[sdiy] tube ladder

media.nai at rcn.com media.nai at rcn.com
Mon Jun 24 19:54:59 CEST 2002

At 11:51 AM +1000 06/23/02, Paul Perry wrote:
>With the classic Moog ladder, it is important to have at least
>the top & bottom pair of transistors matched (I don't know why,
>unless it is to null out 'unfiltered' signal).
>So that might be a problem, if you have to match some of the
>pentodes as well....

My half-baked idea is more like a tube OTA ladder filter -- like the SSM
and Roland chips.

>but if you DO have a supply of matched pentodes,
>go into the lucrative valve compressor business!

I've spent so much time working on cars lately that whenever I hear "valve
compressor" I think of an actual valve compressor :)

At 11:47 PM -0700 06/22/02, Matt Jones wrote:
>Alternatively, one could consider using 6JH8
>(among others..). These are referred to as
>"beam-deflection" tubes, and essentially use
>electric fields to bend the current passing
>through the tube from one anode to another.
>In principle, it should be possible to control
>the total current in one with the gate-cathode
>voltage, while using the deflectors as signal
>inputs. See Eric Barbour's pages for more on the

OK :)

At 1:20 AM -0700 06/23/02, metasonix at earthlink.net wrote:
>>>yes, in fact any device having control grids can be used.
>>Any reason why it hasn't been done??
>I personally haven't had the time. Perhaps disinterest on the
>part of certain experimenters who think they have tried everything
>and know everything?.....

Well, I know next to nothing, so I should be all set :)

>>Are there any particular remote-cutoff pentodes that you would recommend
>>for audio applications such as a VCF??
>Anything will do, there are not huge differences between
>various small-signal pentodes of the same type, except
>you have sharp cutoff, semi-remote cutoff, and remote
>cutoff--the latter will work best as a voltage controlled gain.
>Surplus 6136s are available--the "five-star" premium
>version of 6SK7.
>Lower cost desired, use 6BA6s.

6SK7 seem popular based on what I've found.  If 6136's are both better and
less money then it's a no brainer.  Is that what you're saying??

>If you want to build a big, physically impressive filter,
>type 39/44s are VERY cheap and plentiful.

I'm hoping that if it works, it will be small enough to fit in a 2U rack
box.  It's not like I need to compensate for anything ;)

>On the other hand, if you want low microphony and small
>physical size, look into subminiatures. Types 5633 or 5907.
>Bonus, no sockets needed, just solder them to circuit points.

Do they have lugs or are they some sort of PCB tubes??  I'm thinking
sockets will make the tubes easier to swap and replace.  Being a new
design, it will probably take a few tries to get it right.

>>devices I've seen use brass or steel, couldn't the sockets be mounted on a
>>sheet of plywood in a metal rack box, or is that a fire hazard??
>;) remember, all radios were made of tubes at one time, and
>were usually enclosed in wooden boxes....fires due to hot radios
>didn't become common until the arrival of the plastic cabinet.
>In fact, that's one reason why Catalin radios are
>now scarce and selling for insane prices.
>Catalin is a primitive cast phenol thermosetting resin that
>ages poorly and catches fire somewhat easily.....
>but it's pretty, so radios were made from it....

OK :)  I'm thinking the wood will either be below or perpendicular to the
tubes.  Given that heat rises, it should stay cool enough if I vent the
box.  I'm not set up for metal working, but I can cut a piece of plywood.

At 11:51 AM -0700 06/23/02, Tim Ressel wrote:
>Okay, I'm trying to understand all this. But something
>does not sound right. When using an CA3080 for a
>variable filter, the aspect of the 3080 that is called
>into play is its CURRENT output. You take a 3080 and
>put a cap to ground on the output and you make a
>filter. By changing the 'gain', you vary the amount of
>current that flows into the cap, thus changing the
>center frequency.

That's what I'm thinking -- by essentially varying "R" you vary the time
for "RC".

>Now to use a tube to do this, how would that work?

Allow me to preface the following by saying that I could be completely
@#$%&* wrong.  I don't know much about tubes.  I've never had to fix
anything using a variable mu.  Almost all of my experience comes from
triode preamps and power amplifiers.  I know next to nothing about
small-signal pentodes.

>There is DC current flowing through the tube plus a
>little AC current from the signal. But the plate R
>turns that into voltage.

Yes, this is true for a "resistance-coupled" triode amp.  Yet afaik, Ra
("the plate R") is entirely external.  You have two things connected to the
plate:  you have Ra going to Va, and a DC blocking cap going to the output.
The plate and cathode resistors set the bias point.  So you need Ra, and
yes, the output is a voltage.

>If you put a cap from the plate to ground, you just shunt the voltage
>>variations to ground.

Right, if there is a cap going to ground on the plate side, it is from Va
to ground, on the other side of Ra.

I've looked at a number of AGC circuits using pentodes.  They do have a
resistor going to an external voltage, and an output cap, just like a
typical triode amp.  So I see your point.

However, I found a few circuits that do not use Ra.  I'm looking at a
"cascade RF amplifier"  where the plate of the first triode goes through a
cap and resistor in parallel to the cathode of the following triode.  The
tubes or more or less connected in series -- nothing else is connected to
the cathode of the second tube, or the plate of the first tube.  The plate
voltage of the first triode is held constant while its plate current
varies.  This action is like a pentode tube.  Now if the first tube was
replaced by an actual pentode, and the parallel resistor was removed, then
wouldn't the "variable resistance" of the pentode combined with the cap act
as a VCF??  (As an aside, I do not think using a pentode as the second tube
and putting the cap on the cathode side would work, or if there is any sort
of tube with a variable input impedance.)

If that is true, I think the next question is how one would stack several
poles.  Maybe several pentodes could be connected in series.  Like I said
before, I do not know if this idea would work, it's simply based on my
observation that OTA's and remote-cutoff tubes seem similar in function.

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