The Quad ESL-63 has gone through several revisions of the input protection boards.  The latest design uses a bridge rectifier and zener diode stack similar to the treble panel protection circuit in the Original ESL’s.  This is combined with a simplified arc detection and clamp circuit.  This latest design has been used in all the subsequent Quad ESL models from the 988, 2805, and 2812 models.  

The arcing clamp circuit senses the electrical field from an arc with a short antenna by rectifying the signal with a diode.  This rectified signal turns on a transistor which clamps the input on a 555 timer chip wired as a one-shot.  This then turns on a TRIAC which shorts the input to the speaker.  

The OEM arc clamp board also contains the input ballast resistors as well as a capacitor “zobel” for the input transformers.  In the OEM circuit the input series resistor (1.5ohms) and parallel capacitor (220uf) are mounted to the chassis.  

I have designed a replacement zener clamp board which is much like the OEM unit, but uses surface mount zener diodes.  I have also designed a replacement arc clamp board which uses precision ballast resistors and also incorporates the input resistor and parallel electrolytic capacitor.  And a needed improvement is provisions for adding a healthy sized film bypass capacitor across the input electrolytic.  

The schematics and layouts of both boards are here:

ESL-63 Arc Clamp Board Schematic

ESL-63 Zener Clamp Board Schematic

ESL-63 Arc Clamp Board Layout

ESL-63 Zener Clamp Board Layout

For a number of years, I suggested a gas discharge tube to protect the original Quad ESL treble panels.  and they do work well, but I’ve had them get leaky over time and their striking voltage drops.  I have reverted to the stock Quad ESL treble protection circuit.  The original circuit is a rectifier bridge of 3000 volt diodes, where the input to the treble panel is applied across the AC inputs of the bridge.  A stack of zener diodes is placed across the rectified outputs.  

I’ve designed a little board to fit under the input transformer case.  The schematic and layout are shown here:

Clampboard Schematic

Clampboard Layout

The vast majority of the original Quad ESL’s have rectifier modules that consist of a bakelite box with a circuit board sticking out the top and filled with bee’s wax.  This module can reasonably be rebuilt by heating the blocks in an oven (at 275F) to melt the wax out, and then replacing the components on the circuit board.  When complete, the molten wax is then poured back in the box with the circuit board and allowed to cool.

However, the early Quad ESL’s came with epoxy potted blocks that are not rebuildable.  And occasionally I’ve come across speakers which are not complete, and the rectifiers are missing.  I designed a replacement rectifier block as an upgrade to the original design.

My design uses the same cockroft-walton voltage multiplier as the originals, but rather than running the panels straight off the multiplier, I isolate the panels through a series resistor and a neon bulb (like the Quad ESL-63 circuit).

SDS Labs Quad ESL EHT Schematic

The Citation II is certainly a lovely amplifier, and has been a workhorse for me over the years.  I’ve owned four of them to date.  With every one of these that I have rebuilt, I’ve modified the bias meter circuit.  The Citation II bias meter has two marks on it, an AC balance mark; which is the neutral needle position with no current input to the meter.  The second mark is a “bias mark” which is the desired operating point for the tubes and is adjusted with four bias potentiometers.  The two mark meter makes setting AC balance and DC bias un-ambiguous, but it doesn’t lead to any flexibility when using many modern tubes which require less plate power dissipation.

The DC bias portion of the meter circuit uses 15 ohm resistors inserted between each output tube’s cathode and ground.  This “sense” resistor arrangement produces a voltage at the tube cathode that is proportional to the current through the tube via Ohm’s law (V =I * R).  The meter circuit uses that voltage to move the needle.

The meter itself is really an ammeter, in that the needle displacement is proportional to the current flowing through the meter coil.  The coil resistance is really quite small.  The current required to move the needle to the center of the bias mark in my meter is 3.77 mA.  The meter circuit uses a series resistor to set the voltage versus needle position relationship.  for example, if I wanted 1 volt across the meter to move the needle to the bias mark, I’d put a 265 ohm resistor in series with it.

The very early Citation II’s (which the Sam’s PhotoFacts seem to be based) had a 470 ohm resistor across the meter, so if the original (outside the chassis) meter had the same sensitivity, that would be a voltage of 1.77 volts.  For the 15 ohm cathode resistors, that means that each output tube would have been drawing 118 mA for a plate dissipation (assuming a 460 volt B+) of 54 watts.  I suspect that the original meter was less sensitive, but I have no way of knowing, as I’ve never owned a very early citation.  The later internal meter units had 330 ohm resies resistors installed for a plate dissipation of 38 watts per tube.  However, in an addendum to the assembly manual, HK suggests that the 330 ohm resistor be reduced to a 300 ohm one.  This reduces the quiescent plate dissipation to 34.7 watts per tube.  However this is still very close to the maximum allowable plate dissipation of many modern 6550 or KT88 tubes; for example the Svetlana 6550C has a max plate dissipation of 35 watts.

So in the past I’ve always changed that resistor to a smaller value to run 30-32 watts on the plates of the output tubes.  Other references out on the web suggest changing the cathode resistors on the output tubes rather than change the series resistor in the meter circuit.  Either will accomplish the same goal, but changing the cathode resistors requires changing four resistors instead of one, and they are much more difficult to get to.  The output tube cathode resistors are buried between the power supply bracket and the input tag-boards.

The goal of the circuit shown above is to provide several settings for the bias meter that will not require soldering if the output tube brand or type is changed.  The circuit uses a set of three resistors that are switched in and out of the circuit with a rotary octal switch and a fourth resistor in series.   These resistors provide seven combinations of overall resistance that allow the meter bias point to be set to 6 usable positions from the stock (330 ohm) value down to 30 watts of plate dissipation.

Position 2 is the stock value, and position 3 is very close to the value specified in the addendum.  Positions 4-7 allow a range of values to be selected for modern KT88 class tubes.

The board is held in place using the meter retention bolts and 1/4″ square section aluminum stock which has been tapped at one end and cross-drilled and tapped at the other (both tapped 6-32).  This provides a secure mechanical mount for the meter adjustment circuit, etched onto a small circuit board.  The layout and a simple schematic for this boar is show here:  meterAdjustBoard

Changing of the meter range is as easy as flipping the amp over, removing the bottom cover, and turning the knob to the desired position.  The table shown above was taped to the chassis to assist in setting the proper meter range.  The advantage to having the circuit inside the amplifier is that no metalwork has been modified, and the amplifier can be returned to the stock configuration at any time.

The Harman Kardon Citation II is a great amplifier to be sure, but there are some limitations to the design.  One of the problems that has bothered me over the years are the terminal strips and the poor quality input jacks in the original design.  Many years ago I solved the input jack problem with new FR4 input boards that hold modern high-quality RCA jacks.

As seen in the photo’s above, a set of three layers of 1/16″ thick FR4 fiberglass was used to provide a platform to mount modern RCA jacks.  the four holes at the corners of the boards mate perfectly with the mounting holes in the Citation II chassis.  The bottom layer has smaller holes designed to accept the RCA jack and nut.  The next two layers act to space the mounting nut away from the amplifier chassis to prevent the RCA return connection from being shorted to the chassis.  This simple change allows high quality connections to the amplifier without modifying the metalwork of the amplifier at all.

The second modification is aimed at making the speaker connections more reliable and easier to use with a variety of modern speaker cables.  The original amplifier terminal strip provides four connections for each speaker, a ground return and three different taps on the output transformer for three different nominal speaker impedances (4, 8, and 16 ohms).  The chassis cutout for this terminal strip is fairly small and would at most provide room for three connections.  My solution is to move the terminal strip inside the amplifier chassis and provide a pair of modern 5-way binding posts.  This was accomplished with a set of FR4 fiberglass spacers, much like the RCA jack spacers.  This time two layers of 1/16″ fiberglass was used.  The back layer is the same size as the original terminal strip and mounts tot he chassis using the same mounting points  The front layer acts purely as a spacer to account for the chassis thickness and allows the 5-way binding post bases to sit flush on the spacer rather than straddle the cutout in the chassis.

The set of spacers is held to the chassis by a bolt from the outside, and rather than a nut on the inside, a threaded bar is used.  This bar is made of 1/4″ square section aluminum bar stock, which is drilled and tapped (6-32) on one end and drilled and tap in the side at the opposite end as seen in the picture above.  This allows the bar to act as a nut to hold the 5-way binding post plates on the chassis and also provides a secure mounting point for the terminal strip inside the chassis as seen in the photo above.

The tweeter-saver components as mounted to the back of the terminal strip as they were in the stock amp, and the feedback and ground reference wires are attached to the back of the terminal strips as well.  However, the ground reference lead does not go tot he chassis as it did in the stock form; now it runs back to a common power supply ground.

A set of short leads with spade lugs on the end of them attaches the 5-way binding posts to the original terminal strips inside the amplifier.  The means that to change speaker taps, the amplifier has to be inverted and the 5-way binding post wire moved to the appropriate lug on the terminal strip.  However, in return for the inconvenience, the amplifier speaker connections are more robust and useful with many modern speaker cables.  And the chassis metalwork is never touched.

The Dyna ST-70 is probably the most prolific tube amp, and it’s a solid performer at what used to be a reasonable price.  I got my first one out of a dumpster for free in 1992.  At that time and the decade before, $50 would get you a reasonable sample.  However, today there are a variety of choices that didn’t exist at the time I got my first Dyna amp; choices like Class D and Tripath amps, Chinese tube amps for a fraction of the price of domestic ones, and a large DIY community with many great solid state designs.
The Dyna ST-70 is a very balanced design, with very good output iron for the amps price-point.  But the input section is a compromise and can be improved upon.  I re-drew the Triode electronics design that was lost in their fire many years ago.  I’ve offered up the artwork on my site for many years, and triode electronics has been selling it for many years.  I have never been in love with the design due to the odd number of tubes.  The ST-70 has separate filament windings for each channel.  So with an odd number of tubes, one channel has more filament load than the other channel.
So fast forwarding a couple decades, there are Dyna clones from a couple of manufacturers, and more input board options than ever.  So there’s really no reason for me to have designed and built another one, but I did it anyway.
This input board is slightly different than the others I’ve seen.  First the voltage gain stage is a paralleled 12AT7 dual triode.  That stage feeds a long tail pair phase splitter.  The phase splitter has a balance adjustment potentiometer.  Each output tube has it’s own bias adjustment potentiometer as well.  This input board is a nice compliment to the power supply board that is on my site as well.  The input board area is fairly small to accommodate a four tube circuit and the requisite coupling caps and other parts.  So to get around this limitation, the coupling capacitors are installed under the board.  All the resistors and other components are installed on the top of the board with the tubes.
So how does it sound?  Well, when combined with my capboard and using the Triode Electronics Dynaclone transformers, the amplifier has the typical clean and distortion midrange and treble, with bass authority that the stock Dyna ST-70 circuit cannot match.  I don’t have an actual Dyna ST-70 at the moment, so I can’t do a direct comparison, but this unit is performing remarkably well both on the bench and through my ESL63’s.  In the bench, this amp passes picture perfect square waves at 10Hz (2 watts), and is flat out to 80KHz (2 watts) with a small ringing at the leading and falling edges.

Here is the schematic:   Schematic

Here is the Layout:  Layout