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Hey folks...

I'm curious...have any of you guys screwed around with a Conjunctive filter on your 18 watters?

if you did, what were your results...

-Frank

Here's a thread I saved from another board.



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> QUESTION: conjunctive/corrective/conjugate filter?

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Old Tele man
Techus Maximus
Posts: 930
(8/20/03 6:54 pm)
Reply QUESTION: conjunctive/corrective/conjugate filter?
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Stumbled across info from a Tone-Quest interview of Dr. Z in which he mentioned learning about a "corrective filter" from "...an old RCA book...." Later in the interview, the book is called "RCA Receiver's Handbook."

Anybody know WHICH RCA book he was referring to? I have the 1944 and 1968 RCA Receiving Tube Manauls and neither of them mentions this "corrective filter."

From my understanding it's just a series-RC circuit that connects across the OT-primary -- instead of connecting across the OT-seconday as the Zobel filter does -- which "smooths out" the inductive reactance "rise" that occurs at higher frequencies; this is exactly the same thing that the Zobel filter does! The difference between to two seems to be that:

1) the Zobel filter moderates OT-secondary and the SPEAKER inductance; while...

2) the conjunctive filter moderates the TOTAL inductive reactance seen by the output tubes, which includes OT-primary, OT-secondary and the speakers' inductances.

Someone named "ted" also called them a "conjugate" filter, because they "...present the conjugate (opposite sign) load to the tube to smooth out the signal and machinations in the plate circuit of a power amp."

Has anybody tried or used these circuits on their amps? It's used on the Fender PRO Jr. amp with 6BQ5/EL84 tubes: 270-Ohm and 2200pF.
"...Renovatio, non Exauguratio..."

Edited by: Old Tele man at: 8/21/03 10:03 am

Tremo
No Hack Butchers !!
Posts: 1690
(8/20/03 6:35 pm)
Reply Re: QUESTION: conjunctive/corrective/conjugate filter?
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Ive seen those. It's a series RC network connected between the plates of the output tubes.

SG123
I'm a Picker, I'm a Grinner
Posts: 3875
(8/20/03 7:34 pm)
Reply
ezSupporter
Re: QUESTION: conjunctive/corrective/conjugate filter?
--------------------------------------------------------------------------------
Dr. Z is a guy named Mike Zaite from right here around Cleveland - builds some nice guitar amplifiers.

There has been quite a lot of discussion about those filters over at Ted Weber's amps forum and at Ampage. I've saved some stuff:

From a RCA Receiving Tube Manual[ don't know which one] :

A corrective filter can be used to improve the frequency characteristic of an output stage using a beam power tube or a pentode when inverse feedback is not applicable. The filter consists of a resistor and a capacitor connection in series across the primary of the output transformer. Connected in this way, the filter is in parallel with the plate load impedance reflected from
the voice-coil by the output transformer. The magnitude of this reflected impedance increases with increasing frequency in the middle and upper audio range. The impedance of the filter, however, decreases with increasing frequency. It follows that by use of the proper values for the resistance and the capacitance in the filter, the effective load impedance on the output
tubes can be made practically constant for all frequencies in the middle and upper audio range. The result is an improvement in the frequency characteristic of the output stage.

The resistance to be used in the filter for a push-pull stage is 1.3 times the recommended plate-to-plate load resistance; or, for a single-tube stage, is 1.3 times the recommended plate load resistance. The capacitance in the filter should have a value such that the voltage gain of the output stage at a frequency of 1000 cycles or higher is equal to the voltage gain at 400 cycles.
A method of determining the proper value of capacitance for the filter is to make two measurements of the output voltage across the primary of the output trasformer: first, when a 400-cycle signal is applied to the input, and second, when a 1000-cycle signal of the same voltage as the 400-cycle signal is applied to the input. The correct value of capacitance is the
one which gives equal output voltages for the two signal inputs. In practice, this values is usually found to be in the order of 0.05 microfarad.


from Ampage, as I recall:

"How did you figure out the Resistor and Cap value for the conjugate filter?. I am building a P-P KT88 and would like to try it out.

According to SpeedRacer, who initially hipped me to the idea years ago, you multiply the primary impedance by 1.3, then use the closest standard resistor value to that number. I used a 10W resistor, but even after cranking the amp for a while, it doesn't even get warm, so you can probably get away with a 10W even with KT88s.... You just hook it in series with a 2nF cap across the primary winding. I used a 1600VDC rated cap--can't be too careful here. You can vary the cap to taste, btw. I initially bought 2nF and 3n9F caps to try here, but settled on the 2nF that was suggested....

So if I have a Ot with a primary inductance of 6K6, 6K6x 1.3=8K58. ? Right. .What was the differences you hear with the various cap values?

The difference was more of a "feel" thing than a sound thing. The low end felt better with the 2nF.

Unfortunately, I don't have the equipment to measure why, although I could speculate that it has something to do with the location of the conjugate load's RC corner relative to the location of the speaker's LF resonance.

I imagine it's a "try it and see what works for you" kind of thing, and it might be different with different speakers, tubes, and transformers.

BTW, I used 6k6 primaries in my amp as well (Hammond 1620), with a pair of Sovtek 5881/6L6WGC. The resistor I used was an 8k5, 10W unit."

various other snippets:

"With a single pair of EL84's in push-pull I settled on a 10k/1W resistor (7000 ohms x 1.3). For the cap I tried a few different 1kv ceramic caps I had around and 2200pF sounded the best to my ears."

-------------

"Okay, I was just fooling around... had 5 minutes to spare, before going to work. I used a 5k6 resistor -- working with an 6L6, 4200*1.3. And a .0047uF Orange Drop (sorry, it's all I had to spare).
It seemed to smooth bass response a bit. But it really killed my treble. Sounds like it's rolling off around 4-5Khz. I guess I'll try a couple of other caps.
The RCA comment says .05uF. And Steve tried a 2200pF? So I was somewhere in the middle, the mirky middle I suppose."

---------------

Hope that helps a bit. I saved some schematics from several Hammond organ reverb amps that use the conjunctive/conjugate filter across the OT primary.





Old Tele man
Techus Maximus
Posts: 931
(8/20/03 9:07 pm)
Reply Re: QUESTION: conjunctive/corrective/conjugate filter?
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Yep! That's the quoted referench source, but, just like Paul Harvey, I want to know "...the REST of the story..." before I leap in and "add" things into my amp! Here's why...

Years ago I tried the Zobel filter on my SR (temporarily alligator-clipped across speakers) and got 'smoother' bass but it also lost all the high-end 'chime' that I liked. As I recall, the component values I used were 2.7-Ohms and 20µF, which should've moderated any impedance rise from the nominal 2-Ohms impedance to 4-Ohms (or less) at 5KHz. Apparently, that was TOO much capacitance, inspite of what the equation said...

And, since the Zobel filter didn't seem to work, I'm even more hesitant to consider trying the conjunctive filter, which is WHY I posted this call for help in possibly tracking down the original information source...that RCA book.
"...Renovatio, non Exauguratio..."

Edited by: Old Tele man at: 8/21/03 10:05 am

SG123
I'm a Picker, I'm a Grinner
Posts: 3876
(8/20/03 8:25 pm)
Reply
ezSupporter
Re: QUESTION: conjunctive/corrective/conjugate filter?
--------------------------------------------------------------------------------
The information's there - and that's probably all there is..

If you find more, let us know.

Man o Blues
*
Posts: 571
(8/21/03 10:00 am)
Reply Re: QUESTION: conjunctive/corrective/conjugate filter?
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I have a conjunctive filter accross the primaries in my old Reverberocket and it has definitely helped to balance the frequencies. There were some dead spots in the high end before the filter was installed. IIRC, I used 9K resistor in series with a .002 cap.


Old Tele man
Techus Maximus
Posts: 932
(8/21/03 6:43 pm)
Reply Re: QUESTION: conjunctive/corrective/conjugate filter?
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Found it! The two paragraphs can be found on page 23 of the 1937 RCA Receiving Tube Manual, RC-13.

Remember my earlier posting about how the changing impedance of a speaker would affect tube gain -- WELL -- that's, apparently, exactly what the corrective (conjugate) filter takes care of...

It would be interesting to compare various RC vs. OT primary plate-to-plate (Zpp) values to see how "closely" reality matches with theory. Anybody else got some values to add?

MoB: what was the OT primary Zpp impedance?
"...Renovatio, non Exauguratio..."

Edited by: Old Tele man at: 8/21/03 6:46 pm

EMan
Hell Yes, I'm from Texas.
Posts: 2582
(8/21/03 7:46 pm)
Reply Re: QUESTION: conjunctive/corrective/conjugate filter?
--------------------------------------------------------------------------------
I remember SG and I talking about this very thing a few months ago in one of the other forums here.....
"I haven't failed, I've just found 10,000 ways that don't work."
- Thomas Edison -

Old Tele man
Techus Maximus
Posts: 934
(8/21/03 10:46 pm)
Reply Re: QUESTION: conjunctive/corrective/conjugate filter?
--------------------------------------------------------------------------------
see...I'm only a couple month's behind you guys in the thinking & discussing department. Pretty soon now, I'll discover relativity, you know: mother-in-laws, cousins, etc...
"...Renovatio, non Exauguratio..."

Edited by: Old Tele man at: 8/21/03 11:11 pm

EMan
Hell Yes, I'm from Texas.
Posts: 2583
(8/22/03 8:10 am)
Reply Re: QUESTION: conjunctive/corrective/conjugate filter?
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Seems like it was in the Hammond area. I'll bet SG remembers.

..my memory is excellent..just real short..
"I haven't failed, I've just found 10,000 ways that don't work."
- Thomas Edison -

SG123
I'm a Picker, I'm a Grinner
Posts: 3877
(8/22/03 8:19 am)
Reply
ezSupporter
Re: QUESTION: conjunctive/corrective/conjugate filter?
--------------------------------------------------------------------------------
I remember someone pointed out the use of one in the Hammond amp I mentioned earlier, [ wasn't on this website as I recall ] and I looked up the schematics and saved them for reference. I'd have to go searching for any more of a mental connection than that. Getting old's a bitch.

It would be nice to have the time to tinker with them because I have heard some good things about them, and it's a simple thing to add or remove.

GroundhogKen
*
Posts: 462
(8/22/03 8:20 am)
Reply Re: QUESTION: conjunctive/corrective/conjugate filter?
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Check out the output section of this amp. Is this conjuntive filtering?

theatreorgans.com/hammond...22mn-a.pdf


Ken

SG123
I'm a Picker, I'm a Grinner
Posts: 3878
(8/22/03 8:42 am)
Reply
ezSupporter
Re: QUESTION: conjunctive/corrective/conjugate filter?
--------------------------------------------------------------------------------
It's interesting to look at circuits other than the typical Fender / Marshall. It looks to me like each output tube has its own local negative feedback circuit with the .0047 cap and 390K resistor between plate and control grid. [ Then there's those .0047 to ground on each plate - an ultrasonic filter? ]

I'd say it's not a conjunctive filter in the "classic" sense, but I'm just a student of this stuff.

Old Tele man
Techus Maximus
Posts: 936
(8/22/03 10:11 am)
Reply Re: QUESTION: conjunctive/corrective/conjugate filter?
--------------------------------------------------------------------------------
no, those caps are not a corrective / conjunctive / conjugate filter. The RCA corrective filter is a simple series RC network (1st order low-pass roll-off) thats connected across the primary windings (plate-to-plate) of the OT. It's purpose is to compensate for the "rise" in inductive reactance (XL) at higher frequencies. By matching the rolloff of the RC network to the rise of the inductive reactance, the impedance "seen" by the output tubes becomes much "flatter" and closer to a "straight" load line instead of the normal "elliptical" line.

The "key" to the corrective filter working, is the "matching" of the RC rolloff to the XL increase, which isn't a simple thing. Being located on the primary side of the OT, it and the output tubes (see my earlier posting about speaker effects on gain) "sees" a complex "reflected" load that consists of primary-, secondary-, and speaker inductive reactances. Since each has a 'different' inductance value, it follows that they each has its own different inductive reactance (XL). Thus, a single simple RC corrective filter cannot be 100% effective, but something is always better than nothing!

I wonder what a SPICE simulation would show?


"...Renovatio, non Exauguratio..."

Edited by: Old Tele man at: 8/22/03 2:33 pm

Tremo
No Hack Butchers !!
Posts: 1696
(8/22/03 6:09 pm)
Reply Re: QUESTION: conjunctive/corrective/conjugate filter?
--------------------------------------------------------------------------------
A cap between plate and control grid is a "miller" capacitor. it has the function of drastically increasing the input capacitance of the tube. The intent would be to roll the highs. It's probably for stability. to prevent ultrasonic oscillations.



EMan
Hell Yes, I'm from Texas.
Posts: 2585
(8/23/03 9:46 am)
Reply Re: QUESTION: conjunctive/corrective/conjugate filter?
--------------------------------------------------------------------------------
I think these ramblings are interesting...as mentioned..we spend most of our time musing at the basic guitar amp output stages...when, if fact, there are dozens of different configurations.
"I haven't failed, I've just found 10,000 ways that don't work."
- Thomas Edison -

Old Tele man
Techus Maximus
Posts: 940
(8/24/03 9:29 pm)
Reply Re: QUESTION: conjunctive/corrective/conjugate filter?
--------------------------------------------------------------------------------
FYI -- Dr. Z e-mailed me back and said he uses 10K resistors with 8K-primary OT's and hand-selects the appropriate capacitor value (1000V for safety) for flattest response. Although he didn't say so, he implied doing it just like the RCA manual, ie: same amplitude at 400Hz and 1kHz with swept sinewave input.
"...Renovatio, non Exauguratio..."

Edited by: Old Tele man at: 8/25/03 5:22 pm

SG123
I'm a Picker, I'm a Grinner
Posts: 3885
(8/25/03 5:10 pm)
Reply
ezSupporter
Good info
--------------------------------------------------------------------------------
Just curious - did you e-mail a question to him or did he reply as a response to your posting here?

Perry Roper who is associated with the company in some way posts over at Ted's forum from time to time.
So many frequencies, so little time...

Old Tele man
Techus Maximus
Posts: 947
(8/25/03 5:39 pm)
Reply Re: Good info
--------------------------------------------------------------------------------
e-mailed direct (from home) and got prompt reply.
"...Renovatio, non Exauguratio..."


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I used one in a Firefly type amp to smooth out the harsh high end. At the end of the day I have abandoned that output stage, because the bass never was very good. I'm planning on converting that amp to a SE 6V6 as soon as I get around to buying a new pair of appropriate transformers.

However, I have used a Vox style Cut control on a couple amps. I just use a very high value cap, so it acts more like a Presence control. I generally use a 500p cap and a 500k pot. On a Vox they use a 250k pot and a .0047uf cap, which acts a little more like a treble cut. This goes in parallel with the output tube grids, like PPIMV. It does a good job of smoothing things out. The nice thing is that it gives you a little more control over the high end than just the normal tone controls.

I used a 10k resistor with .002 uF cap.

The 1650F is more of a hi-fi type tranny so the only thing I
noticed was a reduced hi-end. It took the non-musical hi-end
trash out of the speaker. It also seemed to smooth everything out
as well. I suppose I would need an A/B switch to really tell the difference.

I'm using a Weber Ceramic Silver Bell with it.

I really don't care for this tranny.

When I get a real guitar tranny, I'll also probably dump the conjunctive filter.

Hope this helps.

tkroeger wrote:I suppose I would need an A/B switch to really tell the difference.

See Eric Nyberg's site for info on how to switch it out...very simple:

http://www-2.cs.cmu.edu/~ehn/ax84/image ... Layout.gif


I was just wondering if anyone tried it on their 18. The more I think about it, the I come to the conclusion that it's probably not needed on the 18....but I was just curious to hear if anyone's tried it.

-F

Conjunctive filters were basically a cheap way to somewhat flatten out the response of lower cost OTs. The downside is that they suck output power. The WEM Dominator has a conjunctive (aka corrective) filter. I guess because the 18W used a fairly good quality OT, it was considered not to be necessary.

zaphod_phil wrote:Conjunctive filters were basically a cheap way to somewhat flatten out the response of lower cost OTs. The downside is that they suck output power. The WEM Dominator has a conjunctive (aka corrective) filter. I guess because the 18W used a fairly good quality OT, it was considered not to be necessary.

Ah...that was the other thing. I had read somewhere that they tend to impact output power....

Hey rjgtr, you mentioned trying this on a Firefly. I am just starting my firefly build, and could still abandon it for I have only ordered $20 worth of parts. Is it really a harsh amp? Maybe I should just go with the P1 eXtreme.........Thanks,

Chris

I worked with the firefly style output for over a month off and on and could never get a grown up amp sound from it. What I was after was an amp I could soak the output of and put it through a speaker emulator and have a 1U rack mount amp that sounded good direct. I used the channel switching preamp I have used successfully in several builds now. I tweaked and tweaked, but it just doesn't sound like a bigger amp. The 12AU7 is a nice tube, but it just doesn't put out enough low end to get a really full sound. I had to do some pretty severe EQ to get it more balanced. Through a speaker it was sort of Ok, but never great.

I even tried reconfiguring it to parallel single ended. It is a fun little amp through a speaker, but not mature enough sounding. I could never get it to stop from being harsh sounding. And worst of all I could never get a full and beautiful clean sound. It was always a touch harsh and one dimensional.

I am going to convert my build into a 6V6 SE output amp. It will mean a new 2U rack chassis and a new pair of transformers, but I think I'll be a lot happier. You live and learn. I know the single ended 6V6 amp with sound good because I already have built one and liked it. Not being able to leave well enough alone I later turned it into a 2 6V6 amp with a new set of transformers. Since that sounds good, I'm leaving it as is.

So my advice is to build an amp with a Champ or better Gibson GA-5 style output stage. The firefly is fun, but I think 12au7s are better left for reverb drvers, where their lack of a big low end is a plus.

Just my .02. Go to Doug Hammond's site and listen closely. You'll see what I mean about the firefly. Then again you might really dig it. But since you asked...

strangegrey wrote:

zaphod_phil wrote:Conjunctive filters were basically a cheap way to somewhat flatten out the response of lower cost OTs. The downside is that they suck output power. The WEM Dominator has a conjunctive (aka corrective) filter. I guess because the 18W used a fairly good quality OT, it was considered not to be necessary.

Ah...that was the other thing. I had read somewhere that they tend to impact output power....

The Carmen Ghia uses a conjunctive filter, and doesn't lack power. I also doubt it's used to cover for an inferior OT. The power loss may be real but not apparent to the ear. The frequency balancing effect is a matter of taste - to each his own. I bought the parts, but I haven't tried it yet in this amp.

strangegrey wrote:Hey folks...

I'm curious...have any of you guys screwed around with a Conjunctive filter on your 18 watters?

if you did, what were your results...

-Frank

Frank,
Did you search here for previous threads on the Conjunctive filter? Here is a few pages from Tremain's Audiocyclopedia I posted in another thread that address the conjunctive filter.

http://www.18watt.com/files/tremaine-co ... filter.jpg

This is not just some band aid fix for some problems. It really has a sound of it's own. You will have to try it to see if it is something you like.
markd

I was only stating why conjunctive filters were originally used, which is why they're also called "corrective filters", but that doesn't necessarily have to be the same in 100% of all cases today. I don't think the power loss is huge, but you do loose some there. Also, the Carmen Ghia is based on a Hammond organ reverb channel amp, so I'm not really sure what the natural frequency response of its OT would be without a CF. I have seen posts from builders who were using CFs, but once they got their amps voiced right, seem to be glad to be able to dispose of the CF.

I guess as I have the Carmen Ghia clone I should be the one to do the experimentation Currently the conjunctive filter is using a 10K and .0022uF cap (bit suss about the cap as it was out of the original Hammond). Find the amp Darth Vader (a bit on the dark side) for me but I have been using a Tweed Bassman for the last six months. I was going to start playing with the cap values in the tone stack but maybe I should begin with the CF. I shall update this W/E. As far as output is concerned I was a bit taken back by the Ghia clone, it seems louder than my TSL602 although a lot easier on the ear.
Nigel

Your Ghia might not be louder, but is probably fuller and more rich sounding.

Legin wrote:I guess as I have the Carmen Ghia clone I should be the one to do the experimentation Currently the conjunctive filter is using a 10K and .0022uF cap (bit suss about the cap as it was out of the original Hammond). Find the amp Darth Vader (a bit on the dark side) for me but I have been using a Tweed Bassman for the last six months. I was going to start playing with the cap values in the tone stack but maybe I should begin with the CF. I shall update this W/E. As far as output is concerned I was a bit taken back by the Ghia clone, it seems louder than my TSL602 although a lot easier on the ear.
Nigel

Nigel,

see my above post for a link to Eric Nyberg's CF info. it's easy to drop in a switch to remove the CF. break the connection between the two for a quick test...and if it's a sound you might like, simply run a switch between the two, and you've got a switchable CF.

-Frank

LOL, I bought a switch and terminal strip yesterday on the way home from work, I will fit it today after I have tried a selection of caps .0011uf to .0033uf
Nigel

OK, fitted the switch to the conjunctive filter and have been playing for about an hour and a half switching it it and out. Now maybe my ears are just suffering from age but I am finding it very difficult to hear any noticable difference. There again maybe Hammond/Dr Z got this design perfected and the CF is just fine tuning, still I now have another small switch at the back of the amp to play with. The introduction of the 5751 really did make a difference to the sound though, the amp is the most responsive to fingers and changes of guitars and the tone stack has a multitude of wonderfull sounds. I am stll going to do the 18 Watt clone however now I have to buy some trannies as there is no way I am taking this baby apart!
Nigel

It's designed to balance frequency response, so there's shouldn't be much tone change. The effect would show up if you played an arpeggio across six strings, and one string seemed to drop out - or be louder. To tune it for a particular amp, you're supposed to get the signal level the same when feeding in test signals at two different frequencies. the other way is just to try different cap values. Eventually, I'll try it myself just to see what happens. Too much going on right now.

Anyone have any clues as to where to find "Z" ish schematics?

Thanks

Haven't tried it on an 18w, but I have it on my AX84 HO, switched with two different cap values (Can't remember the values & I'm too lazy to go searching through my notes right now ) It's nice on the HO to smooth out some of the fizz & nastiness in the treble, but it definitely does affect the volume level.