Plate choke on a line stage?

Giaime · 9th December 2005, 01:11 PM

Hello people!

I just finished prototyping one of my first tube project.

It's a simple line preamp, made of a cathode follower for each channel, ECC81 by Siemens is the tube. 10k cathode resistor, 220ohm grid stoppers and the usual things. Output cap is a 100uF electrolytic bypassed by a 100nF polyester.

The more interesting part is the PSU. I found 4 220/110V transformers for free, thinking they were autotransformers, useful for powering USA equipment or the like.

Instead they are fully functional transformers, as I measured open connection from the 220V primary to the 110V secondary. So let's use them for an high voltage PSU!

I made a SS voltage doubler with 1N4006 diodes, 2x470uF 200V caps (from old pc smps psu) + 150ohm resistor + 100uF 385V cap bypassed with 10nF cap. This ended out about 322V pretty stable and cool. Of course filaments have a separate 6-0-6 @ 2A tranny.

The question part is: I wanted to add a choke in the psu, because hum was too much, even considering that I build that little thing on a wooden board, no shielding etc etc. So I found a fluorescent lamp reactor that I thought could act like a choke, and I wanted to put it in place of the 150ohm resistor, right?

Wrong. I made a wiring error and I put it after the last filter cap, directly on the plate pins of the ECC81. I thought that before correcting the error, I could see if this worked, and how.

I plugged it in, and it worked, the hum is reduced by a significant amount.

My question is: what's going on in there? What's the role of a choke as a sort of "plate load" in a cathode follower? I mean, no resistor between the choke and the plates...

Or I will gain better hum reduction if I put another cap after the choke?

Giaime · 9th December 2005, 02:55 PM

Just wanted to share the improvements...

I put a 160uF 330V (camera flash cap) after the choke, and an elevated reference for the heaters (about 38V, made with a 1Mohm resistor from the B+, 33uF cap and 120K resistor to ground).

Those measures killed the remaining hum: with full volume I have only a faint hiss.

Giaime · 10th December 2005, 11:56 AM

Now I've got a real problem.

I get random cracking sound on the left channel: what kind of component faliures may do this? Resistors? Capacitors? in the psu or in the audio path? The tube?

I substituted the tube but it's still there... and 2 times yet the preamp has started a strong humming sound throught the speakers, that cured by simply turning off and back on the preamp.

The only components I'm suspect of are:

- the 2 100uF 25V caps in the audio path, on the cathode of the cathode follower. They have only 7V on them, but...

- a 100K 2W resistor as a bleeder on the B+, after the 2nd filter cap and before the choke. It gets quite hot, but still touchable.

Randomly the cracking sound disappears, like now. It has cracked for 5 minutes, and now it has stopped.

I've already excluded things like the power amp (I'm driving the power stage of a Marantz 1060), cables, interconnections...

Somebody has ideas?

Original Burnedfingers · 10th December 2005, 12:47 PM

Check your solder joints and connections.

Cassiel · 10th December 2005, 01:13 PM

I once tried the plate choke thing with the fluorescent ballast and the sound was good but there were cracking noises, they were on like 5 minutes then dissapeared, then back again; really strange. I took the "choke" out and problem resolved, no more noises.

Giaime · 10th December 2005, 01:25 PM

So the choke could be the cause? I will try removing it... maybe I will replace it with a 100ohm resistor.

Btw what's the technical cause for this crackling sound? Is the choke core saturating for DC current? I only have a few mA of load...

Cassiel · 10th December 2005, 03:02 PM

I

Quote:

Btw what's the technical cause for this crackling sound? Is the choke core saturating for DC current? I only have a few mA of load...

I have no idea...i never tried a real choke in its place so i don't know if it was the choke's fault or just the fact that the 6AS7 didn't like that load for unknown reasons. It was a 6AS7 cathode follower line stage. Sound was great much better than with a resistor but in the end i couldn't fix the crackling noise , not until i removed the choke. But i thought to myself one day i shall buy a good choke (i needed more Henries i believe).

Giaime · 10th December 2005, 06:26 PM

Hello people, I replaced the chocke with a 5W 120ohm resistor, and the cracking is still there, just with some more hum.

All the solder joints appear to be well made...

I will recheck anyway.

anatech · 10th December 2005, 07:00 PM

Hi Giaime,
Odd fault. Try running it in the dark. When it starts cracking, look under and over the chassis for small arcs. Just a thought.

-Chris

Edit: one day I'll learn to spell

poobah · 10th December 2005, 07:31 PM

Can you scope the voltage at both ends of the plate resistor <reactor's old location>? Crackling can often be capacitors arcing internally.

Also use a DRY wooden stick and beat/tap on your components... you might find a poor connection INSIDE a compnent... not your soldering.

9th December 2005, 01:11 PM	#1
Giaime diyAudio Member Join Date: Oct 2004	Plate choke on a line stage? Hello people! I just finished prototyping one of my first tube project. It's a simple line preamp, made of a cathode follower for each channel, ECC81 by Siemens is the tube. 10k cathode resistor, 220ohm grid stoppers and the usual things. Output cap is a 100uF electrolytic bypassed by a 100nF polyester. The more interesting part is the PSU. I found 4 220/110V transformers for free, thinking they were autotransformers, useful for powering USA equipment or the like. Instead they are fully functional transformers, as I measured open connection from the 220V primary to the 110V secondary. So let's use them for an high voltage PSU! I made a SS voltage doubler with 1N4006 diodes, 2x470uF 200V caps (from old pc smps psu) + 150ohm resistor + 100uF 385V cap bypassed with 10nF cap. This ended out about 322V pretty stable and cool. Of course filaments have a separate 6-0-6 @ 2A tranny. The question part is: I wanted to add a choke in the psu, because hum was too much, even considering that I build that little thing on a wooden board, no shielding etc etc. So I found a fluorescent lamp reactor that I thought could act like a choke, and I wanted to put it in place of the 150ohm resistor, right? Wrong. I made a wiring error and I put it after the last filter cap, directly on the plate pins of the ECC81. I thought that before correcting the error, I could see if this worked, and how. I plugged it in, and it worked, the hum is reduced by a significant amount. My question is: what's going on in there? What's the role of a choke as a sort of "plate load" in a cathode follower? I mean, no resistor between the choke and the plates... Or I will gain better hum reduction if I put another cap after the choke?

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9th December 2005, 02:55 PM	#2
Giaime diyAudio Member Join Date: Oct 2004	Just wanted to share the improvements... I put a 160uF 330V (camera flash cap) after the choke, and an elevated reference for the heaters (about 38V, made with a 1Mohm resistor from the B+, 33uF cap and 120K resistor to ground). Those measures killed the remaining hum: with full volume I have only a faint hiss.

10th December 2005, 11:56 AM	#3
Giaime diyAudio Member Join Date: Oct 2004	Now I've got a real problem. I get random cracking sound on the left channel: what kind of component faliures may do this? Resistors? Capacitors? in the psu or in the audio path? The tube? I substituted the tube but it's still there... and 2 times yet the preamp has started a strong humming sound throught the speakers, that cured by simply turning off and back on the preamp. The only components I'm suspect of are: - the 2 100uF 25V caps in the audio path, on the cathode of the cathode follower. They have only 7V on them, but... - a 100K 2W resistor as a bleeder on the B+, after the 2nd filter cap and before the choke. It gets quite hot, but still touchable. Randomly the cracking sound disappears, like now. It has cracked for 5 minutes, and now it has stopped. I've already excluded things like the power amp (I'm driving the power stage of a Marantz 1060), cables, interconnections... Somebody has ideas?

10th December 2005, 12:47 PM	#4
Original Burnedfingers diyAudio Member Join Date: Nov 2002	Check your solder joints and connections.

10th December 2005, 01:13 PM	#5
Cassiel diyAudio Member Join Date: Sep 2004 Location: Madrid	I once tried the plate choke thing with the fluorescent ballast and the sound was good but there were cracking noises, they were on like 5 minutes then dissapeared, then back again; really strange. I took the "choke" out and problem resolved, no more noises.

10th December 2005, 01:25 PM	#6
Giaime diyAudio Member Join Date: Oct 2004	So the choke could be the cause? I will try removing it... maybe I will replace it with a 100ohm resistor. Btw what's the technical cause for this crackling sound? Is the choke core saturating for DC current? I only have a few mA of load...

10th December 2005, 06:26 PM	#8
Giaime diyAudio Member Join Date: Oct 2004	Hello people, I replaced the chocke with a 5W 120ohm resistor, and the cracking is still there, just with some more hum. All the solder joints appear to be well made... I will recheck anyway.

10th December 2005, 07:00 PM	#9
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi Giaime, Odd fault. Try running it in the dark. When it starts cracking, look under and over the chassis for small arcs. Just a thought. -Chris Edit: one day I'll learn to spell

10th December 2005, 07:31 PM	#10
poobah diyAudio Member Join Date: Nov 2005	Can you scope the voltage at both ends of the plate resistor <reactor's old location>? Crackling can often be capacitors arcing internally. Also use a DRY wooden stick and beat/tap on your components... you might find a poor connection INSIDE a compnent... not your soldering.

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