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Monday, 17 July 2017

Fine tuning the big amplifier

So I've lived with the big KT88 amplifier for the last few months and have been mostly happy with it, though there were some niggles that I resolved to get around to. 

Specifically, the amp ran hot, and needed more ventilation holes drilled. 

Also, there wasn't enough Negative Feedback (NFB). I wasn't too worried about this until I built the EL84 amp featured elsewhere on this blog, which had more NFB. On hearing the difference, I resolved to correct the situation in the big amplifier, but this would need some equipment I didn't have.

So, first up was a shopping trip online to get some power metal film resistors for a dummy load, it's very important to have a non-inductive load for tuning NFB. These were duly mounted to a large heatsink.

Next I needed a pair of old-style variable capacitors, the kind you'll find in an old valve radio. eBay to the rescue, and these eventually came all the way from Bulgaria.

The method I intended to use for fine tuning the NFB was from Morgan Jones "Building Valve Amplifiers" p.290-291. 

Today, I managed to get the amp back onto the workbench. Pulled out all the valves and attacked it with the drill, to make some new ventilation holes. Problem 1 fixed, and it remains to leave the amp on for several hours to determine its effectiveness.

Problem 2 was also resolved, though this was a good deal more time-consuming. Utilising Morgan Jones' method, and armed with a healthy stock of film capacitors of various values, I started making the necessary modifications to the circuit, first with potentiometers and variable capacitors, before subbing in fixed components.

First order of business was to reduce the NFB resistor from the (fairly useless) 100K to something lower. After experimenting with the input sensitivity, I dropped this to 33K.

This got the amplifier's gain to where it needed to be, and eliminated the problem of the very touchy volume control.
It did however introduce another very serious problem: high-frequency ringing. The Williamson design is prone to this, and adding NFB in any quantity will exacerbate it. 

This was the result (red trace) at the speaker terminals of dropping a 10kHz square wave into the amp, after reducing the NFB resistor from 100K to 33K

Yeah :(

Don't know about you but I don't want to listen to that amplifier like that. Apart from anything else, it'll set all the dogs in the neighbourhood howling. And things will be getting mighty hot with all that high frequency energy to dissipate.

So clearly some compensation needed.

So watching the trace on the scope, using Morgan Jones' method, I arrived at these changes to the circuit:

Added compensating capacitor and resistor parallel to the anode resistor in the first gain stage

Dropped feedback resistor from 100K to 33K
Added compensating capacitor and resistor parallel to feedback resistor

This was the result:

Speaker trace in red

Yeah, I forgot to clip the CH1 probe back on to the input. No matter; it's the same signal at the same amplitude.

So far so good, this is all textbook from Morgan Jones. However in the course of my experiments I discovered something else that Morgan Jones apparently neglected to mention which I pass on here in the hopes that it may help someone.

Specifically. Jones' method calls for the feedback resistor to be bypassed by a variable capacitor and resistor, which I did, and I noted that the resultant waveform at the speakers looked pretty much like the above already. 

Thinking I wouldn't end up needing anything bypassing the anode resistor, I acted on Jones' recommendation and put a 220nF capacitor across the speaker terminals as a test, and watched the output go absolutely crazy. It looked much worse than the amp with no correction at all, and in fact was on the very edge of falling over into uncontrolled oscillation.

I then decided to bypass the anode resistor in the manner suggested, this resulted in some fine tuning of values as these are all inter-dependent. Eventually I got it to approximately the same level of cleanliness on the output as I'd seen with just the feedback resistor bypassed,

Then I tried the capacitor-across-the-output trick again.

Result: The amplifier barely even noticed the capacitor. A complete fix of the problem :)

Conclusion: the anode load resistor bypass doesn't do much to alter the oscillation into a resistive load, but it makes the amplifier much more stable into a capacitive load.

Morgan Jones did not mention this anywhere I could find.

So for reference this is the circuit diagram of the amplifier now (click to see full size)

The power supply implements the timer circuit which is not shown here for clarity, refer the circuit diagram of the EL84 amplifier on this blog for details on that.

This is the last modification or fine tuning I expect to make to this amplifier.


Morgan Jones "Building Valve Amplifiers", Newnes press, 4th. Edition. pp. 290-291

Wednesday, 17 May 2017

Last piece of the EL84 Amp design

So the circuit sketches and various measurements on assorted bits of paper have finally been consolidated into a coherent schematic... thanks in no small part to an incredibly patient and supportive wife who endured being an electronics widow for one final evening on this project!

The schematic is "as built" and there is one part I am less than happy with and if this amp is ever back on my workbench it'll get fixed: I am not happy with the resistance of R3 and the resulting voltage on the low side of it.

This leads to the voltage on the anodes of the RIAA stage for the second valve and the cathode follower. A little low, but notwithstanding, it passes the listening test with flying colours. 

But if it's ever back on my bench, R3 is getting swapped out for a 27K quick-smart.

Apart from that, you can see the implementation of the 555-based delayed HT switch-on circuit, the DC heaters for the first two valves of the RIAA stage, and the sneaky re-use of that voltage for the bias for the EL84s, and the 270K / 68K Voltage Divider for elevating the 12.6V heaters - the cathode follower needs it.

The Gain and phase splitter stages (V4 / V5) are based on the Fisher X100, but with higher quiescent current on the 12AU7.

This is the circuit. You'll need to click it to see full size and download / print / zoom / scroll, or whatever.

You'll probably need to download this to see it clearly

Monday, 15 May 2017

EL84 amp completed

So the EL84 amp is complete and has been delivered to its new owner, who has compared it favourably to his 60wpc Harmon Kardon Solid State amplifier.

The aesthetics of this one are much more favourable than the previous build. In testament to this, the new owner reports a high Spousal Acceptance Factor :) 

Looking good next to the turntable

In the end the chassis was about 5mm too narrow to fit the power transformer and the output transformers across the back in the usual configuration. So it had to be non-symmetrical

Making a virtue out of necessity: The non-symmetrical theme carried through from the transformers to the placement of the valves, and the controls on the front panel.

The front three small-signal valves form the RIAA Phono stage, the rear two are the line-level voltage gain and phase splitter.

The Bias test points sit between the EL84 output valves, with recessed trimmers to adjust, and test points to measure the voltage across the 10Ohm cathode shunt.
The Slovak-made JJ EL84 output valves on this one have quite a pleasing amount of light-leakage from the filaments and cathodes. Unlike the Russian Electro-Harmonix small-signal valves which are hard to see any filament glow from at all

Inside, the amp is crowded. Point to point wiring inside a tight working space.

Polyethylene Film capacitors are used for inter-stage coupling, and also in the RIAA stage, which as at top left in this photo. The power supply board sits under the output transformers, The power supply board has my usual 555 timer-based circuit to delay the B+ turn-on be 30sec giving the valves plenty of time to warm up first. By use of this circuit, combined with heater elevation for the small-signal valves, I can get away with avoiding the diode on the cathode follower in the RIAA stage, which is DC-coupled to the previous gain stage.

6 PCBs inside this case, including 5 home-made ones

Some specs and tech details

Main Amplifier

Line-level amplifier, grounded cathode gain stage, DC-coupled cathodyne phase inverter, push-pull EL84 output in class AB using fixed-bias ultralinear topology, global negative feedback. 370V Plate Voltage.
Valve complement
Gain stage: 1 X 12AX7 (ECC83)
Phase Inverter: 1 X 12AU7 (ECC82)
Output: 4 X EL84
Power Output (measured)
15W RMS both channels driven, 1kHz continuous, resistive load
Distortion (measured)
1% THD at rated power, 1kHz, resistive load
Output Impedance
4Ohm 8Ohm
Input Impedance
50 KOhms
Input Sensitivity
300mV rms for rated power
Frequency Response
6Hz – 55kHz ±3dB
Power consumption
230v 50Hz 190w nominal

RIAA Phono Preamp

Phono-level amplifier, cascaded grounded cathode gain stage, DC-coupled cathode follower, RIAA equalisation
4.5mV for rated power. MM-type cartridge only, 47KOhm load impedance
Valve complement
Gain stage: 2 X 12AX7 (ECC83)
Cathode Follower: 1 X 12AU7 (ECC82)

Listening tests

The sound from this one is clean, detailed and very pleasing. The main amp stage is based on the well-regarded Fisher X100, with the 12AX7 gain and 12AU7 phase splitter, though this design runs that 12AU7 closer to its 5mA sweet spot for linearity from a 300V B+ than the Fisher does.

The output stage is EL84 in fixed bias ultralinear, biased to 8.4W quiescent dissipation (70% of rated maximum)

The RIAA stage works well. The Cathode follower is needed to drive the volume control which represents a 50K load across the input. The noise floor is low, hum is non-existent, and distortion does not occur even on the loudest passages. 

Lessons learned

During the construction of this amp several lessons were learned...

1) Hum was a constant problem
The amp was built backwards, with the output stages and transformers being wired up first, powered on to test, then the preceding stage, right back to the RIAA stage.

The 12AU7 Phase Splitter was putting a nasty hum into the output. After chasing that down and much testing with the oscilloscope etc, it was determined that the hum was on the anode but not the cathode. Many solutions to this common problem are available on the internet, in the end I opted for an additional level of decoupling in the power supply with a 10K resistor and 220µF capacitor, this fixed the problem. The lesson is that Phase Splitters have NO PSRR on the anode side. That power needs to have no trace of ripple on it

2) Grounding needs close attention in a RIAA stage
You can read as many books as you like but it's only when you build an RIAA stage that you truly get to appreciate how to ground the incoming signal... and how not to. This one had a nasty hum which was coming in through the Earth side, it would only manifest when there was a source plugged into the phono input. If the jacks were empty, the stage did not hum. But plug any source in, the hum appeared ... after bridging the input with a 1K2 resistor to simulate the cartridge, it was observed the hum was injecting into the live from the earth through the source. 
After moving all the signal Earth to a common point - which was the star earth off the first valve in the phono stage - suddenly it went dead quiet.

3) Take care with design and placement
There were a few too many near-misses with things fitting much tighter than planned, or almost overlapping other parts, 

There are also some minor changes I'll be making to the design of my bias boards and power supply boards for the next build, which is not currently planned.

Next post... I'll put up the as-implemented circuit schematic.

Monday, 24 April 2017

Some progress on the EL84 amp

The EL84 amp is well underway now. First of all, the name:

Iwa Orotuanaki Wairua

This is is the Maori language (explanation for people outside New Zealand: The Maori are the indigenous people of New Zealand).

It means (approximately) "Nine Echoes" or more accurately "Nine Sound Spirits"

Why this name? Nine because it has nine valves (tubes) and it's going to a friend who is involved in paranormal investigations


So the case has been laser-cut and etched, the front and rear panels have been filled, seats added to the top:

A slight error with the laser etching means we seem to have two sets of "Speakers A" ... oh well, it adds character

Circuit boards made up – the power supply shown in the previous post, plus the relay board for the speaker switcher, the feedback filter board for the Phono stage, and the bias adjusters for the output stage:

Feedback/RC Filter board for the Phono stage
Relay board for the speaker switch (Speakers A/B, 4Ohm and 8Ohm)

The Bias adjust boards, already mounted up. Yes it does say "Star Wars" inside the amp; this was a test of the laser etching before potentially messing up the top!


Bias boards mounted, tag strips, power supply and feedback board... starting to see how much of a squeeze this build is gonna be!

Wiring up the low voltage parts....

So that we can do this!

More later.

Friday, 10 February 2017

Design made pretty

The new amp is not going to live with me... it's being made for a friend. 

Topology-wise, it will be a push-pull running fixed bias, ultralinear, just like the previous one. Except it'll be running EL84s on the output instead of KT88s. And the plate voltage will be 300V instead of 560.

So we're looking at realistically 12-15 watts, which for the intended usage will be ideal.

Owing to this one having a phono stage which includes a cathode follower, we have some fairly rigourous requirements for the power supply:

  • B+ of 300V for the output stage with some hefty reservoir capacitance
  • 2 X separate 280V rails for the phase splitter (DC-coupled, hence the slight voltage drop needed)
  • A separate 300V rail for the phono stage
  • 2 X 250V rails, heavily decoupled to avoid signal feedback through the B+

That's just the high voltage side. On the low voltage side this amp needs

  • 6.3V for the EL84s
  • 12.6V for the input and phase splitter and cathode follower. This rail needs to be elevated ... the 12.6 vac needs to be standing on around 60 VDC to avoid exceeding the Vhk on the follower. (I figured the other valves on that supply wouldn't mind an elevated heater supply too much either)
  • 25 VDC for the heaters on the two phono input valves (12AX7s with heaters in series to absolutely minimise the mA)
  • And last but not least a minus supply of about 20V for the fixed bias

As with my previous amplifier, there's a delay start circuit with a 555 timer and a relay which turns on the HT after about 30 sec from power-up, to allow all the cathodes to be at operating temperature before B+ is applied.

Since I'm challenging myself to build this amp on a small chassis, I decided that the power supply needed to go on the smallest board I could get it on. The design I sketched out in the last entry was the inspiration for the final layout for the board, see below:

Board design... single-layer and NO topside links

Once the design was done, it was time to employ my primitive stone-age PCB manufacturing technique: Print design onto plastic sheet, finely sand copper-coated board, apply plastic sheet face-down to board, then iron it! (Seriously... with a clothes iron).

When all the image is transferred to the board, the board is rinsed then placed into a tray of etchant solution to dissolve away the uncovered copper.

Leaving us with a nice board ready for drilling. The transfer is usually not 100% with this technique, but can be easily touched up with the soldering iron

Out of the etchant, ready for drilling

After the board is drilled it's a case of assembling the components and soldering them on, with a board of this nature I like to start with the smallest/lowest components first (resistors) and work up in size.

The finished product

Count the rectifiers on this board - 4 of them

8 power capacitors on a board not much bigger than a smart phone

Top-down view. At bottom right is the delay circuit

Sunglasses just for a scale prop

Top-down view, rotated

More build images as it progresses...

Friday, 3 February 2017

Design process - the ugly end

So when designing amplifiers, I like to put the power supply on a circuit board.

While I think point-to-point wiring has its place in the amplifier section, the use of a PCB gives a more compact and tidy layout for power supplies.

So in the next amp I'm building the power supply is going to be a little more challenging than the previous, as this one has a phono stage, which adds complexity.

This power supply needs to provide:

  • B+ 300V for the output stage
  • 280V (X2) for the preamp
  • 300V separate regulated supply for phono stage
  • 250V (X2) off the phono stage rail
  • -25V regulated DC for the heaters on the phono stage (and doubling as bias supply for output valves)
  • Elevation voltage of around 60V for the heater supply for the Cathode Follower
  • 555 IC circuit for 30sec B+ delay on power-up

So a number of resistors, capacitors and rails needed. And it all has to fit as compactly as possible onto the PCB since this amp is going into a compact chassis.

So, before starting to design the finished PCB, some thought needs to be given to the layout... in the old days this might have been a pencil-and-paper exercise, but that method isn't particularly flexible when it comes to making corrections or revisions.

Enter a tablet computer with a pen. This is how I make my rough sketches for PCB layouts... from this file I shall design the final PC board

It looks truly awful, but it makes sense to me.

Next step: designing the board from this sketch, when done I'll post the layout here

Thursday, 26 January 2017

Follow the heat waves

Big thanks to a friend who owns a Flir E4 Thermal Imaging camera and lent it to me for the weekend.

After warming up the amp, I used the camera to measure the envelope temperature of the big KT88 valves.

Not surprised at the 172º reading. (340ºF)

Strangely this one runs a little cooler than the rest. Its bias current is the same, 50mA

From the KT88 Datasheet:


The question you may be asking: what was playing through the amplifier during this photo session? Glad you asked. Icehouse... Man of Colours. Since I'm going to be seeing them live tomorrow.