mention what type of tester you were using...
The relevant quantity is the RC product. Either the R or the C can be too large and cause the same LF instability.
Trying to directly measure the voltage on the grid of the cathode follower could create the kind of instability you are seeing. Either way the values you measure will be wrong probe capacitance and meter input resistance will affect the reading. Rather measure the cathode resistors' voltages use those voltages to calculate valve currents and thus the anode and grid voltages plus observe if any instability does occur.