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lndm 18th July 2006 02:21 PM

Mu stage driver
2 Attachment(s)
I have constructed a driver stage and I would like to share the results. It is a variation on the mu-stage. Inspiration for this came from Alan Kimmel and from several threads on diyAudio recently.

Though not an SRPP, the circuit resembles one. It has a pentode as the upper device. Rather than mirroring the lower device in the way an SRPP does, it behaves more like a constant current source due to the increased gain of the pentode.

Some may not like the idea of using a pentode. I was hesitant too.

The voltage gain device I am using is the 46 recommended to me by i_should_coco. I have grown quite fond of this device and it doesn't let me down here.

The pentode is the 6CM5/EL36 which I chose for it's transconductance of around 14,000uS, and because of its wide Vhk capability.

Here is the circuit (grid stoppers have been omitted for clarity).

lndm 18th July 2006 02:22 PM

Constant current
2 Attachment(s)
One thing that makes this circuit stand apart from the SRPP is its relatively constant current. Here is an oscillogram showing the voltage across the triode versus the voltage across the pentode's cathode resistor. It is to scale and the levels have been adjusted to make them proportional.

For those who are interested, I figured that at 8.37mA of quiescent current, the triode with a plate voltage of 177V has an equivalent DCR of 21,150ohms. Since this is 7.8 times higher than the pentode's cathode resistor, I figured the Rk would be that much less sensitive to displaying the current via its voltage. I turned it up on the scope by that much.

Unless I'm mistaken, this amounts to 30dB's worth of 'constant currentness' :)

lndm 18th July 2006 02:23 PM

2 Attachment(s)
According to Alan, the top device is also acting as a cathode follower. He says that this is where a pentode shines. I think this shows in the amount of swing I am able to get despite needing to drive output stage grids.

In this oscillogram, the driver's output is shown at 250Vp-p from a 360V rail, and the KT66's are biased at just 36V!

The distortion shown near the top of the waveforms is due to driving the output stage grids. The clipping near the bottom is the 46's themselves being driven into grid current as is confirmed by looking at the 6SN7's before them (not shown).

I've seen much worse looking driver distortions and at much lower voltages too.

lndm 18th July 2006 02:24 PM

How does it sound?
The 46 for me was already a very articulate driver. Doing this seems to have brought it to a new level. The bass seems a smidge lighter, as if it has had some of the even order harmonics reduced. Bass articulation is better. I speculate that the improved PSRR may have a little to do with this. It really just sounds like there is more control.

But this does not sound like forced control. This circuit seems to have not added anything, just taken away. The mids and treble are also lighter and have improved articulation. There is less congestion. The spectrum remains balanced sounding.

Most importantly, the 46's have not been robbed of their charm. If anything, they sound as if they have just been allowed to perform under better circumstances.

If you've come this far, thanks for reading, I am interested in comments, criticism's and improvements.

Gluca 18th July 2006 03:04 PM

I did something quite similar using 801A+choke+807. 807 was wired as a pent initially to mimick a CCS load for the lower tube (the 801A). I took the output from the upper cathode follower and from the lower plate: as a matter of facts the lower anode sounded better but square waves were sharper when the signal was taken from the upper cathode.

Now the 807 is triode strapped (no more a quasi-CCS then...) and the output is from the upper cathode. I'd say it sounds better to me now. I can switch back and forth (cathode/anode) on the fly but I didnt notice such a difference as in the previous set-up. Again square waves are noticeable better from the upper cathode.

Next I will get rid of the 807 retaining just the choke or using a IXYS Chip a la tubelab. Those staked valves seem too complex for me... I like simplicity.

OH! For your info ... the SRPP is driving a hi-L interstage tranny.


i_should_coco 19th July 2006 09:43 AM

Very cool! I've been tempted by the mu-stage for a while, but never got around to building one - I ended up transformer coupling. I suspect I'm going to have to try this out!

I agree with the use of a pentode - they make better CCSs and cathode followers that triodes. I always thought that the mu-stage had relatively limited swing, but I gues this isn't the case ;-)

BTW, have you read the article on the "Choke Assisted Mu-Stage" in Vacuum Tube Valley a while ago? Very interesting.


Geek 19th July 2006 10:03 AM

Mu-stages/followers are awesome for driving power stages. They can even DC drive a screen driven power tube with little effort and correct tube choice.

Once you try it, you might wonder why you ever used anything else :D

Gluca 19th July 2006 10:06 AM

Not directly that article ... I had the chance to read those ideas reported here and there.

My first amp was transformer coupled (LL1660PP a la Olson) and after a while I tried with a mustage driver 6H30 or 6N6P plus EF184 ... much better IMHO. Measurements were OK in both the amps but I liked the mu-stage more. I am not going back to IT. And as I said, it sounded better (but measured poorer) when the signal was taken from the lower anode.

OH! The swing of the mu-stage is limited as a matter of facts.

The interstage I am using now in this choked-SRPP is actually an autoformer to step-up the signal... stepping up the signal is not my favorite choice but I need that +6db extra gain.


Giaime 19th July 2006 10:09 AM

One little question:

you say you're running the 46 at 8mA, this current obviously has to pass into the EL36. It can't be very happy for such low current, so you might try (see if the sound or measurements get better) to increase the current throught the EL36 without changing 46's current. How? Look at figure 4 of the link you've posted:

i_should_coco 19th July 2006 10:19 AM


Originally posted by Giaime
One little question:

...the current throught the EL36 without changing 46's current. How? Look at figure 4 of the link you've posted:

The 46 will happily take up to 30mA or so, dissipation limits not withstanding. Who knows, might sound better with more current...


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