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lordearl 25th April 2011 05:55 AM

Frank's 6SN7 - plate follower, noise
Recently did a build of Frank's circuit, but modified it to run off the anodes as my power amp has in input impedance of 100k.

I'm using some 10uF mkp capacitors at the output (big I know, but it's all I had to hand), when I plugged it in, although the sound is great, there is noise - like a buzzing sound. I took out the caps and the buzzing is gone, but the gain is also gone - at full volume it's just the line level volume.

How can I begin troubleshooting the buzz/hum? It disappears without the caps, but why should the gain also disappear?

Palustris 25th April 2011 06:07 PM

<but modified it to run off the anodes

What does this mean?

<I took out the caps

What does this mean?

Better post the schematic.

lordearl 30th April 2011 04:15 AM

2 Attachment(s)
No worries - probably a smarter idea to give a schematic.

Schematic attached, however I've made some changes because I think when this line stage was designed it was to run the line pre into a SS power amp (hence need the cathode follower to give a low impedance output). But seeing as my power amp is 100k inoput impedance, I took out the cathode follower and now run the line pre as a plate follower (to lose any dreaded cathode follower effect). Plus it also meant I can take out one of the valves, so the load on the small power transformer is halved. I've wired the valves so that this can be quickly done (one valve shares the two plate followers, the other shares the two cathode followers).

I've also put a picture of the build prior to removing one tube and modifying as follows (not sure how clear the wiring is in the pic):

(1) Removed the bottom tube from its socket.
(2) Removed the two (green) wires from the plates of the top valve going to the grids of the bottom valve.
(3) Removed the two 47k resistors going from the two output caps.
(4) De-soldered the (yellow) wires running from the two output caps to the bottom valve cathodes, and re-soldered them to the junction of the (blue) wires and the top valve plate 47k resistors.

After changing it I have noticed the following; (1) gain appears to be slightly higher, and (2) the buzz is exactly the same. Taking out the 10uF output caps removes all the gain, and the buzz - but I can't work out why this is the case. Clearly the aim is to have no buzz, but a decent amount of gain (theoretically it should be about 10x).

woodturner-fran 30th April 2011 07:41 AM

You should draw out your new schematic even by hand and post that - your description is hard to follow!! Would encourage more responses.....

And while you're at it - also draw in the PS as you have it done too....

lordearl 30th April 2011 08:34 AM

Ha - yes I thought so - unfortunately I don't have a scanner!
The power supply is PS is 220uf - 100R - 220uf - 100R - 220uf/1uf/0.47uf.

By the way, I can see why people have had so much success with Frank's Linestage - the purity of the sound is unrivaled in my experience (I've used half a dozen commercial and DIY preamps).

The only issue in my build is the noise - which seems to happen whether I use it as a cathode follower, or a plate/anode follower circuit. There is no noise without the output caps, but there is also no gain. Likewise, with the caps in place, the gain returns, as does the buzz/hum.

DF96 30th April 2011 11:07 AM

Well designed cathode followers have no effect, dreaded or otherwise, apart from buffering the signal.

From your description, it sounds like you have simply removed the CF - not converted the stage to a plate follower. You take output now directly from the anode of the first stage - this is a common cathode amplifer with cathode degeneration, not a plate follower. It will have high output impedance which means it will not ground any hum picked up on the interconnect. However, you say that removing the output coupling capacitor removes the hum (and signal too, of course!) so the hum is coming from the circuit rather than the interconnect.

The most likely cause of your problem is poor grounding. You are probably injecting charging pulses into your signal ground. I can see a bare ground wire arcing across the amp, but a picture does not give enough detail. What grounding scheme have you adopted - star, bus or some mixture? Exactly how are the negative ends of your smoothing caps grounded? Where does the ground wire attach to incoming signal ground and safety ground/chassis?

john65b 30th April 2011 12:27 PM

No output caps? You are sending full DC plate voltage (wired as a plate follower) to your amp?

I am interested in what you did to get rid of the buzz, as I have a similar problem, but getting rid of the output cap is not possible...what cap are you removing?

woodturner-fran 30th April 2011 12:39 PM

chassis ground looks to be connected to the neg terminal of the cap - could be a cause. back to back diodes, with a 0.01uF cap and 5-30R power resistor in series might work.

And yes - whats this about removing the output cap????

john65b 30th April 2011 01:30 PM

Sorry, clarification needed.

Removing output caps, you no longer have signal out to your amp. If you do have an attenuated signal out, it would mean your active stage is being bypassed and is a simple passive preamp...I can't see how that is by the pic.

Shorting the output caps will send full plate voltage to your amp.

lordearl 30th April 2011 01:41 PM

Thanks for the help folks - no need to worry about the B+ (the input of my power amp is coupled).

My groundings all lead to the chassis ground, I'll try to map it out so it's easier to see.

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