The last EL84 amplifier I built (with the phono stage, tone control and headphone stage) has a Hammond 370FX power transformer. It was noticed this transformer gets uncomfortably hot to touch after about one hour’s use of the amplifier.

Being unfamiliar with how-hot-is-too-hot, I’ve adopted a cautious approach and ordered a higher spec transformer to replace the current unit. However this is a few weeks away (coming from Canada) so in the meantime I decided to measure the temperature rise to gain a deeper insight into the problem.

First thing I tested, before doing any measurements, was to pull the tubes from the headphone stage, thereby relieving the power supply of 44mA of B+ and 1600mA of 6.3volt. As expected, this resulted in a much slower heating up of the transformer.

With all tubes plugged in, the quiescent current on the B+ is 140mAThis is a centre-tapped transformer, so conventional wisdom is that in this mode of usage, the secondary should be rated at 1.2 times the desired DC current, which in this case would be 168mA
In the case of the 370FX the secondary is rated at 173mA

Assessment: OK

On the Low voltage side. the 6.3volt is rated at 5A and the total draw on it is 5.2A so a little over (by 5%)

Assessment: Not ideal
(The replacement unit ordered has an extra 1000mA there).

So. Down to the measurements. I ordered a digital pyrometer (infrared surface temperature measurement gun) and when that arrived, I ran the amplifier for 3 hours, measuring the surface temperature on the top of the transformer, every 5 minutes.

Over three hours, this was the result:

The measurements were made each time at the same spot on the top of the transformer, from the same distance.

After around 45-50 mins the transformer became uncomfortably hot to touch if resting the hand on it. This corresponded to a temp of mid-to-high 40s. Once the temperature was in the low to mid 50s it became uncomfortable even to a fingertip.

Conclusions

1) This is an unscientific test with a cheap uncalibrated instrument from AliExpress. I have some confidence in it because the baseline temperature reported (22ºC) at zero minutes was exactly the ambient temperature in the room reported by multiple other thermometers.

2) Electronic components are rated at 105ºC. I do not know what the temperature difference between the windings and and the outside of the transformer would be, so I am going to make a totally wild and uninformed guess of 20°C. Therefore the windings are at around 80-85°

3) From reviewing others’ experiences with Hammond power transformers, it seems a commonly reported phenomenon that they run hot. Therefore this transformer is behaving as expected, although it is causing considerable unease in doing so.

4) Because of my wild assumption in (2) above, I have no confidence that this transformer will be safe or indeed what detrimental impact sustained running at high temperature will have on it.

It’s been a little while since I posted an update on this project, and there’s been a lot of progress, as well as one or two hiccups.

Thought I’d put up a few photos today since I’ve been taking plenty.

Following the previous post, the next order of business was to get the top panel of the chassis ready. This involved a lot of measuring and drilling – the mounting holes for the boards and transformers, then the chassis punch for the valve sockets. A lot of swarf ended up on the floor during this process.

After getting the top panel ready, it needed to go to the laser etching workshop before I could do anything with it. This is to get the identifiers for the valves etched on – this design uses four different types, so it’s important to know which type goes where!

Once that was back, it was time to begin assembly. Mounting up the transformers and sockets to the top, and circuit boards underneath. A delightful jigsaw puzzle, but everything fit together nicely and it was not necessary to utter any curses.

Next it was time to fit the circuit boards inside, for a photo shoot:

Following this photo it was time to take it all apart again for lasering and drilling on the front and back panels. Once that was done it was time to assemble all the boards together and make up all the various connectors.

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From here it was time to take it to the listening room for some tests.

Connected to the KEF C-95 floorstanding speakers (early 90s vintage) the sound quality was subjectively is clean, pleasantly detailed, no trace of any distortion or harshness in the treble, and with plenty of power to the bass. I would be happy having this sound quality as my daily driver.

The tone control behaved exactly as the previous build of this circuit, this is the second time I’ve built this one.

There was a small amount of ground loop hum on the photo stage which necessitated a bit of re-design of the earthing point ofr the power supply and the addition of a 100µF capacitor in the power supply very close to the photo stage.

Once this was done, the amp was deemed electronically fit for use, and a very enjoyable few days burn-in testing was had. A few cosmetic finishing touches were completed, the cover for the remote sensor, and a bit of cable dress inside

A bit more activity on the amplifier as time permits, and a few trials and errors later… we have a power supply board.

In the previous post, I showed the design. I made the board, using my new hand-made UV exposure box with 120 UV LEDs in it, and after etching and drilling, began to build the circuit.

I began with the low-voltage parts: the delay switch-on and LED colour reverser.
Long story short, it didn’t work. Due to two errors on my part.

1. I’d omitted the reverse-biased diode across the output of the 555 IC (which I’ve never used in previous designs, and thus far gotten away with. This time it didn’t work)
2. I’d accidentally used a relay with a 9V coil voltage instead of the required 5V for the LED colour reverser.

So, it didn’t work. And worse, in soldering and de-soldering components to test it, I ended up stripping some of the tracks off the board.

So it was back to the design. Make up a new board that rectifies these omissions (I ordered a relay with 5V coil and of course its pin spacing was different)

So, a new board was designed and exposed, developed and etched.

I use the Mega / Farnell UV-sensitive boards, and Ammonium Persulphate as an etchant. These boards are not specified for this solution so while it works, it’s very slow, etching a board takes around 30-40 mins. Of course during that time, the etchant bath cools down and the process slows as a result.
//TODO: Buy an etching bath heater!

Anyway, my UV exposure lightbox gives a much more consistent light than my previous approach, which frankly is too embarrassing to describe here. So the boards produced with it look a lot better.

After etching, it was to the drillpress to drill around 145 holes of various sizes, then back to the soldering bench.

First test was to stuff and solder just the components for the low voltage circuit. Make sure the revised delay switch-on and LED colour reverser was working.

Success. It worked as planned! This meant I could then continue to stuff and solder the rest of the components.

This is now done and the results are in the pictures.

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Next steps: Metalwork – case drilling – and a bunch of connectors to make up.

The design of the amplifier has continued since the last post although slower than hoped. This was die to a few random factors (including but not limited to a computer that died and needed replacing and setting up, and some non-electronics projects that came up)

Anyway, the progress this time is the completion of the headphone board, which is based on a White Cathode Follower using ECC99 tubes.

This completes all of the signal handling boards. Below they are shown in their location on the chassis.

This represents a significant achievement: The headphone stage was a late addition requested by the customer after the chassis had already been ordered. I was not at all confident that the entire amp would be possible to build in the small chassis. But I persevered, with the approach of taking a long time on the PCB designs to get them as compact as I could, including manually routing them (which gets more complex as each track and component is added)

The last remaining board needing to be designed was the power supply. This is going to sit inside the chassis under the transformers. This dictated its size, which – as with the rest of the amp – needed to be as small as possible.

The final board design measures 140 X 75mm.

The next step is the careful eyeball check before exposure, etching, drilling (141 holes) and stuffing.

This is the layout

On this board we have:

1. Delay circuit for controlled power-up
2. driver for the 2-colour power LED that starts red and goes green when the main power kicks in after the warmup delay
3. Elevated 6.3vac power supply for heaters, with three sets of output terminals
4. 6V DC power supply for heaters with three sets of output terminals
5. 370V supply rail for output stage
6. 265V rail for phase splitter stage
7. 300V rail for headphone stage
8. 300V isolated rail for phono stage (to avoid feedback through power rail)
9. 280V rail with two outputs for tone control and preamp gain stage
10. double-filtered and isolated negative bias with four outputs (one per output tube)

Regarding point 4: From a technical requirement, it’s only strictly necessary to run the phono stage on DC heaters, but due to the complexity of this project it has a high tube count (13) the number of tubes exceeds the rating of the 6.3vac heater winding on the power transformer. This transformer also has a 5vac winding which will be passed through a diode and thereafter smoothed by a 47,000µF 10V capacitor (it’s huge… has to lie down on the board!) and this rectified DC heater power will be used for other areas besides the phono stage, just to share the load.

Next steps: Make this board, then metalwork for the chassis…

The Christmas/New Year holiday has provided plenty of opportunity to progress the design and construction of this project. My approach to this one is to build a library of discrete PCBs which fit together to complete the project, and which can be produced again for a future project.

So this will allow me to create a menu of sorts, with the customer selecting what features they want.

So, we have a separate board for the RIAA/Phono stage, one for the tone control, one for the preamp stage, and one for the headphone stage.

So far, the RIAA board, tone control board, and preamp board, are built. Here, I’ve laid them out according to where they will sit in the final build

I’ve done them all the same way as the previous post, with the valve sockets on the copper side of the board.

My method for laying out the boards is to use no automation: These are all completely manually routed. Attempting to use the auto-router was like a bad comedy show. 

Top-Right is the RIAA Phono stage. This one:

In front of that is the tone control

This one:

And the preamp gain stage and phase splitter

This is how the boards look from the under side. These are all made using a laser print onto a transparent sheet then exposed onto the photosensitive board under UV light

Remaining to be done: the headphone board (this will be a challenge as it has some large capacitors on it) and the power supply board.

The challenge with this build was to fit everything into a case with an internal dimension of 300mm (11.8″) wide and 225mm (9″) deep. The layout shown will accommodate it… just!

Sadly, one of my suppliers has let me down and as a result, I’ve spent $100 on parts I don’t think I’m ever going to see. I will not name them yet as I am extending them the charity of my silence to give them the opportunity to rectify the matter. In the meantime I’ve had to order some tubes from the regular supplier to replace the missing parts.

Progress is happening with the new amplifier… this design is more modular as I have decided to design standard boards for tone controls, headphone output, and phono RIAA.Having standard boards for these means I can more easily accommodate future builds, shoudl they be requested.

The process has not been entirely smooth sailing, owing to the somewhat hit-and-miss nature of home PCB fabrication. Until now, my method has been to print the PCB design onto an iron-on transfer which then gets pressed onto the board (and then touched up with the etch resist pen) before going into the etchant.

This process has been unreliable and time consuming, and expensive, owing to the high reject rate. So a new technique was called for.

I’ve decided to move to a photosensitive board workflow. The design is printed onto transparency, which is then plaed over a light-sensitive board and exposed under a UV light, thereafter a two-step chemical process: Developing then Etching.

The first board I designed for this project is the tone control. This is using the same circuit as the previous project, except I had two changes:

1. I needed to reduce the size of the board 
2. I needed to put the tubes on the copper side of the board

So, I re-designed it to be 120mm X 65mm (down from 150 X 75) and attempted to fabricate the board… with less than spectacular results

This board failed because I did not expose the photosensitive layer sufficiently.

Lesson learned, I did a second attempt, which looks much better. So I went ahead and drilled and stuffed it.

Result:

The tubes are on the copper side because of the customer’s preferred aesthetic of having the tubes visible. This design will be applied to the other boards in this amp as well.

In the process, I have become a lot more familiar with the operation of my PCB software: namely DesignSpark from RS. Also its quirks and foibles, such as less-than-ideal behaviour when moving things around, and its ability to have “invisible” track that isn’t visible in design but is when you print. As a result of this, the board above needs to have one track cut with the dremel and re-routed with a short jumper on the track side. Yeah I hate doing that!

Lesson: Inspect the board VERY carefully in print-preview before fabrication.

Or, to use an appropriate engineering axiom: Measure Twice, Cut Once!

I also built the bias boards for the EL84s. Owing to the amount of heat these produce, I am not mounting them on boards, but the voltage divider and potentiometers for the negative bias voltage, and the cathode shunt, can be put on a board. So drawing on my earlier design, these are the bias boards, made using the same technique:

Next up: A two-triode RIAA stage, I’m planning this on a board on 100 X 65mm.

There’s a reason I want these boards as small as I can get them: The size of the chassis

This chassis is going to represent a challenge to fit everything into it… this design will have 13 tubes: The RIAA stage, tone controls, headphone stage, as well as the amplifier itself. And size is a consideration since it will be packed up and sent overseas when it’s finished.

Next update when I have more boards to show…

Following an approach from a new customer, a new design has emerged…This customer had a well-defined set of requirements:

• Usage situation dictated an EL84 PP design would be suitable
• MM Phono peamp required
• Line-level inputs required
• Tone controls required
• Headphone output required
• Remote control volume adjustment required

Fairly rapidly I decided this amp could be based on the previous EL84 amp I made at the start of the year, with some additions.

Power Transformer
Firstly, I intend using an off-the-shelf power transformer. The custom-wound transformers are handy, but they’re an industrial product, and as such the aesthetics in their design limit their use in a piece of equipment where they are going to be on display. Sadly the manufacturer was unwilling to work with me on this aspect, so my transformers will have to come from Canada now, instead of being locally made.

The power transformer I selected for this job is the Hammond 370FX. 172mA at 275v, 3A at 5v and 5A at 6.3v, with a 50v bias tap. Everything I need.

Tone Control
The previous tone control worked well enough for it to be included in this project without modification. Except I’ll redesign the circuit board.

Headphone Stage
This customer was adamant this amplifier have a headphone socket. This was a non-negotiable requirement since their musical taste is not shared with other members of their household. This is provided by an ECC99 SRPP-based OTL design borrowed from the internet. It simulates well in LTSpice down to 32 Ohm headhones, and will drive into 16Ohms as well, although with greater distortion and a lower level.

Printed Circuit Boards
This amplifier will be designed on several PCBs:

• Power Supply
• Phono Stage
• Tone Control
• Headphone stage
• Bias adjusters for EL84s

The circuit boards will differ from the previous tone control board in that the tube sockets will be on the opposite side to the discrete components, to facilitate the boards being mounted upside-down in the chassis, allowing the tubes to rise from the top of the chassis as in a point-to-point design.

The EL84s will be chassis-mounted, as in the previous EL84 design on this site.

First stage of development, we have a circuit.

By way of explanation:

V1 + V2 are the MM cartridge phono amplifier stage. RIAA equalisation is given by the RC network giving NFB to the stage

V3 is a Cathode Follower, necessary because the preceding phono stage has a high output impedance, and also to provide additional current capability to any line-level signals at the input, to drive the tone stage.

V4 provides around 20dB gain to compensate for the losses in the tone stage, restoring the entire stage to unity gain. This is the same circuit as the previous “Tone Control” project on this site

V5 is the gain stage for the amplifier properV6 is the concertina phase splitter. This needs an elevated heater.

This stage encompassing V5 and V6 is borrowed from the Fisher X-100.

V7 and V8 are the PP output stage with the EL84s, running in Ultralinear configuration into Hammond 1650E output transformers. Fixed bias is employed with the cathode resistors providing the reference voltage for adjustment.

V9 is the gain stage for the headhone amplifier, V10 and V11 the SRPP current driver stage to power the headhone output.

The power supply will incorporate the same 30-sec startup delay on the B+ as the previous amplifier projects on this site.

Owing to the current capability of the low-voltage secondaries, we have a split, with some of the tubes receiving DC heater voltage and others receiving an elevated AC.

Parts are ordered, next stage is PCB design. to be continued….