Conjunctive filters for EL84's DB: Here is a quote from that thread: ------------------------------------------------------------------------------- From: JaySG Date: 1/12/2001 12:22 AM Subject: Tonequest Dr. Z Interview -- Conjunctive Filters This seems like something to discuss. I'm including some RCA text after the Dr. Z snippet. Please bear with and keep on reading. Q: What other specific features are unique to your amps? A: I probably have a couple of original designs that are unique to my amps, and they were all evolved from a circuit that's called a conjunctive filter. It's a filter that goes across the primary side of the output transformer. The Carmen Ghia has a very traditional conjunctive filter, or corrective filter, as it's described in the RCA Receiver's Handbook. It affects the primary impedance of the transformer and allows frequencies to be very flat, or balanced. From say, 100Hz to 3K, the amplitude is the same. So from the high E to low E strings if your pick attack is the same, you'll get the same volume from the notes. The volume of individual notes isn't frequency dependent. So going back to what we were saying earlier about the touch dynamic of our amps, all that is related to the conjunctive filters that we use, and you're really in control because of them. Q: And this is unique to the Z's? A: No one else uses it. It's something I found in an old RCA book of my dad's, and again, it's referred to as a corrective filter. It was just a little side note on how to make an amplifier more linear within a certain band of frequencies. When you strum a chord, each note makes it's own contribution to the sound without one note overpowering the other. Jazz players love it when they're playing those big, 6-string chords—it really puts a twinkle their eye when they hear it. The Prescription is another one in which I went with a different approach with the conjunctive filter. I have Todd Sharp to thank for that. We were in the tuning room for a Rod Stewart show and I'd sent Todd some parts for a Dr. Z Prescription he was using. We started playing around, and before you know it, we had evolved it into a really cool sound with different values of resistors and caps that we used. It's funny how working with artists, you develop things. --------------------------------------------- From a RCA Receiving Tube Manual: 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. ---------------------------------------------- --submitted by Steve Ahola (thanks to bw for emailing it to me!) I'm sure he means currently but, I've been working on a lot of old Orange amps the last few months and have found a couple with factory installed 1500v .01uF caps with a 2 watt resistor (in series) from each OT plate wire right back to the center tap. Bruce Yes they do run a NFB loop John. bruce You may be surprised what they can accomplish. Don't knock it till you try it. Depending on the amp and OT you can make radical changes in how the thing responds, how the bass reacts, how the highs are and yes, even string balance. Not a panacea, but a heck of a lot cooler IMHO than caps from your plates (which IMHO should be banned as a general rule. ) Speedracer