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Old 4th February 2011, 07:22 PM   #1
RollE2k is offline RollE2k  Sweden
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Default Help modify 6H30 Mu-follower stage

Modifying 6H30 Mu-follower line-stage.

I have a mu-follower line stage that (unfortianly) is single ended. I have been thinking about buying a Pass Labs X250 power-amp, which i was going to run together with this tube-pre.
The pass labs does only have 11k input impedance, so i thought i would try to lower the output impedance on the linestage i have today.
I'm quite new to modifying / building stuff myself so i have tried to understand how everything works to get lower Z-out, and better linearity of the tubes.

Today the amplifier looks like this:
http://img707.imageshack.us/img707/4094/6h30orig.jpg
The tubes are running at ~60V and the bias is -4.2v (~3mA)

And i wouldn't want to modify/Change the PSU, so my question is. would it be possible to change the cathode resistors and the "middle" resistor to the following values?
http://img820.imageshack.us/img820/2842/6h30mod.jpg

This would make the tubes run at 75V and biased at approx -3.7v (~9mA)

And if i have understand it right, the tube curves - Tube Tester Files - 6N30P - 6H30
If running at around 8mV the Rp(Ra?) would be around 1.8K instead of approx 3.4K when running 3mA.
and as i have understand it, the Ra/Ra is directly related to output impedance, and this would make Z-out ~123ohm?
What would be negativ by doing this?!

Any help will be appreciated, since i'm really a noob on this. thanks!
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Old 4th February 2011, 07:38 PM   #2
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Hi. Don't really have time at the moment for anything deeper but here are generally my thoughts.

First i really do not understand why someone will design a power amp with 11k input impedance. Could it be just to make most tube preamps miserable driving it?

Second, you may be slightly confused with the importance of output impedance. While desirable, it is by no means sufficient. However, you are on the right path in raising quiescent current - it will definitely help driving low resistance loads. And if any tube can do it, a 6H30 certainly can.

And finally, running a 6H30 at 3mA is really a waste. Even without challenging loads it only starts sounding alive above 10-12mA.

So, yes, raising the current is the best you can possibly do in the situation.
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Old 4th February 2011, 08:31 PM   #3
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Try this:

R4=R6=270 Ohm;
R3=1K-2K
C2=470uF
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Old 4th February 2011, 10:13 PM   #4
RollE2k is offline RollE2k  Sweden
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Tried that and it looks pretty good with R3=2k.
Approx 84V over the tubes and -4.4v bias, around 12mA.

Is it best to run there or would it be better with R4=R6=220 Ohm, R3=1,5k -4.2v bias at approx 17mA? This considering it will drive such low-impedance poweramp as the pass labs..

Also output impedance on this Z-out=Ra/Mu? R=U/I (R=87/0.017) = 5111Ohm
then Z-out is 5111/15=340 ohm. Have i calculated this right?

Thanks alot for your help so far! :-)
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Old 5th February 2011, 10:03 PM   #5
RollE2k is offline RollE2k  Sweden
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Okay, seems like i have understand everthing wrong with output impedance (Rp) how is it possible to calculate everything around that? is there any formula for that?
Because i see at Tube Tester Files - 6N30P - 6H30 that when the tubes are running around 20mA the impedance seem to vary around 1.1-1.3K, so how can i calculate it at my voltage/current/bias?
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Old 5th February 2011, 10:16 PM   #6
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I use plate curves to estimate output impedance of cathode followers. It works well: look at changes of grid-cathode voltages needed fer given swing of output power.
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Old 6th February 2011, 08:22 PM   #7
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You have a couple of compromises going on here, due mainly to the relatively low power supply voltage for a mu follower. The low distortion characteristics of the mu follower are due largely to the middle resistor being multiplied by the mu of the top tube thus providing a very high effective ra.

The trouble is, as soon as you raise the current, the voltage drop across this resistor increases and you are tempted to reduce its value, as indeed has already been suggested, and you really no longer have a mu follower. The distortion increases and the gain drops as the linearising effect is lost. The normal way to increase the current in a mu follower is to use a higher supply voltage but this is not an option for you.

I have two suggestions:

Do nothing. The output impedance is already low enough to drive an 11K load; the real question is how hard do you want to drive it? Contrary to popular belief, the mu follower is not single ended and with the existing 3mA idle current it can in theory drive 6mA peak to peak into a load which equates to about 20V rms into 11K. I am guessing your power amp needs 2V rms at the most, so this should be quite within the limits of the current circuit.


Alternatively, if you really want to increase the current significantly then change the topology to an SRPP and do without the middle resistor altogether. You will get higher distortion and the stage gain will be about half that of the mu follower but you are moving in that direction anyway by reducing the middle resistor.

Cheers

Ian
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Old 6th February 2011, 09:44 PM   #8
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Quote:
Originally Posted by ruffrecords View Post
Contrary to popular belief, the mu follower is not single ended and with the existing 3mA idle current it can in theory drive 6mA peak to peak into a load which equates to about 20V rms into 11K.
Don't quite agree with that one.
When gain approaches mu because of the high impedance load, the stage will be single ended.
The SRPP functions as a push pull stage.
Everything else in between will function somewhere between SE and PP.
When raising high voltage supply is no option R3 might be replaced by a solid state () current source (DN2540 for instance). That should work with low loss of voltage across the CS. That will give a pure mu follower.
Most important I think is to see what the stage must do in terms of gain and output impedance.
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Old 6th February 2011, 10:12 PM   #9
RollE2k is offline RollE2k  Sweden
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Thanks alot for your answer Ian!

I wouldn't it be ok to run the Cathode resistors at ~220ohm and then drop the middle resistor to about 1k8 - 2k2?

I found another interesting thread with distortion figures about this (although with 12ax7) which seemed to favor lower-value middle resistor, at least when running without the lower valve cathode bypass, and for 2nd harmonics distortion. 3rd order was worse, but isn't the 2nd order more important to keep low?
The tread can be found here - Distortion analysis mu follower
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Old 6th February 2011, 10:31 PM   #10
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Quote:
Originally Posted by pieter t View Post
Don't quite agree with that one.
When gain approaches mu because of the high impedance load, the stage will be single ended.
Excpet the load is an active one and the stage can both sink and source current as witnessed my measuring its load driving capability.

Cheers

Ian
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