Hi,
I have done a first run up of the amp, however I have a loud hum!
With 12ax7 pulled the hum is still there.
On EL34 voltages are
HT 288V
Cathode 16.5V with 220 ohm load
screen 288V.
Tried grounding no luck. increased screen resistor to 470 ohm no difference. Both channels the same. removed cathode feedback and put to gnd no change connected input to EL34 to Gnd no change.
I can get music to play however I cannot get rid of the HUM!
Used 2 220 Ohm across heaters to gnd hum bucking_makes no difference!
Just tried putting another 47uF across the B+ no change
I have not had chance to put a scope on the PSU any thoughts would be helpfull.
All we have is a single EL34 in single ended with no input_ does not make sense!
Regards
M. Gregg
I have done a first run up of the amp, however I have a loud hum!
With 12ax7 pulled the hum is still there.
On EL34 voltages are
HT 288V
Cathode 16.5V with 220 ohm load
screen 288V.
Tried grounding no luck. increased screen resistor to 470 ohm no difference. Both channels the same. removed cathode feedback and put to gnd no change connected input to EL34 to Gnd no change.
I can get music to play however I cannot get rid of the HUM!
Used 2 220 Ohm across heaters to gnd hum bucking_makes no difference!
Just tried putting another 47uF across the B+ no change
I have not had chance to put a scope on the PSU any thoughts would be helpfull.
All we have is a single EL34 in single ended with no input_ does not make sense!
Regards
M. Gregg
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Hi,
I understood that: With both EL34 G1 grounded the hum remains unchanged. Correct ?
This sounds like a power supply or heater supply problem.
Can you make a drawing of your power supply ?
EL34 voltages seem to be fully OK.
When the amplifier is "cold" (been switched off), does the hum begin immediately when power is switched on or just after the tubes have warmed up ?
I understood that: With both EL34 G1 grounded the hum remains unchanged. Correct ?
This sounds like a power supply or heater supply problem.
Can you make a drawing of your power supply ?
EL34 voltages seem to be fully OK.
When the amplifier is "cold" (been switched off), does the hum begin immediately when power is switched on or just after the tubes have warmed up ?
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The power supply is a simple full wave bridge with hex diodes. feeding the 47uf cap with 10K to another 47uf. The EL34 is fed from the first cap.
The hum is after tube warm up about 10 seconds or so. When grid 1 is put to ground the hum get less however only by about a third of the volume.
Regards
M. Gregg
Sounds like it could be mains hum.
1) You using AC heater wiring? If so, make sure the wires are not running alongside signal wiring anywhere.
2) Did you try moving your output transformers around or turning them at different angles? (OR your power transformer relative to them)
3) Your EL34s look as if they are right in the max magnetic field of OPT's. I don't know if the field is strong enough to matter at idle, but you could try slipping some aluminium foil between tubes and trafos. Perhaps also between power and OPT's.
Hope this helps. I know what it feels like!
1) You using AC heater wiring? If so, make sure the wires are not running alongside signal wiring anywhere.
2) Did you try moving your output transformers around or turning them at different angles? (OR your power transformer relative to them)
3) Your EL34s look as if they are right in the max magnetic field of OPT's. I don't know if the field is strong enough to matter at idle, but you could try slipping some aluminium foil between tubes and trafos. Perhaps also between power and OPT's.
Hope this helps. I know what it feels like!
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Hi,
The 12ax7 is out of circuit and Grid 1 of the EL34 is connected to ground for testing. Hum is still there!
Gap between OP tx and tube is 50mm
Gap between mains Tx and OP tx is 50mm
Regards
M. Gregg
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Try looking at the audio output with a scope. No drive at all. The hum will be either 50 or 100hz. If it is 50, it is most likely coming from the filaments, If 100 most likely the power supply. A frequency counter is verry handy for checking.
Correct!
Haven't got a scope with me at the moment _ on loan to a friend! Sounds like 50Hz
Regards
M. Gregg
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Filtering cap seems too small. I would use some 390 uF, though this may not solve all hum problem.
If you have a (6 V) DC power supply capable of delivering the heater current for one EL34, just try to use it instead of AC heater supply.
Difference ?
According to photos sent earlier this can not be the reason. The placement of transformers is correct.
AND, if this were the reason, then the hum would be heard immediately after the power is switched on, i.e. when tubes are still cold.
If it was anything other than mains-induced noise, it would not be a hum, but some other noise (hiss, scratchy sound, annoying buzz etc.) You need to figure out how it is coupling to your el34's. Firechief's suggestion is excellent (if you have a scope). IMHO, 50mm is not a huge gap, but others may disagree....
I will keep grid 1 at Gnd.
I will try larger caps and the heater supply from DC and let you know how it goes!
Thank's for the help.
M. Gregg
Quote:- "AND, if this were the reason, then the hum would be heard immediately after the power is switched on, i.e. when tubes are still cold."
Tubes cannot conduct when cold, therefore will not make any sound. My EL34's take at least 10 to 15 secs before they can conduct.
Tubes cannot conduct when cold, therefore will not make any sound. My EL34's take at least 10 to 15 secs before they can conduct.
OK I have strapped a huge 1000uF 500V across it. The hum is down to a much lower level. Still there at about an 8th of the level.
I find this strange the PYE Mozart uses low levels of cap in the psu?
Regards
M. Gregg
Hi guys!
I just designed this. I intend to use it with KT77 (JJ), E34L (Tesla) and EL34 (RSD). As you can see, there is cathode feedback from the secondary of the output transformer to the cathode of the Power tube. I just wonder if there is enough gain in the driver, to drive it fully from a CD input. Basically, I cannot calculate how much feedback this connection generates. μ=40 for the EF80 as triode.
Also, reality check on the operating points?
I just designed this. I intend to use it with KT77 (JJ), E34L (Tesla) and EL34 (RSD). As you can see, there is cathode feedback from the secondary of the output transformer to the cathode of the Power tube. I just wonder if there is enough gain in the driver, to drive it fully from a CD input. Basically, I cannot calculate how much feedback this connection generates. μ=40 for the EF80 as triode.
Also, reality check on the operating points?
Attachments
What value choke/s does the Pye mozart use? What kind of capacitors?
What type of caps are your 47uF's?
You may gain significant benefit from bypassing your PS caps with smallish value polypropylene fim caps and/or teflon caps. (General rule of thumb is 1% of value) Adding an extra stage or two of filtering would probably have a bigger effect.
What type of caps are your 47uF's?
You may gain significant benefit from bypassing your PS caps with smallish value polypropylene fim caps and/or teflon caps. (General rule of thumb is 1% of value) Adding an extra stage or two of filtering would probably have a bigger effect.
You did not get my point.
The tubes do conduct nor make the hum when cold, but the magnetic field from power transformer to output transformer does.
I have once observed this problem when I placed power transformer too close and parallel to OPT. The hum appeared immediately after power was switched on, i.e. when all tubes were still cold.
When tubes got hot, the hum attenuated some 16 dB.
Why ?
There was 16 dB NFB at the amplifier, and after the amplifier got warm and started, the NFB cut the hum.
I used 16 + 16 uF plus a choke in my version, as per costis' drawing, and there's absolutely no hum at all.
You probably need a pi network of some kind but now have no space for a choke. Try using 47uF then 100 R resistor (10 Watt) then 100uF (or more). Also pay careful attention to wiring as you don't want IR signals from the charging current to sneak in.
Regards
Henry
PS to hihopes, the mozart has no choke at all or even a pi network, but does use a hum cancelling techinique (22MR resistor), see Arto's posting of the mozart circuit.
You probably need a pi network of some kind but now have no space for a choke. Try using 47uF then 100 R resistor (10 Watt) then 100uF (or more). Also pay careful attention to wiring as you don't want IR signals from the charging current to sneak in.
Regards
Henry
PS to hihopes, the mozart has no choke at all or even a pi network, but does use a hum cancelling techinique (22MR resistor), see Arto's posting of the mozart circuit.
Incidentally, I can't really see the point of all those EF80's, a single ECC81 (one section) worked a treat.
Hi,
Thank's for the info I'll get there.
In other projects I have used PI type circuits, should have done it this time. If all else fails I will try winding a choke to fit. It would be interesting to try the type of hum cancelling circuit in the PYE.
Did you hum buck to gnd or lift the heaters? I thought about lifting the heaters as costis_n has and also in simple SE.
Let you know how it goes!
Regards
M. Gregg
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