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Old 2nd Feb 2023, 9:46 am   #1
Gabe001
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Default My first single ended stereo amplifier build

A few months ago I started a thread asking for suggestions on a single ended amplifier build. I've decided to build the mullard 3-3, as I have the appropriate hardware for it, but my el34s are 6L6s in my valve box were looking at me alluringly and I couldn't resist. I do still intend to build the 3-3 as my next project.

This is my very first single ended build from scratch (aluminium plate and wood from B and Q). Its going to my sister in law (if it is allowed on the plane), as she asked me for one after hearing the armstrong 220 and mullard 5-10. It will be paired up with a Raspberry pi 3a + allo boss DAC pre amp for digital streaming which I will provide, and god knows what speakers which she has yet to purchase.

The build is not perfect and there is still some room for improvement. Not being particularly experienced, I botched out the layout slightly, which caused me to chase around for hum for a while. The position of the output transformers was determined using the headphone test, no issues there. The choke is however too close to the OPTs and the pre-amp valves - my mistake - and the el34s should have been closer to the OPTs. In any case, every day is a school day. I'll do better next time.

The iron was all bought from china - cheaply - and the idea was to surpass the chinese commercial offerings in the 300-400 price range in terms of performance, given they they use the same (or broadly similar) iron. The measurements for the Reisong a10 which are shown here:
https://www.audiosciencereview.com/f...-review.19972/

As you can see the Reisong measures horribly - 4.39% THD at 0.78w, with massive 60Hz and 120Hz spikes, and a lot of HF rolloff on the frequency response. Nevertheless, people seem to rate them really highly for some inexplicable reason.

some pictures of the completed build are shown.

my main build requirements were:
1. stability - especially since the load it will be driving is still unknown
2. input sensitivity of about 0.8vrms - although the DAC is capable of 2v, digital recording volume varies quite a bit. This can be "normalised" , but this process can degrade the audio quality
3. sufficient power to be able to drive potentially modern(-ish) speakers


My first circuit was a straightforward 6sn7 cascade driving an el34/6l6, with 18db of global negative feedback. This worked very well, with 0.3% THD at 1w, negligible hum and noise, a very flat frequency response and a good square wave. If I build this amp again in the future, for myself, this is what I will do (with a better layout and probably slightly less feedback). However, I struggled with lead and lag compensation and the amplifier went into oscillation with low-ish capacitive loads across the output. Given that I don't know what load this will be driving, and I don't want to risk blowing up any expensive speakers, I wasn't happy with the overall stability and ended up scrapping this circuit and starting again

As the EL34 is running in UL mode (although the transformer manufacturer does not specify the % of the SG tap!) it requires greater voltage at the grid to drive it c/w pentode mode. The el34 anode voltage is 340v, with a 300ohm cathode resistor (el34 being run quite conservatively), meaning 22-23v on the grid for max drive. The problem was finding an octal valve that could give this amount of drive with a <1v input signal and allow some headroom for feedback.

Google led me to the 6sl7 srpp circuit, which although optimised for a low impedance load drive has been used to successfully drive 300b triodes. It should be sufficient for an el34 in ultralinear (or triode mode)

LT spice calculated the voltage gain of the 6sl7 srpp at 50 using the parameters shown (see picture), meaning that this should allow for almost 10db feedback which is a 3x drop in input sensitivity (i think!). Use of feedback was deemed essential as without it the FR looked like the top of one of Kim Yung Un's missiles. Incidentally, the voltage gain of a 6sl7 in parallel was lower than that of the srpp. 135v on the cathode of the second section of the 6sl7 required elevating the heaters (virtual centre tapped with a humbucking pot), which I did (to 56v). I should have saved myself some grief and installed a noval socket instead, and used an ecc81, but then no plan survives first contact with the enemy.

The final circuit is shown below, along with performance parameters. The 100k grid leak input is actually a concentric volume control. As you can see, it far exceeds the Reisong A10 in terms of measured performance. Hum and noise is virtually non existent. Frequency response between 20-20,000Hz is very good given the cheapness of the OPTs. The only disappointing parameter is the THD (1.1% at 1W), which at least is almost all 2nd harmonic (see picture). This rises to 4.0% at full power, which is 6w per channel. el34 anode plate voltage is 340v and the transformer primary impedance is 3.5k. The % of the ultralinear tap remains unknown. Feedback is around 8db. The amplifier is now completely stable into pure capacitive loads up to 1uf which is the largest non electrolytic cap I've got, so it should be able to handle any speaker/electrostatic tweeter and any length of speaker cable (I hope). the 1kHZ square wave is good, and it sounds good on my mission speakers.

I mentioned before that I had an issue with hum initially. This was due to the choke (magnetically?) coupling with either the OPT or the input valve. It took me a while to figure this out as the hum only appeared once the valves warmed up. In any case, switching from a CLCRC to a CRCLC to reduce the voltage swing in the choke sorted this out.

Any comments and suggestions welcome. Whilst it is unlikely that I will be doing any further changes to this one unless I've done something horribly wrong, I still have a pair of 6l6s and will probably be going down this route again in future.

Note: all measurements done using REW and ARTA on a laptop via a 20db attenuator. i.e. the 50HZ spike at -110db is actually -90db. the 150 Hz spike if from the laptop.

I failed to mention that I'm not actually using 6sl7 valves but their russian copies. I have yet to run it alongside the push pull amps to see if I can hear a difference.

regards,
gabriel
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Old 2nd Feb 2023, 9:51 am   #2
Gabe001
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Default Re: My first single ended stereo amplifier build

More pictures and circuit

Also should have mentioned that to allow airflow, I've done a perforated bottom plate and the amp is raised on legs.
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Old 2nd Feb 2023, 10:19 am   #3
GrimJosef
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Default Re: My first single ended stereo amplifier build

Nice work Gabe, congratulations.

Historically there weren't so many single-ended EL34 amps on the market. The exception, perhaps, was the Pye HF10 from the company's 'Mozart' range. I've never worked on one of those but they claimed that it could deliver 10W from one EL34 and that it had just 0.3% THD at 9W output ! The circuit (given here https://www.valve-radio.co.uk/litera...-instructions/ for example) was unusual and I suspect the output transformer may have been fairly special too.

The one thing I'd say, and you may already have done this already, is that if you're going to hand it over to your sister-in-law then you might want to put a baseplate on it. Otherwise sooner or later someone inexperienced will try to pick it up while it's still plugged into the mains. If their fingers should touch mains live and either earth or neutral at the same time they will get an unwelcome shock. At the least they might drop the amp onto their foot which won't be good either for the amp or for their foot.

EDIT: This post crossed with your second one, so I see you do have a baseplate.

Cheers,

GJ
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Old 2nd Feb 2023, 12:00 pm   #4
Gabe001
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Default Re: My first single ended stereo amplifier build

Hi GJ. Thank you. Yes I do have a baseplate, its just not installed yet in case I need to do some last minute changes.

That pye mozart is interesting. It takes a very brave designer to use an ecc83 in cascade with adjustable feedback (from 15 to infinity?) and still end up (presumably) with a stable amplifier

The problem I found searching single ended circuits online is that most of them have no objective measurements and you rely on the builder saying that its the best thing since sliced cheese and doesn't hum, which are both extremely subjective observations. Most amp circuits which have survived the test of time seem to be push pull, for obvious reasons.
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Old 2nd Feb 2023, 3:08 pm   #5
Edward Huggins
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Default Re: My first single ended stereo amplifier build

You would be surprised at the work that went into design of this. Note the windings on the OPTX, nothing "off the shelf" here. Even after the Mozart went into production, the designers were still looking at further enhancements here and there. Then the Men in Suits noted their escalating time-sheets and all further planned faffing soon ended......
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Old 2nd Feb 2023, 3:13 pm   #6
kalee20
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Default Re: My first single ended stereo amplifier build

Good work!

Quote:
Originally Posted by Gabe001 View Post
Any comments and suggestions welcome.
I have three:

The position of the UL tap! Unknown, though you can determine it by 'scoping the signal at the screen tap and comparing with the signal at the anode.

I designed a low-power UL amplifier a couple of years ago (using 1p24b Russian rod pentodes) - it became clear to me that the farther away from HT the taps are, the better. But with greater signal on the screen, the grid drive requirements increase (as you say). The limiting point comes when, to get sufficient downward swing on the anode to hit the valve's bottoming 'knee' the driving signal has to peak at 0V - you're then in danger of grid current and consequent loading on the first stage.

And... open-loop performance! The overall frequency response looks praiseworthy, but what is it like open-loop? When feedback straightens the frequency response, intermediate points can see larger-than-expected signal levels. So, for instance, of your output transformer is 3db down at 20Hz, and your actual response relative to 1kHz is 2db up at 20Hz (as it looks from your sweep), the EL34 grid must be seeing signal level 5db up at 20Hz... that's 1.8x normal. It's easy to unwittingly overload intermediate stages without realising it, if you expect full power over the whole frequency range.

And the last? On/off switch at the back, when it could so easily have been on the front! But maybe that's just me! (It's a smart-looking piece of equipment!)
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Old 2nd Feb 2023, 4:27 pm   #7
Gabe001
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Default Re: My first single ended stereo amplifier build

Thanks kalee

I'll put a scope on as you suggested. Is the SG tap location directly proportional to the % signal loss c/w signal at plate?
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Old 2nd Feb 2023, 4:41 pm   #8
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Default Re: My first single ended stereo amplifier build

Voltages at tapping points are (if the winding isn't oddly loaded) indeed just proportional to the number of turns involved. You could probably check the tapping point just by feeding a few hundred hertz signal through and measuring the AC voltages with a DMM.

If you're concerned about winding loading effects you could switch the amp off and put a few AC volts into the output transformer's secondary. People have used the 50Hz output from a low voltage mains transformer for this purpose.

Cheers,

GJ
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Old 3rd Feb 2023, 1:18 pm   #9
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Default Re: My first single ended stereo amplifier build

Would be interesting to see the schematic.

I would also have the on/off on the front, partly safety, partly ease of use. I think the layout makes it hard to keep the low signal input supply to the driver tubes out of the way of interference from the power supply or output tubes, so that was a result to solve that potential issue.
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Old 3rd Feb 2023, 5:42 pm   #10
Gabe001
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Default Re: My first single ended stereo amplifier build

Hi Richard

The circuit is shown in the last picture in post 2. It's very minimalistic. It's a bit hard to read component values but happy to answer questions. What is doesn't show is the arrangement to elevate the heaters but that's just a voltage divider. EL34 anodes are at 340v, just to give you an idea of voltages.

I personally like the signal input at the front right next to the volume control, to keep the signal wires short, but she wanted all wires out of sight. Not too fussed about on/off switch but putting this at the front means that the mains primary crosses the width of the amp twice.

The general layout could be improved, I agree. If I build this for myself again I'd have the valves in a 'square' configuration with the el34s behind, next to the OPTs, the 6sn7s/6sl7s in front, and the choke next to the mains transformer. I wasn't aware of the chokes magnetic coupling potential as I had never used one before -or rather I was but underestimated the effect.
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Old 4th Feb 2023, 11:13 am   #11
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Default Re: My first single ended stereo amplifier build

Hi Gabriel, I have been studying lots of configurations of valve amps, old and new, since I like the idea that in design the perfect solution becomes art. Hence the pile of bricks in the Tate Gallery.

But then amplifiers like Quad IIs have their own solutions for valve spacing and mains wiring, so probably some of this stuff is not so critical. My old radios use lots of metal braided wires, soldered to the chassis, which I suppose was belt and braces at that time.

Ultimately it boils down to some basics - wire routing, twisting some pairs, placement of the culprits.

It is a nice looking amp, and great that you solved the hum problem. However by moving from CLCRC to CRCLC you have eliminated some of the benefit of the choke since there is going to be less current flowing through it (assuming B+ goes from after CRC now). If this was me I would be tempted to get another bit of ally plate, and rejig it.

Cheers, Richard
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Old 4th Feb 2023, 12:29 pm   #12
Gabe001
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Default Re: My first single ended stereo amplifier build

Have you got some pictures or a website, so that I can pick up some layout tips?

All HT still goes through choke including the el34 supply. Just the voltage swing in the choke is much less due to preceding CRC. This means the choke is slightly less effective but in reality HT is so well smoothed it doesn't really matter. No re-jig necessary thank goodness given that it was all done with a bog standard hand drill

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Old 4th Feb 2023, 1:05 pm   #13
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Default Re: My first single ended stereo amplifier build

If you're concerned about noise pickup/radiation associated with running wiring to and from the front panel then an effective solution can be to put the actual switch close to the sockets, at the rear, and then run a rod for mechanical operation of the switch to the front panel. This is particularly easy for rotary switches and the same trick can obviously be used for potentiometers (volume and tone controls) too. It keeps the signal wiring short and localised.

Cheers,

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Old 4th Feb 2023, 9:46 pm   #14
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Default Re: My first single ended stereo amplifier build

Hello Gabriel,

That's a cracking build.

The PYE Super Black Box (SBB) has a 33% UL tap and a pair of ‘period’ Gilson SE EL34 transformers I have in the workshop, dating back to the late 50’s early 60’s, have a 40% UL tap.

As pointed out by kalee20, finding the UL ratio is a case of placing a known voltage across the HT and Anode winding and measuring the voltage on the UL tap to ascertain the ratio – this is the method I used to find the UL tap on the two SE o/p t/x’s.

From memory the SBB ran the EL34 with roughly 280V on the anode, 2700Ω anode load and a 150Ω cathode resistor, and this didn’t fall that far short of 10 Watts output – I’ll have to measure it again.

I like the use of the SRPP driver.

Personally, the SRPP is part of the same family as the Bootstrap amplifier and follower, and Jeffery High-Gain Phase splitter. Recently I built a 10-watt push-pull EL84 amplifier using the Jeffery High-Gain Phase splitter (this is the all AC coupled variant of the SRPP), which worked fine, however there were a few problems caused by stray capacitance in the anode circuit of the bottom bootstrapped valve. I suppose this due to action of the bootstrapping increasing the impedance and thus the gain – but as an old Physics teacher said “you don’t get owt for nowt”..

Jeffery, himself, described an EL84 amplifier implementing the ‘Jeffery’ driver using a 6BR8, however I used E180F and a triode connected EF184.

I must admit to having thoughts of using the Jeffery High-Gain Phase splitter (after Bailey and Short) with a singled ended output to drive a SE connected o/p valve. Maybe give the 5B/254M SE a go?

Just thinking out the box here (sorry for the David Brent speak!) regards the hum… Was it the same on both channels? As you said it’s OK now, and best leave it that way, however…, just a thought, could there be some ‘power supply filter noise’ finding its way into the driver ground(s), which was eased by swapping the choke and resistor? However, looking at the grounding, as best I could, all reservoir and smoothing capacitors all seem well anchored to a sound grounding point in the middle of the chassis… I suppose this kind of power supply noise would have been more of a buzz than a hum though.

Terry.
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Old 4th Feb 2023, 11:47 pm   #15
Gabe001
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Default Re: My first single ended stereo amplifier build

Hi terry thanks for the feedback. Somone mentioned the super black box recently too, but i cannot remember the context. The hum/buzz was in the channel closest to the choke only. I cannot remember whether the predominant frequency was 50hz or 100hz.

I've got the el34s running conservatively at about 22w anode dissipation My experiments with the cathode resistor size seem to indicate that it performs better in THD terms if it runs hotter.
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Old 5th Feb 2023, 12:36 am   #16
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Default Re: My first single ended stereo amplifier build

Terry, I wonder if you've read this article about the different phase splitter types, if you're keen on the pentode /triode combo.

https://www.pearl-hifi.com/06_Lit_Ar...e_Splitter.pdf
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Old 5th Feb 2023, 12:34 pm   #17
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Default Re: My first single ended stereo amplifier build

Hello,

Ah, it was just one channel, that explains the choke.

It was probably me that mentioned the Super Black Box as I’d recently taken a pair of SBB amplifiers and fettled them to work as a pair of mono amplifiers for line level inputs.

Yes, I’ve seen the Bailey Pentode/Triode phase splitter which is used in the later Radford and Richardson amplifiers. I have Radford STA15 and a Richardson SA170, both of which use that phase splitter. I suppose the Bailey phase splitter was the final word before it all went ‘transistor’ as regards valve amplifiers. Anyway, thanks for the link.

I suppose the reason I went with the Jeffery phase splitter was purely down to finding an original Wireless World High-Quality Audio Amplifiers reprint book (attached) with the original Jeffery article at a BVWS Harpenden Swapmeet what must be 15-20 years ago. I looked at the Jeffery circuit in this book and thought I’ll have a go at that circuit one day!

The Radiospares mains, output transformers, and chassis for the ‘Jeffery’ build also came from BVWS Swapmeets over the years.

Oh-well, better get into the workshop and put a few more hours into the current guitar amp build – today, it’s wiring input jack sockets and preparing the tag panel with leads to connect to valve bases, controls and suchlike etc., etc.

Terry.
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