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Tubes / Thermionic Valves

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Autobias

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sampleaccurate

Post subject: Autobias

Posted:
21 Jan 2011, 21:27

Joined: 05 Jan 2010, 21:16
Posts:
680
Location: Las Vegas Nevada USA

In considering the construction of an amp that
will use power tubes that are very difficult to match I decided I'd
have a go at an attempt to build an auto-bias circuit. The one I
chose was designed by Johm Broskie (Tube Cad Journal). I modeled it
in PSPICE and it simulates beautifully. I should be able to drop
any tube in the circuit and have the bias current match almost
perfectly. I've got a crude PC board layout that will make the
several I'll need easy to build. Has anybody used this circuit
successfully? Here's the schematic as simulated:

_________________
Stephen

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Geek

Post subject: Re:
Autobias

Posted:
22 Jan 2011, 04:57

Joined: 08 Aug 2009, 03:11
Posts:
1502
Location: Chilliwack, BC

Yup, and simpler than that,
too:
http://geek.scorpiorising.ca/GeeK_ZonE/index.php?topic=1728.0

Cheers!

_________________
-= Gregg
=-
* Ratings
are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve
Design

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sampleaccurate

Post subject: Re:
Autobias

Posted:
22 Jan 2011, 13:05

Joined: 05 Jan 2010, 21:16
Posts:
680
Location: Las Vegas Nevada USA

Interesting. I'll have to try injecting a
signal that causes the tube to draw grid current and see what
happens. Then I'll model your modifications and compare. However,
I can't use a 10 ohm sense resistor, it must be less than 1 ohm, and
what about the diode directly attached to the cathode? Broskie only
has the positive input to an op-amp (10Meg or greater for an FET
input type) and a 1Meg resistor connected to the cathode. Without
looking at the circuit operation in detail, wouldn't a non-linear
component like a diode cause distortion if you connect it directly
to the cathode? Perhaps a voltage follower added between the
cathode and the diode (a third op-amp) would be a good
idea?

Thanks.

_________________
Stephen

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Geek

Post subject: Re:
Autobias

Posted:
22 Jan 2011, 21:37

Joined: 08 Aug 2009, 03:11
Posts:
1502
Location: Chilliwack, BC

sampleaccurate wrote:

Without looking at the circuit operation
in detail, wouldn't a non-linear component like a diode cause
distortion if you connect it directly to the
cathode?

Nah, the time constant in there takes the
average. The fast diode the peak and it's sufficiently forward
biased enough to remove non-linearities.

Quote:

Perhaps a voltage follower added between
the cathode and the diode (a third op-amp) would be a good
idea?

Sure! But that removes the "Muntz"-ness of the
citcuit.

Cheers!

_________________
-= Gregg
=-
* Ratings
are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve
Design

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sampleaccurate

Post subject: Re:
Autobias

Posted:
22 Jan 2011, 21:54

Joined: 05 Jan 2010, 21:16
Posts:
680
Location: Las Vegas Nevada USA

Well, I'll give it a try, in PSPICE at least.
I figure the total cost of a quad op-amp and other parts is about $1
per tube assuming I decided to use a 3 op-amp Broskie version.
Still very small and very cheap. I doubt I'll ever drive the amp
I'm using this for hard enough to make a difference in needing the
foldover circuit, but if it works well I can't see why it would
hurt. This just seems so much better than trying to manually bias
power tubes. I plan to try this in a 6C33C circlotron.

_________________
Stephen

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sampleaccurate

Post subject: Re:
Autobias

Posted:
24 Jan 2011, 13:39

Joined: 05 Jan 2010, 21:16
Posts:
680
Location: Las Vegas Nevada USA

Geek wrote:

Yup, and simpler than that, too:
http://geek.scorpiorising.ca/GeeK_ZonE/index.php?topic=1728.0

Have
you tried this circuit? I understand (I think) your logic, and
assuming perfect diodes it would probably work, but when I try to
simulate it the bias is unstable. The unmodified Broskie circuit
performs beautifully. Either I'm doing something wrong in the
simulation or the circuit doesn't work.

The next step is to
use a better tube model. It may well simply be an error in the
modeling of the grid and plate currents as the grid goes positive,
but I don't really see pushing the amp I'm constructing that hard,
so perhaps I'll just stick with Broskie. I like the idea though.
If over-bias becomes a problem I may have to take a second
look.

_________________
Stephen

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Geek

Post subject: Re:
Autobias

Posted:
24 Jan 2011, 21:27

Joined: 08 Aug 2009, 03:11
Posts:
1502
Location: Chilliwack, BC

Yes, I have used this circuit. I used it in a
trioded EL36, whose curves are so insanely vertical when tied that
way.

My advice: Too much modeling makes for an experimenter
that doesn't experiment - just build it on a breadboard and enjoy
the result

Cheers!

_________________
-= Gregg
=-
* Ratings
are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve
Design

Top

Suncalc

Post subject: Re:
Autobias

Posted:
24 Jan 2011, 22:43

Project Author

Joined: 06 Apr 2009, 10:08
Posts:
977
Location: US Pacific Northwest

Quote:

Too much modeling makes for an
experimenter that doesn't experiment

Awesome quote!

_________________
Matt
It's
all about the Glass!
http://www.cascadetubes.com/
Cascade Tubes Blog

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sampleaccurate

Post subject: Re:
Autobias

Posted:
25 Jan 2011, 15:43

Joined: 05 Jan 2010, 21:16
Posts:
680
Location: Las Vegas Nevada USA

Geek wrote:

Yes, I have used this circuit. I used it
in a trioded EL36, whose curves are so insanely vertical when tied
that way.

My advice: Too much modeling makes for an
experimenter that doesn't experiment - just build it on a breadboard
and enjoy the result

Cheers!

I
guess I should have built the "poor man's compressor" before
simulating it. Hell, it was in a book, it MUST work!!!

Suncalc wrote:

Quote:

Too much modeling makes for an
experimenter that doesn't experiment

Awesome quote!

I
respectfully disagree. First off simulation IS experimentation IMHO
- I'm experimenting with a model of a circuit that will very
accurately predict the behavior of the circuit I will build.
Secondly, I have 3 tube amps constructed under my belt in the last
year, with pics of all of them fully functional and even detailed
measurements of the performance of the KT88 Oddwatt and the 300B SE.
Thirdly, it took me about 10-15 minutes to setup the autobias
circuit in PSPICE and debug, etc. and another 10 minutes to modify
it to the "improved" version to test. It would have taken hours to
locate the parts (assuming I have them), breadboard the whole thing,
debug the circuit, and then take measurements of voltages in a
circuit with 1Meg resistors that could load the circuit and change
the measurements themselves. No such problem with
PSPICE.

Broskie even says in the TCJ that the diode connected
to the cathode is questionable and a non-linear load (I hadn't read
that far when I made the same comment). What's the point of
saving a 25 cent op-amp that's already included in the 4 op-amp DIP
package? I'm not being critical of your
design, just saying I can't get it to work and I think I know
why. The diodes leak forward current even
when biased in the mV range at less than their forward voltage and
also leak reverse current. If the resistor values were all lowered
and tiny currents didn't screw with the voltages the circuit would
probably work in simulation. Glad to hear it works for you, but
again, I'm not seeing the problem with the Broskie circuit that you
say you corrected nor does yours work in PSPICE. The diode
leakage (on the op-amp that is supposed to stop the overbias)
completely screws the circuit up.

The diodes leak - that's
one reason your circuit doesn't work in the simulation.
Also, the Broskie circuit will not work with low impedance
input op-amps for the same reason - current flow into the
inputs. FET input op-amps are required. I found this out a lot
more quickly and less expensively than breadboarding what is a
relatively simple SS circuit but one that will not work if not
properly constructed, and the description in the Broskie article is
silent on some issues that affect performance. I'm
not interested in relying on luck to get my circuits to
work.

Breadboard away, but I can guarantee you my circuits
are much more likely to work and work the way they were intended to
if they are modeled first and constructed second, especially SS
circuits with nothing but op-amps, resistors, caps and
diodes..

I finished my first OTL amp, pic is below!!! I
estimate about 40 WRMS per channel (of heat!).

Actually I'm
breaking in my 6C33C tubes to prepare them for the OLT amp. The
heaters are on with all other pins connected to one side of the
heater to getter all the gas in the 30 year old tubes.

I know
you guys are just ribbing me to get down to actually building a new
amp, but I think 3 amps a year is adequate, and rest assured that
this year you will have the pleasure of witnessing the construction
of two circlotron OTLs, one with 4 6C33C per channel and one with 12
to 15 6H13C per channel, a GM70 PP amp using an Edcor 10K 100 watt
tranny, and first up (due solely to simplicity) a class D SS amp
using 180 WRMS modules from Hypex along with switching power
supplies. The class D amp is super simple – probably a one day
assembly and about as far from a tube amp as you can get, but in the
quest for quality sound I’m not closing my mind to anything. If the
class D amp sounds good I’ll use it.

BTW, I purchased a new
home in November and since then have been unable to do any serious
construction with all the moving, so I’ve been testing circuits in
PSPICE. I finally got moved into my new place with a dedicated
electronics lab (or will be in another week or two) and should soon
have all my tools in my new garage so I can start drilling and
soldering. I’m glad I was forced to delay the builds because the
autobias for one would not be included, I learned how to create a
PSPICE tube model of ANY tube that you can get the plate curves for,
and I made some circuit tweaks to the Atma-Sphere circuit that
reduce distortion by increasing the driver voltage. I also learned
through PSPICE that the Atma-Sphere amps WORK BETTER (less
distortion – more power) with a balanced input. This was confirmed
by the designer of the amp in an interview I subsequently
read.

The point is that I’ve made many discoveries with
PSPICE that proved later to be correct even though I was initially
told “that’s not right – your simulation is wrong”. Yes, many of my
more complex simulations are “wrong” before I correct and tweak
them, but in the end more often than not the simulations are close
to real world performance. And if a circuit doesn’t work in PSPICE
(when properly modeled), chances are it simply doesn’t work, but the
reverse is not true. If it does work in PSPICE it MIGHT work when
you build it.

OK, OK, I get it. Time to start building
instead of designing. The only last comment I’ll make is that
ALMOST NOTHING commercial is ever built or even prototyped before a
computer model is used to simulate the circuit. I’m just applying
that principle to “my” ripped off designs to make sure they at least
have a chance of working and to make minor (or sometimes major)
improvements to the circuits before I build them. And if and when I
see pictures of 3 tube amps that someone else built within the last
year I will not challenge any assertions by that person about
"experimenting" vs. "simulating". Come to think of it there are
people on this forum (not the ones I'm responding to now) that say
they have built a lot of amps yet no pics??? Digital cameras are
ubiquitous, and webspace for pics is free.

_________________
Stephen

Last edited by sampleaccurate
on 25 Jan 2011, 15:59, edited 2 times in total.

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sampleaccurate

Post subject: Re:
Autobias

Posted:
25 Jan 2011, 15:52

Joined: 05 Jan 2010, 21:16
Posts:
680
Location: Las Vegas Nevada USA

PS: What's a good diode that has very small
forward leakage current (and very small reverse bias current)?
Perhaps it's the diode model that's causing the circuit failure.
Maybe I used a rectifier diode instead of a small signal diode.

Let me know what kind of diodes and op-amps you use and I'll
update the model.

_________________
Stephen

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