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DC-coupling of tube stages
DC-coupling of tube stages
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Old 7th May 2009, 12:53 PM   #11
jueic is offline jueic  Taiwan
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Hi, Revintage

I got your point as well..

Not easy to catch your point without pre-reading the ANTI-TRIODE
tread you had a short while back.

Anyway, thanks to point it out.


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Old 7th May 2009, 04:05 PM   #12
revintage is offline revintage
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Hi JueiC,

Yepp, should have referenced to the other thread where also MJKs circuit was shown.
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Old 7th May 2009, 05:24 PM   #13
Michael Koster is offline Michael Koster  United States
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Originally posted by revintage
Hey MJK,
Want your comment on this as it concerns the same topology we are both building.
First I needed to realize that the current loop of your driver supply
goes through the B+ supply ;-) But that pole is isolated by MOSFET.

There is indeed at least a low frequency pole in almost everything that
isn't regulated. Two of your topologies use a regulated driver supply.
and would appear to be mostly DC stable.

Isn't there also a time constant involved in the gate coupling C and
the 1Meg DC reference? If the driver waveform is asymmetric there
will be a small DC shift + time constant with signal level. Keeping the
driver in the symmetric range and small R should mitigate this though.

I think there is a big difference in terms of which current loop the
time constant is in. I would expect more audible artifacts in the
cathode branch vs. the anode branch, the common regulated driver
but unregulated B+ in DC topologies may bear this out.

And what is the goal in direct coupling? For me first and foremost
is to eliminate blocking distortion, where even minute grid current
causes the coupling cap to charge and discharge on peaks. I find
this pretty annoying, and DC is one solution.

Second order is the ability of the output stage to maintain a stable
operating point as signal level changes. This is a side effect of fixed
bias, not DC. Single supply circuits for example have a problem
here, where the big cathode R allows the cathode voltage to
increase at high signal level. Choice of a flat load line can mitigate
this to some extent. A shunt regulator can also be used instead of
a resistor in the cathode branch to simulate fixed-bias using a single

Third is the ability to drive class A2 with these circuits. Referencing
a separate grid drive supply to the cathode keeps the grid current
in a small local loop for cleaner class A2 operation (stay tuned...)

How do you see the difference between series and shunt driver
regulators? The series one using PMOS followers is adaptable to
cathode feedback, which I also want to try with these transmiting
tubes as a way to deal with the high Ri.

One reason for me to use input transformers is that I'm powering
my breadboard with bench supplies that have a 250V max DC
offset and I need a lottle more for the driver. I also have all Low Z
balanced sources (Alesis Masterlink and mixing consoles) so
the transformers are pretty much transparent.

For a real build, I would probably go with stacked/floating supplies
and shunt regulation of the driver, or waste the power and try a
shunt cathode regulator with the single supply. Or the series
follower if I wanted to try cathode feedback.

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Old 20th April 2011, 07:39 AM   #14
hoagje1 is offline hoagje1  United States
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Greetings from Tacoma.

My apologies, revintage for taking things even farther away from your area of interest. I spent last night reading about a related project and I suggest the following: Bobs Hi-Fi Tube Amp Page . If that link doesn't work I will find you another route. You are asking a heavy question here.

My interest in DC coupling is that the cathode follower (ex 12AX7 with 100K plate resistor and 33 - 47K cathode resistor) has always had the reputation for "warming" up a guitar amp. Same for triode SE output. Well, I built my 2A3 amp and cathode followers in the preamps, and I can't hear the difference. I decided that my next project should be to put several cathode followers in series. I have no training in electronics, but it seems logical that 1) all of the followers would need to round up the same half of the wave, so there will have to be 180+180 degrees of phase inversion between each cathode follower and 2) there can be no phase shift from a poor choice of blocking cap/grid resistor. So, jueic's post is interesting to me. Since no one out there is trying to make a guitar amp with lots of 2nd order harmonics there isn't much to look at on the web. If either of you can get me past the grid bias problem for what is coming out of the cathode follower and going into my inverter, I would appreciate it. I don't care if I need to make a bipolar power supply, I just want to hear those second order harmonics before I die.


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Old 20th April 2011, 09:27 AM   #15
SY is offline SY  United States
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DC-coupling of tube stages
What you're describing isn't a cathode follower. A CF has the plate at AC ground (usually connected to B+) and takes the signal from the cathode. Gain is less than unity and there's no polarity inversion.

For high 2nd harmonics, just run a normal common cathode stage with a small plate resistor. For example, using a 30k plate resistor in a 12AX7 stage will give you lower gain (maybe 30) and very high second harmonic.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 21st April 2011, 05:34 AM   #16
tomchr is offline tomchr  Canada
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As far as I see it, the signal out from the driver stage is ground referenced. Hence, the ground connection must be DC coupled as well as the signal connection. As far as I can see, only the top two circuits accomplish this.

I'm mucking with a similar circuit: DC coupled 300B idea - request for comments

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