The Eris 18 Amplifier

[darryl]

This construction thread has developed from some discussion in the "Brown Sound" Thread: http://el34world.com/Forum/index.php?topic=20349.0  To maintain a coherent description in this thread, some of the information in the Brown Sound thread will be repeated here.

A little history. A number of commentators on several different forums have expressed the view that EL34 valves in push-pull do not perform adequately unless their plate voltage is somewhere over 400 volts. This amplifier was designed to investigate the operation of an EL34 amp with a much lower B+ of approximately 250 volts. This probably could have been done using a V.V.R. but the plan was to build an amplifier designed for the lower voltages and output power, rather than using a partially incapacitated larger amp.

The chassis used for this amplifier ( and many of my other amps ) is a Hammond 1550j diecast box. There is sufficient chassis real estate available on these boxes for relatively simple amplifiers up to about 50 watts. Diecast is easy to prepare without specialised tools, and the box lid becomes an effective screened baseplate for the finished amplifier.

These images show a typical example of a chassis ready for component installation. It isn't the actual Eris 18 chassis, but a very close relative. I forgot to take a picture of Eris at the appropriate time during construction . . .







To be continued . . . 

[darryl]

This image and all those following are of the actual Eris 18 build.

Once the chassis had been drilled, punched and filed into submission, protective rubber grommets were fitted to the chassis holes where transformer leads would pass through the chassis. The valve sockets were installed, followed by the tagstrips.

The tagstrips were riveted in place, except for one ground lug in each row, which was attached using a machine screw, nut and several star washers. The riveted ground lugs were then soldered to this secure ground point using 18ga tinned copper wire. This is not the usual star earthing pattern, but rather a ground plane, using the entire chassis. It's unorthodox, but it has always worked in these chassis.

Running the heater wiring was next. The heater supply is 12VAC, so the two EL34 heaters were wired in series across the 12 volt supply, whilst the 12AX7's were connected in their 12-volt configuration, using only pins 4 and 5.

To be continued . . . 

[drew]

This will be interesting to follow along with.  I didn't know they made those stompbox type enclosures in such a big size.

[PRR]

> stompbox type enclosures in such a big size.

Get your nose out of the stomp-parts catalogs.

I have a very similar box in my cellar, whole-house power entrance, 12"x12"x6". And another 6x6x4 which I filed out as a power consumption display and termination box. At that size (and grounding issues) they were cheaper in plastic from another maker; but Hammond makes the pot-metal boxes almost that big.

[kagliostro]

I think that the alloy of those boxes is particularly easily workable (short chipping)

Franco

[shooter]

The tagstrips were riveted in place

I've tried a couple times to *guess* the tag layout, and failed!, do you do *paper* layouts with parts, similar to creating a turret board?
thx

[darryl]

I've tried a couple times to *guess* the tag layout, and failed!, do you do *paper* layouts with parts, similar to creating a turret board?
thx

No.     Most of my layouts are based on earlier designs - some going back to my early amp building days in the 1970's - so the tagstrip layouts all tend to be very similar, with just incremental changes to suit a particular circuit. As sluckey pointed out in the Brown Sound thread, my chassis layout is similar to that used in some vintage Sunn amplifiers - transformers at opposite ends, preamp valves across the front, output valves across the back, and the phase inverter valve close to dead centre. I don't deviate from this layout, so I can start drilling without a drawing.   



Moving along with this project . . .

As has been mentioned before on this and other forums, transformers are a particular problem for Australian amp builders. Once freight and an unfavorable exchange rate are factored in, the landed cost of imported transformers is often impractical. I have been experimenting with small, low voltage toroidal transformers, generally stacked vertically, to conserve chassis real estate:






The transformers intended for the Eris amplifier were somewhat smaller than those in the above image . . .





. . . so it was decided to mount them side by side:



Both transformers were rated at 50VA, one having 12+12 volt secondaries, the other 18+18 volt secondaries. One of the 12 volt secondaries was sufficient for the heater supply, so the other 12 volt secondary was connected in series with the two 18 volt windings, to provide a 48 volt AC supply. It was then calculated that 48VAC into a voltage quadrupler: 48 x 4 x 1.4 = approximately 268VDC B+




Below deck, the transformer leads were attached to their respective termination points. The four capacitors used for the voltage quadrupler had to be split into two pairs because there was a transformer mounting bolt and washer where they would normally be placed. Radial capacitors mounted this way tend to rattle against the chassis if an amplifier is subject to vibration, ( and what amplifier isn't? ), so contact adhesive was used to anchor the caps firmly in place.

The heater supply's hum minimising components were installed next, so the 240 volt primary leads could fly over the tagstrip, to the power inlet socket and power switch.

The speaker sockets were installed, so the output transformer secondary leads could be terminated. The output transformer is another non-standard component. Its design purpose is as a line transformer in distributed sound systems, but its turns ratio and nominal 40 watt rating makes it very suitable for use as an output transformer. There are details of this procedure here: http://www.ozvalveamps.org/optrans.htm




To be continued . . . 

[darryl]

The bias components, including the bias potentiometer were installed alongside the heater hum minimising components. The presence pot was temporarily mounted on the front panel so its screened connecting lead could be run back to the output stage tagstrip.

A tagstrip was mounted inside the chassis back panel and the output valves' screen supply components were installed. Two of the spare tags on this strip were connected to the output valves' cathodes to establish readily accessible bias test points. Each cathode was connected to ground via a 1 ohm resistor, so 10mV measured at a test point established that a current of 10mA was passing through the corresponding valve.

One problem which had appeared in some of my previous experiments with lower output 2204-style circuits was that the phase inverter could seriously overdrive the output valves before the PI itself eventually clipped. The result was crossover distortion, probably caused by the output valves being driven to cutoff. This adds even more distortion to an output stage that is already in hard clipping, so removing it just seemed like the proper thing to do . . .

There are some simple ways to encourage the phase inverter to clip before the output stage is being slammed. One is to reduce the B+ voltage being supplied to the phase inverter. An alternative method is to include voltage dividers between the phase inverter outputs and the output valve grids. This was the procedure used here. The additional resistance between the PI plates and the output valve grids greatly reduces the opportunity for blocking distortion. The standard value of bias feed resistor in a 2204 circuit is 220k ohms, so the divider values initially selected for the Eris amp were 82k and 120k.

To be continued . . .     I'll attempt to have a schematic prepared for the next installment. 
 

[Cliff Rogers]

Was there ever another instalment? 🤔

[darryl]

Was there ever another instalment? 🤔

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Err . . . no.      
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This is a very old thread, but now that I've been called out on it, the remaining construction details are below.
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The front panel was mounted and held in place with the potentiometers. The input sockets and preamp components were next.
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Components around the tonestack driver valve were installed.
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Tonestack components and potentiometer wiring were next. The final construction details were fitting knobs on the front panel and attaching labels.
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A set of valves were installed, and the power was applied. Details of the initial testing have been lost,      but the design 18-watt output was achieved.
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There was another amplifier built at the same time as the Eris, using the same transformer set. This was the Sedna 18 - a Vox themed amp. It was a successful project, although its construction details have been lost . . . 
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The names Eris and Sedna are from two Dwarf Planets in the outer reaches of the Solar system. Eris is also the Greek goddess of strife and discord - so an appropriate name for a guitar amplifier. 
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Both amplifiers were eventually sold - and I never did get to fully investigate the results of running EL34's at reduced B+ voltages. Maybe one day . . . 

[isaac_teller]

I see that you used the Altronics M1130 line transformer. What were the primary and secondary taps? I am building a lower powered Plexi. Thank you.

[darryl]

I see that you used the Altronics M1130 line transformer. What were the primary and secondary taps? I am building a lower powered Plexi. Thank you.

I used the full primary, so the output valve anodes were connected to the "0" and "5W" taps. The mid point of the primary was the "20W" tap. Using the secondaries as labeled - 2, 4, 6, 8 - reflects a primary impedance of 2.1k ohms plate-to-plate. I re-labeled the secondary taps as 4, 8, 12 and 16 ohms which then reflected a primary impedance of 4.2k ohms, which is a useful value for EL34 ( and 6L6 ) valves.
There is more information on repurposing line transformers here: https://ozvalveamps.org/optrans.html

[Cliff Rogers]

Beauty, thanks for that.

Was there ever any feedback from whoever bought the amps?

[darryl]

Was there ever any feedback from whoever bought the amps?

No.   
The Sedna was sold via the AGGH forum, the Eris was sold via a musical instrument retailer.

[Cliff Rogers]

Shame.

I would like to try one at some stage, after all the others I want to try. 🤓