Hallo everybody
This weekend I had to go away. That`s typical when you have so much insperation and willness to fix that amp.
The thing with gridstoppers i`ve totally mised on the Ecc and Ef tube. One thing that I thinking about is:
When I connected the voltagemeter to the anod of the EF86 the "pulsing" disapeared, but the low freq noise was still there. Could it be that I have to types off Osc. in the amplifier? One para I don`t think that the Hum is coused by the filament supply (it`s higher than 100Hz).
Regards
Mattias
ps Roger: Which number of AoE was it posted in. I went to the liberary but I couldn`t find it
DS
This weekend I had to go away. That`s typical when you have so much insperation and willness to fix that amp.
The thing with gridstoppers i`ve totally mised on the Ecc and Ef tube. One thing that I thinking about is:
When I connected the voltagemeter to the anod of the EF86 the "pulsing" disapeared, but the low freq noise was still there. Could it be that I have to types off Osc. in the amplifier? One para I don`t think that the Hum is coused by the filament supply (it`s higher than 100Hz).
Regards
Mattias
ps Roger: Which number of AoE was it posted in. I went to the liberary but I couldn`t find it
DS
And all of this occurring without NFB??
Some kind of mis-wiring or very bad layout for this to happen without negative feedback applied! Your amp should be absolutely stable with no feedback applied, albeit it will be somewhat noisy. So somewhere unwanted coupling or feedback is occurring of the wrong polarity!Remove the EL34's and probe with a scope the anodes of the EF86 and the ECC83, and the cathodes of the latter. Now your B+ will go up without the EL34's installed, so be careful. If everything looks good, reinstall the EL34's and scope probe the B+ lines, the positive and negative terminals of the pwr supply caps. Keep one hand in your pocket! Go slow! On the negative cap terminals you should get a flat line or close to it. Be sure you have your scope neg/gnd at the lowest potential, i.e. star gnd or depending on your grounding scheme (and leave it there!) as this will affect your measurements. Make sure your filament supply is referenced to gnd.
Now it's quite possible that having a single 10' gnd wire IIRC going from the amp to the outboard pwr supply can result in some unwanted coupling, since this a somewhat high gain circuit. As I asked in an earlier post, are you using a pcb or point to point?
Even if the coupling caps to the grids of the EL34's are too large, the pulsing should stop when the NFB is removed. So right now that's at the bottom of my lists of suspects. Is the low frequency noise a steady tone or buzzing/raspy sound? Probing the anode of the EF86, causing the pulsing to cease strongly suggests HF instability! It's difficult to diagnose an amp that's half a world away! This shouldn't be all that hard to figure out. It's more than likely something really simple! Here's a dumb but relevant question: Is the noise present, sound the same in both channels? So with the limited info that I have (since I can't see that far!), go back and take your time and see that everything is connected properly and logically! Then do me a favour and measure the voltages at all of the tube pins. Example:
EF86
(pin numbers)
1:
2:
3:
etc for each tube. And write down any strange or wild readings you get with the scope. Also the voltages present at the power supply filtering/DECOUPLING capacitors. Both pos and neg terminals. And be careful... probes can slip and give you a nasty jolt!
Take your time.Wayne
BTW get those grid-stoppers in thar!
Hi Wayne
The amp is built with a PCB. Similar to the one that Clause used.
The low freq hum is more of a buzzing/raspy sound and when I remove the NFB the hum gets louder. I`ve tried to reverse the secondary then it really started to oscilate. Where could this pos. feedback be?
I`m starting to suspect the grid stoppers and the C2 and R4 anti-ringing network. My primaryimp. of the Outputtransformer is 6500 ohm while clauses is only 5000 ohm. Maby if I`ll connect C2, R4 it will get stable.
The noise is the same in both channels.
Thank you for your ideas
Mattias
The amp is built with a PCB. Similar to the one that Clause used.
The low freq hum is more of a buzzing/raspy sound and when I remove the NFB the hum gets louder. I`ve tried to reverse the secondary then it really started to oscilate. Where could this pos. feedback be?
I`m starting to suspect the grid stoppers and the C2 and R4 anti-ringing network. My primaryimp. of the Outputtransformer is 6500 ohm while clauses is only 5000 ohm. Maby if I`ll connect C2, R4 it will get stable.
The noise is the same in both channels.
Thank you for your ideas
Mattias
The hum gets louder because by removing the NFB the gain of the amp increased. This is called "open loop gain" as opposed to "closed loop gain" with the NFB connected. When you reversed the secondary you changed the NFB into PFB; positive feedback and the amp "howled"!
That buzzy/raspy sound "sounds" like power supply hum/noise! Check those caps and the filament supply. That noise can also be riding along the ground wire from the amp to pwr supply!
Good idea! Do that first off!
Wayne
Hello
I`ve fitted the anti ringing network and moved the grid stoppers close to the tube. The pulsing is gone now and the amp doesn`t sound distorted anymore. Thats great.
The hum is still there. I looked at it with the scope (on the output with no signal present). There is a hum at approx. 2Hz. The hum looks like a sine curve with only the negative half round and the positive side is saturated. Is this motorboating?
I have looked over all the solderings in the PS and they looked okay.
I have tried to fit some temp. caps in parallell with the Caps in the PS, but it didn`t go away. I`ve tried to temp. short out the choke, but that didn`t help either.
All the filament is AC with 120R referenced to gnd.
THIS IS DRIVING ME MAD!
Regards
Mattias
I`ve fitted the anti ringing network and moved the grid stoppers close to the tube. The pulsing is gone now and the amp doesn`t sound distorted anymore. Thats great.
The hum is still there. I looked at it with the scope (on the output with no signal present). There is a hum at approx. 2Hz. The hum looks like a sine curve with only the negative half round and the positive side is saturated. Is this motorboating?
I have looked over all the solderings in the PS and they looked okay.
I have tried to fit some temp. caps in parallell with the Caps in the PS, but it didn`t go away. I`ve tried to temp. short out the choke, but that didn`t help either.
All the filament is AC with 120R referenced to gnd.
THIS IS DRIVING ME MAD!
Regards
Mattias
Hi
Could there be something wrong with my earthing?
I have two PCB in the amp. Each of the PCB has all the circuit grounding on it. exept for the 470n cap that is located between the primary 440V and gnd. The 470n cap is grounded with the minus of the speaker to the chassi. All at the same point in the chassi (both channels). This point goes to an isolated point at the back of the amplifier. At this point I have connected the two PCBs ground, the chassi ground (with the two 470n caps and the neg. speaker output), the cabel to the PS and the filament referenced point.
The PS I have made up like a bus earthing type. On the bus all the earthing is done. It is grounded in the chassi at one point and there I have connected the cable from the amplifier
Should this work?
THANK YOU ALL FOR YOUR TIPS!
Regards
Mattias
ps Sch3mat1c i will by some higher rated Caps and try to remove the last resistor in the PS
Could there be something wrong with my earthing?
I have two PCB in the amp. Each of the PCB has all the circuit grounding on it. exept for the 470n cap that is located between the primary 440V and gnd. The 470n cap is grounded with the minus of the speaker to the chassi. All at the same point in the chassi (both channels). This point goes to an isolated point at the back of the amplifier. At this point I have connected the two PCBs ground, the chassi ground (with the two 470n caps and the neg. speaker output), the cabel to the PS and the filament referenced point.
The PS I have made up like a bus earthing type. On the bus all the earthing is done. It is grounded in the chassi at one point and there I have connected the cable from the amplifier
Should this work?
THANK YOU ALL FOR YOUR TIPS!
Regards
Mattias
ps Sch3mat1c i will by some higher rated Caps and try to remove the last resistor in the PS
2 hz hum?
Did I read this correctly? You can hear a 2 cycle hum?
I am a newbie so take my advice as such but why don't you try tracing the signal back from the output where you measure this hum. Trace it all the way back to the PS until you don't see it anymore. Maybe that will tell you something?
You say hum is equal in both channels--maybe the source of hum is common to both channels?
Did I read this correctly? You can hear a 2 cycle hum?
I am a newbie so take my advice as such but why don't you try tracing the signal back from the output where you measure this hum. Trace it all the way back to the PS until you don't see it anymore. Maybe that will tell you something?
You say hum is equal in both channels--maybe the source of hum is common to both channels?
Y'mean a zobel? For HF crud? Depends on where it is. I'd make it around a quarter of the output stage power if you have it on the OPT. More if you want to be testing HF response at any reasonable power level for any length of time!
Fragman, the problem with chasing down motorboating is, because it's inside a loop, every point in the chain (including the effect it has on the PSU, which IMHO is the cause, phase shift oscillator style as I mentioned previously) shows the signal.
Tim
Fragman, the problem with chasing down motorboating is, because it's inside a loop, every point in the chain (including the effect it has on the PSU, which IMHO is the cause, phase shift oscillator style as I mentioned previously) shows the signal.
Tim
A zobel network is a resistor in series with a capacitor. Its function is to dampen with the resistor when the frequency is above what the capacitor allows. At low frequencies, the reactance of the capacitor is large and the resistor has no effect. At middle frequencies, the resistance and reactance are about equal to the impedance of the circuit and it begins to damp the system, without causing undue phase shift as a plain capacitor would. At high freuquencies the reactance is small and disappears, applying the resistor directly to the circuit, damping it fully.
Tim
Tim
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