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Old 16th Nov 2019, 2:44 pm   #1
Levente
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Default Building a 5 valve A.C. amplifier from scratch

Hey like minded people.

Decided to jump on a project, to build a simple A.C. high fidelity amplifier using the 5 tube schematic from the Practical Amplifier Diagram. Why? Because I want to know more of these, I want to learn more on this and I would like to have a great amplifier.. also, I have many parts lying around. Instead of bin it, would like to use it.

I had a slightly similar amp purchased last year you guys ( Lawrence, Herald, Bill ) helped me a lot with that.

Just to understand this schematic correctly, could you please help me to understand the potentiometer and the diagram attached.

1. R10 variable resistor. The wiper and the pin 3 is connected through the capacitor C6

2. R12 wiper and pin3 are shorted.

I hope this is correct.

If you have any further advise what to be aware of or any good hints, please let me know

Thanks dudes !!!
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Old 16th Nov 2019, 10:37 pm   #2
joebog1
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Default Re: Building a 5 valve A.C. amplifier from scratch

They are both "top cut" tone controls. They both operate by cutting the high frequencies from the signal. They are minimum cost type designs used mainly in radios, although the cheaper record players had them too.

Joe
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Old 17th Nov 2019, 11:24 am   #3
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Default Re: Building a 5 valve A.C. amplifier from scratch

The left hand one could be a treble boost, depending on the rest of the circuit it's in.

Shorting the wiper to one end when a potentiometer is used as a variable resistor can be useful in applications where an open circuit caused by poor wiper contact might be expensive such as output stage bias adjustment. Here it's just good practice.
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Old 19th Nov 2019, 8:59 pm   #4
Levente
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Default Re: Building a 5 valve A.C. amplifier from scratch

Thank you for these suggestions guys ! this is the full schematics attached.
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Old 21st Nov 2019, 2:46 pm   #5
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Default Re: Building a 5 valve A.C. amplifier from scratch

You say you have various parts available but we don't know what they are!

That is an American design which is intended as a Public Address, rather than Hi-Fi amplifier - hence the simple tone control. I assume that any valves you have are more likely to be European, rather than US types although, of course, most types have equivalents in the other range.

However, assuming the parts you have are suitable, I wonder if you would be better off building a true High Fidelity amplifier such as the Mullard 5 - 10? (Mullard being the UK subsidiary of Philips.)

I built several of these excellent amplifiers in the past and they perform very well indeed.

There is a full reprint of the original design as published here:

http://www.r-type.org/articles/art-003e.htm

I think you'll also find the circuit is drawn in a much clearer way which should make it much easier to follow.

There is a full description and a number of options for you to chose from. The version with controls offers treble and bass boost and cut adjustments - much more comprehensive that the US design.

Full constructional details are given and the tag board mounting makes for a very simple and clean layout.

Take careful note of the earth bus bar. Whatever you decide to build, you should use the same earthing arrangement. If you don't, you will live to regret it - probably as soon as you switch it on for the first time!
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Old 24th Nov 2019, 7:44 pm   #6
Alan's MagicEye
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Default Re: Building a 5 valve A.C. amplifier from scratch

Hello Levente

I can understand that if you have the parts already then your proposal may be attractive, but it may not be that high fidelity although you may be content with this.

I have built several single ended triode amplifiers based on the "Decware Zen". Technically they have some shortcomings but they do sound very good in my opinion and sound better than the expensive Quad ll's which I used on my Tannoys originally, although they are not very powerful and need speakers with a reasonable sensitivity, probably in the order of 93dB/W. On the upside the output transformers are relatively affordable due to the low power. The SV83 output valves can be substituted for 6P15P Russian ones, which are fairly equivalent and can be brought very cheaply on eBay etc. The Russian 6N1P valves used on the input are also easily available.

The schematic is very simple and easy to follow as the philosophy behind the design is to have a few components as possible to achieve a decent result.

Here is a link to the webpage giving the user manual, this also has the schematic:

https://www.decware.com/zsowner.pdf

The designer also did a whole series of YouTube videos detailing very concisely the steps involved in building one:

https://youtu.be/n1ENOdYN6iA

If you built one and liked the result, the amplifier can be used bridged to give 6W so building a second monoblock would give you a 6+6W stereo.

This sounds like very low power but with sensitive speakers they are more than loud enough in my average size room.

I have no connection to Decware other than having used the design to make my own versions which I have used happily for at least 5 years.

I hope this helps.

Alan

Last edited by Alan's MagicEye; 24th Nov 2019 at 7:46 pm. Reason: Typo
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Old 25th Nov 2019, 9:11 am   #7
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Default Re: Building a 5 valve A.C. amplifier from scratch

I forgot to say that the Decware amplifier is also available as a kit, or was when I checked last.

Here is a link:
https://www.decware.com/newsite/SE84CDIY.htm

The details in the above can also be useful when building the point to point version as it gives details of suitable transformers etc.

There is also a forum for self build Decware kit:
https://www.decware.com/cgi-bin/yabb...board=SE84CDIY


I hope that I have not butted in with an off track suggestion here, please forgive me if I have, I'm aware that everything comes down to personal preference.

I have detailed this build because I am very pleased with the result and lots of visitors who have heard the results are very impressed. The literature produced by Decware does somewhat blow their own trumpet, but that aside I do find them very good.

Alan
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Old 26th Nov 2019, 10:55 pm   #8
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Default Re: Building a 5 valve A.C. amplifier from scratch

There's more than a hint of audiophoolery about that Decware beast. As a single ended low power class A triode (connected pentode) output amp it's no doubt fair enough but I would expect a Mullard 3-3 would work as well or better without all the gold plated waffle.

All of the stuff about speaker placement and room dynamics is of course equally applicable to any amplifier/speaker combination not just the wondrous Zen.

There's no mention of output power or distortion but Philips reckon on about 2W at 9% for a triode connected EL84! This of course compares very favourably with 3W at 1% for the 3-3
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Old 26th Nov 2019, 10:59 pm   #9
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Default Re: Building a 5 valve A.C. amplifier from scratch

I agree with Herald

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Old 27th Nov 2019, 12:07 am   #10
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Default Re: Building a 5 valve A.C. amplifier from scratch

Yes, I agree about the gold plated claims of the Decware manufactures and the fact that technically it's not the best design. And I did say that Decware blows its own trumpet in my opinion 🙂

I was just commenting that I tried the design and I have put my pair of Quad ll's in the loft, no mean feat I'd say, I know that is all down to personal preference, although 40 plus years of tinkering professionally and as a hobby may count for something?

I have had favourable comments from a number of Dyed in the Wool Hi-Fi buffs, audiofoolery? Possibly but the consensus has been positive.

The Mullard is indeed a fine design and well proven.

My aim in commenting was to help a fellow enthusiast who is starting out and the fact that there was a bit of help from a series of videos on YouTube about the construction was one of the attractions although I suppose they could be useful in general anyway without actually going for the Zen.

With all good intentions.....

Alan
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Old 27th Nov 2019, 9:10 am   #11
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Default Re: Building a 5 valve A.C. amplifier from scratch

Please forgive the further post and at the risk of harping on about it, the power output and spec. for the Zen are presented here as an extratact from a review

"The Zen is a small, simple, stereo, vacuum tube power amplifier. It is a single-ended design with output tubes wired in triode mode. RMS measurements of the Zen indicate an output of 2.4 W/channel at 8 ohms impedance.
The owner's manual states that the Zen will reliably drive speakers rated at only 2 ohms. Additionally, the two channels of this amp may be wired in series with both channel inputs linked (instructions are included with the owner's manual) that will allow it to be used as a monobloc amplifier with 8W to 9W output. The frequency range of the amp is given as 30 to 20,000 Hz at 1.5dB or 25 to 25,000 Hz at 3dB. The Zen Triode measures 15 x 25 x 16 cm, and weighs about 4.5 kg (10 x 6.25 x 6 inches, 10 lbs).

The Zen uses four tubes: a single Svetlana, 6N1P, dual-triode tube in the driver circuit, one Philips 5Y3GT rectifier tube, and one, Svetlana SV83 output tube for each channel. Unlike some single-ended designs at the lower end of the price spectrum, the Zen uses no negative feedback.
Retubing this amplifier will cost as little as $30.00 USD. It should be noted that the driver and rectifier tubes will last for years in normal use; only the power tubes will require regular replacement. Both power tubes in the Zen can be replaced for about $10 USD, no biasing or matching required. Cheap tubes are a good thing with this amplifier as Steve Deckert, designer and builder of the Zen and other products, has biased the output tubes on the high side to improve sound at the expense of tube life.
He recommends replacing the output tubes once per year under normal usage, or every three months, if used continuously."

Full review here at the risk of being accused of more audio silliness ;-)

https://www.tnt-audio.com/ampli/zentriode_e.html

The cheap valves were one attraction for me and possibly for anyone else considering a new valve amplifier build, for example the cost of new KT66's for the Quad amplifiers is in some cases ridiculous.

Anyway, I will be quiet now and let others comment.

Alan
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Old 27th Nov 2019, 11:59 am   #12
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Default Re: Building a 5 valve A.C. amplifier from scratch

An amplifier with output valves that have a claimed service life of as little as four months is ludicrous!

I am in full agreement with others that the Mullard 3-3 is a much more attractive proposal and full constructional details are, of course, included in the design.
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Old 27th Nov 2019, 7:15 pm   #13
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Default Re: Building a 5 valve A.C. amplifier from scratch

Given the continued availability of valves like the ECC83 and the 6BW6 if I was building a 'simple stereo amp' Id use a single dual-triode and a couple of 6BW6 (or 6V6) - and use semiconductor rectification because of its lower forward-resistance (meaning less voltage-sag under load).

A 6BW6 or 6V6 with 315V on the anode will give you 5.5 Watts of audio output, and still sounds good.
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Old 27th Nov 2019, 10:55 pm   #14
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Default Re: Building a 5 valve A.C. amplifier from scratch

One could also use a 7C5 which are quite cheap, and are a fully fledged 6V6. My own stereo at present is 6BW6's in push/pull. It makes about 7 watts RMS per channel.
A slightly smaller "6V6" is the 6AQ5. Also quite cheap.

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Old 28th Nov 2019, 9:26 am   #15
Alan's MagicEye
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Default Re: Building a 5 valve A.C. amplifier from scratch

Quote:
Originally Posted by terrykc View Post
An amplifier with output valves that have a claimed service life of as little as four months is ludicrous!

I am in full agreement with others that the Mullard 3-3 is a much more attractive proposal and full constructional details are, of course, included in the design.
Of course I agree that the Millard design is extremely good and the consensus agrees with this fact

At the risk of annoying everyone though, what Mr Deckert says in his marketing blurb really has nothing to do with what I was trying to say....

I have been using the little Decware designed amplifiers for about 5 years now on an almost daily basis and from memory I have only replaced the output valves twice and they probably didn't need swapping then really, so the comment about replacing ever few months is completely wrong.

My point was, the design is very simple, the bits are reasonably priced and I have been using the resulting amplifiers in a reasonable hifi seup for about 5 years without issue, nothing else.

No offence is intended to anyone who has commented here, I just wanted to make my point clear here as I seem to have irritated some of you with my posts.

With best intentions .....

Alan

Last edited by Alan's MagicEye; 28th Nov 2019 at 9:31 am. Reason: Typo
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Old 29th Nov 2019, 12:25 am   #16
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Default Re: Building a 5 valve A.C. amplifier from scratch

And to be fair, he makes no bones about the fact that the output valves are deliberately overrun to get a bit more oomph.



Now if they were triode connected 807s........
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Old 29th Nov 2019, 7:42 am   #17
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Default Re: Building a 5 valve A.C. amplifier from scratch

I'm afraid I start chuckling when I see the phrase "No feedback"

There is feedback all over the place, intrinsic to many devices. Cathode or emitter degeneration is feedback as well. In triodes, the field from the anode influences the net field in the grid area.... more feedback! and it's where the triode characteristic comes from.

What they mean is no visible feedback.

Holy cows machine-gunned while you wait

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Old 29th Nov 2019, 10:08 am   #18
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Default Re: Building a 5 valve A.C. amplifier from scratch

The Decware circuit shows a common cathode bias resistor for the two output tubes and is not bypassed. Does that result in an out of phase signal on the opposite channel?
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Old 29th Nov 2019, 10:50 am   #19
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Default Re: Building a 5 valve A.C. amplifier from scratch

At the risk of being shot down in flames again��, I personally like the "distortion introduced by single ended triode amplifiers, I am a now an amateur, once professional, (a long time ago) musician. I believe that this is to do with the way that different types of amplifier introduce harmonic distortion?

Here is an excerpt from some research, valid or not, comments welcome...

"It is seen that a SE Triode amp only produces three significant harmonics. Based on listening tests of the fundamental tone plus a single harmonic, the audibility thresholds for the 2nd, 3rd, 4th, 5th, and 7th harmonics are approximately -20 dB, -20 dB, -40 dB, -35 dB, and -50 dB respectively. In general, the higher order harmonics are audible at much lower levels. For the SE Triode amp the 3rd and 4th harmonics are not audible over the range of this figure, and the 2nd harmonic is only audible at a value of alpha of 0.8 or more.

There are no even harmonics for the other two amps.The solid state amp doesn't produce any distortion until the amp clips, but then the levels of the audible distortion products rise very quickly, and the 3rd, 5th, and 7th harmonics are audible for the higher values of alpha. For the push-pull pentode only the 3rd harmonic is audible."

Full article here:
http://www.silcom.com/~aludwig/EARS.htm
And
http://www.silcom.com/~aludwig/Amplifier_distortion.htm

This may also be of interest regarding negative feedback:

http://www.normankoren.com/Audio/FeedbackFidelity.html


I have lit the blue touch paper, now I stand back and admire the firework display

I hope that you find this informative and or amusing.

Alan

Last edited by Alan's MagicEye; 29th Nov 2019 at 11:05 am. Reason: Typo
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Old 29th Nov 2019, 11:02 am   #20
Alan's MagicEye
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Default Re: Building a 5 valve A.C. amplifier from scratch

Quote:
Originally Posted by PJL View Post
The Decware circuit shows a common cathode bias resistor for the two output tubes and is not bypassed. Does that result in an out of phase signal on the opposite channel?
Your post came whilst I was typing my reply, A good question, my knowledge is not sufficient to answer this, what do those with more experience think?

Alan
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